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Home My WebLink AboutDRC-2024-005132DRC-2024-005132 HYDRO GEO CHEM, INC. Environmental Science & Technology CORRECTIVE ACTION COMPREHENSIVE MONITORING EVALUATION (CACME) REPORT WHITE MESA URANIUM MILL NEAR BLANDING, UTAH March 28, 2024 Prepared for: ENERGY FUELS RESOURCES (USA) INC 225 Union Blvd., Suite 600 Lakewood, Colorado 80228 Prepared by: HYDRO GEO CHEM, INC. 51 West Wetmore Road, Suite 101 Tucson, Arizona 85705 (520) 293-1500 Project Number 7180000.00-01.0 Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 i TABLE OF CONTENTS 1. INTRODUCTION .............................................................................................................. 1 2. BACKGROUND AND HISTORICAL OVERVIEW ........................................................ 5 2.1 Perched Groundwater Flow and Chloroform Plume Sources ................................. 6 2.2 Plume Definition, Long Term Pumping Test, and Permeability Distribution ........ 7 2.3 Preliminary CAP and CIR ...................................................................................... 9 2.4 Hydraulic Testing of TW4-4, TW4-6, and TW4-26 and Addition of TW4-4 to the Pumping System ............................................................................... 9 2.5 Impact of Reduced Wildlife Pond Recharge and Nitrate Pumping ...................... 10 2.6 Eastern Nitrate CIR and Delineation of Plume to Southeast ................................ 11 2.7 Addition of TW4-1, TW4-2, TW4-11, TW4-21 and TW4-37 to the Pumping System ................................................................................................................... 13 2.8 Final GCAP ........................................................................................................... 14 2.9 Addition of Compliance Well TW4-38 and Pumping Well TW4-39 ................... 14 2.10 Addition of Compliance Well TW4-42 and Pumping Wells TW4-40 and TW4-41 ............................................................................................ 14 2.11 Abandonment of Pumping Well TW4-20 and Addition of Compliance Well TW4-43 ........................................................................................................ 15 3. SUMMARY OF CHLOROFORM MONITORING AND PUMPING SINCE DECEMBER 31, 2012 ...................................................................................................... 17 3.1 Elements of the Quarterly Chloroform Monitoring Reports and Compliance with Preliminary and Final GCAPs .............................................................................. 17 3.2 Specific Actions Taken Under the Preliminary and Final GCAPs ....................... 20 3.3 Summary of Key Findings and Interpretation of Results Since December 31, 2012 ..................................................................................... 21 3.3.1 Perched Groundwater Flow ...................................................................... 22 3.3.2 Purpose of Chloroform Pumping and Hydraulic Capture......................... 23 3.3.3 Impacts of Pumping Fluctuations and Typical Analytical Error .............. 25 3.3.4 Impacts of Reduced Wildlife Pond Recharge and Nitrate Pumping......... 26 3.3.5 Plume Boundary and Southeast Portion of Plume .................................... 28 3.3.6 Pumping Well Productivity and Plume Control ....................................... 31 4. EVALUATION OF PUMPING SYSTEM EFFECTIVENESS ....................................... 33 4.1 Trends in Plume Area, Mass Removal Rates, Concentrations, Hydraulic Gradients, Saturated Thicknesses, and Residual Mass ......................................... 34 4.2 Capture Effectiveness ........................................................................................... 42 4.3 Pumping Well Productivity................................................................................... 44 4.3.1 Comparison of Pumping and Flow through the Chloroform Plume Over Time and Re-calculation of ‘Background’ Flow in 2015 ......................... 47 4.3.2 Re-calculation of ‘Background’ Flow Based on Changes Since 2015 ..... 50 4.3.3 Evaluation of Interference between Pumping Wells ................................ 52 Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 ii TABLE OF CONTENTS (Continued) 4.3.4 Evaluation of TW4-20 Abandonment ....................................................... 53 4.3.5 Reduced Productivity of Many Chloroform Pumping Wells ................... 53 4.4 Natural Attenuation ............................................................................................... 54 5. EFFECTIVENESS OF GCAP IN PROTECTING PUBLIC HEALTH AND THE ENVIRONMENT ............................................................................................................. 57 6. CONCLUSIONS AND RECOMMENDATIONS ........................................................... 61 7. REFERENCES ................................................................................................................. 67 8. LIMITATIONS ................................................................................................................. 73 TABLES 1 Chloroform Plume Area, Mass Removed/Quarter, Residual Mass, and Average Concentration, First Quarter, 2012 through Fourth Quarter, 2023 2 Hydraulic Gradients within Chloroform Plume, 4th Quarters of 2012, 2015, 2017, 2021 and 2023 3 Plume Areas and Masses Under Capture, Fourth Quarters of 2012 Through 2023 4 Changes in Saturated Thickness Within Northern and Southern Portions of the Chloroform Plume 5 Changes in Hydraulic Gradients Within Northern and Southern Portions of the Chloroform Plume FIGURES 1A White Mesa Plan Showing Locations of Perched Wells and Piezometers 1B White Mesa Site Plan Showing 4th Quarter, 2023 Perched Water Levels and Chloroform and Nitrate Plumes 2 Kriged 4th Quarter, 20231 Chloroform Concentrations, Perched Water Levels, and Chloroform Source Areas, White Mesa Site 3A Kriged 4th Quarter, 2023 Chloroform Concentrations, Perched Water Levels, and Saturated Thicknesses, White Mesa Site 3B Kriged 4th Quarter, 2023 Chloroform Mass Distribution and Perched Water Levels, White Mesa Site 4A Chloroform Concentrations at TW4-6 4B Chloroform Concentrations in TW4-6 and TW4-26 Since the 1st Quarter 2012 Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 iii TABLE OF CONTENTS (Continued) FIGURES (Continued) 5 Comparison of Kriged 4th Quarter, 2014 and 1st Quarter, 2012 Chloroform Plumes, White Mesa Site 6 Comparison of Kriged 4th Quarter, 2023 and 4th Quarter, 2014 Chloroform Plumes, White Mesa Site 7A Chloroform Plume Area and Quarterly Chloroform Mass Removed Since 1st Quarter 2012 7B Chloroform Plume Area and Quarterly Chloroform Mass Removed Since 3rd Quarter 2015 8A Chloroform Plume Area and Average Plume Chloroform Concentration Since 1st Quarter 2012 8B Chloroform Plume Area and Average Plume Chloroform Concentration Since 3rd Quarter 2015 8C Chloroform Plume Mass Removed/Quarter and Average Plume Concentrations Since 3rd Quarter 2015 9 Chloroform in Non-Pumping Wells within Plume (Includes TW4-6*, TW4-8*, TW4-9*, TW4-30* and TW4-33*, no Longer Within Plume) 10 Chloroform in Pumping Wells within Plume (Including Nitrate Pumping Wells TW4-22* and TW4-24*) 11 Change in Perched Water Elevation within Chloroform Plume, 4th Quarter, 2012 to 4th Quarter, 2023, White Mesa Site 12A Water Levels in Wells Marginal to Plume (Includes Subset of GCAP Compliance Monitoring Wells Closest to Plume) 12B Saturated Thickness in Wells Marginal to Plume (Includes Subset of GCAP Compliance Monitoring Wells Closest to Plume) 13A Water Levels in Non-Pumping Wells within Plume (Includes TW4-6*, TW4-8*, TW4- 9*, TW4-30* and TW4-33*, no Longer Within Plume) 13B Saturated Thickness in Non-Pumping Wells within Plume (Includes TW4-6*, TW4-8*, TW4-9*, TW4-30* and TW4-33*, no Longer Within Plume) 14A Water Levels in Pumping Wells within Plume (Including Nitrate Pumping Well TW4-22) 14B Saturated Thickness in Pumping Wells within Plume (Including Nitrate Pumping Well TW4-22) 15 Percentage Change in Saturated Thickness within Chloroform Plume 4th Quarter, 2012 to 4th Quarter, 2023, White Mesa Site 16A Chloroform Plume Area and Residual Mass Since 1st Quarter 2012 16B Chloroform Plume Area and Residual Mass Since 3rd Quarter 2015 17A Chloroform Plume Residual Mass and Quarterly Mass Removed Since 1st Quarter 2012 17B Chloroform Plume Residual Mass and Quarterly Mass Removed Since 3rd Quarter 2015 18 MW-26, TW4-19, -20, -22, and -37 Chloroform Concentration and Residual Plume Mass, 2012 to 2023 19 Chloroform Plume Residual Mass Estimates, 2006 to 2023 Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 iv TABLE OF CONTENTS (Continued) FIGURES (Continued) 20 Kriged 4th Quarter, 2023 Water Level Contours, Chloroform Plume Boundary, and Estimated Total Capture, White Mesa Site 21 Kriged 4th Quarter, 2022 Water Level Contours, Chloroform Plume Boundary, and Estimated Total Capture, White Mesa Site 22 Kriged 4th Quarter, 2021 Water Level Contours, Chloroform Plume Boundary, and Estimated Total Capture, White Mesa Site 23 Kriged 4th Quarter, 2020 Water Level Contours, Chloroform Plume Boundary, and Estimated Total Capture, White Mesa Site 24 Kriged 4th Quarter, 2019 Water Level Contours, Chloroform Plume Boundary, and Estimated Total Capture, White Mesa Site 25 Kriged 4th Quarter, 2018 Water Level Contours, Chloroform Plume Boundary, and Estimated Total Capture, White Mesa Site 26 Kriged 4th Quarter, 2017 Water Level Contours, Chloroform Plume Boundary, and Estimated Total Capture, White Mesa Site 27 Kriged 4th Quarter, 2016 Water Level Contours, Chloroform Plume Boundary, and Estimated Total Capture, White Mesa Site 28 Kriged 4th Quarter, 2015 Water Level Contours, Chloroform Plume Boundary, and Estimated Total Capture, White Mesa Site 29 Kriged 4th Quarter, 2014 Water Level Contours, Chloroform Plume Boundary, and Estimated Total Capture, White Mesa Site 30 Kriged 4th Quarter, 2013 Water Level Contours, Chloroform Plume Boundary, and Estimated Total Capture, White Mesa Site 31 Kriged 4th Quarter, 2012 Water Level Contours, Chloroform Plume Boundary, and Estimated Total Capture, White Mesa Site 32 Chloroform Pumping Well Productivity 33 Nitrate Pumping Well Productivity 34A TW4-19 Chloroform Mass Removal and Total Chloroform Mass Removal Since 1st Quarter 2012 34B TW4-19 Chloroform Mass Removal and Total Chloroform Mass Removal Since 3rd Quarter 2015 35A Chloroform Pumping Wells Having Declining Productivity Since 3rd Quarter 2015. 35B Total Pumping and Average Saturated Thickness for Wells Having Declining Productivity Since 3rd Quarter 2015. APPENDICES A Historic Chloroform Plume Comparison Maps (Figures A.1-A.6) B Well Productivity and Background Flow Analysis Tables and Figures (Tables B.1-B.4; Figures B.1-B.13) C Natural Chloroform Degradation Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 1 1. INTRODUCTION This is the fifth Corrective Action Comprehensive Monitoring Evaluation (CACME) report for chloroform in perched groundwater at the White Mesa Uranium Mill (the Mill or the site) located near Blanding, Utah. The report is prepared as required by Part III.H of the Groundwater Corrective Action Plan (GCAP) found in Attachment 1, of the final Stipulation and Consent Order (“SCO”) Docket No. UGW20-01, approved on September 14, 2015 by the Utah Department of Environmental Quality Division of Waste Management and Radiation Control (DWMRC) [Utah Department of Environmental Quality Division of Solid Waste and Radiation Control, 2015]. As required under the GCAP, the CACME will: 1. Summarize and interpret the results of all past quarterly groundwater monitoring performed after December 31, 2012 in accordance with Parts IILA through E of the GCAP; 2. Review chloroform mass removal rates resulting from pumping to evaluate the performance of the Pumping Wells. In the event that the mass removal rates have dropped substantially, such evaluation shall include a determination whether the removal rates have dropped as a result of reduced concentrations within the plume, lost well productivities or a general reduction in saturated thickness; 3. Demonstrate how and why the GCAP continues to be protective of public health and the environment; and 4. Bear the seal of a Professional Engineer or Professional Geologist, pursuant to UAC R3l7- 6-6.15.D.3. This report meets the above requirements of the GCAP and focuses on quarterly data collected between December 31, 2012 and the fourth quarter of 2023. The first CACME (HGC 2016a) focused on quarterly data collected between December 31, 2012 and the fourth quarter of 2015; the second CACME (HGC, 2018a) focused on quarterly data collected between December 31, 2012 and the fourth quarter of 2017; the third CACME (HGC, 2020) focused on quarterly data collected between December 31, 2012 and the fourth quarter of 2019; and the fourth CACME (HGC, 2022a) focused on quarterly data collected between December 31, 2012 and the fourth quarter of 2021. However, in past and present CACME reports, monitoring data collected prior to December 31, 2012 are discussed as needed for interpretive context. In particular, data collected both before and after December 31, 2012 are important because significant changes to the chloroform plume are attributable to actions taken by the Mill in the first quarters of 2012 and 2013 in response to regulatory requirements. Specifically, changes in Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 2 groundwater flow dynamics, chloroform concentrations, and plume boundaries have resulted from two actions: 1) cessation of water delivery to the northern wildlife ponds (Figures 1A and 1B) in the first quarter of 2012; and 2) the initiation of nitrate pumping in the first quarter of 2013. These two actions were expected to result in at least temporary expansion of plume boundaries, increases in chloroform concentrations within and marginal to the plume, and corresponding increases in the calculated residual chloroform masses within the plume. Reduced hydraulic gradients and reduced dilution attributable to reduced wildlife pond recharge did indeed cause the expected lateral expansion of the plume and increases in concentrations in wells within the plume. Some wells at the margins of the plume, such as TW4-16 (Figure 1B), were re-incorporated into the plume. In addition, initiation of nitrate pumping in the first quarter of 2013 changed hydraulic gradients, causing re-distribution of chloroform, increases in chloroform concentrations at wells such as nitrate pumping wells TW4-22 and TW4-24, and westerly expansion of the plume. By the latter half of 2014, more westerly hydraulic gradients resulting from nitrate pumping likely contributed to re-incorporation of wells TW4-6 and TW4-16 into the plume. Since 2014, TW4-16 has been both within and outside the plume; while TW4-6 has been outside the plume since the third quarter of 2018. Although the chloroform plume boundaries expanded between 2012 and 2014, and concentrations generally increased as a result of the above two actions (cessation of water delivery to the northern wildlife ponds and commencement of nitrate pumping), these conditions were addressed by doubling the number of chloroform pumping wells (from 5 to 10) during 2015, which more than tripled chloroform mass removal rates by the fourth quarter of 2015. The pumping system was further enhanced by the addition of chloroform pumping well TW4-39 during 2016; TW4-41 during 2018; and TW4-40 during 2019. Pumping of TW4-40 and TW4-41 has substantially expanded hydraulic capture to the south. The increase in the number of pumping wells has also compensated for reduced productivity at some of the wells. Since the end of 2012, total pumping from the chloroform plume, including pumping from nitrate wells TW4-22 and TW4-24 (located within and just inside the margin of the chloroform plume, respectively) has increased from approximately 2.8 gallons per minute (gpm) to approximately 4.3 gpm. The failure of pumping well TW4-20 and abandonment during October, 2020 had little or no impact on capture or total pumping rates in the vicinity as discussed in HGC (2022a) and as will be discussed in Section 4.3.4. Increased pumping at TW4-19 subsequent to TW4-20 failure more than compensated for the loss in pumping at TW4-20. Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 3 Since the fourth quarter of 2021 (the end of the period covered by the previous CACME) the plume area has increased only slightly (by less than 1%); and the plume boundaries have been relatively stable except for cross-gradient contraction near TW4-9 TW4-30 and TW4-33; and slight cross- gradient expansion near TW4-21 and TW4-24. In addition, since 2015, both average chloroform concentrations within the plume and residual mass estimates are trending downward. Furthermore, the proportion of the plume mass under capture has remained high, ranging between approximately 79% and 99% since the fourth quarter of 2021. Finally, chloroform detected in the vicinity of TW4-26 and TW4-40 appears to be largely within the hydraulic capture zone of TW4-40. Because the area of the plume has expanded by less than 1% since 2021; the proportion of the mass of the plume under hydraulic capture has remained high; average chloroform concentrations within the plume and residual mass estimates continue to trend downward; and pumping system capture appears to include much of the chloroform detected in the vicinity of TW4-26 and TW4- 40; it can be concluded that the chloroform pumping system has performed well in light of the above two actions, has maintained its effectiveness, and continues to be protective of public health and the environment. Therefore, continued implementation of the GCAP and the current pumping system is recommended. Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 4 Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 5 2. BACKGROUND AND HISTORICAL OVERVIEW An extensive description of the site hydrogeology, which focuses on the perched groundwater zone, is provided in HGC (2018c and 2022b). Perched groundwater is the shallowest groundwater encountered beneath the site and is the primary focus of all groundwater monitoring and corrective action (chloroform and nitrate pumping) activities. Perched groundwater is hosted primarily by the Burro Canyon Formation. Where saturated thicknesses are large, perched water extends into the overlying Dakota Sandstone. The perched water is supported within the Burro Canyon Formation by the underlying Brushy Basin Member of the Morrison Formation. The Brushy Basin Member is a bentonitic shale that is considered an aquiclude. The generally low permeability of the perched zone limits well yields. Although sustainable yields of a few gpm have been achieved in site wells penetrating higher transmissivity zones near wildlife ponds (Figures 1A and 1B), yields are typically low (<1/2 gpm). Many of the perched monitoring wells purge dry and take several hours to more than a day to recover sufficiently for groundwater samples to be collected. In extreme cases, wells require several weeks to recover sufficiently for groundwater samples to be collected. During redevelopment (HGC, 2011a) many of the wells went dry during surging and bailing and required several sessions on subsequent days to remove the proper volumes of water. The chloroform plume, defined by groundwater concentrations exceeding the chloroform Groundwater Corrective Action Limit (GCAL) of 70 µg/L, occurs within the perched groundwater zone and was first discovered in 1999 at MW-4, a perched monitoring well located east and cross- gradient of the tailings management system (TMS) which encompasses cells 1 through 4B (Figures 1A and 1B). Specifically, MW-4 is located east of cell 2. The chloroform plume appears to have resulted from the operation of a temporary laboratory facility that was located at the site prior to and during the construction of the Mill, and from septic drain fields that were used for laboratory and sanitary wastes prior to construction of the Mill’s TMS. Between the time of discovery and the fourth quarter of 2023, the plume has been delineated by installation of 43 TW4-series wells and placed under remediation by pumping. The northwestern portion of the chloroform plume commingles with a nitrate plume which is also under remediation by pumping (Figure 1B). Elevated nitrate concentrations that are not considered part of the nitrate plume occur to the east of the nitrate plume within and near the chloroform plume. Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 6 Physical factors that have influenced the transport of chloroform (and the size and shape of, and concentration distribution within, the chloroform plume) include the following: 1) the nature of the source(s); 2) perched groundwater flow in the vicinity of the plume; 3) the permeability distribution; 4) natural attenuation; 5) initiation of long term pumping within the plume at wells MW-4, MW-26, and TW4-19 in 2003; 6) the addition of TW4-20 to the pumping system in 2005; 7) the addition of TW4-4 to the pumping system in 2010; 8) reduced wildlife pond recharge (since the first quarter of 2012); 9) nitrate pumping in TW4-22, TW4-24, TW4-25, and TWN-2 (since the first quarter of 2013); 10) the addition of TW4-1, TW4-2, TW4-11, TW4-21, and TW4-37 to the pumping system during the first half of 2015; and 11) the addition of TW4-39, TW4-40 and TW4-41 to the pumping system since 2015. Although pumping well TW4-20 failed and was abandoned during 2020, as will be discussed in Section 4.3.4, little or no impact to chloroform mass removal rates and capture effectiveness has occurred (HGC, 2022a). Increased pumping at TW4-19 subsequent to TW4-20 failure more than compensated for the loss in pumping at TW4-20. The following Sections provide a brief chronological history of chloroform plume characterization and remediation through the fourth quarter of 2023 and discuss the above physical factors affecting the plume. 2.1 Perched Groundwater Flow and Chloroform Plume Sources Figure 1B displays the fourth quarter 2023 kriged boundaries of the chloroform and nitrate plumes, and fourth quarter 2023 perched water level contours for the Mill. As indicated, flow is generally to the southwest. Flow beneath the millsite and TMS ranges from generally west-southwest to southwest and is influenced by perched water discharge points Westwater Seep, located west to west-southwest of the TMS, and Ruin Spring, located southwest of the TMS. The overall southwesterly flow pattern is locally influenced by past seepage from the wildlife ponds. Perched groundwater flow within the chloroform plume ranges from southwesterly in the western portion of the plume to generally south in the eastern portion of the plume. This flow pattern is strongly influenced by seepage from the two formerly used northern wildlife ponds (shown in Figures 1A and 1B), which, prior to 2012, received a relatively continuous supply of water. The groundwater mound associated with these ponds (the northern mound) has been decaying since the cessation of water delivery to the ponds in the first quarter of 2012 but is still evident in Figure 1B. Flow in the southeastern portion of the chloroform plume is also influenced by seepage from the formerly used southern wildlife pond (shown in Figures 1A and 1B). The groundwater mound Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 7 associated with the southern pond (the southern mound) has also been decaying due to reduced water delivery. Figure 2 is a detail map showing fourth quarter 2023 kriged chloroform concentrations and perched water levels, and the locations of the likely sources of the chloroform plume. The abandoned scale house leach field is the likely source of the southeastern portion of the plume and the former office leach field is the likely source of the northwestern portion of the plume (HGC, 2007a and HGC, 2007b, discussed in Section 2.3). Both of these sources received laboratory wastes prior to operation of the TMS (circa 1980), and in the case of the abandoned scale house leachfield, prior to construction of the Mill. Laboratory wastes prior to 1980 were first disposed to the abandoned scale house leach field, and later to the former office leach field. Chloroform migration has been generally southwesterly in the northwestern portion of the plume and generally south to locally southeasterly in the southeastern portion of the plume in response to perched groundwater flow that was strongly influenced by the groundwater mounds associated with the formerly used wildlife ponds. The northern mound caused relatively steep hydraulic gradients that enhanced historic chloroform migration rates especially within higher permeability materials generally coincident with the plume as discussed below. As a result of past recharge from the northern ponds and the presence of the northern groundwater mound, the largest saturated thicknesses within the chloroform plume occur in the northwestern portion of the plume as shown in Figure 3A. 2.2 Plume Definition, Long Term Pumping Test, and Permeability Distribution From the fourth quarter of 1999 through the third quarter of 2002, nineteen TW4-series temporary chloroform monitoring wells (TW4-1 through TW4-19) were installed to define and characterize the plume. TW4-15 and TW4-17 were later renamed MW-26 and MW-32, respectively. Data collected from these and other MW-series wells are used in conjunction with data from the chloroform wells to define and characterize the plume. A long-term (7-month long) pumping test was initiated in 2003 to provide data regarding the permeability distribution within the vicinity of the plume (HGC, 2004). Pumping was initiated at MW-4, followed by pumping at TW4-19, then MW-26 (formerly TW4-15). As discussed in HGC (2007a), chloroform migration was facilitated by a relatively continuous higher permeability zone generally coincident with much of the plume that appeared to ‘pinch out’ in the vicinity of TW4- 6. Hydraulic test data collected during this test and from subsequent tests were consistent with this zone ‘pinching out’ between TW4-4 and TW4-6. Hydraulic test and well productivity data also Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 8 indicate that higher permeability materials penetrated by TW4-19 ‘pinch out’ to the southwest of TW4-19 (between TW4-19 and TW4-20 [now abandoned]). The ‘pinching out’ of the higher permeability materials helps to reduce rates of chloroform migration in the downgradient portions of the plume. Pumping of MW-4, MW-26, and TW4-19 has continued to the present day in an effort to control the plume boundaries and reduce concentrations within the plume. The primary purpose of the pumping was to remove chloroform mass within the plume as rapidly as practical. This was accomplished by pumping at locations having both high chloroform concentrations and relatively high productivities (due to large saturated thicknesses and generally moderate to high permeabilities). TW4-19 and MW-26 were located in the northwest portion of the plume which had relatively high concentrations and saturated thicknesses, and therefore a relatively large proportion of the chloroform mass. At the start-up of MW-4 pumping, the portion of the plume in the vicinity of MW-4 also had relatively high concentrations and saturated thicknesses. As of the fourth quarter of 2023, the northwest portion of the plume still encompasses relatively high concentrations and saturated thicknesses, and a large proportion of the plume mass; however, the proportion of plume mass in the vicinity of MW-4 has been substantially reduced. Figures 3A and 3B show that saturated thickness and plume mass in the vicinity of MW-4 are relatively small compared to areas within the northwestern portion of the plume. Based on the results of the long term pumping test (HGC, 2004) and subsequent hydraulic tests, pumping at downgradient locations (in the vicinity of TW4-6) was considered impractical because of low permeability and small saturated thickness and consequent low well productivity. As shown in Figures 3A and 3B, saturated thicknesses as of the fourth quarter of 2023 are less than 20 feet at the southeastern extremity of the plume, and the proportion of the total chloroform mass encompassed by this portion of the plume is relatively small. Between 2005 and 2007, six additional chloroform monitoring wells (TW4-20 through TW4-25) were installed and tested to better define and characterize the plume. TW4-20 (now abandoned) was installed in 2005. Because sampling of TW4-20 indicated that it was located within an area having the highest chloroform concentrations, it was converted to a pumping well to augment mass removal in the vicinities of TW4-19 and MW-26. Mass removal in the vicinity of TW4-20 was then augmented by adding TW4-37 to the pumping system in 2015 (as discussed in Section 2.7) and by adding TW4-39 to the pumping system in 2016 (as discussed in Section 2.9). Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 9 2.3 Preliminary CAP and CIR A preliminary Corrective Action Plan (PCAP) and a preliminary Contamination Investigation Report (PCIR) were submitted in 2007 (HGC, 2007a and HGC, 2007b). These documents summarized information available to date and formed the basis for continued chloroform pumping, quarterly monitoring, and quarterly reporting. Additional chloroform monitoring wells (TW4-26 and TW4-27) were subsequently installed in consultation with DWMRC, and generally in conformance with the rationale outlined in the PCAP, to better characterize the chloroform plume. These documents discussed the relative stability of the chloroform plume boundaries at that time as reflected in Appendix A Figure A.1 which compares the first quarter 2006 and first quarter 2007 plume boundaries. Appendix A Figures A.2 through A.6 compare chloroform plume boundaries at various other times. The PCAP and PCIR also discussed natural attenuation and provided calculations of natural chloroform biodegradation rates based on parent and daughter product concentrations and a first order decay model. These calculations indicated that chloroform concentrations everywhere within the plume were expected to be reduced to less than the chloroform GCAL of 70 µg/L within less than 200 years solely by natural biodegradation, without taking into consideration any pumping. Other natural attenuation processes expected to augment the reduction in chloroform concentrations included hydrodynamic dispersion, volatilization, and dilution. Biodegradation rate calculations provided in Appendix C represent updates of calculations included in the previous CACME report (HGC, 2022). Appendix C provides the original PCAP (HGC, 2007a) chloroform biodegradation rate calculations as well as calculations updated based on chloroform and daughter product concentrations from the first quarter of 2013 through the third quarter of 2023. Appendix C is summarized in Section 4.4. The calculated degradation rate based on data from the first quarter of 2013 through the third quarter of 2023 is similar to the rate presented in the PCAP. 2.4 Hydraulic Testing of TW4-4, TW4-6, and TW4-26 and Addition of TW4-4 to the Pumping System Hydraulic testing of TW4-4, TW4-6, and TW4-26 was performed in 2010 (HGC, 2010). At that time TW4-4 and TW4-6 were within the downgradient portion of the plume and TW4-26 was downgradient of the southern boundary of the plume. Data from these tests indicated that the relatively high permeability zone associated with the chloroform plume ‘pinches out’ between TW4-4 and TW4-6. Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 10 Pumping of TW4-4 was also initiated in the first quarter of 2010 to improve mass removal rates and reduce the downgradient migration of chloroform to the extent possible in the southernmost portion of the plume (in particular near TW4-6). TW4-4 was the southernmost of the existing wells within the plume that was completed in materials sufficiently permeable to be effective as a pumping well. The impact of TW4-4 pumping on chloroform concentrations at TW4-6 is shown in Figure 4A. Chloroform concentrations at TW4-6 were reduced from 1,000 µg/L to approximately 10 µg/L within 5 quarters. As shown in Figures 4A and 4B, concentrations at TW4- 6 subsequently increased, likely due to reduced wildlife pond recharge and possibly nitrate pumping as discussed below; however since peaking in 2015, concentrations have dropped to less than 5 µg/L. 2.5 Impact of Reduced Wildlife Pond Recharge and Nitrate Pumping Significant changes to the distribution of chloroform and to chloroform concentrations within the plume resulted from the cessation of water delivery to the northern wildlife ponds in the first quarter of 2012 and the initiation of nitrate pumping at wells TW4-22, TW4-24, TW4-25, and TWN-2 during the first quarter of 2013. Between 2007 and 2011 the kriged chloroform plume boundaries were relatively stable as shown in Figures A.2 through A.6 (Appendix A). Figure A.2 compares first quarter 2007 and first quarter 2008 plume boundaries; Figure A.3 compares first quarter 2008 and first quarter 2009 plume boundaries; Figure A.4 compares first quarter 2009 and first quarter 2010 plume boundaries; and Figure A.5 compares first quarter 2010 and first quarter 2011 plume boundaries. As shown, the plume boundary was relatively stable except for expansion to the south to incorporate TW4-6, which reached a maximum concentration of 1,000 µg/L prior to initiation of TW4-4 pumping (Figures 4A and A.4) and the subsequent shrinkage back toward TW4-4 (Figure A.5). Between the first quarter of 2007 and the first quarter of 2011 there was almost no change in the plume boundary, as shown in Figure A.6. Seepage from the two northern wildlife ponds provided a significant source of dilution by chloroform-free water, which helped to limit chloroform concentrations prior to 2012. However, once water delivery ceased, and wildlife pond recharge was reduced, chloroform concentrations in many wells began to increase and some wells that had been outside the plume for many years were reincorporated into the plume (for example TW4-8 and TW4-16). TW4-8 had been within the plume for most of the time prior to the fourth quarter of 2004; and TW4-16 had been within the plume prior to the third quarter of 2005. Both wells were within the plume prior to initiation of long term pumping in 2003. Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 11 Temporary increases in chloroform concentrations, and potential expansion of the plume boundaries, were anticipated to result from reduced dilution once water delivery to the northern ponds ceased. However, reduced pond seepage has resulted in the decay of the associated northern perched groundwater mound, which reduces hydraulic gradients and chloroform migration rates within the plume. The reduction in chloroform migration rates resulting from reduced pond seepage is judged more beneficial to long-term chloroform remediation than the relatively short- term negative impact of increased concentrations. In addition, nitrate pumping at TW4-22 and TW4-24 caused chloroform to migrate to the west from the vicinity of TW4-20 (now abandoned), the well having the highest chloroform concentrations historically. Chloroform concentrations at TW4-22 increased substantially, and concentrations at TW4-24, which had historically been near the detection limit, increased sufficiently to bring it within the plume twelve times since nitrate pumping began. The increases in concentration at TW4-16 are also likely related in part to more westerly hydraulic gradients in that portion of the plume resulting from initiation of nitrate pumping. Figure 5 compares chloroform plume boundaries from the first quarter of 2012 and the fourth quarter of 2014. As discussed above, water delivery to the northern wildlife ponds ceased in the first quarter of 2012, approximately one year prior to nitrate pumping. The fourth quarter of 2014 was nearly two years after initiation of nitrate pumping and is reflective of the combined impacts of reduced recharge and nitrate pumping. As shown in Figure 5, the plume boundaries expanded over this period. The presumed extension of the plume to the southeast in the first quarter of 2012 is estimated by a hand-drawn boundary. The plume likely extended to the southeast in the first quarter 2012 but had not yet been defined by monitoring wells installed after this date. As discussed above, the expansion of the plume boundaries between early 2012 and late 2014 likely resulted from reduced wildlife pond recharge and the commencement of nitrate pumping. 2.6 Eastern Nitrate CIR and Delineation of Plume to Southeast TW4-27 was installed south of TW4-14 in the fourth quarter of 2011 to better define chloroform concentrations in the southernmost portion of the plume and to better characterize water level conditions in that portion of the site. TW4-14 had a historically low water level. Prior to the second quarter of 2015, the water level at TW4-14 was more than 10 feet below that of TW4-4 even after TW4-4 began pumping. TW4-14 appeared to be directly downgradient of TW4-4 and TW4-6, yet chloroform was not detected at TW4-14. Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 12 As the persistently low water level at TW4-14 appeared to be anomalous, water level data from TW4-27 were to be used to better understand water level behavior at TW4-14. Because the water level at TW4-27 was similar to that of TW4-14, chloroform was not detected at TW4-27 after the initial sampling, and the hydraulic conductivity measured at TW4-27 was low, both TW4-14 and TW4-27 were interpreted to be completed in low permeability materials. Water levels at both wells generally increased until reaching relative stability during 2018. The relatively slow pre-2012 increase in water level at TW4-14 compared to nearby well TW4-6 (resulting from previous wildlife pond recharge) and the absence of chloroform at TW4-14, were also consistent with low permeability. Water levels at TW4-14 and TW4-27 were eventually expected to ‘catch up’ and chloroform was eventually expected to migrate to these wells (HGC, 2014a). Detectable chloroform reached TW4-14 and TW4-27 in 2014 and 2015, respectively, although both wells have remained outside the plume; and concentrations at TW4-14 again dropped below detection during the second quarter of 2021. Concentrations at TW4-14 have remained below detection except for a detection of 1.1 ug/L during the third quarter of 2022. However, unexpectedly high nitrate concentrations were detected at TW4-27 which necessitated delineation of the extent of nitrate exceeding 10 milligrams per liter (mg/L) in this area. The areal extent of relatively high nitrate concentrations at TW4-12 was also undefined. In response, wells TW4-28 through TW4-31 were installed and tested (HGC 2013a). Based on water level and analytical data from these new wells, elevated nitrate at TW4-27 was bounded, but chloroform exceeding 70 µg/L that was not bounded to the south was detected at TW4-29. Elevated nitrate at TW4-28 was also not bounded to the southeast (downgradient). In response, wells TW4-32 through TW4-34 were installed and tested (HGC 2013b) and a nitrate Contamination Investigation Report (CIR) was prepared (HGC 2014a). Elevated nitrate southeast of TW4-28 was bounded by TW4-32. Chloroform exceeding 70 µg/L was detected at TW4-33, located between TW4-4 and TW4-29, and chloroform exceeding 70 µg/L at TW4-29 appeared to be bounded to the south and east by TW4-30 and TW4-34. During the first quarter of 2014, chloroform at TW4-8 exceeded 70 µg/L for the first time since the fourth quarter of 2004. To provide a nearby bounding well to the east of TW4-8, and a bounding well to the southeast of TW4-29, TW4-35 and TW4-36 were installed and tested (HGC 2014b). TW4-35 bounded the chloroform plume to the southeast and TW4-36 bounded the chloroform plume to the east. Because concentrations at TW4-8 dropped below and have remained below 70 µg/L since the first quarter of 2020, TW4-8 again bounds the plume to the east. Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 13 The presence of chloroform exceeding 70 µg/L at TW4-29 and TW4-33 indicated a relatively narrow extension of the chloroform plume to the southeast of TW4-4. Based on historic water level data, this chloroform had apparently migrated in a nearly cross-gradient direction around the low permeability zone penetrated by TW4-14 and TW4-27. Although chloroform at TW4-29 continues to exceed 70 µg/L, chloroform at TW4-33 dropped below 70 µg/L during the first quarter of 2021, and has remained below 70 µg/L through the fourth quarter of 2023. The water level at TW4-30, located immediately east of TW4-29, remained lower than the water level at TW4-29 until the end of 2020; however, since the end of 2020, water levels at the two wells have been nearly identical; and since the second quarter of 2021, the water level at TW4-30 has been slightly higher than at TW4-29. As of the fourth quarter of 2023, TW4-30 is located generally cross-gradient of TW4-29. TW4-30 served as a bounding well for the southeastern extremity of the plume until the fourth quarter of 2020, when concentrations first exceeded 70 µg/L. As will be discussed in Section 2.11, TW4-43 was subsequently installed east-southeast of TW4-30 to serve as a new bounding well. However, TW4-30 again serves as a bounding well since chloroform concentrations dropped below 70 µg/L in the third quarter of 2022. 2.7 Addition of TW4-1, TW4-2, TW4-11, TW4-21 and TW4-37 to the Pumping System The general increase in concentrations at many of the chloroform wells and apparent areal expansion to re-incorporate wells that had been outside the plume for many years as a result of reduced wildlife pond recharge and nitrate pumping led to a decision to add wells to the chloroform pumping system. TW4-1, TW4-2 and TW4-11 were converted to pumping wells during the first quarter of 2015 and TW4-21 was converted to pumping during the following quarter. In addition, TW4-37 was installed in the first quarter of 2015 and began pumping during the second quarter of 2015. Addition of these five wells had a measurable impact on the chloroform plume, resulting in stabilization of the plume boundary and increased mass removal rates. Figure 6 compares the fourth quarter, 2014 and fourth quarter, 2023 chloroform plume boundaries. As shown, little change in the plume boundary occurred over this period other than cross-gradient contraction near TW4-6, TW4-8, TW4-9 and TW4-33; cross-gradient expansion near TW4-24; slight cross-to downgradient expansion near TW4-30; and southerly (downgradient) expansion in the vicinity of TW4-26. The expansion to the south at TW4-26 likely results from changes in groundwater flow Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 14 induced by reduced recharge from the southern wildlife pond and decay of the associated (southern) groundwater mound. 2.8 Final GCAP A final GCAP for chloroform was approved by DWMRC on September 14, 2015 (DWMRC, 2015). The third quarter 2015 Chloroform Monitoring report (EFRI, 2015d) was the first quarterly report prepared under the requirements of the final GCAP. Additional information required under the final GCAP that did not appear in previous quarterly reports is discussed in Section 3. 2.9 Addition of Compliance Well TW4-38 and Pumping Well TW4-39 Concentrations at TW4-9 exceeded the chloroform GCAL of 70 µg/L during the first and second quarters of 2016, requiring the installation and testing of new compliance well TW4-38 (Figure 1B) as per the GCAP. In addition, as per the GCAP, because no existing wells were considered suitable, new pumping well TW4-39 was installed (Figure 1B) and tested. TW4-39 pumping helps to remove chloroform from the northern portion of the plume and has likely contributed to the downward trend in chloroform concentrations at TW4-9 since the first quarter of 2018. As a result of this downward trend, chloroform concentrations dropped below 70 µg/L during the second quarter of 2019; and, except for the second quarter of 2021, TW4-9 has remained outside the plume. 2.10 Addition of Compliance Well TW4-42 and Pumping Wells TW4-40 and TW4-41 Concentrations at TW4-26 exceeded the chloroform GCAL of 70 µg/L during the second and third quarters of 2017, requiring the installation and testing of new compliance well TW4-40 (Figure 1B) as per the GCAP (HGC, 2018b). In addition, as per the GCAP, a new chloroform pumping well was installed. Because no existing wells were considered suitable, and productivity at existing pumping well TW4-4 had diminished, new pumping well TW4-41 was installed (Figure 1B) and tested (HGC, 2018b). TW4-41, located adjacent to TW4-4 in an area known to have relatively high permeability, augments pumping at TW4-4. To improve productivity, TW4-41 was constructed using 6-inch (rather than 4-inch) diameter casing. TW4-40, located approximately 200 feet south (downgradient) of TW4-26, was intended to be a new bounding compliance well. However, chloroform at TW4-40 exceeded 70 µg/L, so that new bounding well TW4-42 (Figure 1B) was installed approximately 200 feet south (downgradient) of TW4-40 as per the GCAP (HGC 2019). TW4-40 was then converted to a pumping well. TW4-40 Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 15 is valuable in that it is located within the downgradient (southern) toe of the plume and is relatively productive. Pumping of TW4-40 is expected to more effectively reduce or prevent further downgradient plume migration than can be expected by pumping at the more upgradient locations. 2.11 Abandonment of Pumping Well TW4-20 and Addition of Compliance Well TW4-43 Pumping well TW4-20 collapsed during July, 2020 and was abandoned during October, 2020. The collapse was caused by failure of the pump timing device during the second quarter of 2020. The timing device cycles the pump on and off to prevent drying up of the well and damage to the pump. Cycling is necessitated by the relatively low productivity of wells at the Mill caused by low permeability. Pumps cannot be operated continuously at low enough pumping rates to prevent drying up of the wells and consequent damage to the pumps. Failure of the timing device at TW4- 20 caused nearly continuous pumping that dried up the well and irreparably damaged both the pump and the well casing. Both pump and well failed early in the third quarter of 2020; efforts to rehabilitate the well were unsuccessful and the well was subsequently abandoned. As will be discussed in Section 4.3.4, little or no impact to pumping, chloroform mass removal rates or capture effectiveness occurred as a result of TW4-20 abandonment. Chloroform concentrations at TW4-30 first exceeded 70 ug/L during the fourth quarter of 2020 and remained above 70 ug/L through the second quarter of 2022. Because TW4-30 no longer bounded the chloroform plume to the east of TW4-29, new compliance well TW4-43 was installed approximately 200 feet east-southeast of TW4-30 during September 2021. As chloroform has not been detected at TW4-43, it served as the new bounding compliance well to the south-southeast of the plume until chloroform concentrations at TW4-30 again dropped below 70 ug/L in the third quarter of 2022. Hydraulically, TW4-43 is located generally cross- to downgradient of TW4-30. Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 16 Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 17 3. SUMMARY OF CHLOROFORM MONITORING AND PUMPING SINCE DECEMBER 31, 2012 The following subsections discuss elements included in the quarterly Chloroform Monitoring reports, and summarize and interpret key findings and results. Since December 31, 2012, forty four (44) quarterly Chloroform Monitoring reports were submitted (Energy Fuels Resources (USA) Inc [EFRI], 2013a; EFRI, 2013b; EFRI, 2013c; EFRI, 2014a; EFRI, 2014b; EFRI, 2014c; EFRI, 2014d; EFRI, 2015a; EFRI, 2015b; EFRI, 2015c; EFRI, 2015d; EFRI, 2016a; EFRI, 2016b; EFRI, 2016c; EFRI, 2016d; EFRI, 2017; EFRI, 2017b; EFRI, 2017c; EFRI, 2017d; EFRI, 2018a; EFRI, 2018b; EFRI, 2018c; EFRI, 2018d; EFRI, 2019a; EFRI, 2019b; EFRI, 2019c; EFRI, 2019d; EFRI, 2020a; EFRI, 2020b; EFRI, 2020c; EFRI, 2020d; EFRI, 2021a; EFRI, 2021b; EFRI, 2021c; EFRI, 2021d; EFRI, 2022a; EFRI, 2022b; EFRI, 2022c; EFRI, 2022d; EFRI, 2023a; EFRI, 2023b; EFRI, 2023c; EFRI, 2023d; EFRI, 2024). The first ten (10) were submitted prior to the final GCAP. Actions taken under the PCAP and final GCAP are consistent with objectives specified in Part I of the final GCAP to “permanently restore groundwater quality in all pumping wells and performance monitoring wells completed in the White Mesa shallow aquifer for all contaminants of concern in accordance with the Ground Water Corrective Action Objectives”. Objectives will be met by reducing contaminant of concern (COC) concentrations to levels at or below the GCALs specified in Table 2 of the GCAP. Through reducing chloroform concentrations to the chloroform GCAL (70 µg/L), concentrations of other COCs associated with the chloroform plume are also expected to be reduced to their respective GCALs. 3.1 Elements of the Quarterly Chloroform Monitoring Reports and Compliance with Preliminary and Final GCAPs Quarterly Chloroform Monitoring reports covering quarters from December 31, 2012 through the second quarter of 2015 (EFRI, 2013a; EFRI, 2013b; EFRI, 2013c; EFRI, 2014a; EFRI, 2014b; EFRI, 2014c; EFRI, 2014d; EFRI, 2015a; EFRI, 2015b; EFRI, 2015c) were submitted prior to the final GCAP. Elements of quarterly reports submitted during this period were guided by the August, 2007 PCAP (HGC 2007a). The PCAP proposed semi-annual corrective action reporting; nevertheless, reports were prepared and submitted quarterly. First quarter 2013 through second quarter 2015 quarterly Chloroform Monitoring reports included the following elements: • description of the chloroform program monitoring • quality assurance and data validation • data interpretation Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 18 • description of long-term pumping operation • description of any corrective action taken • conclusions and recommendations • electronic analytical data files Data interpretation included generation and discussion of perched water elevation and chloroform concentration contour maps, and a map estimating capture zones resulting from chloroform pumping. Subsequent to the fourth quarter of 2013, this map also provided estimated capture resulting from nitrate pumping. Data interpretation also included discussions of perched water level and chloroform concentration changes and changes in plume boundaries and capture between the quarter of interest and the previous quarter. Graphs of perched water levels and chloroform concentrations were provided. The third quarter 2015 quarterly Chloroform Monitoring report (EFRI, 2015d) was the first report submitted under the final GCAP and is consistent with the requirements of the final GCAP. Quarterly Chloroform Monitoring reports submitted under the final GCAP comply with Parts II through VI of the final GCAP and include the following elements: • description of chloroform program monitoring • quality assurance and data validation • data interpretation • description of long-term pumping operation • description of any corrective action taken • current compliance status • conclusions and recommendations • electronic data files (including residual chloroform plume mass estimate files and analytical data files) Data interpretation encompassed all of the elements submitted under the PCAP (including water level and concentration maps and graphs, maps showing estimated capture zones, and quarter to quarter comparisons of water level and chloroform concentration changes and changes in capture) as well as additional elements. The third quarter, 2015 chloroform report (EFRI, 2015d) and all subsequent reports contain required water level time-series graphs for all pumping wells, performance monitoring wells, and compliance monitoring wells (as specified in Table 1A of the GCAP), and time-series chloroform concentration graphs for all wells sampled during the third Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 19 quarter of 2015 and all subsequent quarters. Similar concentration and water level graphs appeared in quarterly reports submitted prior to approval of the GCAP. Some of the elements required under the final GCAP that were not incorporated previously include: • quarterly calculation of volume of water extracted from each pumping well • quarterly calculation of chloroform mass removed at each pumping well • quarterly calculation of residual chloroform plume mass and trend analysis • submission of a chloroform concentration contour map based on the ‘chloroform greater than or equal to 70 µg/L’ grid file used in residual chloroform plume mass calculations. • discussion of current compliance status • submission of residual chloroform mass calculation electronic files These elements are included in EFRI (2015d) and all subsequent reports. Current compliance status is addressed within Section 7 (introduced in EFRI, 2015d) that includes discussion of: • long term chloroform plume control • well construction, maintenance, and operation • disposal of extracted groundwater • compliance well performance • chloroform plume monitoring for (any) wells within 500 feet of the Mill property boundary The final GCAP specifies actions to be taken should the chloroform plume boundary migrate within 500 feet of a property boundary (in particular the eastern property boundary which is closest to the southeastern extremity of the plume). Currently, the southeastern extremity of the plume is approximately 1,200 feet from the eastern property boundary (Figure 1B). Site water level data suggest that the plume is unlikely to reach the eastern property boundary as perched water flow along the boundary to the east of the southeastern extremity of the plume appears to be generally south-southwesterly and sub-parallel to the boundary (Figure 1B; HGC, 2014a). The southern property boundary on the east side of the site is more than three miles to the south of the plume. The nearest downgradient perched water discharge point (Ruin Spring) is nearly two miles to the south-southwest of the plume. Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 20 3.2 Specific Actions Taken Under the Preliminary and Final GCAPs Some of the specific work performed after December 31, 2012 but before approval of the final GCAP included: • all required quarterly sampling, monitoring, quality control, pumping, and reporting activities; • submission of required quarterly electronic analytical data files; • installation and testing of wells TW4-28 through TW4-36 and associated reporting (HGC. 2013a; HGC. 2013b; HGC, 2014b); • preliminary evaluation of reduced productivity at TW4-19 (and TW4-24) [EFRI, 2015d]; • addition of TW4-1, TW4-2, TW4-4, TW4-11, and TW4-21 to the chloroform pumping system; and • installation and testing of TW4-37, associated reporting (HGC, 2015), and inclusion in the chloroform pumping system. Wells TW4-28 through TW4-36 were installed (with the concurrence of DWMRC) to bound the chloroform plume and to bound relatively isolated occurrences of elevated nitrate within and near the chloroform plume (HGC 2014a). The addition of TW4-1, TW4-2, TW4-4, TW4-11, TW4-21 and TW4-37 to the chloroform pumping system was proactive as it preceded a formal evaluation of reductions in well productivity and mass extraction rates submitted with the third quarter 2015 Chloroform Monitoring report (EFRI, 2015d) under the final GCAP. The productivity evaluation, included as Attachment N (Tab N), was also proactive as this type of analysis is not required except as part of the CACME every two years. Some of the specific work performed as of the fourth quarter of 2023 to comply with the final GCAP includes: • all required quarterly sampling, monitoring, quality control, pumping, and reporting activities; • graphical presentation of volumes pumped and masses removed from each pumping well. • calculation of chloroform mass removed per quarter; • calculation of residual chloroform plume mass per quarter; • contouring of ‘chloroform greater than or equal to 70 µg/L’ grid file used in residual chloroform plume mass calculations; • submission of all required quarterly electronic data files (including the additional residual plume mass calculation files); Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 21 • installation and testing of new compliance well TW4-38, new pumping well TW4-39, and associated reporting (HGC, 2016b). • installation and testing of new compliance well TW4-42, new pumping wells TW4-40 and TW4-41, and associated reporting (HGC, 2018b; HGC, 2019). • Installation and testing of new compliance well TW4-43, and associated reporting (HGC, 2021). 3.3 Summary of Key Findings and Interpretation of Results Since December 31, 2012 Overall, quarterly monitoring indicates that the chloroform plume is completely bounded by the existing monitoring network. Although increases and decreases in concentration occur within the plume from quarter to quarter, since the fourth quarter of 2014, except for cross-gradient contraction near TW4-6, TW4-8, TW4-9 and TW4-33; slight cross- to downgradient expansion near TW4-30; cross-gradient expansion near TW4-24; and southerly (downgradient) expansion near TW4-26; the plume boundary has been relatively stable (Figure 6). The highest chloroform concentrations have historically occurred at TW4-20 (now abandoned), located immediately downgradient of the former office leach field source; and at subsequently installed adjacent well TW4-37 (Figure 2). The historic maximum chloroform concentration (61,000 µg/L) was detected at TW4-20 during the second quarter of 2006; the final sampling of TW4-20 (second quarter of 2020) yielded a chloroform concentration of approximately 5,800 µg/L. At that time, the concentration at adjacent pumping well TW4-37 was approximately 11,700 µg/L. At the distal (southern) end of the plume, relatively stable to decreasing chloroform at TW4-33; and previously increasing to stable concentrations at TW4-29; suggested that chloroform migration had been arrested at TW4-33 by TW4-4 (and TW4-41) pumping and that increasing to stable chloroform at downgradient well TW4-29 resulted from a remnant of the plume that migrated downgradient from TW4-33 to TW4-29; then toward TW4-30, which was temporarily incorporated into the plume (from the fourth quarter of 2020 through the second quarter of 2022). Generally decreasing chloroform at TW4-29 since the end of 2020; and a hydraulic gradient that is no longer directed from TW4-29 towards TW4-30 (which is now generally cross-gradient of TW4-29); is consistent with this interpretation. Similarly, since 2016, increases in concentration at TW4-26, located in the southern extremity of the plume, accompanied by decreases in concentration at TW4-6 (located immediately upgradient [north] of TW4-26), suggest that chloroform at TW4-26 represents a remnant of the plume that migrated downgradient to the south. Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 22 Although chloroform at the southeastern extremity of the plume may temporarily continue to migrate to the southeast (from TW4-29 generally toward TW4-35), the southeastern extremity of the plume is approximately 1,200 feet from the closest (eastern) property boundary (Figure 1B). As discussed in Section 3.1, site water level data suggest that the plume is unlikely to reach the eastern property boundary as perched water flow along the boundary to the east of the southeastern extremity of the plume appears to be generally south-southwesterly and sub-parallel to the boundary (Figure 1B; HGC, 2014a). The southern property boundary on the east side of the site is more than three miles to the south of the plume and the nearest downgradient discharge point (Ruin Spring) is nearly two miles to the south-southwest of the plume. Therefore, because of the large distance to the southern property boundary, it is unlikely that any chloroform within the southern or southeastern extremities of the plume will ever reach the southern property boundary. 3.3.1 Perched Groundwater Flow The water level contours for the site (Figure 1B) indicate that perched water flow ranges from generally southwesterly beneath the Mill site and TMS to generally southerly along the eastern and western margins of White Mesa. Flow beneath the millsite and TMS ranges from generally west-southwest to southwest and is influenced by perched water discharge points Westwater Seep, located west to west-southwest of the TMS, and Ruin Spring, located southwest of the TMS. The overall southwesterly flow is locally influenced by seepage from the formerly used wildlife ponds. Perched groundwater mounding associated with the wildlife ponds locally changes the generally southwesterly perched water flow patterns. For example, northeast of the Mill site, relict mounding associated with the two northern wildlife ponds results in locally northwesterly flow near MW-19 and PIEZ-1. The impact of the mounding associated with the northern ponds, to which water has not been delivered since March 2012, is diminishing and is expected to continue to diminish as the associated northern mound decays due to reduced recharge. Likewise, the impact of mounding associated with reduced water delivery to the southern wildlife pond is diminishing and is expected to continue to diminish as the southern mound decays due to reduced recharge. Since the first quarter of 2012, water levels have declined within the northern mound by nearly 27 feet (at PIEZ- 2), and within the southern mound by more than 23 feet (at PIEZ-5). Flow within both the chloroform and nitrate plumes is influenced by the past water delivery to the wildlife ponds. Under the influence of past water delivery to the northern wildlife ponds, flow in the northwestern portion of the chloroform plume is generally southwesterly (the typical pattern for the site) and in the southeastern portion of the plume, southerly. Flow in the southern extremity of the plume is generally to the south and locally to the south-southeast. In addition, the Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 23 diminishing groundwater mound associated with the southern wildlife pond has changed hydraulic gradients and contributed to downgradient expansion of the chloroform plume near TW4-26. Continued decay of the southern mound is expected to result in more typical south-southwesterly flow within the southern extremity of the plume. Furthermore, flow within both the chloroform and nitrate plumes is locally influenced by chloroform pumping wells MW-4, MW-26, TW4-1, TW4-2, TW4-4, TW4-11, TW4-19, TW4-21, TW4-37, TW4-39, TW4-40 and TW4-41; and nitrate pumping wells TW4-22, TW4-24, TW4-25, and TWN-2. Nitrate pumping wells TW4-22 and TW4-24 are within and just within the boundary of the chloroform plume, respectively. Flow is also influenced by the permeability distribution. Hydraulic testing demonstrates that wells TW4-14, TW4-27, and TW4-36 are located in materials that are one to three orders of magnitude lower in hydraulic conductivity than surrounding materials. In addition, wells south and southeast of TW4-4 that include TW4-6, TW4-26, TW4-29, TW4-30, TW4-31, TW4-33, TW4-34, and TW4-35 are completed in materials having conductivities one to two orders of magnitude lower than those penetrated by TW4-4. Water level responses to changing recharge and pumping conditions are influenced by distance from pumping wells, distance from the wildlife ponds, and permeability. Overall, pumping and cessation of water delivery to the wildlife ponds has reduced saturated thicknesses and hydraulic gradients within both the chloroform and nitrate plumes. 3.3.2 Purpose of Chloroform Pumping and Hydraulic Capture The purpose of chloroform pumping is to reduce the chloroform mass within the perched zone as rapidly as practical. This has been accomplished by pumping at upgradient locations having both relatively high transmissivities (and productivities) and relatively high chloroform concentrations. Although the entire plume is not within hydraulic capture, most of the area and nearly all the plume mass are under capture, as will be discussed in Section 4.2. Perched water containing chloroform has been removed by operating chloroform pumping wells MW-4 (since the second quarter of 2003), MW-26 (since the third quarter of 2003), TW4-4 (since the first quarter of 2010), TW4-19 (since the second quarter of 2003), TW4-20 (from the third quarter of 2005 to the second quarter of 2020); and more recently by TW4-1, TW4-2, and TW4-11 (since the first quarter of 2015), TW4-21 and TW4-37 (since the second quarter of 2015), TW4-39 (since the fourth quarter of 2016), TW4-41 (since the second quarter of 2018) and TW4-40 (since the second quarter of 2019). TW4-40 is the most downgradient pumping well. Prior to installation and operation of TW4-40 and TW4-41, TW4-4 was the most downgradient well within the plume having sufficient productivity to make pumping practical. As discussed in Section 2.10, TW4-40 is valuable because Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 24 it 1) is located within the downgradient (southern) toe of the plume and 2) is relatively productive. Pumping of TW4-40 is likely to more effectively reduce or prevent further downgradient plume migration than can be expected by pumping at the more upgradient locations. However, pumping at TW4-4 and adjacent well TW4-41 augments the beneficial impacts of pumping at TW4-40 by helping to reduce chloroform mass and rates of migration within the southern portion of the plume. The separation of the kriged southern extremity of the plume from the main body of the plume since the first quarter of 2020 is attributable to TW4-4 and TW4-41 pumping (Figure 3A). Drawdown patterns and overall capture associated with pumping of the original chloroform pumping wells MW-4, MW-26, and TW4-19 have changed as additional groups of wells have been added to the pumping system. Relatively large capture zones became evident at MW-4, MW- 26 and TW4-19 as of the third quarter of 2003; and at TW4-20 (now abandoned) as of the fourth quarter of 2005. A large expansion in capture occurred within a year of the initiation of pumping at nitrate pumping wells TW4-22, TW4-24, TW4-25, and TWN-2 in the first quarter of 2013. Additional large expansions occurred once chloroform pumping wells TW4-1, TW4-2, TW4-11, TW4-21 and TW4-37 became operational in 2015, and once TW4-39 became operational in the fourth quarter of 2016. Significant expansion of capture to the south has resulted from pumping of TW4-41 since the second quarter of 2018 and from pumping at TW4-40 since the second quarter of 2019. As a result, chloroform detected in the vicinity of TW4-26 and TW4-40 appears to be largely within the hydraulic capture zone of TW4-40. Prior to operation of TW4-41, a well-defined capture zone was not evident at TW4-4. The lack of a well-defined capture zone near TW4-4 likely resulted from its position at a transition from relatively high to relatively low permeability conditions and, prior to the second quarter of 2018, from the persistent relatively low water level at adjacent non-pumping well TW4-14 (Figure 1B). Even though TW4-4 was pumping, prior to the second quarter of 2015, the water level at TW4-4 was typically more than 10 feet higher, and as of the first quarter of 2018, remained approximately 5 feet higher, than the water level at TW4-14. Nevertheless, as discussed in Section 2.4, water level and chloroform concentration behavior at TW4-6 (located immediately downgradient of TW4-4) suggest that, although influenced by reduced wildlife pond recharge, TW4-6 is also under the hydraulic influence of TW4-4. In addition, since the second quarter of 2018, the water level at TW4-4 has been lower than the water level at TW4-14 more than 80% of the time as a result of the addition TW4-41 to the pumping system.. The beneficial impact of TW4-4 pumping is demonstrated by chloroform decreases in TW4-6 (Figure 4A) between startup in 2010 and the end of 2013. After 2013, the chloroform concentration at TW4-6 again increased, most likely due to its position near the plume boundary, reduced dilution Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 25 from wildlife pond recharge, and more westerly flow induced by nitrate pumping and decay of the southern mound. However, the subsequent downward trend in TW4-6 chloroform concentrations that coincided with the initiation of pumping at TW4-1, TW4-2 and TW4-11 in the first quarter of 2015, has again taken TW4-6 outside the plume since the third quarter of 2018 (Figure 4A). Although concentrations at TW4-6 have generally decreased since the first quarter of 2015, concentrations at TW4-26 have generally increased since the first quarter of 2016, and have exceeded 70 µg/L since the second quarter of 2017 (Figure 4B). As discussed above, the diminishing groundwater mound associated with the southern wildlife pond has changed hydraulic gradients and contributed to downgradient expansion of the plume in the vicinity of TW4-26. Pumping of TW4-40, and continued pumping of TW4-4 and adjacent well TW4-41 is expected to slow or halt plume expansion in this area. Although chloroform mass removal is enhanced by operation of nitrate pumping wells TW4-22 and TW4-24, nitrate pumping created temporary undesirable expansion of the chloroform plume to the west, from the area of TW4-20 (now abandoned) towards TW4-24, and to the west of TW4- 16, by inducing more westerly flow. However, capture associated with nitrate pumping wells has slowly increased over time and enhanced chloroform capture as the impacts from former wildlife pond recharge have dissipated. The relatively slow development of hydraulic capture associated with nitrate pumping was expected based on the generally low permeability of the perched zone. 3.3.3 Impacts of Pumping Fluctuations and Typical Analytical Error Water level fluctuations at pumping wells are typical and occur in part because of fluctuations in pumping conditions. Pumps operate intermittently because low permeability prevents sufficient water from entering wells for continuous pump operation. Although most pumping wells are purposely situated at locations having relatively high permeability and saturated thickness (yielding relatively high transmissivity), the transmissivities of the perched zone even in these areas are low enough to limit average well productivities to at most a few gpm. Fluctuations in chloroform concentrations at pumping wells and adjacent wells likely result in part from changes in pumping conditions. These fluctuations impact plume residual mass calculations from quarter to quarter. Through the conversion of additional non-pumping wells to pumping wells in order to improve mass capture and control plume boundaries, the proportion of pumping to non- pumping wells within the plume has increased, which may contribute to unavoidable variability in the residual mass calculations. An additional significant source of variability in the residual mass calculations is concentration fluctuations resulting from normal sampling and analytical error. Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 26 Sampling and analytical error typically result in concentration fluctuations of +/- 20%, which could result in similar fluctuations in residual mass estimates from quarter to quarter. In addition, fluctuations in concentrations at wells located near plume boundaries are also expected to result in quarter to quarter fluctuations in the locations of the plume boundaries and the area of the plume. 3.3.4 Impacts of Reduced Wildlife Pond Recharge and Nitrate Pumping The two northern wildlife ponds are located upgradient of the chloroform and nitrate plumes (Figure 1B). Historic recharge from these unlined ponds generated a perched groundwater mound that acted to increase hydraulic gradients within the plumes and facilitate downgradient migration. Conversely, the recharge from these ponds acted to dilute constituent concentrations within both plumes which helped to limit the areal extents of the plumes. As shown in Figure A.6, there was almost no areal expansion of the chloroform plume between 2007 and 2011, largely the result of pumping and dilution by chloroform-free wildlife pond recharge. Reduced dilution resulting from reduced wildlife pond recharge was expected to increase chloroform (and nitrate) concentrations as discussed in EFRI (2013d). These effects were expected to propagate generally downgradient over time, and to potentially impact wells completed in higher permeability materials prior to those completed in lower permeability materials. Therefore, wells completed in higher permeability materials could be impacted sooner than those completed in lower permeability materials even if those completed in lower permeability materials are closer to the ponds. Beginning in the first quarter of 2012, once water was no longer delivered to the two northern ponds, the impacts of dilution were reduced as recharge diminished, and constituent concentrations at many locations within both chloroform and nitrate plumes began to increase, consistent with expectations. This resulted in areal expansion of the chloroform plume as chloroform concentrations at wells that had been outside the plume for many years increased sufficiently to bring the wells again within the plume (as discussed in Section 2.5). Examples of wells that had been outside the plume for many years prior to the reduction in wildlife pond recharge include TW4-6, TW4-8, and TW4-16. TW4-6 was inside the plume between the third quarter of 2009 and the fourth quarter of 2010 and was reincorporated into the plume in the third quarter of 2014. As discussed above, since the third quarter of 2018, TW4-6 is once again outside the plume due to increased pumping. Wells TW4-8 and TW4-16 were inside the plume prior to the initiation of pumping at MW-4, MW-26, and TW4- 19 in 2003. TW4-8 was inside the plume prior to the first quarter of 2005, outside the plume from the first quarter of 2005 through the fourth quarter of 2013, was reincorporated into the plume in Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 27 the first quarter of 2014, and since the first quarter of 2020 is again outside the plume. TW4-16 was inside the plume prior to the fourth quarter of 2005, was outside the plume from the fourth quarter of 2005 through the second quarter of 2014, and was reincorporated into the plume by the third quarter of 2014. Since then, TW4-16 has been both within and outside the plume, and has been within the plume since the second quarter of 2019. Although the reduction in wildlife pond recharge was expected to increase plume constituent concentrations due to reduced dilution, the decay of the perched groundwater mound associated with historic northern pond recharge was expected to decrease plume movement as hydraulic gradients decreased. As discussed in Section 3.3.1, hydraulic gradients within the plume have indeed been reduced. Decreases in saturated thicknesses at many wells and piezometers have resulted primarily from cessation of wildlife pond water deliveries and perched mound decay. To the extent that saturated thickness reduction results from dewatering of higher permeability layers receiving low constituent concentration pond water, wells intercepting such layers are expected to exhibit increases in dissolved constituent concentrations. These effects may be especially evident at pumping wells. However, as discussed above, the net impact of reduced wildlife pond recharge is expected to be beneficial, as temporary increases in chloroform concentrations are judged less important than reduced chloroform migration rates. Nitrate pumping initiation in the first quarter of 2013 has also impacted chloroform concentrations and plume boundaries. As discussed in Section 2.5, nitrate pumping at TW4-22 and TW4-24 caused chloroform to migrate to the west from the vicinity of TW4-20 (now abandoned), the well having the highest chloroform concentrations historically. Chloroform concentrations at TW4-22 increased substantially and concentrations at TW4-24, which had historically been near the detection limit, have increased sufficiently to bring it within the plume twelve times since nitrate pumping began. As discussed above, past increases at TW4-6 and TW4-16 are also likely related in part to more westerly hydraulic gradients resulting from initiation of nitrate pumping. Other factors include reduced dilution and the proximity of these wells to the plume boundary. Furthermore, reduced recharge at the southern wildlife pond (located south-southwest and generally downgradient of the plume), and decay of the associated groundwater mound, has changed hydraulic gradients within the southern extremity of the plume. Since the first quarter of 2016, this change likely contributed to the increase in chloroform concentrations at TW4-26, and, since the second quarter of 2017, incorporation of TW4-26 within the plume. Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 28 3.3.5 Plume Boundary and Southeast Portion of Plume The chloroform plume is entirely within the Mill property. The closest boundary is the eastern property boundary, located approximately 1,200 feet from the southeastern extremity of the plume, as shown in Figure 1B. The plume is completely surrounded by wells having chloroform concentrations that are below 70 ug/L. The majority of the wells immediately surrounding the plume are non-detect for chloroform or have chloroform concentrations that are less than 5 µg/L. As of the second quarter of 2017, when downgradient well TW4-26 was first incorporated into the plume, the plume was no longer bounded immediately to the south, although MW-22 and MW-40 bounded the plume far to the south. TW4-40 (Figure 1B) was installed in the first quarter of 2018 to bound the plume immediately to the south; and TW4-41 was installed adjacent to TW4-4 to augment pumping from the relatively high permeability materials known to exist at TW4-4. However, because chloroform at TW4-40 exceeded 70 ug/L, TW4-42 was installed approximately 200 feet south of TW4-40 to serve as the new bounding compliance well. Although current water level data suggest that the extreme southeastern tip of the plume is likely to migrate temporarily to the southeast (toward TW4-35), as discussed in Section 3.1, site water level data also suggest that the plume is unlikely to reach the eastern property boundary because perched water flow along the boundary to the east of the southeastern extremity of the plume appears to be generally south-southwesterly and sub-parallel to the boundary (Figure 1B; HGC, 2014a). The southern property boundary on the east side of the site is more than three miles to the south of the plume and the nearest downgradient discharge point (Ruin Spring) is nearly two miles to the south-southwest of the plume. Past easterly migration of chloroform from TW4-29 toward TW4-30 was a consequence primarily of the permeability distribution. Although historic hydraulic gradients suggested that TW4-14 was almost directly downgradient of TW4-4, the concentrations at TW4-29 and TW4-33 suggest that chloroform migrated along a narrow path to the southeast from the vicinity of TW4-4 to TW4-33 then TW4-29 in a direction nearly cross-gradient with respect to the expected direction of groundwater flow in this area. Such migration was possible because prior to 2019, the water levels at TW4-29 were consistently lower than the water levels at TW4-4 (and have remained lower than the water levels at TW4-6). The hydraulic conductivities of TW4-29, TW4-30, TW4-31, TW4-33, and TW4-34 are one to two orders of magnitude lower than the conductivity of TW4-4, and one to two orders of magnitude higher than the conductivities of TW4-27 (and TW4-36). The permeability and historic water level distributions are generally consistent with the apparent nearly cross-gradient migration of chloroform from TW4-4 around the low permeability zone defined by TW4-36, TW4-14, and TW4-27. Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 29 Although TW4-4 was impacted by the chloroform plume by 2001; TW4-14 appeared to be almost directly downgradient of TW4-4; and, prior to 2018, relatively steep hydraulic gradients existed from TW4-4 to TW4-14; chloroform was not detected at TW4-14 until the fourth quarter of 2014. The slow rate of migration from TW4-4 to TW4-14 is a result of the low permeability near TW4- 14. Chloroform at TW4-14 has remained below 10 µg/L; dropped below 2 µg/L during the first quarter of 2021; and, except for the third quarter of 2022, has not been detected since. Data from wells located near the southern extremity of the chloroform plume indicate that: 1. Chloroform exceeding 70 µg/L at TW4-29 is bounded by concentrations below 70 µg/L at wells TW4-6, TW4-23, TW4-27, TW4-33, TW4-34, TW4-35 and TW4-42; and, as discussed above, TW4-30 (with a fourth quarter, 2023 concentration of approximately 40 µg/L) again bounds the plume immediately to the east. Although previously downgradient of TW4-29, due to long-term changes in water levels, TW4-30 is now generally cross- gradient of TW4-29. TW4-6, TW4-23, TW4-27 and TW4-33 are generally cross- to upgradient of TW4-29; TW4-34 and TW4-35 are generally cross- to downgradient of TW4-29; TW4-42 is generally cross- to downgradient of TW4-29; and TW4-43 is generally cross- to downgradient of TW4-30. 2. Chloroform concentrations at TW4-33 that are lower than concentrations at TW4-29, and the likelihood that a pathway exists from TW4-4 to TW4-33 to TW4-29, suggest that concentrations in the vicinity of TW4-33 were likely higher prior to initiation of TW4-4 pumping, and that lower concentrations currently detected at TW4-33 are due to its closer proximity to TW4-4. 3. Chloroform concentrations at TW4-26 exceeded 70 µg/L for the first time during the second quarter of 2017. Chloroform at TW4-26 is bounded by concentrations below 70 µg/L at TW4-6 and TW4-23 (located up- to cross-gradient of TW4-26); and at TW4-34 (located generally cross- gradient of TW4-26). Chloroform has not been detected at TW4- 23; and, although historically non-detect, chloroform was first detected at TW4-34 during the third quarter of 2022. Since that first detection, chloroform was detected twice, including during the second quarter of 2023 (at a concentration of 1.2 µg/L). Although chloroform exceeding 70 µg/L was detected at well TW4-40, installed approximately 200 feet south of TW4-26 in February, 2018, chloroform has not been detected at TW4-42, installed approximately 200 feet south of TW4-40 in April, 2019. TW4-42 is generally down to cross-gradient of TW4-26 and downgradient of TW4-40 and bounds the chloroform plume to the south. TW4-4 pumping, enhanced by pumping at TW4-41 (Figure 1A), has been expected to eventually reduce chloroform at both TW4-33 and TW4-29 by cutting off the source. The decrease at TW4- 33 has been anticipated to be faster than at TW4-29 because TW4-33 is in closer proximity to TW4-4 pumping. Such behavior would be analogous to the temporary decreases in chloroform concentrations that occurred at TW4-6 and TW4-26 once TW4-4 pumping began. Since Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 30 installation in 2013, concentrations at TW4-33 have been relatively stable to decreasing (and have generally been decreasing since 2018); however, from the third quarter of 2014 until the second quarter of 2020, concentrations at TW4-29 generally increased before subsequently stabilizing and decreasing. This behavior suggests that chloroform migration has been arrested at TW4-33 by TW4-4 (and TW4-41) pumping and that previously increasing chloroform at TW4-29 resulted from a remnant of the plume that migrated past TW4-33 and generally toward TW4-30 (which was previously downgradient of TW4-29). The influence of TW4-4 pumping at the distal end of the plume is consistent with generally decreasing water levels at both TW4-29 and TW4-33. Pumping at TW4- 41 since the second quarter of 2018 has been expected to help maintain or enhance this decline. Decreasing water level trends at TW4-29 and TW4-33 are also consistent with reduced wildlife pond seepage. The decay of the groundwater mound associated with the southern wildlife pond, which is 3 to 4 times closer to the southern extremity of the chloroform plume than the northern ponds, is expected to impact water levels within and adjacent to this portion of the plume. Reduced wildlife pond seepage, in particular, reduced seepage from the southern wildlife pond, likely contributes to decreasing water level trends at both wells (since about the fourth quarter of 2013); temporarily increased concentrations at TW4-6 subsequent to the first quarter of 2014; and increased concentrations at TW4-26 since the third quarter of 2016. As the groundwater mound associated with the southern pond decays, groundwater flow directions in the southern extremity of the plume have become more southerly, and plume migration has turned more to the south. An increasingly southerly direction of plume migration is consistent with water level changes in this area and increased concentrations at TW4-26. In addition, generally decreasing concentrations at TW4-6 since the third quarter of 2015, and generally increased concentrations at TW4-26 since the third quarter of 2016, suggest that TW4-4 pumping has arrested chloroform migration between TW4-4 and TW4-6, and that increased chloroform at TW4-26 results from a remnant of the plume that continues to migrate south from TW4-6 to TW4-26. The enhancement of pumping in the vicinity of TW4-4 by the start-up of TW4- 41 pumping in the second quarter of 2018 has been expected to increase this apparent separation and to have resulted in concentrations at TW4-6 dropping below 70 µg/L after the second quarter of 2018. Furthermore, the initiation of pumping at TW4-40 during the second quarter of 2019 is expected to reduce or prevent chloroform migration to the south of TW4-40. Small but detectable chloroform concentrations at TW4-14 (from the fourth quarter of 2014 through the first quarter of 2021; and during the third quarter of 2022) and at TW4-27 (since the Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 31 third quarter of 2015) suggest ongoing, but slow, downgradient migration of chloroform from the southeastern extremity of the plume (near TW4-29 and TW4-33) into the low permeability materials penetrated by TW4-14 and TW4-27. Pumping at TW4-41 is expected to reduce or prevent future migration of chloroform toward these wells. 3.3.6 Pumping Well Productivity and Plume Control Overall, the productivities of chloroform pumping well TW4-19 and nitrate pumping well TW4- 24 have decreased since the third quarter of 2014; productivity at TW4-4 has decreased since the third quarter of 2016; and pumping well TW4-20 failed during the third quarter of 2021 and was subsequently abandoned. Decreased productivity at TW4-19 and TW4-24 were addressed in Attachment N (Tab N) of the third quarter, 2015 Chloroform Monitoring report (EFRI, 2015d). That analysis concluded that chloroform pumping and associated hydraulic capture were nevertheless adequate at that time primarily due to the doubling of the number of pumping wells during the first half of 2015. The addition of pumping well TW4-39 during the fourth quarter of 2016 also helps mitigate the impacts of reduced productivity at TW4-19 and TW4-24. Reduced productivity at TW4-4 is likely in part related to reduced saturated thicknesses at that location. Reduced productivity has been mitigated by the installation of pumping well TW4-41 adjacent to TW4-4 (Figure 1A). As discussed in Section 2.10, TW4-41 is constructed using a 6- inch diameter casing to improve productivity. As discussed in Section 2.11, pumping well TW4-20 collapsed during July, 2020 and was abandoned during October, 2020. The collapse was caused by failure of the pump timing device during the second quarter of 2020. The timing device cycles the pump on and off to prevent drying up of the well and damage to the pump. Cycling is necessitated by the relatively low productivity of wells at the Mill caused by low permeability. Pumps cannot be operated continuously at low enough pumping rates to prevent drying up of the wells and consequent damage to the pumps. Failure of the timing device at TW4-20 caused nearly continuous pumping that dried up the well and irreparably damaged both the pump and the well casing. Both pump and well failed early in the third quarter of 2020; efforts to rehabilitate the well were unsuccessful and the well was subsequently abandoned. As discussed in HGC (2022a) and below in Section 4.3.4, little or no impact to chloroform mass removal rates and capture effectiveness has occurred as a result of TW4-20 abandonment. Increased pumping at TW4-19 subsequent to TW4-20 failure more than compensated for the loss of pumping at TW4-20. Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 32 However, although the productivity of TW4-19 increased subsequent to the abandonment of TW4- 20, as will be discussed in Section 4.3.5, since the third quarter of 2015, in addition to TW4-4, the productivities of chloroform pumping wells TW4-1, TW4-2, TW4-11, TW4-21, TW4-37, TW4- 39, TW4-40 and TW4-41 have gradually declined. This general decline correlates to a general decline in saturated thickness in these wells. However, as will be discussed in Section 4.3.1, ongoing decreases in saturated thicknesses (and hydraulic gradients) reduce the ‘background’ flow through the plume and consequently reduce the rate of pumping needed to control the plume. Calculations to be presented in Section 4.3.2 indicate that, based on reduced ‘background’ flow through the plume, chloroform pumping is more than adequate for plume control. Overall, regardless of the gradual declines in many well productivities, data collected to date indicate that pumping is sufficient and that sufficient pumping and non-pumping wells exist to effectively define, control and monitor the plume. A detailed discussion of well productivity is provided in Section 4.3. Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 33 4. EVALUATION OF PUMPING SYSTEM EFFECTIVENESS Factors that are relevant to pumping system effectiveness are discussed in Sections 4.1 through 4.4. Section 4.1 discusses conditions that are in part impacted by pumping including trends in chloroform plume area, pumped mass removal rates, concentrations, hydraulic gradients, saturated thicknesses, and residual mass estimates. These conditions are also impacted by reduced dilution resulting from reduced wildlife pond recharge which tends to increase chloroform concentrations, plume area, and residual mass estimates. The goal of the pumping is to stabilize or reduce chloroform plume area, chloroform concentrations, residual mass, hydraulic gradients, and saturated thicknesses, while maintaining mass removal rates as high as is practical. Time-series perched water level and chloroform concentration graphs presented in Section 4.1 are focused on the immediate vicinity of the chloroform plume. Complete sets of time-series water level graphs for all wells (pumping wells, performance monitoring wells, and compliance monitoring wells), and time-series contaminant concentration graphs for all wells, are available in the fourth quarter, 2023 Chloroform Monitoring report (EFRI, 2024). Section 4.2 discusses hydraulic capture effectiveness. As will be discussed in Section 4.2, approximately 77% of the plume area, and 92% of the plume mass are under hydraulic capture as of the fourth quarter of 2023; and the proportion of the plume mass under hydraulic capture has remained between 79% and 99% since the previous CACME (HGC, 2022a). Section 4.3 discusses pumping well productivity. The productivities of chloroform pumping well TW4-19 and nitrate pumping well TW4-24 declined after the third quarter of 2014. Although productivity declines are partly due to reductions in transmissivities and interference between pumping wells, reduced well efficiency may also contribute to reduced productivity. However, the impact of reduced productivity at TW4-19 on chloroform mass removal was mitigated by adding TW4-1, TW4-2, TW4-11, TW4-21, and TW4-37 to the pumping system in the first half of 2015. The substantial increase in chloroform mass removal that subsequently occurred indicates that adding these wells more than compensated for reduced TW4-19 productivity. Since the third quarter of 2016, productivity at TW4-4 has also declined, in part due to reduced saturated thicknesses (as will be discussed below). Operation of chloroform pumping well TW4- 41 (installed adjacent to TW4-4 in 2018) has mitigated the reduced productivity at TW4-4. Adding TW4-39 and TW4-40 to the chloroform pumping system since 2015 has also helped to expand overall pumping system effectiveness. In particular, operation of TW4-39 has helped shrink the plume boundary away from TW4-9 which is again outside the plume. Operation of Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 34 TW4-40 (and TW4-41) has expanded hydraulic capture to the south, and chloroform detected in the vicinity of TW4-26 and TW4-40 appears to be largely within the hydraulic capture zone of TW4-40. In addition, although pumping well TW4-20 collapsed during the third quarter of 2020 and was subsequently abandoned, as will be discussed in Section 4.3.4, little or no impact to chloroform mass removal rates and capture effectiveness has resulted. As the analysis presented in the previous CACME (HGC, 2022a) indicated, increased pumping at TW4-19 since TW4-20 failure has more than compensated for the loss in pumping at TW4-20. Furthermore, reduced saturated thicknesses within the plume have generally reduced the productivity of many plume pumping wells other than those specifically noted above. As will be discussed in Section 4.1, saturated thicknesses within some portions of the plume have declined by as much as 99%. However, even though productivity has generally decreased, as will be discussed in Section 4.3.1, the ‘background’ flow through the plume has also declined due to the reductions in saturated thicknesses and hydraulic gradients. Because of the decrease in ‘background’ flow through the plume, pumping, albeit reduced, still exceeds ‘background’ flow by a substantial margin. Section 4.3.5 discusses generally reduced productivity in more detail. Section 4.4 discusses natural attenuation of chloroform. Natural attenuation enhances the effectiveness of the pumping system by independently reducing chloroform concentrations within the plume. 4.1 Trends in Plume Area, Mass Removal Rates, Concentrations, Hydraulic Gradients, Saturated Thicknesses, and Residual Mass Table 1 summarizes chloroform plume area, mass removed by pumping, residual mass, and average concentrations from the first quarter of 2012 through the fourth quarter of 2023. The first quarter of 2012 (rather than the first quarter of 2013) was used as a starting point because water delivery to the wildlife ponds ceased in the first quarter of 2012. As discussed previously, the resulting reduction in chloroform-free seepage and dilution created substantial (but anticipated) changes in the chloroform plume. Over the final year covered by the first CACME (between the fourth quarters of 2014 and 2015 [HGC, 2016a]), the plume area was relatively stable and mass removal rates increased substantially, while average concentrations within the plume and calculated residual masses increased. As will be discussed in Section 4.2, the proportion of the plume area under hydraulic capture also increased. Subsequently, over the last nine years (between the fourth quarters of 2014 Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 35 and 2023) the plume area maintained relative stability (increasing by less than 6%) although average concentrations, mass removal rates, and calculated residual masses have decreased. Average chloroform concentrations within the plume were calculated two ways; as the average of the chloroform concentrations of all wells within the plume and as an area-weighted average based on the gridded (kriged) concentration data. The estimates using the gridded data are lower because they are a more accurate area-weighted representation of the data and because they account for the plume margins in which concentrations depend on data from wells both within and outside the plume. Figure 5 compares first quarter 2012 and fourth quarter 2014 plume boundaries; Figure 6 compares fourth quarter 2014 and 2023 plume boundaries; Figure 7A compares graphs of plume area and mass removed per quarter from the first quarter of 2012 through the fourth quarter of 2023; and Figure 8A compares graphs of plume area and average chloroform concentrations (calculated as described above) within the plume from the first quarter of 2012 through the fourth quarter of 2023. Figures 7B and 8B compare graphs of the same quantities as Figures 7A and 8A, but over the time period beginning with implementation of the GCAP (third quarter of 2015) through the fourth quarter 2023. Figure 8C compares graphs of mass removed per quarter and average concentrations within the plume from the third quarter of 2015 through the fourth quarter of 2023. As shown in Figure 5, between the first quarter of 2012 and fourth quarter of 2014, the plume boundary expanded and the number of wells within the plume increased from 12 to 18 (Table 1). As shown in Figure 6, between the fourth quarters of 2014 and 2023, the plume boundary was relatively stable except for (cross-gradient) contraction in the vicinities of TW4-6, TW4-8, TW4- 9, TW4-16 and TW4-33; cross-gradient expansion in the vicinities of TW4-21 and TW4-24; slight cross-to downgradient expansion near TW4-30; and southerly (downgradient) expansion in the vicinity of TW4-26. The number of wells within the plume increased from 18 to 19 (Table 1), the net result of: the installation of four new wells (TW4-37, TW4-39, TW4-40 and TW4-41) within the plume; the incorporation of TW4-24 and TW4-26 into the plume; plume shrinkage away from TW4-6, TW4-8, TW4-9 and TW4-33; and the abandonment of TW4-20. As indicated in Figure 7A, since the first quarter of 2012, the overall trend in the plume area and the mass removal rates (pumped mass removed per quarter) are upward and comparatively flat, respectively. However, the rate of increase in plume area decreased substantially after the fourth quarter of 2014, and, overall, the plume area has been relatively stable since that time. In addition, although the plume area was relatively stable (rather than increasing) during 2015, the mass removal rate more than tripled. The increase in chloroform mass removal rate during 2015 resulted in large part from the addition of the five new pumping wells, in particular TW4-37. The mass Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 36 removal rate reached a peak in the second quarter of 2016, correlating with a peak in average concentrations within the plume (Table 1; Figures 8A and 8C). Figure 7B shows that although the trend in plume area was essentially flat between the third quarter of 2015 and fourth quarter of 2023, mass removal rates have trended downward. The downward trend is attributable primarily to reductions in the average chloroform concentrations within the plume as shown in Table 1 and Figure 8B. As concentrations decrease in pumping wells within the plume, the mass removal rates also decrease (Figure 8C), even if pumping rates remain relatively stable. Although Figure 8A demonstrates that between the first quarter of 2012 and the fourth quarter of 2023 the plume area has an increasing trend overall, the average chloroform concentrations within the plume have decreasing trends overall. Figure 8B shows that between the third quarter of 2015 and the fourth quarter of 2023 the downward trends in average plume chloroform concentrations are stronger, while the trend in plume area is essentially flat. Figures 9 and 10 graph chloroform concentrations at wells within the plume from the first quarter of 2012 through the fourth quarter of 2023. Non-pumping wells are considered in Figure 9 and pumping wells in Figure 10. Included are wells that were inside the plume during more than two quarters over this period (note that TW4-6 has been outside the plume since the third quarter of 2018; TW4-8 has been outside the plume since the first quarter of 2020; except for the second quarter of 2021, TW4-9 has been outside the plume since the second quarter of 2019; TW4-30 has been outside the plume since the third quarter of 2022; and TW4-33 has been outside the plume since the first quarter of 2021). As shown in Figure 9, concentrations are relatively stable at TW4-7; stable to decreasing at TW4- 10 (since 2013) and TW4-33; and variable at TW4-6, TW4-8, TW4-9, TW4-16, TW4-26, TW4- 29 and TW4-30. However, concentrations at TW4-26 have been relatively stable to decreasing since the first quarter of 2018 (and have generally decreased since the end of 2020); and concentrations at TW4-30 have generally decreased since the fourth quarter of 2021.In addition, although concentrations at TW4-29 generally increased between the third quarter of 2014 and second quarter of 2020, concentrations have been stable to decreasing since then. Increases at TW4-6, TW4-8, and TW4-16 during 2013 and 2014 resulted from lateral expansion of the plume to re-encompass these wells during 2014. Increases at TW4-9 also resulted from lateral expansion to re-incorporate this well during 2016. As discussed previously, these wells are likely impacted by close proximity to the plume boundary and decreased dilution from reduced wildlife pond recharge. The increases at TW4-6 and TW4-16 in 2013 and 2014 are also likely Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 37 attributable to more westerly flow induced by nitrate pumping. The general decrease at TW4-6 since mid-2015 and at TW4-9 since early 2018 is attributable to increased pumping; and the increase at TW4-30 that caused its incorporation into the plume from the fourth quarter of 2020 through the second quarter of 2022 is attributable to the former hydraulic gradient that was directed from TW4-29 to TW4-30. The concentration behavior at TW4-6, TW4-26, TW4-29, TW4-30 and TW4-33 is generally consistent with downgradient migration of a remnant of the plume. The downgradient expansion at TW4-26 likely results from changes in hydraulic gradients caused by reduction of the groundwater mound associated with the southern wildlife pond as discussed in Section 3.3.4. Although concentrations at many non-pumping wells within (or formerly within) the plume displayed increasing trends prior to stabilizing, concentrations at nearly all wells have been generally decreasing since the end of 2020, including wells TW4-6, TW4-7, TW4-8, TW4-9, TW4-10, TW4-16, TW4-26, TW4-29 and TW4-33. The exception is TW4-30, at which concentrations began to decrease subsequent to the end of 2021. As shown in Figure 10, concentrations at many of the pumping wells, in particular, at TW4-19 and TW4-39, are relatively variable (noisy). Concentrations at TW4-2 and TW4-11 were relatively stable until pumping began in 2015; and concentrations at TW4-1 were relatively stable until 2017 (approximately two years after commencement of pumping). The highest concentrations have been measured at TW4-20 (now abandoned) and TW4-37. Concentrations at TW4-22 and TW4-24 increased substantially as a result of initiation of nitrate pumping in the first quarter of 2013, as chloroform from the vicinity of TW4-20 was drawn to the west. TW4-24 has remained at the margin of (primarily outside of but periodically just within) the chloroform plume since the first quarter of 2014. Although noisy, since the first quarter of 2012, concentrations at the majority of the pumping wells within the chloroform plume have overall decreasing trends, including wells MW-4, MW-26, TW4-4, TW4-20 (now abandoned), TW4-22, TW4-37, TW4-39 and TW4-41. In addition, concentrations at TW4-1 are relatively flat to decreasing; and concentrations at TW4-19 have generally decreased since the second quarter of 2016. The overall decreasing trends at wells typically having the highest concentrations (such as TW4-20 [now abandoned], TW4-22, TW4-37 and TW4-39) are consistent with decreasing mass estimates for the plume as these wells are located within areas having the highest concentrations and saturated thicknesses. The impacts of pumping and reduced wildlife pond recharge on water levels are illustrated in Figure 11 and Figures 12A through 14B. Figure 11 is a contour map based on kriged water level Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 38 data showing the fourth quarter, 2023 chloroform plume boundary and water level changes between the fourth quarter of 2012 (just prior to nitrate pumping) and the fourth quarter of 2023. Changes in the southern extremity of the plume are not shown because wells TW4-29, TW4-33 and TW4-40 were installed after 2012, and data from this portion of the plume were therefore not available for comparison. Figures 12A through 14B graph water levels and saturated thicknesses at selected wells within and marginal to the plume between the first quarter of 2012 and the fourth quarter of 2023. Figures 12A and 12B graph water levels and saturated thicknesses at non-pumping wells marginal to the plume; Figures 13A and 13B graph water levels and saturated thicknesses at non-pumping wells within the plume; and Figures 14A and 14B graph water levels and saturated thicknesses at pumping wells within the plume. TW4-6, TW4-8, TW4-9, TW4-30 and TW4-33 are included in Figures 13A and 13B although they are no longer within the plume. As indicated in Figure 11, since the first quarter of 2012 nearly the entire area of the plume experienced water level decreases. Based on the kriged water level data, the average decrease in water level within the plume between the first quarter of 2012 and the fourth quarter of 2023 was more than 17 feet. As shown in Figure 12A, there were both increases and decreases in water levels at non-pumping wells marginal to the plume, As shown in Figure 13A (except for TW4-30), there were net decreases in water levels at all non-pumping wells within the plume, including TW4-29 and TW4- 40 which were installed after 2012. At wells marginal to the plume, larger decreases typically occurred closer to the northern wildlife ponds (for example, at TW4-5 and TW4-18, adjacent to the northern portion of the plume) in response to reduced wildlife pond recharge. Exceptions are TW4-7 and TW4-8 which experienced relatively large declines once pumping began at adjacent wells TW4-1 and TW4-2 during the first half of 2015. Note that TW4-8 has been outside the plume since the first quarter of 2020. At non-pumping wells within and marginal to the southern extremity of the plume (for example TW4-6, TW4-14, TW4-26, and TW4-27), water level changes were generally smaller (Figures 12A and 13A). Net increases in water levels since 2012 at downgradient locations such as TW4- 14, TW4-27, and newer wells TW4-30 and TW4-31 are the result of past water delivery to the wildlife ponds and the expansion of the associated groundwater mounds. Because of their greater distance from the wildlife ponds these wells are generally expected to respond more slowly to changes in pond recharge compared to wells closer to the ponds (such as TW4-18, close to the northern ponds). In addition, water level responses at wells TW4-14, TW4-27, and TW4-36 are expected to be delayed because of low permeability conditions, and responses at TW4-30 and Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 39 TW4-31 are expected to be delayed because of their relative hydraulic isolation from both the two northern and the southern wildlife ponds. Hydraulic testing and water level behavior (HGC, 2014a and HGC, 2014b) demonstrate that wells TW4-14, TW4-27, and TW4-36 are located in materials that are one to three orders of magnitude lower in conductivity than surrounding materials, in addition, wells south and southeast of TW4- 4 (that include TW4-6, TW4-26, TW4-29, TW4-30, TW4-31, TW4-33, TW4-34, and TW4-35) are completed in materials having conductivities one to two orders of magnitude lower than those penetrated by TW4-4. Eventually, water levels at downgradient wells TW4-14, TW4-27, TW4-30 and TW4-31 have been expected to trend downward in response to reduced wildlife pond recharge as they ‘catch up’ with wells that are closer to the ponds, are completed in higher permeability materials, or both. As shown in Figure 14A, water levels at pumping wells within the chloroform plume decreased over the period, and were generally more variable than at non-pumping wells. Substantial water level decreases occurred in wells TW4-1, TW4-2, TW4-11, and TW4-21 as pumping was initiated in the first and second quarters of 2015. As shown in Figure 13A, substantial water level decreases also occurred in non-pumping wells TW4-7 and TW4-8 in response to initiation of pumping at TW4-1, TW4-2, and TW4-11. Decreases in water levels at TW4-22 since the fourth quarter of 2012 coincide with initiation of nitrate pumping in the first quarter of 2013. Overall, between the fourth quarter of 2012 and the fourth quarter of 2023, hydraulic gradients within the chloroform plume decreased (Table 2). The average hydraulic gradient in the southeastern portion of the plume can be approximated between non-pumping wells TW4-10 and TW4-6 and between TW4-5 and TW4-27 by assuming straight flow paths between the upgradient (TW4-10 and TW4-5) and downgradient wells (TW4-6 and TW4-27). Although TW4-5 and TW4- 27 are located just outside the plume, water level data are available for both quarters. Assuming straight flow paths will underestimate the actual lengths of the flow paths between wells and result in overestimation of the gradients. However, this is considered acceptable for comparison purposes. Assuming straight flow paths, between the fourth quarter of 2012 and the fourth quarter of 2023, the average hydraulic gradient between TW4-10 and TW4-6 decreased from approximately 0.0286 ft/ft to 0.0256 ft/ft, a reduction of more than 10%, and the hydraulic gradient between TW4-5 and TW4-27 decreased from approximately 0.0318 ft/ft to approximately 0.0207 ft/ft, a reduction of 35%. Likewise, the hydraulic gradient within the northwestern portion of the plume can be estimated between non-pumping wells TW4-5 and TW4-16. Between the fourth quarter of 2012 and the Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 40 fourth quarter of 2023, the hydraulic gradient in the northwestern portion of the plume decreased from approximately 0.0203 ft/ft to 0.0134 ft/ft, a reduction of 34%. Figure 15 displays percentage changes in saturated thickness within the chloroform plume between the fourth quarter of 2012 (just prior to nitrate pumping) and the fourth quarter of 2023. As with Figure 11, Figure 15 is based on kriged water level data. Based on data shown in Figure 15, the average percentage decrease in saturated thickness within the plume between the fourth quarter of 2012 and the fourth quarter of 2023 is more than 37%. Changes in the southern extremity of the plume are not shown because wells TW4-29, TW4-30 and TW4-40 were installed after 2012 and data from this portion of the plume are not available for comparison. As indicated in Figure 15 (and Figures 13B and 14B), saturated thicknesses between the fourth quarters of 2012 and 2023 decreased throughout nearly the entire area of the plume. Relatively large decreases in the central portion of the plume are primarily related to addition of pumping wells TW4-1, TW4-2, and TW4-11. The screen of TW4-11 extends approximately 10 feet below the top of the Brushy Basin Member which defines the base of the perched water zone hosted by the overlying Burro Canyon Formation. Pumping of TW4-11 maintains the water level at or below the base of the Burro Canyon Formation (top of the Brushy Basin Member), which yields a saturated thickness approximately equal to or less than zero, even though there may be as much as approximately 10 feet of water in the screen. Similarly, as of the fourth quarter of 2023, water levels in TW4-1 and TW4-2 were below the top of the Brushy Basin Member. The decreases in water levels and saturated thicknesses within the northern portion of the plume, which is closest to the northern wildlife ponds, result primarily from cessation of water delivery to the ponds in March, 2012. As discussed above, water level increases within and bordering the southeastern extremity of the plume result from past wildlife pond recharge and expansion of the associated perched groundwater mounds. Wells within this area that are completed within lower permeability materials (such as TW4-14 and TW4-27), or that are relatively isolated from the influence of the northern two ponds and/or the southern pond by intervening lower permeability materials (such as TW4-30 and TW4-31), have a delayed response to changes in recharge from the ponds. Decreases in saturated thicknesses as of the second quarter of 2015 were discussed in Attachment N (Tab N) of the third quarter, 2015 Chloroform Monitoring report (EFRI 2015d). Water level trends in wells within and bordering the southeastern extremity of the plume, such as TW4-14, TW4-27, TW4-30 and TW4-31, were eventually expected to ‘catch up’ with other nearby wells and begin to decrease. Since 2018, stabilization to slightly decreasing water levels at TW4- Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 41 14, TW4-27 and TW4-30; and since the first quarter of 2021, stabilization of water levels at TW4- 31; are consistent with this expectation. In contrast to many wells located at the plume margins where water levels generally increased between 2012 and 2018 to 2021 (Figure 12A), water levels decreased in all non-pumping wells within the plume (except TW4-30 which is no longer within the plume) as shown in Figure 13A. Such behavior is consistent with the chloroform plume following higher permeability zones that respond more rapidly to changes in pumping and wildlife pond recharge. As shown in Figure 16A, since the first quarter of 2012, the overall trend in calculated plume area is upward while the trend in chloroform residual mass is downward. However, although both were generally increasing until the second quarter of 2016, both were relatively stable between the fourth quarter of 2014 and third quarter of 2015, likely the result of adding TW4-1, TW4-2, TW4- 11, TW4-21, and TW4-37 to the pumping system. There is a general apparent correlation between residual mass and plume area between the first quarter of 2012 and second quarter of 2016 which suggests that expansions occurred in areas having both high concentrations and large saturated thicknesses as well as areas having both low concentrations and small saturated thicknesses (such as the southeast area). Should expansion have occurred only in areas having relatively low concentrations and small saturated thicknesses, the impact on calculated residual masses should also be small to negligible. Subsequent to the second quarter of 2016, the plume area remained relatively stable while residual mass estimates trended strongly downward (although the plume area has trended generally downward since the fourth quarter of 2020). As shown in in Figure 16B which plots plume area and residual mass since implementation of the GCAP in the third quarter of 2015, the overall trend in plume area is essentially flat whereas the trend in residual mass estimates trends downward. The decrease in residual mass estimates since the third quarter of 2015 is attributable to the decreasing trend in average chloroform concentrations within the plume as shown in Figure 8B and to decreases in saturated thicknesses within the plume as shown in Figures 13B, 14B, and 15. Since the first quarter of 2012, the overall trend in residual chloroform mass estimates is downward, while the overall trend in chloroform mass removal rates is slightly downward (Figure 17A). Mass removal rates increased substantially between the fourth quarter of 2014 and third quarter of 2015 (Table 1) even though residual mass estimates were relatively stable over the same period. As discussed above, mass removal rate increases over this period were substantially the result of adding pumping wells TW4-1, TW4-2, TW4-11, TW4-21, and, in particular TW4-37. Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 42 As shown in Figure 17B, since the third quarter of 2015, both chloroform mass removal rates and residual mass estimates are trending relatively strongly downward. As discussed above, residual mass estimates are trending downward due to the decreasing trend in average chloroform concentrations and saturated thicknesses within the plume. As concentrations decrease in pumping wells within the plume, the mass removal rates also decrease (Figure 8C), even if pumping rates remain relatively stable. Figure 18 displays chloroform plume residual mass estimates since the first quarter of 2012, chloroform concentrations from pumping wells MW-26, TW4-19, TW4-20 (now abandoned), TW4-22, and TW4-37, and the average chloroform concentration from these pumping wells. These wells are located in the northwest portion of the plume. The general correlation between the mass estimates and the concentrations in most of these wells indicates the substantial influence of these wells in calculating the mass estimates. Figure 19 displays residual chloroform plume mass estimates since the second quarter of 2006. Prior to the first quarter of 2010, estimates are calculated annually rather than quarterly. As shown, mass estimates were generally stable prior to 2012. Increases after the first quarter of 2012 are attributable to the impacts of reduced wildlife pond recharge (since the first quarter of 2012) and nitrate pumping (since the first quarter of 2013). Decreases subsequent to the peak in the second quarter of 2016 result from reduced concentrations and saturated thicknesses within the plume, attributable to pumping and natural attenuation. 4.2 Capture Effectiveness As discussed in Section 2.2, chloroform pumping is designed to remove chloroform mass from the plume as rapidly as practical by pumping in generally upgradient, more productive locations that also have high chloroform concentrations. Until the installation of TW4-40, pumping in the downgradient toe of the plume was impractical because no wells were completed in materials having sufficient permeability and saturated thickness to make non-negligible extraction rates possible. TW4-40 is valuable in that it is 1) located within the downgradient (southern) toe of the plume and 2) is relatively productive. Pumping of TW4-40 is expected to more effectively reduce or prevent further downgradient plume migration than can be expected by pumping at the more upgradient locations. Pumping at locations downgradient of the plume is not considered desirable. Even if productive wells could be installed, such pumping would, at least temporarily, increase the rate of downgradient plume migration. Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 43 Even before the operation of TW4-40, pumping resulted in hydraulic capture of a substantial portion of the plume. Table 3 summarizes the calculated plume areas, masses, areas under capture and masses under capture for the fourth quarters of 2012 through 2023. Figures 20 through 31 show the areas of the chloroform plume under hydraulic capture from the fourth quarter of 2023 (Figure 20) to the fourth quarter of 2012 (Figure 31). As of the fourth quarter of 2018, prior to operation of TW4-40, approximately 71% of the plume area (Figure 25) and approximately 93% of the residual plume mass were under hydraulic capture. As of the fourth quarter of 2019, subsequent to initiation of pumping of TW4-40, approximately 90% of the plume area (Figure 24) and approximately 99% of the residual plume mass were under hydraulic capture, in part due to expansion of hydraulic capture to the south. The area under hydraulic capture also expanded substantially after initiation of pumping at TW4- 1, TW4-2, TW4-11, TW4-21, and TW4-37 in the first half of 2015. Figures 28 and 29 show the areas of the chloroform plume under hydraulic capture in the fourth quarter of 2015 and fourth quarter of 2014, respectively. The area under capture increased by approximately 30% (from approximately 27 acres to approximately 35 acres [Table 3]) between the fourth quarter of 2014 and fourth quarter of 2015. However, the increase in the proportion of total plume mass under hydraulic capture did not change significantly and was approximately 90% in both cases. This is a consequence of the concentration and saturated thickness distributions within the plume, and is expected based on the historic focus of pumping those portions of the plume having both high concentrations and large saturated thicknesses (and therefore large masses). Figure 30 shows the area of the chloroform plume under hydraulic capture in the fourth quarter of 2013 after three quarters of nitrate pumping were underway. The plume area expanded from approximately 26 to 38 acres (46%) between the fourth quarters of 2012 and 2013 [Table 3] as a result of both reduced wildlife pond recharge and nitrate pumping. Although in the fourth quarter of 2013 only about 55% of the plume area was under capture, about 84% of the plume mass was under capture. Historically, the portions of the plume under hydraulic capture represented areas generally having the highest concentrations and saturated thicknesses and therefore the majority of the plume mass. That a relatively high proportion of the plume mass was under hydraulic capture historically is consistent with conditions prior to the initiation of nitrate pumping. Figure 31 shows the portion of the chloroform plume under hydraulic capture in the fourth quarter of 2012, just prior to the initiation of nitrate pumping. At that time approximately 73% of the plume area was under capture Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 44 (approximately 19 of the total 26 acres of the plume) yet approximately 93% of the plume mass was under capture [Table 3]. 4.3 Pumping Well Productivity An analysis of chloroform pumping well productivity was presented in Attachment N (Tab N) of the third quarter, 2015 Chloroform Monitoring Report (EFRI, 2015d). The analysis discussed the impacts of reduced productivity at chloroform pumping well TW4-19 (and nitrate pumping well TW4-24). Reductions in pumping rates at these wells (primarily TW4-19) caused rebounds in water levels at nearby non-pumping wells TW4-5, TW4-9, TW4-10, TW4-16, and TW4-18. Water level data through the second quarter of 2015 at these non-pumping wells were judged sufficiently unaffected by the startup of new pumping wells TW4-1, TW4-2, and TW4-11 to allow quantitative analysis of the water level changes that resulted from reduced pumping at TW4-19 (and TW4-24). The analysis presented below (including Section 4.3.1) is a revised version of Attachment N. Some of the calculations presented in Attachment N have been refined to be more representative of site conditions, and additional supporting data is provided. Chloroform plume ‘background’ flow calculations are based on data collected through the second quarter of 2015. Because of ongoing reductions in hydraulic gradients and saturated thicknesses within the plume since 2015, calculated ‘background’ flows through the plume are conservatively large with respect to current conditions, and therefore over-represent the actual pumping needed for plume control. Figures and Tables from Attachment N are reproduced in Appendix B as Figures B.1 through B.13 and Tables B.1 through B.4. As shown in Figures B.1 and B.2 (Appendix B), pumping well productivities between the second quarter of 2013 and fourth quarter of 2015 were relatively stable except for chloroform pumping well TW4-19 and nitrate pumping well TW4-24. The productivities of TW4-19 and TW4-24 dropped after the third quarter of 2014. Temporary reductions in pumped chloroform and nitrate masses from TW4-19 and TW4-24 between the third quarter of 2014 and second quarter of 2015 likely resulted from decreased pumping at these wells. Figures 32 and 33 indicate that productivities of these wells have remained at reduced levels though the fourth quarter of 2023 (although pumping at TW4-19 increased after failure of TW4-20 in mid-2020). As per the GCAP and Nitrate CAP (HGC 2012), reductions in productivity of chloroform and nitrate pumping wells requires an evaluation to determine the likely causes and, depending on the results of the evaluation, a decision to either take no additional action, or to take action that may include rehabilitation or replacement of the affected wells, or installation of additional wells. Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 45 Although under the final chloroform GCAP such an evaluation is only required as part of the two- year review process (CACME), to be proactive, and due to chloroform and nitrate pumping system overlap, the evaluation of both systems commenced prior to approval of the final chloroform GCAP. Lost productivity may result from several causes. Likely causes at the Mill include: interference between relatively large numbers of closely spaced extraction wells; reductions in hydraulic gradients resulting from reduced wildlife pond recharge; reduced transmissivities as saturated thicknesses decline due to reduced wildlife pond recharge and increases in the number of pumping wells; potentially lower average hydraulic conductivity related to saturated thickness declines (that presumably have resulted in dewatering of relatively shallow zones of higher permeability); and losses in well efficiency. Reduced productivity at TW4-24 doesn’t significantly affect chloroform mass removal because TW4-24 is primarily a nitrate pumping well and because of relatively low chloroform concentrations. The impact of reduced productivity at TW4-19 on total quarterly chloroform mass removal is mitigated by factors that include: 1) reduced wildlife pond recharge that reduces non-pumping ‘background’ flow through the chloroform plume as a result of decreases in saturated thicknesses, decreases in hydraulic gradients, and potentially lower average hydraulic conductivities; 2) chloroform concentrations at TW4-19 are on average lower than concentrations at nearby chloroform pumping wells; 3) the addition to the chloroform pumping system of TW4-1, TW4-2, TW4-11, TW4-21and TW4-37 during the first half of 2015; and 4) the subsequent addition to the chloroform pumping system of TW4-39, TW4-40 and TW4-41. Pumping these eight additional wells, in particular TW4-37, increased total chloroform pumping and mass removal rates (as discussed above) and reduced the relative importance of TW4-19. The addition of TW4-37 is particularly important because the average mass removal rate at TW4-37 is higher than at TW4- 19 both before and after the reduction in TW4-19 productivity. As discussed in HGC (2020), between startup of TW4-37 in the second quarter of 2015 and the fourth quarter of 2019, the approximately 222 lbs. of chloroform removed by TW4-37 was more than two and one-half times the 87 lbs. removed by TW4-19 over the same nearly 5-year period. In addition, the 87 lb. removed by TW4-19 over this same nearly five year period was only slightly smaller than the mass of chloroform removed by TW4-19 over the same length of time prior to the loss in productivity. From the first quarter of 2010 through the third quarter of 2014, before the reduction in productivity, approximately 90 lbs. of chloroform was removed by TW4-19, only Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 46 slightly more than the 87 lbs. removed after the reduction in productivity (from the second quarter of 2015 through the fourth quarter of 2019). Furthermore, over the longer term, mass removal rates at TW4-37 have remained larger than mass removal rates at TW4-19. Between startup of TW4-37 in the second quarter of 2015 and the fourth quarter of 2023, approximately 311 lbs. of chloroform have been removed by TW4-37 compared with approximately 150 lbs. removed by TW4-19. Figures 34A and 34B compare chloroform mass removal rates at TW4-19 and total chloroform mass removal rates. The stronger downward trends since the third quarter of 2015 (Figure 34B) primarily result from generally reduced concentrations. Furthermore, because of continued reductions in saturated thicknesses and hydraulic gradients resulting from reduced wildlife pond recharge, ‘background’ flows through both chloroform and nitrate plumes are expected to continue to diminish, thereby reducing the pumping needed to control both plumes. An updated calculation of ‘background‘ flow that accounts for changes in saturated thicknesses and hydraulic gradients since the 2015 recalculation is presented in Section 4.3.2. As discussed above, the impact of reduced productivity at TW4-19 was mitigated by the beneficial impacts of adding eight wells to the chloroform pumping system since the beginning of 2015, in particular TW4-37, which reduced the relative importance of TW4-19. Therefore no additional action regarding reduced productivity at TW4-19 has been deemed necessary, especially considering the productivity increase after TW4-20 abandonment. As shown in Figure 32, productivity at TW4-4 since the third quarter of 2016 has also dropped. Reduced productivity at TW4-4 is likely related in part to reduced saturated thicknesses at this location. In response to increased chloroform concentrations at TW4-26, and reduced productivity at TW4-4, new pumping well TW4-41 was installed during February, 2018 (Figure 1B) in accordance with EFRI (2017e). Operation of TW4-41 has not only mitigated the impact of reduced productivity at TW4-4, it has resulted in the development of a well-defined capture zone in the vicinity of TW4-4 and contributed to an increase in hydraulic capture in the southern portion of the plume. Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 47 4.3.1 Comparison of Pumping and Flow through the Chloroform Plume Over Time and Re-calculation of ‘Background’ Flow in 2015 Reduced productivity at TW4-19 since the third quarter of 2014 is likely the result of four factors other than potential losses in well efficiency: 1) smaller hydraulic gradients related to reduced wildlife pond recharge; 2) smaller saturated thicknesses, also related to reduced wildlife pond recharge; 3) smaller average hydraulic conductivities (presumably as a result of dewatering relatively shallow zones of higher permeability); and 4) interference between pumping wells. ‘Background’ flow through the chloroform plume will be affected by the first three factors because it is meant to represent the condition that would arise in the absence of pumping. The pre-chloroform pumping hydraulic gradient within the chloroform plume can be calculated using water levels at wells TW4-5 and TW4-16 and wells TW4-10 and TW4-6. The pair TW4- 5/TW4-16 can be used to represent the hydraulic gradient within the northwestern portion of the plume and the pair TW4-10/TW4-6 can be used to represent the hydraulic gradient within the southeastern portion of the plume. Using these well pairs also allows representative comparison to current gradients because these wells have not been pumped. As discussed in Section 4.1, the calculated hydraulic gradients are likely overestimated because straight flow paths are assumed between upgradient and downgradient wells. Using the above well pairs and water level data from September 2002 (Figure B.3), a pre-pumping hydraulic gradient of approximately 0.022 ft/ft for the northwestern portion of the plume and a pre-pumping hydraulic gradient of approximately 0.039 ft/ft for the southeastern portion of the plume were calculated. As of the second quarter of 2015, the hydraulic gradient within the northwestern and southeastern portions of the plume were approximately 0.017 ft/ft (a 23% reduction) and 0.026 ft/ft (a 33% reduction), respectively. Hydraulic gradient calculations are summarized in Table B.1. The hydraulic gradient within the chloroform plume has been reduced by decay of the groundwater mound resulting from cessation of water delivery to the northern wildlife ponds combined with pumping. Reliable separation of these effects within much of the chloroform plume was problematic because pumping of the plume had been underway for more than 12 years. However, pre-pumping flow through the plume based on pre-pumping gradients and saturated thicknesses can be compared with the pumping during the 2003 7-month long pumping test (HGC, 2004). Representative hydraulic conductivities within and adjacent to the northwestern and southeastern portions of the plume are provided in Table B.2. The geometric average hydraulic conductivity for the northwest portion of the plume is calculated as 0.52 feet per day (ft/day), and for the Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 48 southeastern portion of the plume as 0.20 ft/day. Data used in the calculations are based on the results of the 7-month long pumping test (HGC, 2004) and on slug tests performed at individual wells. Estimates provided in Table B.2 assumed unconfined conditions to be compatible with the use of the full saturated thicknesses in the flow calculations. In addition, the result of the analysis of well MW-4A based on early-time data was used because it yielded a conservatively large conductivity (0.32 ft/day compared to 0.15 ft/day). Slug test results for wells TW4-4, TW4-20 through TW4-22, and TW4-33 are based on analysis of automatically logged data using the KGS solution provided in AQTESOLVE (HydroSOLVE, 2000). For the northwestern portion of the plume, assuming that the average hydraulic conductivity was 0.52 ft/day, the pre-pumping plume width was approximately 1,200 feet (northern cross-section in Figure B.3), the hydraulic gradient was approximately 0.022 ft/ft, and the saturated thickness was approximately 65 feet (based on September 2002 data from wells TW4-5, TW4-9, TW4-10, TW4- 16, TW4-18, and TW4-19) a pre-pumping flow of approximately 4.6 gpm is calculated. For the southeastern portion of the plume, assuming that the average hydraulic conductivity is 0.20 ft/day, the pre-pumping plume width was approximately 600 feet (southern cross-section in Figure B.3), the hydraulic gradient was approximately 0.039 ft/ft, and the saturated thickness was approximately 29 feet (based on September 2002 data from wells TW4-1, TW4-2, TW4-6, TW4- 7, TW4-8, and TW4-11) a pre-pumping flow of approximately 0.71 gpm is calculated. The total pre-pumping flow through the plume was therefore approximately 5.3 gpm. During the 7-month long pumping test, the average pumping within the plume was approximately 6.4 gpm (from wells MW-4, MW-26 [TW4-15], and TW4-19). Therefore, pumping within the plume was approximately 1.1 gpm (21%) higher than the calculated total pre-pumping flow. Because calculated pre-pumping flow through the plume was likely overestimated due to overestimation of hydraulic gradients, pumping may have exceeded pre-pumping flow by more than 1.1 gpm. Saturated thicknesses within the chloroform plume declined between initiation of pumping in 2003 and 2015. Changes between 2002 and 2015 can be estimated using data from wells within and adjacent to the plume that were installed prior to 2003 but were not been pumped until 2015. Up to the fourth quarter of 2014 (the last quarter prior to addition of wells TW4-1, TW4-2, TW4-11, and TW4-21 to the pumping system), wells TW4-1, TW4-2, TW4-5, TW4-6, TW4-7, TW4-8, TW4-9, TW4-10, TW4-11, TW4-16, and TW4-18 meet these criteria. Changes in saturated thickness within and adjacent to the northwest portion of the plume can be estimated using data from wells TW4-5, TW4-9, TW4-10, TW4-16, and TW4-18, and changes in the southeast portion of the plume can be estimated using data from wells TW4-1, TW4-2, TW4-6, TW4-7, TW4-8, and TW4-11. Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 49 Using data from the above listed wells, between September 2002 and the fourth quarter of 2014, the reduction in saturated thickness in the northwestern portion of the chloroform plume is approximately 11%. Conversely, the saturated thickness in the southeastern portion of the plume increased by approximately 12% over this time period. These results are summarized in Table B.3 As discussed above, the potential exists for average hydraulic conductivity to change as a result of reductions in saturated thickness. An assessment of the potential change in transmissivity (product of saturated thickness and hydraulic conductivity) was performed based on changes in water levels in non-pumping observation wells TW4-5, TW4-9, TW4-10, TW4-16, and TW4-18 that resulted from reduced pumping at TW4-19 and TW4-24. Water levels at these wells, measured monthly since the first quarter of 2013 as part of the nitrate program, clearly responded to the reductions in pumping at TW4-19 and TW4-24. As shown in Figures B.4 through B.8, the downward trends in water levels in these wells were halted or reversed once pumping was reduced. These same wells responded to pumping of TW4-19 during the 2003 7-month long pumping test. By superposition, the reduced pumping at TW4-19 and TW4-24 can be simulated as injection of water at these locations at rates equivalent to the decreases in rates of pumping at these locations. Water level changes (displacements) at non-pumping observation wells in response to reduced pumping were calculated by de-trending the water level data at wells TW4-5, TW4-9, TW4-10, TW4-16, and TW4-18 to eliminate water level reductions attributable primarily to reduced wildlife pond recharge. The de-trended data were analyzed as an equivalent injection test using the well hydraulics interpretation software WHIP (HGC, 1998). The previous use of WHIP at the Mill is described in HGC (2002). WHIP was chosen for the analysis because it is designed to interpret both pumping and injection tests. Figures B.9 through B.13 provide the results and the fits between measured and simulated displacements at observation wells TW4-5, TW4-9, TW4-10, TW4-16, and TW4-18. Transmissivity estimates are similar, but lower, than estimates derived from the 7-month long pumping test (HGC, 2004). The reduction in transmissivity appears to be primarily related to reduced saturated thickness; however, as shown in Table B.4, compared to the year 2003 analysis, the transmissivity is reduced on average by approximately 27% whereas the average saturated thickness (representing at each observation well the average of the saturated thicknesses at pumping well TW4-19 and the observation well) is reduced on average by only about 20%. This implies a reduction in average conductivity of approximately 9%. Assuming that the following changes occurred in the northwestern portion of the chloroform plume between September 2002 and the second quarter of 2015: the conductivity was reduced by 9%; Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 50 the hydraulic gradient was reduced by 23%; and the saturated thickness was reduced by 11%; the ‘background’ flow through the plume would be approximately 62% of its initial value. As of the second quarter of 2015, these changes imply a reduced ‘background’ flow of approximately 2.9 gpm, which is approximately 37% lower than the pre-pumping calculated flow of 4.6 gpm. For the southeastern portion of the plume, assuming no change in hydraulic conductivity, a reduction in hydraulic gradient of 33%, and an increase in saturated thickness of approximately 12%, a new ‘background’ flow of approximately 0.53 gpm is calculated. The total ‘background’ flow through the plume as of the second quarter of 2015 is therefore approximately 3.4 gpm. As with the pre- pumping flow calculation, because of the assumption of straight flow paths in calculating hydraulic gradients, the new ‘background’ flow is likely overestimated. Total chloroform pumping is assumed to include pumping from all chloroform pumping wells and pumping from nitrate pumping wells TW4-22 and TW4-24. Total pumping from the chloroform plume from the third quarter of 2014 through the fourth quarter of 2023 has exceeded the 2015 ‘background’ flow estimate of 3.4 gpm each quarter except the first quarter of 2015. Temporarily reduced pumping during the first quarter of 2015 resulted in part from down time related to upgrading discharge lines to accommodate the addition of wells TW4-1, TW4-2, TW4-11, TW4- 21, and TW4-37 to the pumping system. Since 2015, total pumping (from wells MW-4, MW-26, TW4-1, TW4-2, TW4-4, TW4-11, TW4-19, TW4-20, TW4-21, TW4-22, TW4-24, TW4-37 and [since the fourth quarter of 2016] TW4-39) exceeded the 2015 ‘background’ flow estimate by between approximately 0.82 and 2.46 gpm (averaging 1.63 gpm), indicating that pumping has been adequate even with reduced productivity at TW4-19. Furthermore, as discussed in Section 4.1, hydraulic gradients and saturated thicknesses within the plume have continued to decrease since 2015. The ongoing decreases in hydraulic gradients and saturated thicknesses reduce the ‘background’ flow through the plume and consequently reduce the rate of pumping needed to control the plume. Therefore, pumping since 2015 likely exceeds the actual ‘background’ flow on average by more than 1.63 gpm. 4.3.2 Re-calculation of ‘Background’ Flow Based on Changes Since 2015 As discussed above, reductions in hydraulic gradients and saturated thicknesses within the chloroform plume since 2015 have reduced the rate of groundwater flow through the plume. A recalculation of ‘background’ flow based on data collected since 2015 is presented below. As will be shown, the current ‘background’ flow is more than 30 % lower than the value of 3.4 gpm calculated based on the 2015 data. The revised ‘background’ flow calculation is based on the average saturated thicknesses of the northern and southern portions of the chloroform plume and Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 51 the hydraulic gradients through both portions of the plume. The average saturated thicknesses are based on the fourth quarter, 2023 saturated thicknesses of non-pumping wells located either within or immediately adjacent to the relevant portions of the plume. Likewise, the average hydraulic gradients are based on fourth quarter, 2023 water levels of non-pumping wells either within or immediately adjacent to the relevant portions of the plume. Saturated thickness and hydraulic gradient calculations are summarized, respectively, in Tables 4 and 5. As shown in Table 4, the average saturated thickness of the northern portion of the plume, approximately 48 ft, is based on the saturated thicknesses of non-pumping wells TW4-5, TW4-9, TW4-10, TW4-16 and TW4-18; and the average saturated thickness of the southern portion of the plume, approximately 11 feet, is based on. the saturated thicknesses of non-pumping wells TW4- 6, TW4-26, TW4-27, TW4-29 and TW4-33. Based on the same sets of wells, the average saturated thickness for the northern portion of the plume, 48 ft, is nearly 17% smaller than the second quarter, 2015 average saturated thickness of approximately 57.6 ft; and the average saturated thickness for the southern portion of the plume, 11 ft, is approximately 37 % smaller than the second quarter, 2015 average saturated thickness of approximately 17.5 ft (Table 4). As shown in Table 5, the hydraulic gradient for the northern portion of the plume, 0.0134 ft/ft is based on the water levels and distance between non-pumping wells TW4-5 and TW4-16. This is approximately 21 % smaller than the second quarter, 2015 hydraulic gradient of approximately 0.017 ft/ft. The hydraulic gradient for the southern portion of the plume, 0.0256 ft/ft is based on the water levels and distance between non-pumping wells TW4-10 and TW4-6. This is approximately 1.6 % smaller than the second quarter, 2015 hydraulic gradient of approximately 0.0260 ft/ft (Table 5). Using these changes in saturated thicknesses and hydraulic gradients, and assuming the same plume widths as used in the second quarter, 2015 calculations, the second quarter, 2015 calculation of 2.9 gpm for the northern portion of the plume is reduced by approximately 34%, to 1.91 gpm; and the second quarter, 2015 calculation of 0.53 gpm for the southern portion of the plume is reduced by approximately 38%, to 0.33 gpm. The total flow though the plume is therefore reduced from 3.4 gpm (as calculated for the second quarter of 2015) to 2.24 gpm as of the fourth quarter of 2023. An even lower ‘background’ flow though the plume would be calculated if 1) the reduction in plume width since 2015 was taken into account; and 2) if the reduction in hydraulic gradient for the southern portion of the plume was calculated based on the change in hydraulic gradient between wells TW4-5 and TW4-27 (rather than between wells TW4-10 and TW4-6, as shown in Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 52 Table 5). The choice of using wells TW4-10 and TW4-6 for the hydraulic gradient calculations ensures a conservatively large ‘background’ flow estimate for the plume. Based on the new ‘background’ flow of 2.24 gpm, 2023 chloroform pumping (averaging approximately 4.35 gpm per quarter) has exceeded background by an average of approximately 2.1 gpm, or 94%. 4.3.3 Evaluation of Interference between Pumping Wells Closely spaced pumping wells will ‘interfere’ with one another as they ‘compete’ for groundwater. This ‘interference’ reduces the productivities of the individual wells. While adding wells will likely increase total pumping, a point will be reached where the gains are negligible. Reduced productivity at individual wells results in part from reduced saturated thicknesses as overall pumping increases with the addition of wells. Addition of wells also creates stagnation points between wells; by superposition, an effective no-flow boundary is created between pumping wells. Because of the effective creation of a no-flow boundary between pumping wells, rectangular grids of wells or triangular patterns of wells may be undesirable. The creation of effective no-flow boundaries increases the rates of drawdowns at individual wells as well as the rates of reductions in saturated thicknesses within pumped areas; both reduce individual well productivities. A quantitative analysis of interference within the chloroform and nitrate pumping systems is considered unnecessary at this time; both chloroform and nitrate pumping appear adequate even with reduced productivity at TW4-4, TW4-19 and TW4-24; with abandonment of TW4-20 due to collapse; and as a result of generally decreased productivities of many other chloroform pumping wells due to the general decline in saturated thicknesses within the plume as will be discussed in Section 4.3.5. Reduced chloroform pumping at TW4-19 after the third quarter of 2014 was mitigated by the addition of TW4-1, TW4-2, TW4-11, TW4-21, TW4-37 and TW4-39 to the chloroform pumping system in 2015 and 2016. Operation of pumping well TW4-41 has mitigated reduced pumping at TW4-4; and the addition of both TW4-40 and TW4-41 has resulted in significant expansion of pumping system capture to the south, apparently incorporating much of the chloroform plume detected in the vicinity of TW4-26 and TW4-40. Although pumping at TW4-19 decreased substantially subsequent to the third quarter of 2014, then remained relatively stable from the third quarter of 2015 until 2020, pumping increased between mid-2020 and the fourth quarter of 2021 subsequent to the collapse and abandonment of TW4-20, Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 53 as discussed in the 2022 CACME (HGC, 2022a). Furthermore, even though the general decline in saturated thicknesses has reduced productivity in many wells, as discussed in Section 4.3.2, pumping exceeds ‘background’ flow through the plume. 4.3.4 Evaluation of TW4-20 Abandonment As discussed above, TW4-20 collapsed during July, 2020 and was abandoned during October, 2020. Details regarding the failure and abandonment of TW4-20 are provided in EFRI (2020d). The collapse was caused by failure of the pump timing device during the second quarter of 2020. The timing device cycles the pump on and off to prevent drying up of the well and damage to the pump. Cycling is necessitated by the relatively low productivity of wells at the Mill caused by low permeability. Pumps cannot be operated continuously at low enough pumping rates to prevent drying up of the wells and consequent damage to the pumps. Failure of the timing device at TW4- 20 caused nearly continuous pumping that dried up the well and irreparably damaged both the pump and the well casing. Both pump and well failed early in the third quarter of 2020; efforts to rehabilitate the well were unsuccessful and the well was subsequently abandoned. An analysis of the impact of abandoning TW4-20 on mass removal rates and capture effectiveness was presented in the 2022 CACME (HGC, 2022a). Analysis of analytical, groundwater level, and pumping rate data from adjacent pumping wells TW4-19, TW4-37 and TW4-39 from the third quarter of 2020 through the fourth quarter of 2021 demonstrated that TW4-20 abandonment had little or no impact on total pumping, mass removal rates, groundwater levels, hydraulic gradients, or flow directions in the vicinity of TW4-20. The analysis concluded that, due to: 1) the negligible impact of TW4-20 abandonment on mass removal rates and capture zone extent in this area of the plume; 2) the increase in pumping rates in this area subsequent to TW4-20 abandonment; and 3) the likelihood that interference between pumping wells would counteract any potential benefits that may accrue should TW4-20 be replaced; there was no technical justification for replacing TW4-20. In addition, due to anticipated reductions in achievable pumping in nearby wells as a result of interference, replacing TW4-20 was considered unlikely to achieve any significant benefit with regard to control of the chloroform plume. 4.3.5 Reduced Productivity of Many Chloroform Pumping Wells As shown in Figure 35A, since the third quarter of 2015, the productivities of wells TW4-1, TW4- 2, TW4-4, TW4-11, TW4-21, TW4-37, TW4-39, TW4-40 and TW4-41 have declined. Similarly, Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 54 prior to abandonment, the productivity of TW4-20 had also declined. In contrast, the productivity of chloroform pumping well MW-26 has been stable to increasing; and, subsequent to the approximately 80% productivity decline at chloroform pumping well TW4-19 between the third quarter of 2014 and the first quarter of 2015, the productivity of TW4-19 has increased. A portion of the increase occurred between the first and third quarters of 2015, after which productivity generally stabilized at nearly double the first quarter, 2015 low. An additional increase in TW4-19 productivity during 2020 is attributable to the failure and abandonment of adjacent chloroform pumping well TW4-20 (HGC, 2022a). As discussed in Section 4 and as shown in Figures 35A and 35B, generally declining chloroform pumping well productivity correlates to the general decline in saturated thicknesses at pumping wells having declining productivities. Figure 35B displays the total pumping from wells having declining productivities and the change in saturated thickness using an arithmetic (rather than logarithmic) scale. Since the third quarter of 2015, the total productivity of wells shown in Figures 35A and 35B has declined by approximately 38% (from approximately 374,163 to 232,981 gallons per quarter), while the average saturated thickness at these wells has declined by more than 46% (from approximately 30.1 ft to approximately 16 ft). Based on the gridded data (shown in Figure 15), the average plume saturated thickness has declined by more than 37%, similar to the 38% reduction in total productivity. Because declines in saturated thicknesses also reduce the volumetric rate of flow through the chloroform plume, less pumping is needed to control the plume. Calculations presented in Section 4.3.2 indicate that, based on the new ‘background’ flow of 2.24 gpm through the plume, chloroform pumping during 2023 has exceeded background by an average of approximately 2.10 gpm (or 94%), indicating that pumping is more than adequate for plume control. 4.4 Natural Attenuation Natural attenuation processes that include dilution, hydrodynamic dispersion, volatilization, and biodegradation enhance the effectiveness of the pumping system by reducing chloroform concentrations within and at the margins of the plume. Calculations of chloroform biodegradation rates based on daughter product concentrations using a first order decay model were provided in the PCAP (HGC, 2007a). These calculations (reproduced in Appendix C) indicated that chloroform concentrations everywhere within the plume were expected to be reduced to the action level of 70 µg/L within less than 200 years solely by natural attenuation (including biodegradation), without taking into consideration any pumping. Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 55 In addition to the calculations provided in the PCAP, Appendix C provides updated calculations based on chloroform and daughter product concentrations over the period covering the first quarter of 2013 through the third quarter of 2023. These calculations are based on concentrations of chloroform and methylene chloride in wells within and marginal to the plume that had at least one detection of both chloroform and methylene chloride within the same sample. Data from wells MW-26, TW4-14, TW4-19, TW4-20, TW4-22, TW4-24, TW4-37 and TW4-39 met these criteria. The largest data set was derived from MW-26 (sampled both quarterly and monthly), which had detectable and reportable chloroform and methylene chloride concentrations during 121 sampling events. TW4-20 (now abandoned) met these criteria 10 consecutive times from the first quarter of 2013 through the second quarter of 2015; once during the first quarter of 2016; twice during the first and second quarters of 2018; and once during the second quarter of 2020. TW4-37 met these criteria 5 consecutive times from the second quarter of 2015 through the second quarter of 2016; and during the fourth quarter of 2021. TW4-22 met these criteria during the first quarter of 2014; and for four consecutive quarters from the fourth quarter of 2022 through the third quarter of 2023. TW4-14 met these criteria during each quarter of 2016. TW4-39 met these criteria during the first through third quarters of 2017. Finally TW4-19 met these criteria once during the third quarter of 2021; and TW4-24 met these criteria once during the first quarter of 2014. Using the same approach described in the PCAP, rates of chloroform degradation were calculated for each well. Because MW-26 had the most methylene chloride detections, and because the average quarterly chloroform concentration at MW-26 since the end of 2012 (approximately 1,650 µg/L) was similar to the average chloroform concentration within the plume (approximately 1,360 µg/L based on gridded quarterly concentration data and approximately 2,745 µg/L based on the average quarterly concentrations at wells within the plume), calculations using MW-26 data are likely to be the most reliable and representative. However, to be conservative, calculations using data from all wells having both chloroform and methylene chloride detections are provided. Based on the rates calculated for each well (Table C.1), the geometric average first order degradation rate is -1.01 x 10-4/day. To reduce the highest 2023 concentration of 14,000 µg/L to the GCAL of 70 µg/L would take approximately 52,459 days or 144 years, even in the absence of pumping. If the rate of -3.17 x 10-4/day calculated using MW-26 data were used, the time to reduce the highest 2023 concentration of 14,000 µg/L to the GCAL of 70 µg/L would take approximately 16,714 days or only 46 years. Furthermore, direct mass removal by pumping, dilution, volatilization, and hydrodynamic dispersion are expected to independently reduce concentrations and plume remediation times. Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 56 Calculated remediation times are therefore conservative and likely underestimate actual concentration reduction rates, overestimate plume remediation times in the absence of pumping, and significantly overestimate plume remediation times considering the beneficial impacts of pumping. Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 57 5. EFFECTIVENESS OF GCAP IN PROTECTING PUBLIC HEALTH AND THE ENVIRONMENT Actions carried out under the PCAP and the final GCAP have maintained control of the chloroform plume, and have been consistent with the objectives specified in Part I of the final GCAP to “permanently restore groundwater quality in all pumping wells and performance monitoring wells completed in the White Mesa shallow aquifer for all contaminants of concern in accordance with the Ground Water Corrective Action Objectives”. Continued implementation of the GCAP is expected to reduce COC concentrations to levels at or below the GCALs specified in Table 2 of the GCAP. As chloroform concentrations are reduced to the chloroform GCAL (70 µg/L), concentrations of other COCs associated with the chloroform plume are also expected to be reduced to their respective GCALs. Control of the chloroform plume, and the eventual reduction of chloroform concentrations to levels at or below the GCAL, are supported by the following: 1. The plume is currently bounded by twenty compliance wells having concentrations that are below the GCAL of 70 µg/L: seventeen of these wells are specified in Table 1A of the GCAP and TW4-38, TW4-42 and TW4-43 are new compliance wells installed since implementation of the GCAP. Fifteen of these twenty bounding wells are non-detect for chloroform as of the fourth quarter of 2023. The thirteen of these non-detect wells that are closest to the plume are MW-32, TW4-3, TW4-12, TW4-13, TW4-14, TW4-23, TW4-25, TW4-28, TW4-34, TW4-36, TW4-38, TW4-42 and TW4-43. In addition, non-detect wells MW-28 and MW-31 bound the plume to the west; and non-detect well MW-27 bounds the plume to the north-northwest. The plume is also bounded far to the south-southwest by non-detect well MW-17; and far to the south by non-detect well MW-40. 2. TW4-42 bounds the plume immediately to the south. TW4-40 was installed once concentrations at TW4-26 exceeded 70 µg/L for two consecutive quarters; likewise, TW4- 42 was installed once concentrations at TW4-40 exceeded 70 µg/L for two consecutive quarters. TW4-40 has been converted to a pumping well. The plume is bounded far to the south by MW-22 and MW-40 (both non-detect for chloroform). 3. Chloroform concentrations at TW4-30 first exceeded 70 ug/L during the fourth quarter of 2020 and remained above 70 ug/L until the third quarter of 2022. Because TW4-30 no longer bounded the chloroform plume to the east of TW4-29, new compliance well TW4- 43 was installed approximately 200 feet east-southeast of TW4-30 during September 2021. As chloroform has not been detected at TW4-43, it served as the new bounding compliance well to the south-southeast of the plume until concentrations at TW4-30 again dropped below 70 ug/L during the third quarter of 2022 and TW4-30 again bounds the plume to the south-southeast. Hydraulically, TW4-43 is located generally cross- to downgradient of TW4-30. Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 58 4. The plume is approximately 1,200 feet from the closest (eastern) property boundary (as of the fourth quarter of 2023), and perched water flow is sub-parallel to that boundary. In addition, continued migration of chloroform to the east in the vicinity of TW4-30 is unlikely as the hydraulic gradient near TW4-30 is now more southerly than easterly, due to long term changes in water levels in this area that are attributable to both TW4-4 and TW4-41 pumping and reduced wildlife pond recharge. 5. Both saturated thicknesses and hydraulic gradients within the plume have diminished. Since 2012, decreases in saturated thickness average more than 37%; and decreases in hydraulic gradients range between more than 10% and 35%. 6. The plume boundary has been relatively stable since 2014 except for cross-gradient contraction near TW4-6, TW4-8, TW4-9, TW4-16 and TW4-33; cross-gradient expansion near TW4-21 and TW4-24; slight cross-to downgradient expansion near TW4-30; and southerly (downgradient) expansion in the vicinity of TW4-26. Between the fourth quarters of 2014 and 2023 the plume area maintained relative stability (increasing by less than 6%); and, since the third quarter of 2015, the trend in plume area has been essentially flat. Between the fourth quarters of 2021 and 2023 the plume area increased only slightly (by less than 1%). 7. Average chloroform concentrations within the plume and residual mass estimates have both trended downward since inception of the GCAP in the third quarter of 2015. The downward trend in residual mass estimates results from reductions in both average concentrations and saturated thicknesses. 8. Approximately 92% of the plume mass and 77% of the plume area are under hydraulic capture as of the fourth quarter of 2023. The plume mass under capture has remained between 79% and 99% since the fourth quarter of 2021 (the end of the period covered by the previous CACME). 9. The area of the plume under hydraulic capture increased by 30% between the fourth quarters of 2014 and 2015 due to the addition of pumping wells TW4-1, TW4-2, TW4-11, TW4-21, and TW4-37 during the first half of 2015. 10. Pumping exceeds calculated total flow through the plume, indicating that pumping is adequate. Since 2015, excluding the anomalously large pumping that occurred during the second quarter of 2020 (when the pump timer at TW4-20 failed), chloroform pumping has exceeded the conservatively large calculated ‘background flows’ through the plume. Pumping has exceeded the 2015 calculated ‘background’ flow of 3.4 gpm by between approximately 0.82 gpm and 2.46 gpm (24% to 72%), or on average 1.63 gpm (48%). Pumping during 2023 has exceeded the most recently updated ‘background’ flow of 2.24 gpm (Section 4.3.2) by between approximately 1.98 gpm and 2.36 gpm (88% to 105%), or on average 2.1 gpm (94%). 11. Although the productivities of TW4-19 and nitrate pumping well TW4-24 have diminished since the third quarter of 2014, total chloroform mass removal rates more than tripled between the third quarter of 2014 (10.2 lbs/quarter) and fourth quarter of 2015 (32.3 lbs/quarter), primarily the result of adding TW4-1, TW4-2, TW4-11, TW4-21, and TW4- Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 59 37 to the pumping system. In addition, 5-year average TW4-19 chloroform mass removal rates were similar before and after the loss in productivity after the third quarter of 2014 (Section 4.3). Furthermore, since 2015, the pumping system has been additionally augmented by pumping TW4-39, TW4-40 and TW4-41. 12. Reductions in productivity at TW4-4 have been addressed by operation of pumping well TW4-41 since the second quarter of 2018. Pumping of TW4-41 and TW4-40 (since the second quarter of 2019) has expanded hydraulic capture to the south. Chloroform detected in the vicinity of TW4-26 and TW4-40 appears largely to be within the hydraulic capture zone of TW4-40. 13. TW4-40 is particularly valuable because it is located within the downgradient (southern) toe of the plume and is relatively productive. Pumping of TW4-40 is likely to more effectively reduce or prevent further downgradient plume migration than can be expected by pumping at the more upgradient locations. 14. The abandonment of TW4-20 had little to no measurable impact on pumping, mass removal rates, and capture in the vicinity of TW4-20 (Section 4.3.4). Pumping increases at TW4-19 subsequent to TW4-20 failure more than compensated for the loss in pumping at TW4-20. In addition, as discussed in Section 4.3.5, since inception of the GCAP in the third quarter of 2015, a general decrease in productivity in most pumping wells within the plume is the result of the decreased saturated thicknesses within the plume caused by pumping and reduced wildlife pond recharge. However, based on the new ‘background’ flow of 2.24 gpm through the plume (calculated in Section 4.3.2), chloroform pumping during 2023 has exceeded background by an average of approximately 2.1 gpm (or 94%), indicating that pumping is more than adequate for plume control. 15. Although partly due to the decreasing productivity of many pumping wells, the decreasing trend in pumped mass removal rate since inception of the GCAP (third quarter of 2015) results primarily from decreases in average chloroform concentrations within the plume. As concentrations decrease in pumping wells within the plume, the mass removal rates also decrease, even if pumping rates remain relatively stable. 16. Calculations of natural degradation of chloroform (based on daughter product concentrations as discussed in Appendix C) indicate that, using an overall average degradation rate, reducing the highest 2023 concentration of 14,000 µg/L to the GCAL of 70 µg/L would take approximately 52,459 days or 144 years, even in the absence of pumping. If the degradation rate is based only on MW-26 data, the time to reduce the highest 2023 concentration of 14,000 µg/L to the GCAL of 70 µg/L would take approximately 16,714 days or only 46 years. As discussed above, the plume boundary is approximately 1,200 feet from the nearest (eastern) property boundary, and perched water flow appears to be sub-parallel to that boundary, making it unlikely that chloroform would migrate across that boundary. Regardless, GCAP procedures are in place to ensure that the plume will not reach a property boundary (in particular, the eastern Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 60 property boundary). In addition, although the plume has expanded and concentrations at many wells at least temporarily increased (as anticipated) after cessation of water delivery to the northern wildlife ponds and initiation of nitrate pumping, the highest concentration detected during 2023 (14,000 µg/L at TW4-37) is less than one-quarter of the historic maximum of approximately 61,000 µg/L (at TW4-20 during the second quarter of 2006). Furthermore, as discussed above, natural attenuation calculations provided in Appendix C and HGC (2007a) suggest that all chloroform concentrations will be below the GCAL within less than 200 years (and possibly in as short a time as 46 years), not taking into account the effects of any pumping. Therefore, the GCAP is considered effective in protecting public health and the environment. Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 61 6. CONCLUSIONS AND RECOMMENDATIONS Actions taken by the Mill in response to regulatory requirements include cessation of water delivery to the northern wildlife ponds in the first quarter of 2012 and the initiation of nitrate pumping in the first quarter of 2013. The northern wildlife ponds are located immediately upgradient of the chloroform and nitrate plumes at the site. The above two actions have changed perched groundwater flow dynamics, thereby impacting chloroform concentrations and chloroform plume boundaries. Chloroform pumping since 2003, reduced wildlife pond recharge since 2012, and nitrate pumping since 2013 have reduced hydraulic gradients and saturated thicknesses, and consequently the rate of groundwater flow through the chloroform plume. Because the rate of groundwater flow through the plume has been reduced, the rate of groundwater pumping needed to control the plume has been reduced. Between the fourth quarters of 2012 and 2023, the calculated hydraulic gradient within the northwestern portion of the plume, between non-pumping wells TW4-5 and TW4-16, decreased from approximately 0.0203 ft/ft to 0.0134 ft/ft, a reduction of 34%. Likewise, the calculated hydraulic gradients within the southeastern portion of the plume decreased: the calculated hydraulic gradient between non-pumping wells TW4-10 and TW4-6 decreased from approximately 0.0286 ft/ft to 0.0256 ft/ft, a reduction of more than 10%; and the calculated hydraulic gradient between TW4-5 and TW4-27 (both adjacent to the plume) decreased from approximately 0.0318 ft/ft to approximately 0.0207 ft/ft, a reduction of 35%. Based on kriged water level data, the average percentage decrease in saturated thickness within the plume between the fourth quarters of 2012 and 2023 is more than 37%. Since 2012, although recharge has been reduced, water levels in many wells marginal to the chloroform plume are increasing to stable, while water levels in all wells within the plume are decreasing. Water levels in TW4-30, within the plume from the fourth quarter of 2020 through the second quarter of 2022, were increasing until about the first quarter of 2020; subsequently water levels stabilized and began to decrease. Water level behavior in wells within the plume is consistent with the chloroform plume following higher permeability zones that respond more rapidly to chloroform pumping and changes in wildlife pond recharge. The number of chloroform pumping wells doubled (from five to ten) during the first half of 2015. Doubling the number of chloroform pumping wells more than tripled the short-term rate of chloroform mass removal from approximately 10.2 lbs/quarter in the third quarter of 2014 to 32.3 lbs/quarter by the fourth quarter of 2015. Since 2015, the chloroform pumping system has Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 62 continued to expand through the addition of TW4-39, TW4-40 and TW4-41. Chloroform pumping since 2015 has exceeded the conservatively large calculated ‘background flow’ through the plume (3.4 gpm, based on 2015 data) by between approximately 0.82 gpm and 2.46 gpm (24% to 72%), or on average 1.63 gpm (48%), indicating that pumping is adequate. Chloroform pumping during 2023 has exceeded the 2023 calculated ‘background flow’ through the plume (2.24 gpm as presented in Section 4.3.2) by between approximately 1.98 gpm and 2.36 gpm (88% to 105%), or on average 2.1 gpm (94%), indicating that pumping is adequate by an even larger margin. Since implementation of the GCAP in the third quarter of 2015, chloroform plume residual mass estimates have trended downward. The decrease in residual mass estimates is a consequence of decreases in average concentrations and reduced saturated thicknesses within the plume. In addition, since implementation of the GCAP, pumped mass removal rates have trended downward. Decreasing mass removal rates are attributable primarily to the reduced concentrations within the plume. As concentrations decrease in pumping wells within the plume, the pumped mass removal rates also decrease, even if pumping rates remain relatively stable. The chloroform plume boundary expanded between the first quarter of 2012 and fourth quarter of 2014 as a result of reduced dilution (caused by reduced wildlife pond recharge since the first quarter of 2012) and nitrate pumping since the first quarter of 2013. Operation of nitrate pumping wells TW4-22 and TW4-24 in particular caused westerly migration of chloroform from the vicinity of TW4-20 (now abandoned) and westerly expansion of the plume boundary. However, expansion was nearly halted by the fourth quarter of 2014. Plume boundaries were relatively stable between the fourth quarters of 2014 and 2015. Since 2014 the increase in plume area is less than 6% and plume boundaries have remained relatively stable except for cross-gradient contraction near TW4-6, TW4-8, TW4-9, TW4-16 and TW4-33; cross-gradient expansion near TW4-21 and TW4-24; southerly (downgradient) expansion in the vicinity of TW4-26; and slight cross- to downgradient expansion near TW4-30. The contraction near TW4-6 and TW4-33 is attributable to pumping at TW4-4 enhanced by pumping at adjacent well TW4-41. The downgradient expansion near TW4-26 is attributable primarily to changes in hydraulic gradients resulting from decay of the groundwater mound associated with the southern wildlife pond. Doubling the number of chloroform pumping wells during 2015 also expanded the area of the plume under hydraulic capture by approximately 30%. As of the fourth quarter of 2015, approximately 74% of the plume area and approximately 89% of the residual plume mass were under hydraulic capture. Although the area under capture increased by approximately 30% (from Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 63 approximately 27 acres to approximately 35 acres) between the fourth quarters of 2014 and 2015, the increase in the proportion of total plume mass under hydraulic capture did not change significantly and remained at approximately 90%. This is a consequence of the concentration and saturated thickness distributions within the plume, and is expected based on the historic focus of pumping those portions of the plume having both high chloroform concentrations and large saturated thicknesses (and therefore large chloroform masses). Additional expansion of capture since 2015 has resulted from adding TW4-39, TW4-40 and TW4- 41 to the pumping system. As of the fourth quarter of 2023, approximately 77% of the plume area and approximately 92% of the plume mass are under capture. Since the end of 2012, total pumping from the chloroform plume (including pumping from nitrate wells TW4-22 and TW4-24, located within and just inside the margin of the chloroform plume, respectively) has increased from approximately 2.8 gpm to approximately 4.3 gpm. The chloroform plume is completely bounded by twenty compliance wells having concentrations that are below the GCAL of 70 µg/L: seventeen of these wells are specified in Table 1A of the GCAP and TW4-38, TW4-42 and TW4-43 are new compliance wells installed since implementation of the GCAP. Fifteen of these twenty bounding wells are non-detect for chloroform as of the fourth quarter of 2023. The thirteen of these non-detect wells that are closest to the plume are MW-32, TW4-3, TW4-12, TW4-13, TW4-14, TW4-23, TW4-25, TW4-28, TW4- 34, TW4-36, TW4-38, TW4-42 and TW4-43. TW4-43 was installed as a new compliance well once concentrations at TW4-30 exceeded 70 µg/L for two consecutive quarters. However, concentrations at TW4-30 again dropped below 70 µg/L during the third quarter of 2022 and TW4- 30 again bounds the plume to the southeast. In addition, non-detect wells MW-28 and MW-31 bound the plume to the west; and non-detect well MW-27 bounds the plume to the north-northwest. The plume is also bounded far to the south- southwest by non-detect well MW-17; and far to the south by non-detect well MW-40. The plume is also approximately 1,200 feet from the closest (eastern) property boundary (as of the fourth quarter of 2023). Because perched water flow is approximately parallel to that boundary, any chloroform encroaching on that boundary is unlikely to cross the boundary. The southern portion of the plume is bounded immediately to the south by TW4-42, and far to the south by MW-22 and MW-40 (all non-detect for chloroform). TW4-40 was installed once concentrations at TW4-26 exceeded 70 µg/L for two consecutive quarters; likewise, TW4-42 was installed once concentrations at TW4-40 exceeded 70 µg/L for two consecutive quarters. TW4-40 has been converted to a pumping well. Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 64 At the southeast extremity of the plume, chloroform concentrations at TW4-33 (no longer within the plume) that are relatively stable to decreasing; and concentrations at downgradient well TW4- 29 that were generally increasing until concentrations stabilized and began to decrease; suggest that chloroform migration has been arrested at TW4-33 by TW4-4 (and TW4-41) pumping and that chloroform concentration behavior at TW4-29 results from a remnant of the plume that migrated downgradient from TW4-33 to TW4-29; then toward TW4-30. As discussed above, once chloroform concentrations at TW4-30 exceeded 70 ug/L for two consecutive quarters, new compliance well TW4-43 was installed approximately 200 feet east-southeast of TW4-30; and because concentrations at TW4-30 again dropped below 70 µg/L during the third quarter of 2022, TW4-30 again bounds the plume to the southeast. Likewise, generally decreasing chloroform concentrations at downgradient well TW4-6 since the first quarter of 2015 and increasing to relatively stable concentrations at TW4-26 since the first quarter of 2016 suggest that chloroform migration has been arrested at TW4-6 by TW4-4 (and TW4-41) pumping and that increasing to relatively stable chloroform at TW4-26 results from a remnant of the plume that migrated downgradient to the south. Generally decreasing concentrations at TW4-26 since the end of 2020 are consistent with this interpretation. That chloroform trends at wells within and near the southern extremity of the plume are largely influenced by TW4-4 and TW4-42 pumping is also consistent with the ‘pinching off’ of the southern extremity of the plume by the kriging algorithm since 2019. In addition, continued migration of chloroform to the east in the vicinity of TW4-30 is unlikely as the hydraulic gradient near TW4-30 is now more southerly than easterly, due to long term changes in water levels in this area that are attributable to both TW4-4 and TW4-41 pumping and reduced wildlife pond recharge. The generally decreasing concentration trend at TW4-30 since the end of 2021 that has once again brought TW4-30 outside the plume (since the third quarter of 2022) is consistent with the changing hydraulic gradient. TW4-40 began pumping during the second quarter of 2019. TW4-40 is particularly valuable because it is located within the downgradient (southern) toe of the plume, south of TW4-26, and is relatively productive. Pumping of TW4-40 is likely to more effectively reduce or prevent further downgradient plume migration than can be expected by pumping at the more upgradient locations. Chloroform detected in the vicinity of TW4-26 and TW4-40 appears largely to be within the hydraulic capture zone of TW4-40. Based on the above factors, and as discussed in Section 5, the chloroform plume is under control and the GCAP has been effective in protecting public health and the environment. In particular, Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 65 current pumping system effectiveness is demonstrated by 1) the slowing to near halting of plume boundary expansion attributable to reduced dilution from reduced wildlife pond recharge and redistribution of chloroform resulting from nitrate pumping; and 2) maintaining a large proportion of the plume mass under hydraulic capture (approximately 92% as of the fourth quarter of 2023). High rates of capture have been maintained even considering reduced productivities at some of the pumping wells and the failure and subsequent abandonment of TW4-20. As discussed in Section 4.3.4, because the abandonment of TW4-20 had little to no measurable impact on pumping, mass removal rates, and capture in the vicinity of TW4-20, there is no technical justification for replacing TW4-20. Increases in pumping at TW4-19 subsequent to TW4-20 failure more than compensated for the loss of pumping at TW4-20. Furthermore, natural attenuation calculations provided in Appendix C and HGC (2007a) suggest that all chloroform concentrations will be below the GCAL within less than 200 years, not taking into account the effects of any pumping. Specifically, using the average calculated chloroform degradation rate (Appendix C), reducing the highest 2023 chloroform concentration of 14,000 µg/L to the GCAL of 70 µg/L would take approximately 52,459 days or 144 years, even in the absence of pumping. If the degradation rate is based only on MW-26 data, the time to reduce the highest 2023 concentration of 14,000 µg/L to the GCAL of 70 µg/L would take approximately 16,714 days or only 46 years Therefore, continued implementation of the GCAP and the current pumping system is recommended. Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 66 Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 67 7. REFERENCES Energy Fuels Resources (USA) Inc. (EFRI). 2013a. White Mesa Uranium Mill Chloroform Monitoring Report, State of Utah Notice of Violation and Groundwater Corrective Action Order UDEQ Docket No. UGW-20-01, 1st Quarter (January through March) 2013. June 1, 2013. 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White Mesa Uranium Mill Chloroform Monitoring Report, State of Utah Notice of Violation and Groundwater Corrective Action Order UDEQ Docket No. UGW-20-01, 1st Quarter (January through March) 2014. June 1, 2014. EFRI. 2014c. White Mesa Uranium Mill Chloroform Monitoring Report, State of Utah Notice of Violation and Groundwater Corrective Action Order UDEQ Docket No. UGW-20-01, 2nd Quarter (April through June) 2014. September 1, 2014. EFRI. 2014d. White Mesa Uranium Mill Chloroform Monitoring Report, State of Utah Notice of Violation and Groundwater Corrective Action Order UDEQ Docket No. UGW-20-01, 3rd Quarter (July through September) 2014. December 1, 2014. EFRI. 2015a. White Mesa Uranium Mill Chloroform Monitoring Report, State of Utah Notice of Violation and Groundwater Corrective Action Order UDEQ Docket No. UGW-20-01, 4th Quarter (October through December) 2014. March 1, 2015. EFRI. 2015b. 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White Mesa Uranium Mill Chloroform Monitoring Report, State of Utah Notice of Violation and Groundwater Corrective Action Order UDEQ Docket No. UGW-20-01, 4th Quarter (October through December) 2016. March 1, 2017. EFRI. 2017b. White Mesa Uranium Mill Chloroform Monitoring Report, State of Utah Notice of Violation and Groundwater Corrective Action Order UDEQ Docket No. UGW-20-01, 1st Quarter (January through March) 2017. June 1, 2017. EFRI. 2017c. White Mesa Uranium Mill Chloroform Monitoring Report, State of Utah Notice of Violation and Groundwater Corrective Action Order UDEQ Docket No. UGW-20-01, 2nd Quarter (April through June) 2017. September 1, 2017. EFRI. 2017d. White Mesa Uranium Mill Chloroform Monitoring Report, State of Utah Notice of Violation and Groundwater Corrective Action Order UDEQ Docket No. UGW-20-01, 3rd Quarter (July through September) 2017. December 1, 2017. EFRI. 2017e. Plan and Time Schedule Under Part II.H.2 For Exceedances in TW4-26 in the Third Quarter of 2017. January 10, 2018. EFRI. 2018a. White Mesa Uranium Mill Chloroform Monitoring Report, State of Utah Notice of Violation and Groundwater Corrective Action Order UDEQ Docket No. UGW-20-01, 4th Quarter (October through December) 2017. March 1, 2018. EFRI. 2018b. White Mesa Uranium Mill Chloroform Monitoring Report, State of Utah Notice of Violation and Groundwater Corrective Action Order UDEQ Docket No. UGW-20-01, 1st Quarter (January through March) 2018. June 1, 2018. EFRI. 2018c. White Mesa Uranium Mill Chloroform Monitoring Report, State of Utah Notice of Violation and Groundwater Corrective Action Order UDEQ Docket No. UGW-20-01, 2nd Quarter (April through June) 2018. September 1, 2018. EFRI. 2018d. White Mesa Uranium Mill Chloroform Monitoring Report, State of Utah Notice of Violation and Groundwater Corrective Action Order UDEQ Docket No. UGW-20-01, 3rd Quarter (July through September) 2018. December 1, 2018. Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 69 EFRI. 2019a. White Mesa Uranium Mill Chloroform Monitoring Report, State of Utah Notice of Violation and Groundwater Corrective Action Order UDEQ Docket No. UGW-20-01, 4th Quarter (October through December) 2018. March 1, 2019. EFRI. 2019b. White Mesa Uranium Mill Chloroform Monitoring Report, State of Utah Notice of Violation and Groundwater Corrective Action Order UDEQ Docket No. UGW-20-01, 1st Quarter (January through March) 2019. June 1, 2019. EFRI. 2019c. White Mesa Uranium Mill Chloroform Monitoring Report, State of Utah Notice of Violation and Groundwater Corrective Action Order UDEQ Docket No. UGW-20-01, 2nd Quarter (April through June) 2019. September 1, 2019. EFRI. 2019d. White Mesa Uranium Mill Chloroform Monitoring Report, State of Utah Notice of Violation and Groundwater Corrective Action Order UDEQ Docket No. UGW-20-01, 3rd Quarter (July through September) 2019. December 1, 2019. EFRI. 2020a. White Mesa Uranium Mill Chloroform Monitoring Report, State of Utah Notice of Violation and Groundwater Corrective Action Order UDEQ Docket No. UGW-20-01, 4th Quarter (October through December) 2019. March 1, 2020. EFRI. 2020b. White Mesa Uranium Mill Chloroform Monitoring Report, State of Utah Notice of Violation and Groundwater Corrective Action Order UDEQ Docket No. UGW-20-01, 1st Quarter (January through March) 2020. June 1, 2020. EFRI. 2020c. White Mesa Uranium Mill Chloroform Monitoring Report, State of Utah Notice of Violation and Groundwater Corrective Action Order UDEQ Docket No. UGW-20-01, 2nd Quarter (April through June) 2020. September 1, 2020. EFRI. 2020d. White Mesa Uranium Mill Chloroform Monitoring Report, State of Utah Notice of Violation and Groundwater Corrective Action Order UDEQ Docket No. UGW-20-01, 3rd Quarter (July through September) 2020. December 1, 2020. EFRI. 2021a. White Mesa Uranium Mill Chloroform Monitoring Report, State of Utah Notice of Violation and Groundwater Corrective Action Order UDEQ Docket No. UGW-20-01, 4th Quarter (October through December) 2021. March 1, 2021. EFRI. 2021b. White Mesa Uranium Mill Chloroform Monitoring Report, State of Utah Notice of Violation and Groundwater Corrective Action Order UDEQ Docket No. UGW-20-01, 1st Quarter (January through March) 2021. June 1, 2021. EFRI. 2021c. White Mesa Uranium Mill Chloroform Monitoring Report, State of Utah Notice of Violation and Groundwater Corrective Action Order UDEQ Docket No. UGW-20-01, 2nd Quarter (April through June) 2021. September 1, 2021. EFRI. 2021d. White Mesa Uranium Mill Chloroform Monitoring Report, State of Utah Notice of Violation and Groundwater Corrective Action Order UDEQ Docket No. UGW-20-01, 3rd Quarter (July through September) 2021. December 1, 2021. Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 70 EFRI. 2022a. White Mesa Uranium Mill Chloroform Monitoring Report, State of Utah Notice of Violation and Groundwater Corrective Action Order UDEQ Docket No. UGW-20-01, 4th Quarter (October through December) 2021. March 1, 2022. EFRI. 2022b. White Mesa Uranium Mill Chloroform Monitoring Report, State of Utah Notice of Violation and Groundwater Corrective Action Order UDEQ Docket No. UGW-20-01, 1st Quarter (January through March) 2022. June 1, 2022. EFRI. 2022c. White Mesa Uranium Mill Chloroform Monitoring Report, State of Utah Notice of Violation and Groundwater Corrective Action Order UDEQ Docket No. UGW-20-01, 2nd Quarter (April through June) 2022. September 1, 2022. EFRI. 2022d. White Mesa Uranium Mill Chloroform Monitoring Report, State of Utah Notice of Violation and Groundwater Corrective Action Order UDEQ Docket No. UGW-20-01, 3rd Quarter (July through September) 2022. December 1, 2022. EFRI. 2023a. White Mesa Uranium Mill Chloroform Monitoring Report, State of Utah Notice of Violation and Groundwater Corrective Action Order UDEQ Docket No. UGW-20-01, 4th Quarter (October through December) 2022. March 1, 2023. EFRI. 2023b. White Mesa Uranium Mill Chloroform Monitoring Report, State of Utah Notice of Violation and Groundwater Corrective Action Order UDEQ Docket No. UGW-20-01, 1st Quarter (January through March) 2023. June 1, 2023. EFRI. 2023c. White Mesa Uranium Mill Chloroform Monitoring Report, State of Utah Notice of Violation and Groundwater Corrective Action Order UDEQ Docket No. UGW-20-01, 2nd Quarter (April through June) 2023. September 1, 2023. EFRI. 2023d. White Mesa Uranium Mill Chloroform Monitoring Report, State of Utah Notice of Violation and Groundwater Corrective Action Order UDEQ Docket No. UGW-20-01, 3rd Quarter (July through September) 2023. December 1, 2023. EFRI. 2024. White Mesa Uranium Mill Chloroform Monitoring Report, State of Utah Notice of Violation and Groundwater Corrective Action Order UDEQ Docket No. UGW-20-01, 4th Quarter (October through December) 2023. March 1, 2024. Hydro Geo Chem, Inc. (HGC). 1988. WHIP. Well Hydraulics Interpretation Program, Version 3.22, User’s Manual. July, 1988 HGC. 2002. Hydraulic Testing at the White Mesa Uranium Mill Near Blanding, Utah During July 2002. Submitted to International Uranium Corporation. August 22, 2002 HGC. 2004. Final Report. Long Term Pumping at MW-4, TW4-10, and TW4-15. White Mesa Uranium Mill Near Blanding, Utah. May 26, 2004. HGC. 2007a. Preliminary Corrective Action Plan. White Mesa Uranium Mill Site Near Blanding, Utah. August 20, 2007. HGC. 2007b. Preliminary Contamination Investigation Report. White Mesa Uranium Mill Site Near Blanding, Utah. November 20, 2007. Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 71 HGC. 2010. Hydraulic Testing of TW4-4, TW4-6, and TW4-26. White Mesa Uranium Mill. July 2010. September 20, 2010. HGC. 2011a. Redevelopment of Existing Perched Monitoring Wells. White Mesa Uranium Mill Near Blanding, Utah. September 30, 2011 HGC. 2011b. Installation, Hydraulic Testing, and Perched Zone Hydrogeology of Perched Monitoring Well TW4-27. White Mesa Uranium Mill Near Blanding Utah. November 28, 2011. HGC. 2012. Corrective Action Plan for Nitrate. White Mesa Uranium Mill Near Blanding, Utah. May 7, 2012 HGC. 2013a. Installation and Hydraulic Testing of Perched Monitoring Wells TW4-28 through TW4-31. White Mesa Uranium Mill Near Blanding Utah. April 30, 2013. HGC. 2013b. Installation and Hydraulic Testing of Perched Monitoring Wells TW4-32 through TW4-34. White Mesa Uranium Mill Near Blanding Utah.As Built Report. October 30, 2013. HGC. 2014a. Contamination Investigation Report. TW4-12 and TW4-27 Areas. White Mesa Uranium Mill Near Blanding, Utah. January 23, 2014. HGC. 2014b Installation and Hydraulic Testing of Perched Monitoring Wells TW4-35 and TW4- 36. White Mesa Uranium Mill Near Blanding Utah. As Built Report. July 1, 2014. HGC. 2015. Installation and Hydraulic Testing of Perched Monitoring Well TW4-37. White Mesa Uranium Mill Near Blanding Utah. As Built Report. May 12, 2015. HGC, 2016a. Corrective Action Comprehensive Monitoring Evaluation (CACME) Report, White Mesa Uranium Mill Near Blanding, Utah. March 31, 2016. HGC. 2016b. Installation and Hydraulic Testing of Perched Monitoring Wells TW4-38 and TW4-39. White Mesa Uranium Mill Near Blanding Utah. As Built Report. December 8, 2016. HGC, 2018a. Corrective Action Comprehensive Monitoring Evaluation (CACME) Report, White Mesa Uranium Mill Near Blanding, Utah. March 31, 2018. HGC. 2018b. Installation and Hydraulic Testing of Perched Monitoring Wells TW4-40 and TW4-41. White Mesa Uranium Mill Near Blanding Utah. As Built Report. April 10, 2018. HGC. 2018c. Hydrogeology of the White Mesa Uranium Mill and Recommended Locations of New Perched Wells to Monitor Proposed Cells 5A and 5B. July 11, 2018. HGC. 2019. Installation and Hydraulic Testing of Perched Monitoring Well TW4-42. White Mesa Uranium Mill Near Blanding Utah. As Built Report. June 11, 2019. HGC, 2020. Corrective Action Comprehensive Monitoring Evaluation (CACME) Report, White Mesa Uranium Mill Near Blanding, Utah. March 30, 2020. Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 72 HGC. 2021. Installation and Hydraulic Testing of Perched Monitoring Well TW4-43. White Mesa Uranium Mill Near Blanding Utah. As Built Report. November 8, 2021. HGC, 2022a. Corrective Action Comprehensive Monitoring Evaluation (CACME) Report, White Mesa Uranium Mill Near Blanding, Utah. March 30, 2022. HGC. 2022b. Hydrogeology of the White Mesa Uranium Mill. July 13, 2022. HydroSOLVE, Inc. 2000. AQTESOLVE for Windows. User=s Guide. Utah Department of Environmental Quality Division of Solid Waste and Radiation Control, 2015. Letter to Mr. David Frydenlund, Energy Fuels Resources (USA) Inc. September 16, 2015. Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 73 8. LIMITATIONS The information and conclusions presented in this report are based upon the scope of services and information obtained through the performance of the services, as agreed upon by HGC and the party for whom this report was originally prepared. Results of any investigations, tests, or findings presented in this report apply solely to conditions existing at the time HGC’s investigative work was performed and are inherently based on and limited to the available data and the extent of the investigation activities. No representation, warranty, or guarantee, express or implied, is intended or given. HGC makes no representation as to the accuracy or completeness of any information provided by other parties not under contract to HGC to the extent that HGC relied upon that information. This report is expressly for the sole and exclusive use of the party for whom this report was originally prepared and for the particular purpose that it was intended. Reuse of this report, or any portion thereof, for other than its intended purpose, or if modified, or if used by third parties, shall be at the sole risk of the user. Corrective Action Comprehensive Monitoring Evaluation Report White Mesa Uranium Mill, Near Blanding, Utah https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/CACME_March 2024.docx March 28, 2024 74 TABLES TABLE 1 Plume Area, Mass Removed/Quarter,Residual Mass, and Average Concentration, First Quarter 2012 through Fourth Quarter 2023 Number Plume Total Mass Residual 1Average 2Average of Plume Area Removed/Quarter Plume Mass Chloroform Chloroform Quarter Wells (m2)(lb)(lb)Concentration (ug/L)Concentration (ug/L) Q1 2012 12 1.01E+05 10.6 492 1946 843 Q2 2012 12 9.59E+04 10.4 623 3884 1125 Q3 2012 12 1.09E+05 6.6 640 2195 1000 Q4 2012 12 1.07E+05 7.2 649 2598 1029 Q1 2013 12 1.20E+05 13.7 1373 3755 1784 Q2 2013 13 1.24E+05 13.5 1480 3993 1920 Q3 2013 13 1.41E+05 31.1 1858 4464 2125 Q4 2013 14 1.54E+05 10.3 1206 3060 1390 Q1 2014 16 1.59E+05 8.6 1321 2749 1477 Q2 2014 15 1.62E+05 9.9 1334 3152 1518 Q3 2014 18 1.88E+05 10.2 1416 2175 1330 Q4 2014 18 1.89E+05 14.6 1677 2990 1632 Q1 2015 17 1.93E+05 9.9 1638 3047 1549 Q2 2015 17 1.77E+05 15.3 1670 4273 1661 Q3 2015 18 1.86E+05 33.4 1712 3541 1576 Q4 2015 18 1.91E+05 32.3 1869 3767 1756 Q1 2016 18 1.86E+05 30.6 1946 3843 1847 Q2 2016 19 2.00E+05 39.4 2261 4599 2012 Q3 2016 18 1.87E+05 24.7 1717 3766 1757 Q4 2016 20 1.89E+05 25.6 1704 3301 1652 Q1 2017 19 1.87E+05 27.2 1271 3602 1535 Q2 2017 20 1.92E+05 19.6 1372 2935 1339 Q3 2017 21 2.00E+05 28.5 1948 3834 1794 Q4 2017 21 1.94E+05 16.9 884 2153 948 Q1 2018 22 2.05E+05 18.0 1271 2432 1234 Q2 2018 22 2.04E+05 20.1 1271 2484 1318 Q3 2018 21 1.99E+05 17.6 1208 2511 1231 Q4 2018 21 1.96E+05 20.8 1107 2546 1289 Q1 2019 21 1.86E+05 16.2 833 2307 1093 Q2 2019 21 1.98E+05 26.2 1408 3316 1632 Q3 2019 21 1.97E+05 18.3 1050 2390 1251 Q4 2019 21 1.98E+05 13.0 770 2043 992 Q1 2020 20 1.98E+05 18.9 1106 2757 1298 Q2 2020 20 1.96E+05 25.2 1056 2404 1240 Q3 2020 19 2.00E+05 13.4 909 2104 1092 Q4 2020 21 2.02E+05 9.6 715 1396 803 Q1 2021 20 2.01E+05 16.8 1273 1986 1338 Q2 2021 20 1.97E+05 11.8 784 1790 965 Q3 2021 20 1.98E+05 13.4 933 1637 1045 Q4 2021 20 1.99E+05 16.1 981 1945 1136 Q1 2022 20 1.93E+05 8.4 711 1556 840 Q2 2022 20 1.93E+05 21.8 1124 3215 1307 Q3 2022 19 1.95E+05 12.3 850 1772 1004 Q4 2022 19 1.88E+05 15.0 893 1796 1089 Q1 2023 19 1.85E+05 5.9 444 1202 680 Q2 2023 19 1.89E+05 11.4 928 1797 1134 Q3 2023 18 1.75E+05 7.2 663 1644 870 Q4 2023 19 2.00E+05 10.6 845 1971 1010 Notes: 1 = average of concentrations in wells within plume 2 = average plume concentrations based on gridded data (weighted average) lb = pounds ug/L = micrograms per liter m2 = square meters https://hgcinc.sharepoint.com/VOL4/718000/feb24/Chl_WL_1Q12_4Q23.xlsx: Table 1 TABLE 1 Plume Area, Mass Removed/Quarter,Residual Mass, and Average Concentration, First Quarter 2012 through Fourth Quarter 2023 https://hgcinc.sharepoint.com/VOL4/718000/feb24/Chl_WL_1Q12_4Q23.xlsx: Table 1 TABLE 2 Hydraulic Gradients Within Chloroform Plume 4th Quarters of 2012, 2015, 2017, 2019, 2021 and 2023 Q4 2012 Q4 2015 Q4 2017 Q4 2019 Q4 2021 Q4 2023 Well Water Level Water Level Water Level Water Level Water Level Water Level (ft amsl) (ft amsl) (ft amsl) (ft amsl) (ft amsl) (ft amsl) TW4-5 5583.9 5576.3 5573.5 5570.7 5569.0 5567.1 TW4-6 5538.7 5537.4 5534.5 5531.7 5529.8 5528.4 TW4-10 5577.8 5572.4 5569.4 5566.8 5565.0 5563.2 TW4-16 5563.8 5560.8 5558.5 5556.9 5555.2 5553.8 TW4-27 5524.9 5527.9 5528.9 5529.1 5529.1 5528.7 approximate approximate approximate distance (ft)distance (ft)distance (ft) TW4-10 to TW4-6 TW4-5 to TW4-27 TW4-5 to TW4-16 1365 1855 990 Q4 2012 hydraulic Q4 2012 hydraulic Q4 2012 hydraulic gradient (ft/ft)gradient (ft/ft)gradient (ft/ft) TW4-10 to TW4-6 TW4-5 to TW4-27 TW4-5 to TW4-16 0.0286 0.0318 0.0203 Q4 2015 hydraulic Q4 2015 hydraulic Q4 2015 hydraulic gradient (ft/ft)gradient (ft/ft)gradient (ft/ft) TW4-10 to TW4-6 TW4-5 to TW4-27 TW4-5 to TW4-16 0.0256 0.0261 0.0157 Q4 2017 hydraulic Q4 2017 hydraulic Q4 2017 hydraulic gradient (ft/ft)gradient (ft/ft)gradient (ft/ft) TW4-10 to TW4-6 TW4-5 to TW4-27 TW4-5 to TW4-16 0.0255 0.0241 0.0151 Q4 2019 hydraulic Q4 2019 hydraulic Q4 2019 hydraulic gradient (ft/ft)gradient (ft/ft)gradient (ft/ft) TW4-10 to TW4-6 TW4-5 to TW4-27 TW4-5 to TW4-16 0.0258 0.0224 0.0140 Q4 2021 hydraulic Q4 2021 hydraulic Q4 2021 hydraulic gradient (ft/ft)gradient (ft/ft)gradient (ft/ft) TW4-10 to TW4-6 TW4-5 to TW4-27 TW4-5 to TW4-16 0.0258 0.0215 0.0139 Q4 2023 hydraulic Q4 2023 hydraulic Q4 2023 hydraulic gradient (ft/ft)gradient (ft/ft)gradient (ft/ft) TW4-10 to TW4-6 TW4-5 to TW4-27 TW4-5 to TW4-16 0.0256 0.0207 0.0134 Notes: ft amsl = feet above mean sea level ft/ft = feet per foot https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Tables&Graphs/Chl_WL_1Q12_4Q23_CACME.xlsx: Table 2 TABLE 3 Plume Areas and Masses Under Capture Fourth Quarters of 2012 Through 2023 Plume Residual Plume Area Chloroform Mass Percentage Mass Area Plume Mass Under Capture Under Capture Under Capture Quarter (acres) (lbs) (acres) (lbs) (%) Q4 2012 26 649 19 602 93 Q4 2013 38 1206 21 1009 84 Q4 2014 47 1677 27 1507 90 Q4 2015 47 1869 35 1672 89 Q4 2016 47 1704 34 1432 84 Q4 2017 48 884 35 766 87 Q4 2018 48 1107 34 1031 93 Q4 2019 49 770 44 764 99 Q4 2020 50 715 42 639 89 Q4 2021 49 981 42 952 97 Q4 2022 46 893 41 883 99 Q4 2023 49 845 38 777 92 Notes: lbs = pounds % = percent https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Tables&Graphs/Chl_WL_1Q12_4Q23_CACME.xlsx: Table 3 Table 4 Changes in Saturated Thickness Within Northern and Southern Portions of the Chloroform Plume Q4 2012 Q2 2015 Q4 2023 change (feet) portion Well Saturated Thickness Saturated Thickness Saturated Thickness Q2 2015 to of (feet) (feet) (feet) Q4 2023 plume TW4-5 47.6 41.2 30.8 -10.4 northern TW4-9 50.8 44.4 34.1 -10.3 northern TW4-10 51.8 47.3 37.2 -10.1 northern TW4-16 82.6 79.4 72.6 -6.9 northern TW4-18 83.1 75.5 65.3 -10.3 northern TW4-6 26.4 25.5 16.0 -9.4 southern TW4-26 21.2 19.2 9.4 -9.8 southern TW4-27 9.7 12.7 13.5 0.9 southern TW4-29 NA 18.4 11.7 -6.7 southern TW4-33 NA 11.8 4.2 -7.5 southern average (north portion)NA 57.6 48.0 -9.6 change since 2Q 2015 -16.6 % change since 2Q 2015 average (south portion) NA 17.5 11.0 -6.5 change since 2Q 2015 -37.3 % change since 2Q 2015 Table 5 Changes in Hydraulic Gradients Within Northern and Southern Portions of the Chloroform Plume Q4 2012 Q2 2015 Q4 2023 Well Water Level Water Level Water Level (ft amsl) (ft amsl) (ft amsl) TW4-5 5583.9 5577.5 5567.1 TW4-9 5582.9 5576.5 5566.2 TW4-10 5577.8 5573.3 5563.2 TW4-16 5563.8 5560.6 5553.8 TW4-18 5584.7 5577.2 5566.9 TW4-6 5538.7 5537.8 5528.4 TW4-26 5538.2 5536.2 5526.3 TW4-27 5524.9 5527.9 5528.7 TW4-29 NA 5532.9 5526.3 TW4-33 NA 5535.0 5527.5 southern portion southern portion northern portion approximate approximate approximate distance (ft) distance (ft) distance (ft) TW4-10 to TW4-6 TW4-5 to TW4-27 TW4-5 to TW4-16 1365 1855 990 Q2 2015 hydraulic Q2 2015 hydraulic Q2 2015 hydraulic gradient (ft/ft) gradient (ft/ft) gradient (ft/ft) TW4-10 to TW4-6 TW4-5 to TW4-27 TW4-5 to TW4-16 0.0260 0.0267 0.0170 Q4 2023 hydraulic Q4 2023 hydraulic Q4 2023 hydraulic gradient (ft/ft) gradient (ft/ft) gradient (ft/ft) TW4-10 to TW4-6 TW4-5 to TW4-27 TW4-5 to TW4-16 0.0256 0.0207 0.0134 Notes: ft = feet ft/ft = feet per foot ft amsl = feet above mean sea level FIGURES HYDRO GEO CHEM, INC. 1 mile WHITE MESA Mill Site CORRAL CANYON CORRAL SPRINGS COTTONWOOD ENTRANCE SPRING RUIN SPRING WESTWATER Cell 1 Cell 2 Cell 3 Cell 4A Cell 4B MW-01 MW-02 MW-3A MW-11 MW-14MW-15 MW-17 MW-18 MW-19 MW-20 MW-21 MW-22 MW-23 MW-24 MW-25 MW-27 MW-28 MW-29 MW-30 MW-31 MW-32 MW-33 MW-34MW-37 MW-38 MW-39 MW-40 TW4-01 TW4-03 TWN-01 TWN-02 TWN-03 TWN-04 TWN-05 TWN-06 TWN-07 TWN-08 TWN-09 TWN-10 TWN-11 TWN-12 TWN-13 TWN-14 TWN-15 TWN-16 TWN-17 TWN-18 TWN-19 TWN-20 TWN-21 PIEZ-01 PIEZ-02 PIEZ-3A PIEZ-04 PIEZ-05 TW4-05 TW4-12 TW4-13 TW4-31 TW4-32 MW-12 TW4-11TW4-16 TW4-18 TW4-27 MW-26 MW-35 MW-36 TW4-04 TW4-07 TW4-09 TW4-19 TW4-21 TW4-24 TW4-25 TW4-26 TW4-40 TW4-06 TW4-42 TW4-02 TW4-08 MW-04 MW-05 TW4-22 TW4-23 TW4-20 TW4-28 TW4-29 TW4-30 TW4-10 TW4-33 TW4-35 TW4-36 TW4-41TW4-14 DR-05 DR-06 DR-07 DR-08 DR-09 DR-10 DR-11 DR-12 DR-13 DR-14 DR-15 DR-17 DR-19 DR-20 DR-21 DR-22 DR-23 DR-24 TW4-37 TW4-38 TW4-39 MW-41B MW-24A abandoned abandoned abandoned abandoned abandoned abandoned abandoned abandoned abandoned abnd wildlife pond wildlife pond wildlife pond TW4-43 EXPLANATION perched monitoring well perched piezometer seep or spring WHITE MESA SITE PLAN SHOWING LOCATIONS OF PERCHED WELLS AND PIEZOMETERS H:/718000/71801/ CACME2024/maps/Uwelloc1223.srf MW-5 PIEZ-1 RUIN SPRING temporary perched monitoring well temporary perched nitrate monitoring well TW4-12 TWN-7 TW4-19 perched chloroform or nitrate pumping well TW4-40 perched chloroform pumping well installed February 2018 temporary perched monitoring well installed April 2019 TW4-42 MW-24A perched monitoring well installed December 2019 TWN-20 temporary perched nitrate monitoring well installed April, 2021 temporary perched monitoring well installed September, 2021 TW4-43 MW-41B perched monitoring well installed April, 2023 1A HYDRO GEO CHEM, INC. EXPLANATION perched monitoring well perched piezometer seep or spring MW-5 PIEZ-1 RUIN SPRING temporary perched monitoring well temporary perched nitrate monitoring well TW4-12 TWN-7 TW4-42 temporary perched nitrate monitoring well installed April, 2021 temporary perched monitoring well installed September, 2021 TW4-43 TWN-20 MW-41B perched monitoring well installed April, 2023 WHITE MESA SITE PLAN SHOWING 4th QUARTER 2023 PERCHED WATER LEVELS AND CHLOROFORM AND NITRATE PLUMES H:/718000/ 71801/cacme2024/maps/UwlNchl4Q23.srf 5500 4th quarter 2023 water level contour and label in feet amsl 4th quarter 2023 nitrate plume 4th quarter 2023 chloroform plume chloroform or nitrate pumping well 1B HYDRO GEO CHEM, INC. 55 20 5525 5530 5 5 4 5 5 550 5 5 6 0 5 5 6 5 55 7 0 5 5 7 5 5 5 8 0 3 5 35 3 5 7 0 70 7 0 250 25 0 2 5 0 5 0 0 5 0 0 1 0 0 0 1 0 0 0 2 0 0 0 perched monitoring well temporary perched monitoring well MW-32 TW4-7 PIEZ-2 kriged chloroform isocon and label70 2 temporary perched monitoring well installed September, 2021 temporary perched monitoring well installed April, 2019 KRIGED 4th QUARTER, 2023 CHLOROFORM CONCENTRATIONS, PERCHED WATER LEVELS, AND CHLOROFORM SOURCE AREAS WHITE MESA SITE 5550 kriged perched water level (feet amsl) abandoned scale house leach field source former office leach field source SJS HYDRO GEO CHEM, INC. 5520 5525 5 5 2 5 5530 5 545 5 5 5 0 5 5 6 0 5 5 6 5 5 5 7 0 5 5 7 5 5 5 8 0 5585 70 25 0 5 0 0 1 0 0 0 2 0 0 0 EXPLANATION perched monitoring well temporary perched monitoring well perched piezometer MW-32 TW4-7 PIEZ-2 KRIGED 4th QUARTER, 2023 CHLOROFORM CONCENTRATIONS, PERCHED WATER LEVELS, AND SATURATED THICKNESSES WHITE MESA SITE kriged chloroform isocon (ug/L)70 H:/718000/71801/ CACME2024/maps/Uchlsat4Q23.srf 3A 5550 kriged perched water level (feet amsl)1 10 20 30 40 50 60 70 saturated thickness (feet) perched pumping well MW-4 TW4-40 TW4-43 temporary perched monitoring well installed September, 2021 temporary perched monitoring well installed April, 2019 SJS HYDRO GEO CHEM, INC. 5520 5525 5 5 2 5 5530 5 545 5 5 5 0 5 5 6 0 5 5 6 5 5 5 7 0 5 5 7 5 5 5 8 0 5585 EXPLANATION perched monitoring well temporary perched monitoring well perched piezometer MW-32 TW4-7 PIEZ-2 kriged chloroform plume boundary H:/718000/71801/ CACME2024/maps/Uchlmass4Q23.srf 3B 5550 kriged perched water level (feet amsl) chloroform mass (lb) per grid cell perched pumping well MW-4 TW4-42 TW4-43 temporary perched monitoring well installed September, 2021 KRIGED 4th QUARTER, 2023 CHLOROFORM MASS DISTRIBUTION AND PERCHED WATER LEVELS WHITE MESA SITE grid cell for mass calculation0.031250.06250.1250.250.51251015temporary perched monitoring well installed April, 2019 SJS https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Draft_Tables&Graphs/F4_tw6chl_4Q23.xls: Fig 4A 0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1/1/2002 1/1/2004 1/1/2006 1/1/2008 1/1/2010 1/1/2012 1/1/2014 1/1/2016 1/1/2018 1/1/2020 1/1/2022 1/1/2024 Ch l o f o r o m C o n c e n t r a t i o n ( µ g / L ) date TW4-6 TW4-4 pumping additional pumping wells CHLOROFORM CONCENTRATIONS AT TW4-6 HYDRO GEO CHEM, INC.Approved FigureDateAuthorDate File Name SJS 4A F4_tw6chl.xlsSJS https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Draft_Tables&Graphs/F4_tw6chl_4Q23.xls: Fig 4B 0 100 200 300 400 500 600 700 800 900 1000 1100 1200 1/1/2012 1/1/2013 1/1/2014 1/1/2015 1/1/2016 1/1/2017 1/1/2018 1/1/2019 1/1/2020 1/1/2021 1/1/2022 1/1/2023 1/1/2024 Ch l o r o f o r m C o n c e n t r a t i o n ( µ g / L ) date TW4-6 TW4-26 additional pumping wells CHLOROFORM CONCENTRATIONS AT TW4-6 AND TW4-26 SINCE THE FIRST QUARTER OF 2012 HYDRO GEO CHEM, INC.Approved FigureDateAuthorDate File Name SJS 4B F4_tw6chl.xlsSJS HYDRO GEO CHEM, INC. 5 EXPLANATION perched monitoring well temporary perched monitoring well temporary perched nitrate monitoring well perched piezometer temporary perched monitoring well installed May, 2014 MW-4 TW4-7 TWN-1 PIEZ-1 TW4-36 hand drawn Q1 2012 chloroform isocon kriged Q4 2014 chloroform plume boundary kriged Q1 2012 chloroform plume boundary H:/718000/71801/ CACME2024/maps/chlcomp_12_14.srf COMPARISON OF KRIGED 4th QUARTER 2014 AND 1st QUARTER 2012 CHLOROFORM PLUMES WHITE MESA SITE SJS HYDRO GEO CHEM, INC. EXPLANATION temporary perched monitoring well temporary perched nitrate monitoring well perched piezometer TW4-7 TWN-1 PIEZ-1 6 perched monitoring well MW-32 MW-38 TW4-43 temporary perched nitrate monitoring well installed April, 2021 COMPARISON OF KRIGED 4th QUARTER 2023 AND 4th QUARTER 2014 CHLOROFORM PLUMES WHITE MESA SITE Q4 2014 chloroform plume boundary Q4 2023 chloroform plume boundary SJS https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Tables&Graphs/Chl_WL_1Q12_4Q23_CACME.xlsx: F7A chl area+mr CHLOROFORM PLUME AREA AND QUARTERLY CHLOROFORM MASS REMOVED SINCE 1st QUARTER 2012 HYDRO GEO CHEM, INC.Approved FigureDateAuthorDate File Name SJS 7Achl_wl_1q12_4q21.xls 0 50000 100000 150000 200000 250000 Q1 12 Q2 12 Q3 12 Q4 12 Q1 13 Q2 13 Q3 13 Q4 13 Q1 14 Q2 14 Q3 14 Q4 14 Q1 15 Q2 15 Q3 15 Q4 15 Q1 16 Q2 16 Q3 16 Q4 16 Q1 17 Q2 17 Q3 17 Q4 17 Q1 18 Q2 18 Q3 18 Q4 18 Q1 19 Q2 19 Q3 19 Q4 19 Q1 20 Q2 20 Q3 20 Q4 20 Q1 21 Q2 21 Q3 21 Q4 21 Q1 22 Q2 22 Q3 22 Q4 22 Q1 23 Q2 23 Q3 23 Q4 23 Pl u m e A r e a ( m 2) plume area Linear (plume area) 0 5 10 15 20 25 30 35 40 45 Q1 12 Q2 12 Q3 12 Q4 12 Q1 13 Q2 13 Q3 13 Q4 13 Q1 14 Q2 14 Q3 14 Q4 14 Q1 15 Q2 15 Q3 15 Q4 15 Q1 16 Q2 16 Q3 16 Q4 16 Q1 17 Q2 17 Q3 17 Q4 17 Q1 18 Q2 18 Q3 18 Q4 18 Q1 19 Q2 19 Q3 19 Q4 19 Q1 20 Q2 20 Q3 20 Q4 20 Q1 21 Q2 21 Q3 21 Q4 21 Q1 22 Q2 22 Q3 22 Q4 22 Q1 23 Q2 23 Q3 23 Q4 23 Ch l o r o f o r m M a s s R e m o v e d ( l b s ) Quarter total lbs pumped Linear (total lbs pumped) https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Tables&Graphs/Chl_WL_1Q12_4Q23_CACME.xlsx: F 7B CHLOROFORM PLUME AREA AND QUARTERLY CHLOROFORM MASS REMOVED SINCE 3rd QUARTER 2015 HYDRO GEO CHEM, INC.Approved FigureDateAuthorDate File Name SJS 7Bchl_wl_1q12_4q21xls 0 50000 100000 150000 200000 250000 Q3 15 Q4 15 Q1 16 Q2 16 Q3 16 Q4 16 Q1 17 Q2 17 Q3 17 Q4 17 Q1 18 Q2 18 Q3 18 Q4 18 Q1 19 Q2 19 Q3 19 Q4 19 Q1 20 Q2 20 Q3 20 Q4 20 Q1 21 Q2 21 Q3 21 Q4 21 Q1 22 Q2 22 Q3 22 Q4 22 Q1 23 Q2 23 Q3 23 Q4 23 Pl u m e A r e a ( m 2) plume area Linear (plume area) 0 5 10 15 20 25 30 35 40 45 Q3 15 Q4 15 Q1 16 Q2 16 Q3 16 Q4 16 Q1 17 Q2 17 Q3 17 Q4 17 Q1 18 Q2 18 Q3 18 Q4 18 Q1 19 Q2 19 Q3 19 Q4 19 Q1 20 Q2 20 Q3 20 Q4 20 Q1 21 Q2 21 Q3 21 Q4 21 Q1 22 Q2 22 Q3 22 Q4 22 Q1 23 Q2 23 Q3 23 Q4 23 Ch l o r o f o r m M a s s R e m o v e d ( l b s ) Quarter total lbs pumped Linear (total lbs pumped) https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Tables&Graphs/Chl_WL_1Q12_4Q23_CACME.xlsx: F8A chl area+conc CHLOROFORM PLUME AREA AND AVERAGE PLUME CHLOROFORM CONCENTRATION SINCE 1st QUARTER 2012 HYDRO GEO CHEM, INC.Approved FigureDateAuthorDate File Name SJS 8Achl_wl_1q12_4q21.xls 0 50000 100000 150000 200000 250000 Q1 12 Q2 12 Q3 12 Q4 12 Q1 13 Q2 13 Q3 13 Q4 13 Q1 14 Q2 14 Q3 14 Q4 14 Q1 15 Q2 15 Q3 15 Q4 15 Q1 16 Q2 16 Q3 16 Q4 16 Q1 17 Q2 17 Q3 17 Q4 17 Q1 18 Q2 18 Q3 18 Q4 18 Q1 19 Q2 19 Q3 19 Q4 19 Q1 20 Q2 20 Q3 20 Q4 20 Q1 21 Q2 21 Q3 21 Q4 21 Q1 22 Q2 22 Q3 22 Q4 22 Q1 23 Q2 23 Q3 23 Q4 23 Pl u m e A r e a ( m 2) plume area Linear (plume area) 0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 Q1 12 Q2 12 Q3 12 Q4 12 Q1 13 Q2 13 Q3 13 Q4 13 Q1 14 Q2 14 Q3 14 Q4 14 Q1 15 Q2 15 Q3 15 Q4 15 Q1 16 Q2 16 Q3 16 Q4 16 Q1 17 Q2 17 Q3 17 Q4 17 Q1 18 Q2 18 Q3 18 Q4 18 Q1 19 Q2 19 Q3 19 Q4 19 Q1 20 Q2 20 Q3 20 Q4 20 Q1 21 Q2 21 Q3 21 Q4 21 Q1 22 Q2 22 Q3 22 Q4 22 Q1 23 Q2 23 Q3 23 Q4 23 Av e r a g e C h l o r o f o r m C o n c e n t r a t i o n ( µ g / L ) Quarter average plume concentration, plume wells average plume concentration, gridded data Linear (average plume concentration, plume wells) Linear (average plume concentration, gridded data) https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Tables&Graphs/Chl_WL_1Q12_4Q23_CACME.xlsx: F 8B CHLOROFORM PLUME AREA AND AVERAGE PLUME CHLOROFORM CONCENTRATION SINCE 3rd QUARTER 2015 HYDRO GEO CHEM, INC.Approved FigureDateAuthorDate File Name SJS 8Bchl_wl_1q12_4q21.xls 0 50000 100000 150000 200000 250000 Q3 15 Q4 15 Q1 16 Q2 16 Q3 16 Q4 16 Q1 17 Q2 17 Q3 17 Q4 17 Q1 18 Q2 18 Q3 18 Q4 18 Q1 19 Q2 19 Q3 19 Q4 19 Q1 20 Q2 20 Q3 20 Q4 20 Q1 21 Q2 21 Q3 21 Q4 21 Q1 22 Q2 22 Q3 22 Q4 22 Q1 23 Q2 23 Q3 23 Q4 23 Pl u m e A r e a ( m 2) plume area Linear (plume area) 0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 Q3 15 Q4 15 Q1 16 Q2 16 Q3 16 Q4 16 Q1 17 Q2 17 Q3 17 Q4 17 Q1 18 Q2 18 Q3 18 Q4 18 Q1 19 Q2 19 Q3 19 Q4 19 Q1 20 Q2 20 Q3 20 Q4 20 Q1 21 Q2 21 Q3 21 Q4 21 Q1 22 Q2 22 Q3 22 Q4 22 Q1 23 Q2 23 Q3 23 Q4 23 Av e r a g e C h l o r o f o r m C o n c e n t r a t i o n ( µ g / L ) Quarter average plume concentration, plume wells average plume concentration, gridded data Linear (average plume concentration, plume wells) Linear (average plume concentration, gridded data) CHLOROFORM PLUME MASS REMOVED/QUARTER AND AVERAGE PLUME CHLOROFORM CONCENTRATIONS SINCE 3rd QUARTER 2015 HYDRO GEO CHEM, INC.Approved FigureDateAuthorDate File Name SJS 8Cchl_wl_1q12_4q21.xls 0 5 10 15 20 25 30 35 40 45 50 Q3 15 Q4 15 Q1 16 Q2 16 Q3 16 Q4 16 Q1 17 Q2 17 Q3 17 Q4 17 Q1 18 Q2 18 Q3 18 Q4 18 Q1 19 Q2 19 Q3 19 Q4 19 Q1 20 Q2 20 Q3 20 Q4 20 Q1 21 Q2 21 Q3 21 Q4 21 Q1 22 Q2 22 Q3 22 Q4 22 Q1 23 Q2 23 Q3 23 Q4 23 Ch l o r o f o r m M a s s R e m o v e d ( l b s ) lbs pumped Linear (lbs pumped) 0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 Q3 15 Q4 15 Q1 16 Q2 16 Q3 16 Q4 16 Q1 17 Q2 17 Q3 17 Q4 17 Q1 18 Q2 18 Q3 18 Q4 18 Q1 19 Q2 19 Q3 19 Q4 19 Q1 20 Q2 20 Q3 20 Q4 20 Q1 21 Q2 21 Q3 21 Q4 21 Q1 22 Q2 22 Q3 22 Q4 22 Q1 23 Q2 23 Q3 23 Q4 23 Av e r a g e C h l o r o f o r m C o n c e n t r a t i o n ( µ g / L ) Quarter average plume concentration, plume wells average plume concentration, gridded data Linear (average plume concentration, plume wells) Linear (average plume concentration, gridded data) https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Tables&Graphs/Chl_WL_1Q12_4Q23_CACME.xlsx: F 9 chlpl non-P 1 10 100 1000 10000 100000 Q1 12 Q2 12 Q3 12 Q4 12 Q1 13 Q2 13 Q3 13 Q4 13 Q1 14 Q2 14 Q3 14 Q4 14 Q1 15 Q2 15 Q3 15 Q4 15 Q1 16 Q2 16 Q3 16 Q4 16 Q1 17 Q2 17 Q3 17 Q4 17 Q1 18 Q2 18 Q3 18 Q4 18 Q1 19 Q2 19 Q3 19 Q4 19 Q1 20 Q2 20 Q3 20 Q4 20 Q1 21 Q2 21 Q3 21 Q4 21 Q1 22 Q2 22 Q3 22 Q4 22 Q1 23 Q2 23 Q3 23 Q4 23 Ch l o r o f o r m C o n c e n t r a t i o n ( µ g / L ) Quarter TW4-6*TW4-7 TW4-8*TW4-9* TW4-10 TW4-16 TW4-26 TW4-29 TW4-30*TW4-33* Note: Plume defined by concentrations greater than or equal to 70 ug/L CHLOROFORM IN NON-PUMPING WELLS WITHIN PLUME (INCLUDES TW4-6*, TW4-8*, TW4-9*, TW4-30* AND TW4-33*. NO LONGER WITHIN PLUME) HYDRO GEO CHEM, INC.Approved FigureDateAuthorDate File Name SJS 9 chl_wl_1Q12_4Q21xls https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Tables&Graphs/Chl_WL_1Q12_4Q23_CACME.xlsx: F 10 chlpl-P CHLOROFORM IN PUMPING WELLS WITHIN PLUME (INCLUDING NITRATE PUMPING WELLS TW4-22* and TW4-24*) HYDRO GEO CHEM, INC.Approved FigureDateAuthorDate File Name SJS 10 chl_wl_1Q12_4Q21.xls 1 10 100 1000 10000 100000 Q1 12 Q2 12 Q3 12 Q4 12 Q1 13 Q2 13 Q3 13 Q4 13 Q1 14 Q2 14 Q3 14 Q4 14 Q1 15 Q2 15 Q3 15 Q4 15 Q1 16 Q2 16 Q3 16 Q4 16 Q1 17 Q2 17 Q3 17 Q4 17 Q1 18 Q2 18 Q3 18 Q4 18 Q1 19 Q2 19 Q3 19 Q4 19 Q1 20 Q2 20 Q3 20 Q4 20 Q1 21 Q2 21 Q3 21 Q4 21 Q1 22 Q2 22 Q3 22 Q4 22 Q1 23 Q2 23 Q3 23 Q4 23 Ch l o r o f o r m C o n c e n t r a t i o n ( µ g / L ) Quarter MW-4 MW-26 TW4-1 TW4-2 TW4-4 TW4-11 TW4-19 TW4-20 TW4-21 TW4-22* TW4-24*TW4-37 TW4-39 TW4-40 TW4-41 Note: plume defined by concentrations greater than or equal to 70 ug/L HYDRO GEO CHEM, INC. EXPLANATION perched monitoring well temporary perched monitoring well perched piezometer MW-32 TW4-7 PIEZ-2 Q4 2023 kriged chloroform plume boundary H:/718000/71801/ CACME2024/maps/Udelwl_4Q12_4Q23.srf 11 -40 -35 -30 -25 -20 -15 -10 -5 0 change in perched water elevation (feet) perched pumping well MW-4 TW4-42 TW4-43 temporary perched monitoring well installed September, 2021 CHANGE IN PERCHED WATER ELEVATION WITHIN CHLOROFORM PLUME 4th QUARTER 2012 TO 4th QUARTER 2023 WHITE MESA SITE temporary perched monitoring well installed April, 2019 SJS https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Tables&Graphs/Chl_WL_1Q12_4Q23_CACME.xlsx: F12A WL-marg WATER LEVELS IN WELLS MARGINAL TO PLUME (INCLUDES SUBSET OF GCAP COMPLIANCE MONITORING WELLS CLOSEST TO PLUME HYDRO GEO CHEM, INC.Approved FigureDateAuthorDate File Name SJS 12A chl_wl_1Q12_4Q21.xls 5520 5530 5540 5550 5560 5570 5580 5590 Q1 12 Q2 12 Q3 12 Q4 12 Q1 13 Q2 13 Q3 13 Q4 13 Q1 14 Q2 14 Q3 14 Q4 14 Q1 15 Q2 15 Q3 15 Q4 15 Q1 16 Q2 16 Q3 16 Q4 16 Q1 17 Q2 17 Q3 17 Q4 17 Q1 18 Q2 18 Q3 18 Q4 18 Q1 19 Q2 19 Q3 19 Q4 19 Q1 20 Q2 20 Q3 20 Q4 20 Q1 21 Q2 21 Q3 21 Q4 21 Q1 22 Q2 22 Q3 22 Q4 22 Q1 23 Q2 23 Q3 23 Q4 23 Wa t e r L e v e l E l e v a t i o n ( f e e t a m s l ) Quarter TW4-5 TW4-6 TW4-8 TW4-9 TW4-13 TW4-14 TW4-18 TW4-23 TW4-27 TW4-30 TW4-31 TW4-33 TW4-34 TW4-35 TW4-36 TW4-42 TW4-43 https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Tables&Graphs/Chl_WL_1Q12_4Q23_CACME.xlsx: F12B sat marg 0 10 20 30 40 50 60 70 80 90 Q1 12 Q2 12 Q3 12 Q4 12 Q1 13 Q2 13 Q3 13 Q4 13 Q1 14 Q2 14 Q3 14 Q4 14 Q1 15 Q2 15 Q3 15 Q4 15 Q1 16 Q2 16 Q3 16 Q4 16 Q1 17 Q2 17 Q3 17 Q4 17 Q1 18 Q2 18 Q3 18 Q4 18 Q1 19 Q2 19 Q3 19 Q4 19 Q1 20 Q2 20 Q3 20 Q4 20 Q1 21 Q2 21 Q3 21 Q4 21 Q1 22 Q2 22 Q3 22 Q4 22 Q1 23 Q2 23 Q3 23 Q4 23 Sa t u r t a e d T h i c k n e s s ( f e e t ) Quarter TW4-5 TW4-6 TW4-8 TW4-9 TW4-13 TW4-14 TW4-18 TW4-23 TW4-27 TW4-30 TW4-31 TW4-33 TW4-34 TW4-35 TW4-36 TW4-42 TW4-43 SATURATED THICKNESS IN WELLS MARGINAL TO PLUME (INCLUDES SUBSET OF GCAP COMPLIANCE MONITORING WELLS CLOSEST TO PLUME HYDRO GEO CHEM, INC.Approved FigureDateAuthorDate File Name SJS 12B chl_wl_1Q12_4Q21.xls https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Tables&Graphs/Chl_WL_1Q12_4Q23_CACME.xlsx: F13A WL non-P 5520 5530 5540 5550 5560 5570 5580 5590 Q1 12 Q2 12 Q3 12 Q4 12 Q1 13 Q2 13 Q3 13 Q4 13 Q1 14 Q2 14 Q3 14 Q4 14 Q1 15 Q2 15 Q3 15 Q4 15 Q1 16 Q2 16 Q3 16 Q4 16 Q1 17 Q2 17 Q3 17 Q4 17 Q1 18 Q2 18 Q3 18 Q4 18 Q1 19 Q2 19 Q3 19 Q4 19 Q1 20 Q2 20 Q3 20 Q4 20 Q1 21 Q2 21 Q3 21 Q4 21 Q1 22 Q2 22 Q3 22 Q4 22 Q1 23 Q2 23 Q3 23 Q4 23 Wa t e r L e v e l E l e v a t i o n ( f e e t a m s l ) Quarter TW4-6*TW4-7 TW4-8*TW4-9* TW4-10 TW4-16 TW4-26 TW4-29 TW4-30*TW4-33* WATER LEVELS IN NON-PUMPING WELLS WITHIN PLUME (INCLUDES TW4-6*, TW4-8*, TW4-9*, TW4-30* and TW4-33*, NO LONGER WITHIN PLUME) HYDRO GEO CHEM, INC.Approved FigureDateAuthorDate File Name SJS 13A chl_wl_1Q12_4Q21xls https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Tables&Graphs/Chl_WL_1Q12_4Q23_CACME.xlsx: F13B sat non-P 0 10 20 30 40 50 60 70 80 90 Q1 12 Q2 12 Q3 12 Q4 12 Q1 13 Q2 13 Q3 13 Q4 13 Q1 14 Q2 14 Q3 14 Q4 14 Q1 15 Q2 15 Q3 15 Q4 15 Q1 16 Q2 16 Q3 16 Q4 16 Q1 17 Q2 17 Q3 17 Q4 17 Q1 18 Q2 18 Q3 18 Q4 18 Q1 19 Q2 19 Q3 19 Q4 19 Q1 20 Q2 20 Q3 20 Q4 20 Q1 21 Q2 21 Q3 21 Q4 21 Q1 22 Q2 22 Q3 22 Q4 22 Q1 23 Q2 23 Q3 23 Q4 23 Sa t u r a t e d T h i c k n e s s ( f e e l ) Quarter TW4-6*TW4-7 TW4-8*TW4-9* TW4-10 TW4-16 TW4-26 TW4-29 TW4-30*TW4-33* SATURATED THICKNESS IN NON-PUMPING WELLS WITHIN PLUME (INCLUDES TW4-6*, TW4-*8, TW4-9*, TW4-30* and TW4-33*, NO LONGER WITHIN PLUME) HYDRO GEO CHEM, INC.Approved FigureDateAuthorDate File Name SJS 13B chl_wl_1Q12_4Q21.xls https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Tables&Graphs/Chl_WL_1Q12_4Q23_CACME.xlsx: F14A WL-P 5500 5510 5520 5530 5540 5550 5560 5570 5580 5590 Q1 12 Q2 12 Q3 12 Q4 12 Q1 13 Q2 13 Q3 13 Q4 13 Q1 14 Q2 14 Q3 14 Q4 14 Q1 15 Q2 15 Q3 15 Q4 15 Q1 16 Q2 16 Q3 16 Q4 16 Q1 17 Q2 17 Q3 17 Q4 17 Q1 18 Q2 18 Q3 18 Q4 18 Q1 19 Q2 19 Q3 19 Q4 19 Q1 20 Q2 20 Q3 20 Q4 20 Q1 21 Q2 21 Q3 21 Q4 21 Q1 22 Q2 22 Q3 22 Q4 22 Q1 23 Q2 23 Q3 23 Q4 23 Wa t e r L e v e l E l e v a t i o n ( f e e t a m s l ) Quarter MW-4 MW-26 TW4-1 TW4-2 TW4-4 TW4-11 TW4-19 TW4-20 TW4-21 TW4-22*TW4-24*TW4-37 TW4-39 TW4-40 TW4-41 WATER LEVELS IN PUMPING WELLS WITHIN PLUME (INCLUDING NITRATE PUMPING WELLS TW4-22* and TW4-24*) HYDRO GEO CHEM, INC.Approved FigureDateAuthorDate File Name SJS 14A chl_wl_1Q12_4Q21.xls https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Tables&Graphs/Chl_WL_1Q12_4Q23_CACME.xlsx: F14B sat-P -10 0 10 20 30 40 50 60 70 80 90 Q1 12 Q2 12 Q3 12 Q4 12 Q1 13 Q2 13 Q3 13 Q4 13 Q1 14 Q2 14 Q3 14 Q4 14 Q1 15 Q2 15 Q3 15 Q4 15 Q1 16 Q2 16 Q3 16 Q4 16 Q1 17 Q2 17 Q3 17 Q4 17 Q1 18 Q2 18 Q3 18 Q4 18 Q1 19 Q2 19 Q3 19 Q4 19 Q1 20 Q2 20 Q3 20 Q4 20 Q1 21 Q2 21 Q3 21 Q4 21 Q1 22 Q2 22 Q3 22 Q4 22 Q1 23 Q2 23 Q3 23 Q4 23 Sa t u r a t e d T h i c k n e s s ( f e e t ) Quarter MW-4 MW-26 TW4-1 TW4-2 TW4-4 TW4-11 TW4-19 TW4-20 TW4-21 TW4-22*TW4-24*TW4-37 TW4-39 TW4-40 TW4-41 SATURATED THICKNESS IN PUMPING WELLS WITHIN PLUME (INCLUDING NITRATE PUMPING WELLS TW4-22* and TW4-24*) HYDRO GEO CHEM, INC.Approved FigureDateAuthorDate File Name SJS 14B chl_wl_1Q12_4Q21.xls Note: Pumping water levels for TW4-1, TW4-2 and TW4-11 are often below the base of the Burro Canyon Formation yielding the 'negative' saturated thicknesses displayed above HYDRO GEO CHEM, INC. EXPLANATION perched monitoring well temporary perched monitoring well perched piezometer MW-32 TW4-7 PIEZ-2 Q4 2023 kriged chloroform plume boundary H:/718000/71801/ CACME2024/maps/U%delsat_4Q12_4Q23.srf 15 -99 -50 -35 -25 -20 -15 -10 -5 0 % change in saturated thickness (feet) perched pumping well MW-4 TW4-42 TW4-43 temporary perched monitoring well installed September, 2021 PERCENTAGE CHANGE IN SATURATED THICKNESS WITHIN CHLOROFORM PLUME 4th QUARTER 2012 TO 4th QUARTER 2023 WHITE MESA SITE temporary perched monitoring well installed April, 2019 SJS https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Tables&Graphs/Chl_WL_1Q12_4Q23_CACME.xlsx: F 16A chl area+mass CHLOROFORM PLUME AREA AND RESIDUAL MASS SINCE 1st QUARTER 2012 HYDRO GEO CHEM, INC.Approved FigureDateAuthorDate File Name SJS 16Achl_wl_1q12_4q21.xls 0 500 1000 1500 2000 2500 Q1 12 Q2 12 Q3 12 Q4 12 Q1 13 Q2 13 Q3 13 Q4 13 Q1 14 Q2 14 Q3 14 Q4 14 Q1 15 Q2 15 Q3 15 Q4 15 Q1 16 Q2 16 Q3 16 Q4 16 Q1 17 Q2 17 Q3 17 Q4 17 Q1 18 Q2 18 Q3 18 Q4 18 Q1 19 Q2 19 Q3 19 Q4 19 Q1 20 Q2 20 Q3 20 Q4 20 Q1 21 Q2 21 Q3 21 Q4 21 Q1 22 Q2 22 Q3 22 Q4 22 Q1 23 Q2 23 Q3 23 Q4 23 Ch l o r o f o r m P l u m e R e s i d u a l M a s s ( l b s ) Quarter residual mass (lbs) Linear (residual mass (lbs)) 0 50000 100000 150000 200000 250000 Q1 12 Q2 12 Q3 12 Q4 12 Q1 13 Q2 13 Q3 13 Q4 13 Q1 14 Q2 14 Q3 14 Q4 14 Q1 15 Q2 15 Q3 15 Q4 15 Q1 16 Q2 16 Q3 16 Q4 16 Q1 17 Q2 17 Q3 17 Q4 17 Q1 18 Q2 18 Q3 18 Q4 18 Q1 19 Q2 19 Q3 19 Q4 19 Q1 20 Q2 20 Q3 20 Q4 20 Q1 21 Q2 21 Q3 21 Q4 21 Q1 22 Q2 22 Q3 22 Q4 22 Q1 23 Q2 23 Q3 23 Q4 23 Pl u m e A r e a ( m 2) plume area Linear (plume area) https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Tables&Graphs/Chl_WL_1Q12_4Q23_CACME.xlsx: F 16B CHLOROFORM PLUME AREA AND RESIDUAL MASS SINCE 3rd QUARTER 2015 HYDRO GEO CHEM, INC.Approved FigureDateAuthorDate File Name SJS 16Bchl_wl_1q12_4q21.xls 0 500 1000 1500 2000 2500 Q3 15 Q4 15 Q1 16 Q2 16 Q3 16 Q4 16 Q1 17 Q2 17 Q3 17 Q4 17 Q1 18 Q2 18 Q3 18 Q4 18 Q1 19 Q2 19 Q3 19 Q4 19 Q1 20 Q2 20 Q3 20 Q4 20 Q1 21 Q2 21 Q3 21 Q4 21 Q1 22 Q2 22 Q3 22 Q4 22 Q1 23 Q2 23 Q3 23 Q4 23 Ch l o r o f o r m P l u m e R e s i d u a l M a s s ( l b s ) Quarter residual mass (lbs) Linear (residual mass (lbs)) 0 50000 100000 150000 200000 250000 Q3 15 Q4 15 Q1 16 Q2 16 Q3 16 Q4 16 Q1 17 Q2 17 Q3 17 Q4 17 Q1 18 Q2 18 Q3 18 Q4 18 Q1 19 Q2 19 Q3 19 Q4 19 Q1 20 Q2 20 Q3 20 Q4 20 Q1 21 Q2 21 Q3 21 Q4 21 Q1 22 Q2 22 Q3 22 Q4 22 Q1 23 Q2 23 Q3 23 Q4 23 Pl u m e A r e a ( m 2) plume area Linear (plume area) https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Tables&Graphs/Chl_WL_1Q12_4Q23_CACME.xlsx: F17A chl mass+mr CHLOROFORM PLUME RESIDUAL MASS AND QUARTERLY MASS REMOVED SINCE 1st QUARTER 2012 HYDRO GEO CHEM, INC.Approved FigureDateAuthorDate File Name SJS 17Achl_wl_1q12_4q21.xls 0 5 10 15 20 25 30 35 40 45 Q1 12 Q2 12 Q3 12 Q4 12 Q1 13 Q2 13 Q3 13 Q4 13 Q1 14 Q2 14 Q3 14 Q4 14 Q1 15 Q2 15 Q3 15 Q4 15 Q1 16 Q2 16 Q3 16 Q4 16 Q1 17 Q2 17 Q3 17 Q4 17 Q1 18 Q2 18 Q3 18 Q4 18 Q1 19 Q2 19 Q3 19 Q4 19 Q1 20 Q2 20 Q3 20 Q4 20 Q1 21 Q2 21 Q3 21 Q4 21 Q1 22 Q2 22 Q3 22 Q4 22 Q1 23 Q2 23 Q3 23 Q4 23 Ch l o r o f o r m M a s s R e m o v e d ( l b s ) Quarter total lbs pumped Linear (total lbs pumped) 0 500 1000 1500 2000 2500 Q1 12 Q2 12 Q3 12 Q4 12 Q1 13 Q2 13 Q3 13 Q4 13 Q1 14 Q2 14 Q3 14 Q4 14 Q1 15 Q2 15 Q3 15 Q4 15 Q1 16 Q2 16 Q3 16 Q4 16 Q1 17 Q2 17 Q3 17 Q4 17 Q1 18 Q2 18 Q3 18 Q4 18 Q1 19 Q2 19 Q3 19 Q4 19 Q1 20 Q2 20 Q3 20 Q4 20 Q1 21 Q2 21 Q3 21 Q4 21 Q1 22 Q2 22 Q3 22 Q4 22 Q1 23 Q2 23 Q3 23 Q4 23 Ch l o r o f o r m P l u m e R e s i d u a l M a s s ( l b s ) residual mass (lbs) Linear (residual mass (lbs)) https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Tables&Graphs/Chl_WL_1Q12_4Q23_CACME.xlsx: F 17B CHLOROFORM PLUME RESIDUAL MASS AND QUARTERLY MASS REMOVED SINCE 3rd QUARTER 2015 HYDRO GEO CHEM, INC.Approved FigureDateAuthorDate File Name SJS 17Bchl_wl_1q12_4q21.xls 0 5 10 15 20 25 30 35 40 45 Q3 15 Q4 15 Q1 16 Q2 16 Q3 16 Q4 16 Q1 17 Q2 17 Q3 17 Q4 17 Q1 18 Q2 18 Q3 18 Q4 18 Q1 19 Q2 19 Q3 19 Q4 19 Q1 20 Q2 20 Q3 20 Q4 20 Q1 21 Q2 21 Q3 21 Q4 21 Q1 22 Q2 22 Q3 22 Q4 22 Q1 23 Q2 23 Q3 23 Q4 23 Ch l o r o f o r m M a s s R e m o v e d ( l b s ) Quarter total lbs pumped Linear (total lbs pumped) 0 500 1000 1500 2000 2500 Q3 15 Q4 15 Q1 16 Q2 16 Q3 16 Q4 16 Q1 17 Q2 17 Q3 17 Q4 17 Q1 18 Q2 18 Q3 18 Q4 18 Q1 19 Q2 19 Q3 19 Q4 19 Q1 20 Q2 20 Q3 20 Q4 20 Q1 21 Q2 21 Q3 21 Q4 21 Q1 22 Q2 22 Q3 22 Q4 22 Q1 23 Q2 23 Q3 23 Q4 23 Ch l o r o f o r m P l u m e R e s i d u a l M a s s ( l b s ) residual mass (lbs) Linear (residual mass (lbs)) https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Tables&Graphs/F18_PlumeMass_4Q23.xlsx: F18 MW-26, TW4-19, -20, -22 AND -37 CHLOROFORM CONCENTRATIONS AND RESIDUAL PLUME MASS 2012 to 2023 HYDRO GEO CHEM, INC.Approved FigureDateAuthorDate File Name SJS 18PlumeMass192022.xls 0 500 1000 1500 2000 2500 0 5000 10000 15000 20000 25000 30000 35000 40000 45000 50000 Q1 1 2 Q2 1 2 Q3 1 2 Q4 1 2 Q1 1 3 Q2 1 3 Q3 1 3 Q4 1 3 Q1 1 4 Q2 1 4 Q3 1 4 Q4 1 4 Q1 1 5 Q2 1 5 Q3 1 5 Q4 1 5 Q1 1 6 Q2 1 6 Q3 1 6 Q4 1 6 Q1 1 7 Q2 1 7 Q3 1 7 Q4 1 7 Q1 1 8 Q2 1 8 Q3 1 8 Q4 1 8 Q1 1 9 Q2 1 9 Q3 1 9 Q4 1 9 Q1 2 0 Q2 2 0 Q3 2 0 Q4 2 0 Q1 2 1 Q2 2 1 Q3 2 1 Q4 2 1 Q1 2 2 Q2 2 2 Q3 2 2 Q4 2 2 Q1 2 3 Q2 2 3 Q3 2 3 Q4 2 3 Ch l o r o f o r m C o n c e n t r a t i o n ( u g / l ) Quarter MW-26 TW4-19 TW4-20 TW4-22 TW4-37 Chloroform Plume Mass (lbs) pl u m e m a s s ( l b s ) 0 500 1000 1500 2000 2500 0 2000 4000 6000 8000 10000 12000 14000 Q1 1 2 Q2 1 2 Q3 1 2 Q4 1 2 Q1 1 3 Q2 1 3 Q3 1 3 Q4 1 3 Q1 1 4 Q2 1 4 Q3 1 4 Q4 1 4 Q1 1 5 Q2 1 5 Q3 1 5 Q4 1 5 Q1 1 6 Q2 1 6 Q3 1 6 Q4 1 6 Q1 1 7 Q2 1 7 Q3 1 7 Q4 1 7 Q1 1 8 Q2 1 8 Q3 1 8 Q4 1 8 Q1 1 9 Q2 1 9 Q3 1 9 Q4 1 9 Q1 2 0 Q2 2 0 Q3 2 0 Q4 2 0 Q1 2 1 Q2 2 1 Q3 2 1 Q4 2 1 Q1 2 2 Q2 2 2 Q3 2 2 Q4 2 2 Q1 2 3 Q2 2 3 Q3 2 3 Q4 2 3 Av e r a g e C h l o r o f o r m C o n c e n t r a t i o n ( u g / l ) Quarter MW-26, TW4-19, 20, 22, 37 Average Chloroform Plume Mass (lbs) pl u m e m a s s ( l b s ) https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Tables&Graphs/F19_resid_chl_mass_4Q23.xls: F19 Resid CHL Mass 0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 Re s i d u a l C h l o r o f o r m M a s s E s t i m a t e ( l b ) mass estimate CHLOROFORM PLUME RESIDUAL MASS ESTIMATES 2006 TO 2021 HYDRO GEO CHEM, INC.Approved FigureDateAuthorDate File Name SJS 19 MassEstTimeSeries.xlsSJS RE D U C E D R E C H A R G E NI T R A T E P U M P I N G HYDRO GEO CHEM, INC. 5520 5525 5526.25 5527.5 5530 5 54 5 5 5 5 05552.5 5 5 6 0 5 5 6 5 5 5 7 0 5 5 7 5 5 5 8 0 EXPLANATION perched monitoring well showing elevation in feet amsl temporary perched monitoring well showing elevation in feet amsl perched piezometer showing elevation in feet amsl MW-25 TW4-7 PIEZ-2 5530 5538 5581 5523 TW4-43 5524 TW4-42 temporary perched monitoring well installed April, 2019 showing elevation in feet amsl temporary perched monitoring well installed September, 2021 showing elevation in feet amsl KRIGED 4th QUARTER, 2023 WATER LEVEL CONTOURS, CHLOROFORM PLUME BOUNDARY AND ESTIMATED TOTAL CAPTURE WHITE MESA SITE 20 HYDRO GEO CHEM, INC. 5520 5525 5526.25 5527.5 5530 55 45 5 5 5 0 5 5 5 2 .5 5 5 6 0 5 5 6 5 5 5 7 0 5 5 7 5 5 5 8 0 5 5 8 2 .5 EXPLANATION perched monitoring well showing elevation in feet amsl temporary perched monitoring well showing elevation in feet amsl perched piezometer showing elevation in feet amsl MW-25 TW4-7 PIEZ-2 5531 5539 5583 5524 TW4-43 5523 TW4-42 temporary perched monitoring well installed April, 2019 showing elevation in feet amsl temporary perched monitoring well installed September, 2021 showing elevation in feet amsl KRIGED 4th QUARTER, 2022 WATER LEVEL CONTOURS, CHLOROFORM PLUME BOUNDARY AND ESTIMATED TOTAL CAPTURE WHITE MESA SITE 21 HYDRO GEO CHEM, INC. 5520 5525 5526.25 5527.5 5530 5 5 4 5 5 5 5 0 5 55 2 .5 5 5 5 7 .5 5560 5 5 6 5 5 5 7 0 5 5 7 5 5 5 8 0 5 5 8 2 .5 5 5 8 5 EXPLANATION perched monitoring well showing elevation in feet amsl temporary perched monitoring well showing elevation in feet amsl perched piezometer showing elevation in feet amsl MW-25 TW4-7 PIEZ-2 5532 5539 5583 22 5525 TW4-43 5524 TW4-42 temporary perched monitoring well installed April, 2019 showing elevation in feet amsl temporary perched monitoring well installed September, 2021 showing elevation in feet amsl KRIGED 4th QUARTER, 2021 WATER LEVEL CONTOURS, CHLOROFORM PLUME BOUNDARY AND ESTIMATED TOTAL CAPTURE WHITE MESA SITE SJS HYDRO GEO CHEM, INC. 552 0 5525 5527.5 5530 5532.5 5 5 4 5 5 5 5 0 5 5 5 2.5 55 55 5 5 6 0 5 5 6 5 5 5 7 0 5 5 7 5 5 5 8 0 5 5 8 2 .5 5 5 8 5 EXPLANATION perched monitoring well showing elevation in feet amsl temporary perched monitoring well showing elevation in feet amsl perched piezometer showing elevation in feet amsl MW-25 TW4-7 PIEZ-2 KRIGED 4th QUARTER, 2020 WATER LEVEL CONTOURS, CHLOROFORM PLUME BOUNDARY AND ESTIMATED TOTAL CAPTURE WHITE MESA SITE 5532 5539 5583 23 5523 temporary perched monitoring well installed February, 2018 showing elevation in feet amsl TW4-42 5526 TW4-40 temporary perched monitoring well installed April, 2019 showing elevation in feet amsl SJS HYDRO GEO CHEM, INC. 5 5 2 0 5527.5 5530 5530 5532.5 5 5 4 5 5 5 5 0 5 5 5 2 .5 5 5 5 5 5 5 6 0 5 5 6 5 5 5 7 0 5 5 7 5 5 5 8 0 5 5 8 2 .5 5 5 8 5 55 8 7.5 EXPLANATION perched monitoring well showing elevation in feet amsl temporary perched monitoring well showing elevation in feet amsl perched piezometer showing elevation in feet amsl MW-25 TW4-7 PIEZ-2 KRIGED 4th QUARTER, 2019 WATER LEVEL CONTOURS, CHLOROFORM PLUME BOUNDARY AND ESTIMATED TOTAL CAPTURE WHITE MESA SITE 5533 5539 5585 24 5526 temporary perched monitoring well installed February, 2018 showing elevation in feet amsl TW4-42 5527 TW4-40 temporary perched monitoring well installed April, 2019 showing elevation in feet amsl SJS HYDRO GEO CHEM, INC. 5520 5525 5530 5 5 4 5 5 5 5 0 5 5 5 5 55 60 5 5 6 5 5 5 7 0 5 5 7 5 5 5 8 0 5 5 8 4 5 5 8 5 5 5 8 7 .5 EXPLANATION perched monitoring well showing elevation in feet amsl temporary perched monitoring well showing elevation in feet amsl perched piezometer showing elevation in feet amsl MW-25 TW4-7 PIEZ-2 5534 5538 5586 25 PIEZ-3A 5583 May, 2016 replacement of perched piezometer Piez-03 showing elevation in feet amsl temporary perched monitoring well installed February, 2018 showing elevation in feet amsl TW4-40 5530 KRIGED 4th QUARTER, 2018 WATER LEVEL CONTOURS, CHLOROFORM PLUME BOUNDARY AND ESTIMATED TOTAL CAPTURE WHITE MESA SITE SJS HYDRO GEO CHEM, INC. 5 5 2 0 5525 5 52 5 5530 5 5 3 5 5 5 4 5 5 5 5 0 5 5 5 5 5 5 6 0 5 5 6 5 5 5 7 0 5 5 7 5 5 5 8 0 5 5 8 5 5 5 8 7 .5 5590 EXPLANATION perched monitoring well showing elevation in feet amsl temporary perched monitoring well showing elevation in feet amsl perched piezometer showing elevation in feet amsl MW-25 TW4-7 PIEZ-2 KRIGED 4th QUARTER, 2017 WATER LEVEL CONTOURS, CHLOROFORM PLUME BOUNDARY, AND ESTIMATED TOTAL CAPTURE WHITE MESA SITE 5534 5541 5587 5585 NOTES: MW-4, MW-26, TW4-1, TW4-2, TW4-4, TW4-11, TW4-19, TW4-20, TW4-21, TW4-37 and TW4-39 are chloroform pumping wells; TW4-22, TW4-24, TW4-25, and TWN-2 are nitrate pumping wells; TW4-11 water level is below the base of the Burro Canyon Formation temporary perched pumping well installed October, 2016 showing elevation in feet amsl TW4-38 5576 26PIEZ-3A May, 2016 replacement of perched piezometer Piez-03 showing elevation in feet amsl SJS HYDRO GEO CHEM, INC. 5 5 2 0 5525 5 5 2 5 5530 5 545 5 5 5 0 5 5 5 5 5 5 6 0 5 5 6 5 5 56 5 5 5 7 055755580 5 5 8 5 5 58 7 .5 5590 EXPLANATION perched monitoring well showing elevation in feet amsl temporary perched monitoring well showing elevation in feet amsl perched piezometer showing elevation in feet amsl MW-25 TW4-7 PIEZ-2 KRIGED 4th QUARTER, 2016 WATER LEVEL CONTOURS, CHLOROFORM PLUME BOUNDARY, AND ESTIMATED TOTAL CAPTURE WHITE MESA SITE 5535 5544 5588 5587 NOTES: MW-4, MW-26, TW4-1, TW4-2, TW4-4, TW4-11, TW4-19, TW4-20, TW4-21, TW4-37 and TW4-39 are chloroform pumping wells; TW4-22, TW4-24, TW4-25, and TWN-2 are nitrate pumping wells; TW4-11 water level is below the base of the Burro Canyon Formation temporary perched pumping well installed October, 2016 showing elevation in feet amsl TW4-38 5577 27PIEZ-3A May, 2016 replacement of perched piezometer Piez-03 showing elevation in feet amsl SJS HYDRO GEO CHEM, INC. 5 5 2 0 5525 5 5 2 5 5530 5 5 3 0 5535 5 5 4 5 5 5 5 0 5 5 5 5 5 5 6 0 5 5 6 5 5 5 6 5 5 5 7 0 5 5 7 5 5 5 8 0 5 5 8 5 5 5 8 7.5 5 5 9 0 5 5 9 5 EXPLANATION perched monitoring well showing elevation in feet amsl temporary perched monitoring well showing elevation in feet amsl perched piezometer showing elevation in feet amsl temporary perched monitoring well installed May, 2014 showing elevation in feet amsl MW-25 TW4-7 PIEZ-2 TW4-35 KRIGED 4th QUARTER, 2015 WATER LEVEL CONTOURS, CHLOROFORM PLUME BOUNDARY, AND ESTIMATED TOTAL CAPTURE WHITE MESA SITE 5536 5540 5590 5526 NOTES: MW-4, MW-26, TW4-1, TW4-2, TW4-4, TW4-11, TW4-19, TW4-20, TW4-21 and TW4-37 are chloroform pumping wells; TW4-22, TW4-24, TW4-25, and TWN-2 are nitrate pumping wells; TW4-11 water level is below the base of the Burro Canyon Formation perched pumping well installed March, 2015 showing elevation in feet amsl TW4-37 5568 28SJS HYDRO GEO CHEM, INC. 5525 5 5 2 5 5530 5 5 3 0 5 5 4 5 5 5 5 0 55 5 5 5 5 6 0 5 56 5 5 5 6 5 5 5 7 0 5570 5 5 7 5 5575 5 5 8 0 5 5 8 5 55905590 5595 EXPLANATION perched monitoring well showing elevation in feet amsl temporary perched monitoring well showing elevation in feet amsl perched piezometer showing elevation in feet amsl temporary perched monitoring well installed May, 2014 showing elevation in feet amsl MW-4 TW4-1 PIEZ-2 TW4-35 KRIGED 4th QUARTER, 2014 WATER LEVEL CONTOURS, CHLOROFORM PLUME BOUNDARY, AND ESTIMATED TOTAL CAPTURE WHITE MESA SITE 5552 5551 5593 5526 NOTE: MW-4, MW-26, TW4-4, TW4-19, and TW4-20 are chloroform pumping wells; TW4-22, TW4-24, TW4-25, and TWN-2 are nitrate pumping wells 29SJS HYDRO GEO CHEM, INC. 5 5 2 5 5 5 2 5 5530 5 5 3 0 5 5 3 5 5 5 50 5 5 5 5 5 5 6 0 5 5 6 5 5 5 6 5 5 57 0 5570 5 57 0 5 5 7 5 5 5 7 5 558 0 5580 5585 5590 5595 5595 EXPLANATION perched monitoring well showing elevation in feet amsl temporary perched monitoring well showing elevation in feet amsl perched piezometer showing elevation in feet amsl temporary perched monitoring well installed September, 2013 showing elevation in feet amsl MW-4 TW4-1 PIEZ-2 TW4-32 5553 5554 5595 5564 NOTE: MW-4, MW-26, TW4-4, TW4-19, and TW4-20 are chloroform pumping wells; TW4-22, TW4-24, TW4-25, and TWN-2 are nitrate pumping wells 30 KRIGED 4th QUARTER, 2013 WATER LEVEL CONTOURS, CHLOROFORM PLUME BOUNDARY, AND ESTIMATED TOTAL CAPTURE WHITE MESA SITE estimated total capture resulting from chloroform and nitrate pumping estimated portion of chloroform plume within capture zone chloroform plume boundary SJS HYDRO GEO CHEM, INC. 5 5 2 5 5 5 3 0 5 5 3 0 5535 5 5 3 5 5 5 4 0 5 5 5 5 5 5 6 0 5 5 6 5 5 5 6 5 5 5 7 0 5 5 7 0 5 5 7 5 5575 5 5 8 0 5580 5 5 8 5 5585 5 5 9 0 5 5 9 5 56005605 5 6 0 5 EXPLANATION perched monitoring well showing elevation in feet amsl temporary perched monitoring well showing elevation in feet amsl perched piezometer showing elevation in feet amsl temporary perched monitoring well installed October, 2011 showing elevation in feet amsl MW-4 TW4-1 PIEZ-2 TW4-27 NOTE: MW-4, MW-26, TW4-4, TW4-19 and TW4-20 are pumping wells KRIGED 4th QUARTER, 2012 WATER LEVEL CONTOURS, CHLOROFORM PLUME BOUNDARY, AND ESTIMATED TOTAL CAPTURE WHITE MESA SITE 5549 5554 5609 5525 estimated total capture resulting from chloroform pumping estimated portion of chloroform plume within capture zone chloroform plume boundary 31SJS https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Tables&Graphs/F32_33_35_Pvolume_4Q23_Chl.xls: F 32 chl pmp (renumbered) 0 50,000 100,000 150,000 200,000 250,000 300,000 350,000 400,000 450,000 Q2 13 Q3 13 Q4 13 Q1 14 Q2 14 Q3 14 Q4 14 Q1 15 Q2 15 Q3 15 Q4 15 Q1 16 Q2 16 Q3 16 Q4 16 Q1 17 Q2 17 Q3 17 Q4 17 Q1 18 Q2 18 Q3 18 Q4 18 Q1 19 Q2 19 Q3 19 Q4 19 Q1 20 Q2 20 Q3 20 Q4 20 Q1 21 Q2 21 Q3 21 Q4 21 Q1 22 Q2 22 Q3 22 Q4 22 Q1 23 Q2 23 Q3 23 Q4 23 Ga l l o n s P u m p e d p e r Q u a r t e r MW-4 MW-26 TW4-1 TW4-2 TW4-4 TW4-11 TW4-19 TW4-20 TW4-21 TW4-37 TW4-39 TW4-40 TW4-41 PRODUCTIVITY OF CHLOROFORM PUMPING WELLS HYDRO GEO CHEM, INC.Approved FigureDateAuthorDate File Name SJS 32Pvolume_4Q23SJS https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Tables&Graphs/F32_33_35_Pvolume_4Q23_Chl.xls: F 33 N pump (renumbered) 0 50,000 100,000 150,000 200,000 250,000 300,000 Q2 13 Q3 13 Q4 13 Q1 14 Q2 14 Q3 14 Q4 14 Q1 15 Q2 15 Q3 15 Q4 15 Q1 16 Q2 16 Q3 16 Q4 16 Q1 17 Q2 17 Q3 17 Q4 17 Q1 18 Q2 18 Q3 18 Q4 18 Q1 19 Q2 19 Q3 19 Q4 19 Q1 20 Q2 20 Q3 20 Q4 20 Q1 21 Q2 21 Q3 21 Q4 21 Q1 22 Q2 22 Q3 22 Q4 22 Q1 23 Q2 23 Q3 23 Q4 23 Ga l l o n s P u m p e d p e r Q u a r t e r TW4-22 TW4-24 TW4-25 TWN-2 PRODUCTIVITY OF NITRATE PUMPING WELLS HYDRO GEO CHEM, INC.Approved FigureDateAuthorDate File Name SJS 33Pvolume_4Q23SJS https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Tables&Graphs/Chl_WL_1Q12_4Q23_CACME.xlsx: F34A (renumbered) mr+tw19mr TW4-19 CHLOROFORM MASS REMOVED/QUARTER AND TOTAL CHLOROFORM MASS REMOVED/QUARTER SINCE 1st QUARTER 2012 HYDRO GEO CHEM, INC.Approved FigureDateAuthorDate File Name SJS 34Achl_wl_1q12_4q21.xls 0 5 10 15 20 25 30 35 40 45 Q1 12 Q2 12 Q3 12 Q4 12 Q1 13 Q2 13 Q3 13 Q4 13 Q1 14 Q2 14 Q3 14 Q4 14 Q1 15 Q2 15 Q3 15 Q4 15 Q1 16 Q2 16 Q3 16 Q4 16 Q1 17 Q2 17 Q3 17 Q4 17 Q1 18 Q2 18 Q3 18 Q4 18 Q1 19 Q2 19 Q3 19 Q4 19 Q1 20 Q2 20 Q3 20 Q4 20 Q1 21 Q2 21 Q3 21 Q4 21 Q1 22 Q2 22 Q3 22 Q4 22 Q1 23 Q2 23 Q3 23 Q4 23 Ch l o r o f o r m M a s s R e m o v e d ( l b s ) TW4-19 lbs pumped Linear (TW4-19 lbs pumped) 0 5 10 15 20 25 30 35 40 45 Q1 12 Q2 12 Q3 12 Q4 12 Q1 13 Q2 13 Q3 13 Q4 13 Q1 14 Q2 14 Q3 14 Q4 14 Q1 15 Q2 15 Q3 15 Q4 15 Q1 16 Q2 16 Q3 16 Q4 16 Q1 17 Q2 17 Q3 17 Q4 17 Q1 18 Q2 18 Q3 18 Q4 18 Q1 19 Q2 19 Q3 19 Q4 19 Q1 20 Q2 20 Q3 20 Q4 20 Q1 21 Q2 21 Q3 21 Q4 21 Q1 22 Q2 22 Q3 22 Q4 22 Q1 23 Q2 23 Q3 23 Q4 23 Ch l o r o f o r m M a s s R e m o v e d ( l b s ) Quarter total lbs pumped Linear (total lbs pumped) https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Tables&Graphs/Chl_WL_1Q12_4Q23_CACME.xlsx: F 34B (renumbered) mr+tw19mr TW4-19 CHLOROFORM MASS REMOVED/QUARTER AND TOTAL CHLOROFORM MASS REMOVED/QUARTER SINCE 3rd QUARTER 2015 HYDRO GEO CHEM, INC.Approved FigureDateAuthorDate File Name SJS 34Bchl_wl_1q12_4q21.xls 0 5 10 15 20 25 30 35 40 45 Q3 15 Q4 15 Q1 16 Q2 16 Q3 16 Q4 16 Q1 17 Q2 17 Q3 17 Q4 17 Q1 18 Q2 18 Q3 18 Q4 18 Q1 19 Q2 19 Q3 19 Q4 19 Q1 20 Q2 20 Q3 20 Q4 20 Q1 21 Q2 21 Q3 21 Q4 21 Q1 22 Q2 22 Q3 22 Q4 22 Q1 23 Q2 23 Q3 23 Q4 23 Ch l o r o f o r m M a s s R e m o v e d ( l b s ) TW4-19 lbs pumped Linear (TW4-19 lbs pumped) 0 5 10 15 20 25 30 35 40 45 Q3 15 Q4 15 Q1 16 Q2 16 Q3 16 Q4 16 Q1 17 Q2 17 Q3 17 Q4 17 Q1 18 Q2 18 Q3 18 Q4 18 Q1 19 Q2 19 Q3 19 Q4 19 Q1 20 Q2 20 Q3 20 Q4 20 Q1 21 Q2 21 Q3 21 Q4 21 Q1 22 Q2 22 Q3 22 Q4 22 Q1 23 Q2 23 Q3 23 Q4 23 Ch l o r o f o r m M a s s R e m o v e d ( l b s ) Quarter total lbs pumped Linear (total lbs pumped) 0 1 10 100 1,000 10,000 100,000 1,000,000 Q3 15 Q4 15 Q1 16 Q2 16 Q3 16 Q4 16 Q1 17 Q2 17 Q3 17 Q4 17 Q1 18 Q2 18 Q3 18 Q4 18 Q1 19 Q2 19 Q3 19 Q4 19 Q1 20 Q2 20 Q3 20 Q4 20 Q1 21 Q2 21 Q3 21 Q4 21 Q1 22 Q2 22 Q3 22 Q4 22 Q1 23 Q2 23 Q3 23 Q4 23 Av e r a g e S a t u r a t e d T h i c k n e s s ( f e e t ) Ga l l o n s P u m p e d p e r Q u a r t e r TW4-1 TW4-2 TW4-4 TW4-11 TW4-20 TW4-21 TW4-37 TW4-39 TW4-40 TW4-41 Total Avg Sat Thick CHLOROFORM PUMPING WELLS HAVING DECLINING PRODUCTIVITY SINCE 3rd QUARTER 2015 HYDRO GEO CHEM, INC.Approved FigureDateAuthorDate File Name SJS 35APvolume_4Q21SJS TW 4 - 2 0 p u m p t i m e r f a i l u r e 0 5 10 15 20 25 30 35 40 45 50 0 50,000 100,000 150,000 200,000 250,000 300,000 350,000 400,000 450,000 Q3 15 Q4 15 Q1 16 Q2 16 Q3 16 Q4 16 Q1 17 Q2 17 Q3 17 Q4 17 Q1 18 Q2 18 Q3 18 Q4 18 Q1 19 Q2 19 Q3 19 Q4 19 Q1 20 Q2 20 Q3 20 Q4 20 Q1 21 Q2 21 Q3 21 Q4 21 Q1 22 Q2 22 Q3 22 Q4 22 Q1 23 Q2 23 Q3 23 Q4 23 Av e r a g e S a t u r a t e d T h i c k n e s s ( f e e t ) Ga l l o n s P u m p e d p e r Q u a r t e r Total Pumping from Declining Wells Average Saturated Thickness Linear (Total Pumping from Declining Wells) Linear (Average Saturated Thickness) TOTAL PUMPING AND AVERAGE SATURATED THICKNESS FOR WELLS HAVING DECLINING PRODUCTIVITY SINCE 3rd QUARTER 2015 HYDRO GEO CHEM, INC.Approved FigureDateAuthorDate File Name SJS 35BPvolume_4Q21SJS TW 4 - 2 0 p u m p t i m e r f a i l u r e APPENDIX A HISTORIC CHLOROFORM PLUME COMPARISON MAPS (FIGURES A.1-A.6) HYDRO GEO CHEM, INC. APPROVED DATE REFERENCE FIGURE CELL NO. 2 CELL NO. 4A 3332 MW-21 3000 BOUNDARY PROPERTY SCALE IN FEET 0 CELL NO. 1 MILL SITE MW-01 MW-02 MW-03 MW-05 MW-11 MW-12 MW-14 MW-15 MW-17 MW-18 MW-19 MW-20 MW-22 MW-23 MW-24 MW-25 MW-27 MW-28 MW-29 MW-30 MW-31 MW-32 MW-26 MW-16 PIEZ-1 PIEZ-2 PIEZ-3 PIEZ-4 PIEZ-5 TW4-1 TW4-2 TW4-3 TW4-4 TW4-5 TW4-6 TW4-9 TW4-11 TW4-12 TW4-13 TW4-14 TW4-16 TW4-18 TW4-20 TW4-21 MW-04TW4-7 TW4-8 TW4-10 TW4-22 TW4-19 MW-20 PIEZ-1 perched monitoring well perched piezometer temporary perched monitoring well COMPARISON OF KRIGED 1st QUARTER 2007 AND 1st QUARTER 2006 CHLOROFORM PLUMES WHITE MESA SITE TW4-1 EXPLANATION chloroform plume boundaries (solid = 1st Quarter, 2007 dashed = 1st Quarter, 2006) A.1SJS H:/718000/71801/ CACME2016/maps/chlcomp_06_07.srf Note: MW-4, MW-26, TW4-19, and TW4-20 are pumping wells HYDRO GEO CHEM, INC. APPROVED DATE REFERENCE FIGURE CELL NO. 2 CELL NO. 4A 3332 MW-21 3000 BOUNDARY PROPERTY SCALE IN FEET 0 CELL NO. 1 MILL SITE MW-01 MW-02 MW-03 MW-05 MW-11 MW-12 MW-14 MW-15 MW-17 MW-18 MW-19 MW-20 MW-22 MW-23 MW-24 MW-25 MW-27 MW-28 MW-29 MW-30 MW-31 MW-32 MW-26 MW-16 PIEZ-1 PIEZ-2 PIEZ-3 PIEZ-4 PIEZ-5 TW4-1 TW4-2 TW4-3 TW4-4 TW4-5 TW4-6 TW4-9 TW4-11 TW4-12 TW4-13 TW4-14 TW4-16 TW4-18 TW4-20 TW4-21 MW-04TW4-7 TW4-8 TW4-10 TW4-22 TW4-19 TW4-23 TW4-24 TW4-25 MW-20 PIEZ-1 perched monitoring well perched piezometer temporary perched monitoring well COMPARISON OF KRIGED 1st QUARTER 2008 AND 1st QUARTER 2007 CHLOROFORM PLUMES WHITE MESA SITE H:/718000/71801/ CACME2016/maps/chlcomp_07_08.srf TW4-1 EXPLANATION chloroform plume boundaries (solid = 1st Quarter, 2008 dashed = 1st Quarter, 2007) SJS A.2 Note: MW-4, MW-26, TW4-19, and TW4-20 are pumping wells HYDRO GEO CHEM, INC. APPROVED DATE REFERENCE FIGURE CELL NO. 2 CELL NO. 4A 3332 MW-21 3000 BOUNDARY PROPERTY SCALE IN FEET 0 CELL NO. 1 MILL SITE MW-01 MW-02 MW-03 MW-05 MW-11 MW-12 MW-14 MW-15 MW-17 MW-18 MW-19 MW-20 MW-22 MW-23 MW-24 MW-25 MW-27 MW-28 MW-29 MW-30 MW-31 MW-32 MW-26 MW-16 PIEZ-1 PIEZ-2 PIEZ-3 PIEZ-4 PIEZ-5 TW4-1 TW4-2 TW4-3 TW4-4 TW4-5 TW4-6 TW4-9 TW4-11 TW4-12 TW4-13 TW4-14 TW4-16 TW4-18 TW4-20 TW4-21 MW-04TW4-7 TW4-8 TW4-10 TW4-22 TW4-19 TW4-23 TW4-24 TW4-25 MW-20 PIEZ-1 perched monitoring well perched piezometer temporary perched monitoring well COMPARISON OF KRIGED 1st QUARTER 2009 AND 1st QUARTER 2008 CHLOROFORM PLUMES WHITE MESA SITE H:/718000/71801/ CACME2016/maps/chlcomp_08_09.srf TW4-1 EXPLANATION chloroform plume boundaries (solid = 1st Quarter, 2009 dashed = 1st Quarter, 2008) SJS A.3 Note: MW-4, MW-26, TW4-19, and TW4-20 are pumping wells HYDRO GEO CHEM, INC. APPROVED DATE REFERENCE FIGURE CELL NO. 2 CELL NO. 4A 3332 MW-21 3000 BOUNDARY PROPERTY SCALE IN FEET 0 CELL NO. 1 MILL SITE MW-01 MW-02 MW-03 MW-05 MW-11 MW-12 MW-14 MW-15 MW-17 MW-18 MW-19 MW-20 MW-22 MW-23 MW-24 MW-25 MW-27 MW-28 MW-29 MW-30 MW-31 MW-32 MW-26 MW-16 PIEZ-1 PIEZ-2 PIEZ-3 PIEZ-4 PIEZ-5 TW4-1 TW4-2 TW4-3 TW4-4 TW4-5 TW4-6 TW4-9 TW4-11 TW4-12 TW4-13 TW4-14 TW4-16 TW4-18 TW4-20 TW4-21 MW-04TW4-7 TW4-8 TW4-10 TW4-22 TW4-19 TW4-23 TW4-24 TW4-25 TWN-1 TWN-2 TWN-3 TWN-4 TWN-5 TWN-6 TWN-7 TWN-8 TWN-9 TWN-10 TWN-11 TWN-12 TWN-13 TWN-14 TWN-15 TWN-16 TWN-17 TWN-18 TWN-19 MW-20 PIEZ-1 perched monitoring well perched piezometer temporary perched monitoring well COMPARISON OF KRIGED 1st QUARTER 2010 AND 1st QUARTER 2009 CHLOROFORM PLUMES WHITE MESA SITE H:/718000/71801/ CACME2016/maps/chlcomp_09_10.srf TW4-1 EXPLANATION chloroform plume boundaries (solid = 1st Quarter, 2010 dashed = 1st Quarter, 2009) SJS A.4 temporary perched nitrate monitoring well TWN-1 Note: MW-4, MW-26, TW4-4 (as of Q1 2010), TW4-19, and TW4-20 are pumping wells HYDRO GEO CHEM, INC. APPROVED DATE REFERENCE FIGURE CELL NO. 2 CELL NO. 4A 3332 MW-21 3000 BOUNDARY PROPERTY SCALE IN FEET 0 CELL NO. 1 MILL SITE MW-01 MW-02 MW-03 MW-05 MW-11 MW-12 MW-14 MW-15 MW-17 MW-18 MW-19 MW-20 MW-22 MW-23 MW-24 MW-25 MW-27 MW-28 MW-29 MW-30 MW-31 MW-32 MW-26 MW-33 MW-34 MW-35 PIEZ-1 PIEZ-2 PIEZ-3 PIEZ-4 PIEZ-5 TW4-1 TW4-2 TW4-3 TW4-4 TW4-5 TW4-6 TW4-9 TW4-11 TW4-12 TW4-13 TW4-14 TW4-16 TW4-18 TW4-20 TW4-21 MW-04TW4-7 TW4-8 TW4-10 TW4-22 TW4-19 TW4-23 TW4-24 TW4-25 TW4-26 TWN-1 TWN-2 TWN-3 TWN-4 TWN-5 TWN-6 TWN-7 TWN-8 TWN-9 TWN-10 TWN-11 TWN-12 TWN-13 TWN-14 TWN-15 TWN-16 TWN-17 TWN-18 TWN-19 MW-20 PIEZ-1 perched monitoring well perched piezometer temporary perched monitoring well COMPARISON OF KRIGED 1st QUARTER 2011 AND 1st QUARTER 2010 CHLOROFORM PLUMES WHITE MESA SITE H:/718000/71801/ CACME2016/maps/chlcomp_10_11.srf TW4-1 EXPLANATION chloroform plume boundaries (solid = 1st Quarter, 2011 dashed = 1st Quarter, 2010) SJS A.5 TWN-1 temporary perched nitrate monitoring well CELL NO. 4B Note: MW-4, MW-26, TW4-4, TW4-19, and TW4-20 are pumping wells HYDRO GEO CHEM, INC. APPROVED DATE REFERENCE FIGURE CELL NO. 2 CELL NO. 4A 3332 MW-21 3000 BOUNDARY PROPERTY SCALE IN FEET 0 CELL NO. 1 MILL SITE MW-01 MW-02 MW-03 MW-05 MW-11 MW-12 MW-14 MW-15 MW-17 MW-18 MW-19 MW-20 MW-22 MW-23 MW-24 MW-25 MW-27 MW-28 MW-29 MW-30 MW-31 MW-32 MW-26 TW4-1 TW4-2 TW4-3 TW4-4 TW4-5 TW4-6 TW4-9 TW4-11 TW4-12 TW4-13 TW4-14 TW4-16 TW4-18 TW4-20 TW4-21 MW-04TW4-7 TW4-8 TW4-10 TW4-22 TW4-19 PIEZ-1 PIEZ-2 PIEZ-3 PIEZ-4 PIEZ-5 TW4-23 TW4-24 TW4-25 TWN-1 TWN-2 TWN-3 TWN-4 TWN-5 TWN-6 TWN-7 TWN-8 TWN-9 TWN-10 TWN-11 TWN-12 TWN-13 TWN-14 TWN-15 TWN-16 TWN-17 TWN-18 TWN-19 TW4-26 MW-33 MW-34 MW-35 MW-20 PIEZ-1 perched monitoring well perched piezometer temporary perched monitoring well COMPARISON OF KRIGED 1st QUARTER 2011 AND 1st QUARTER 2007 CHLOROFORM PLUMES WHITE MESA SITE H:/718000/71801/ CACME2016/maps/chlcomp_07_11.srf TW4-1 EXPLANATION chloroform plume boundaries (solid = 1st Quarter, 2011 dashed = 1st Quarter, 2007) SJS A.6 CELL NO. 4B temporary perched nitrate monitoring well TWN-1 Note: MW-4, MW-26, TW4-4 (as of Q1 2010), TW4-19, and TW4-20 are pumping wells APPENDIX B WELL PRODUCTIVITY AND BACKGROUND FLOW ANALYSIS TABLES AND FIGURES (TABLES B.1-B.4; FIGURES B.1-B.13) H:\718000\71801\CACME2016\Pvolume_4Q15: F B1 N pump 0 50,000 100,000 150,000 200,000 250,000 300,000 Q2 13 Q3 13 Q4 13 Q1 14 Q2 14 Q3 14 Q4 14 Q1 15 Q2 15 Q3 15 Q4 15 ga l l o n s p u m p e d TW4-22 TW4-24 TW4-25 TWN-2 PRODUCTIVITY OF NITRATE PUMPING WELLS HYDRO GEO CHEM, INC.Approved FigureDateAuthorDate File Name SJS 2/18/16 B.1Pvolume_4Q15.xls2/5/16SJS H:\718000\71801\CACME2016\Pvolume_4Q15: F B2 chl pmp 0 50,000 100,000 150,000 200,000 250,000 300,000 350,000 400,000 450,000 Q2 13 Q3 13 Q4 13 Q1 14 Q2 14 Q3 14 Q4 14 Q1 15 Q2 15 Q3 15 Q4 15 ga l l o n s p u m p e d MW-4 MW-26 TW4-4 TW4-19 TW4-20 TW4-1 TW4-2 TW4-11 PRODUCTIVITY OF CHLOROFORM PUMPING WELLS HYDRO GEO CHEM, INC.Approved FigureDateAuthorDate File Name SJS 2/18/16 B.2Pvolume_4Q15.xls2/5/16SJS N.3 H:\718000\71801\CACME2016\CACME_DTW_TimeSeries_chl: F B4 TW4-5 TIME SERIES OF DEPTHS TO WATER AT TW4-5 SINCE Q1 2012 HYDRO GEO CHEM, INC.Approved FigureDateAuthorDate File Name SJS 2/18/16 B.4DTW_TimeSeries.xls2/4/16GEM 50 55 60 65 70 De p t h t o W a t e r ( f t ) Date H:\718000\71801\CACME2016\CACME_DTW_TimeSeries_chl: F B5 TW4-9 TIME SERIES OF DEPTHS TO WATER AT TW4-9 SINCE Q1 2012 HYDRO GEO CHEM, INC.Approved FigureDateAuthorDate File Name SJS 2/18/16 B.5DTW_TimeSeries.xls2/4/16GEM 50 55 60 65 70 De p t h t o W a t e r ( f t ) Date H:\718000\71801\CACME2016\CACME_DTW_TimeSeries_chl: F B6 TW4-10 TIME SERIES OF DEPTHS TO WATER AT TW4-10 SINCE Q1 2012 HYDRO GEO CHEM, INC.Approved FigureDateAuthorDate File Name SJS 2/18/16 B.6DTW_TimeSeries.xls2/4/16GEM 50 55 60 65 70 De p t h t o W a t e r ( f t ) Date H:\718000\71801\CACME2016\CACME_DTW_TimeSeries_chl: F B7 TW4-16 TIME SERIES OF DEPTHS TO WATER AT TW4-16 SINCE Q1 2012 HYDRO GEO CHEM, INC.Approved FigureDateAuthorDate File Name SJS 2/18/16 B.7DTW_TimeSeries.xls2/4/16GEM 50 55 60 65 70 De p t h t o W a t e r ( f t ) Date H:\718000\71801\CACME2016\CACME_DTW_TimeSeries_chl: F B8 TW4-18 TIME SERIES OF DEPTHS TO WATER AT TW4-18 SINCE Q1 2012 HYDRO GEO CHEM, INC.Approved FigureDateAuthorDate File Name SJS 2/18/16 B.8DTW_TimeSeries.xls2/4/16GEM 50 55 60 65 70 De p t h t o W a t e r ( f t ) Date H:\718000\71801\CACME2016\CACME_WHIP_Fits_chl: F B9 TW4-5 0 1 2 3 4 5 6 0.01 0.1 1 10 100 1000 10000 100000 1000000 Di s p l a c e m e n t ( f t ) Elapsed Time (minutes) Observed Simulated Results Transmissivity = 45.9 ft 2/d Storativity = 2.86E-04 OBSERVED AND SIMULATED WATER LEVEL DISPLACEMENTS IN TW4-5 SINCE Q4 2014 HYDRO GEO CHEM, INC.Approved FigureDateAuthorDate File Name SJS 2/18/16 B.9CACME_WHIP_fits_chl.xls2/4/16GEM H:\718000\71801\CACME2016\CACME_WHIP_Fits_chl: Fig B10 TW4-9 0 1 2 3 4 5 6 0.01 0.1 1 10 100 1000 10000 100000 1000000 Di s p l a c e m e n t ( f t ) Elapsed Time (minutes) Observed Simulated OBSERVED AND SIMULATED WATER LEVEL DISPLACEMENTS IN TW4-9 SINCE Q4 2014 HYDRO GEO CHEM, INC.Approved FigureDateAuthorDate File Name SJS 2/18/16 B.10CACME_WHIP_fits_chl.xls2/4/16GEM Results Transmissivity = 50.8 ft 2/d Storativity = 1.23E-04 H:\718000\71801\CACME2016\CACME_WHIP_Fits_chl: Fig B11 TW4-10 0 1 2 3 4 5 6 0.01 0.1 1 10 100 1000 10000 100000 1000000 Di s p l a c e m e n t ( f t ) Elapsed Time (minutes) Observed Simulated Results Transmissivity = 47.4 ft 2/d Storativity = 8.98E-04 OBSERVED AND SIMULATED WATER LEVEL DISPLACEMENTS IN TW4-10 SINCE Q4 2014 HYDRO GEO CHEM, INC.Approved FigureDateAuthorDate File Name SJS 2/18/16 B.11CACME_WHIP_fits_chl.xls2/4/16GEM H:\718000\71801\CACME2016\CACME_WHIP_Fits_chl: Fig B12 TW4-16 0 1 2 3 4 5 6 0.01 0.1 1 10 100 1000 10000 100000 1000000 Di s p l a c e m e n t ( f t ) Elapsed Time (minutes) Observed Simulated Results Transmissivity = 9.2 ft 2/d Storativity = 7.23E-04 OBSERVED AND SIMULATED WATER LEVEL DISPLACEMENTS IN TW4-16 SINCE Q4 2014 HYDRO GEO CHEM, INC.Approved FigureDateAuthorDate File Name SJS 2/18/16 B.12CACME_WHIP_fits_chl.xls2/4/16GEM H:\718000\71801\CACME2016\CACME_WHIP_Fits_chl: Fig B13 TW4-18 0 1 2 3 4 5 6 0.01 0.1 1 10 100 1000 10000 100000 1000000 Di s p l a c e m e n t ( f t ) Elapsed Time (minutes) Observed Simulated Results Transmissivity = 65.7 ft 2/d Storativity = 1.29E-05 OBSERVED AND SIMULATED WATER LEVEL DISPLACEMENTS IN TW4-18 SINCE Q4 2014 HYDRO GEO CHEM, INC.Approved FigureDateAuthorDate File Name SJS 2/18/16 B.13CACME_WHIP_fits_chl.xls2/4/16GEM TABLE B.1 Hydraulic Gradients Within the Chloroform Plume 3rd Quarter 2002 and 2nd Quarter 2015 Q3 2002 Q2 2015 well water level water level (ft amsl) (ft amsl) TW4-5 5585.0 5577.5 TW4-6 5524.4 5537.8 TW4-10 5578.2 5573.3 TW4-16 5563.4 5560.6 approximate approximate distance (ft)distance (ft) TW4-5 to TW4-16 TW4-10 to TW4-6 990 1365 Q3 2002 hydraulic Q3 2002 hydraulic gradient (ft/ft)gradient (ft/ft) TW4-5 to TW4-16 TW4-10 to TW4-6 0.022 0.039 Q2 2015 hydraulic Q2 2015 hydraulic gradient (ft/ft)gradient (ft/ft) TW4-5 to TW4-16 TW4-10 to TW4-6 0.017 0.026 Notes: ft amsl = feet above mean sea level ft/ft = feet per foot H:\718000\71801\CACME2016\Table_B1_B3.xls: Table B1 TABLE B.2 Hydraulic Conductivities Within and Adjacent to the Northwestern and Southeastern Portions of the Chloroform Plume location k (cm/s) k (ft/d) location k (cm/s) k (ft/d) TW4-5 4.60E-04 1.29 TW4-1 8.20E-05 0.23 TW4-9 3.90E-04 1.09 TW4-2 4.30E-05 0.12 TW4-10 2.60E-04 0.73 TW4-4 1.66E-03 4.65 TW4-16 1.00E-04 0.28 TW4-6 1.15E-05 0.03 TW4-18 3.90E-04 1.09 TW4-7 4.30E-05 0.12 TW4-19 2.40E-04 0.67 TW4-8 4.30E-05 0.12 TW4-20 5.90E-05 0.17 TW4-33 5.51E-05 0.15 TW4-21 1.90E-04 0.53 MW-4A 1.14E-04 0.32 TW4-22 1.30E-04 0.36 MW-26 7.90E-05 0.22 geomean:1.86E-04 0.52 geomean:7.27E-05 0.20 Notes: k = hydraulic conductivity cm/s = centimeters per second ft/d = feet per day northwest southeast H:\718000\71801\CACME2016\Table_B1_B3.xls: Table B2 TABLE B.3 Changes in Saturated Thickness Between 3rd Quarter 2002 and 4th Quarter 2014 Brushy Basin Q3 2002 Q4 2014 Q3 2002 Q4 2014 well elevation water level water level saturated saturated (ft amsl) (ft amsl) (ft amsl) thickness (ft) thickness (ft) TW4-5 5536.3 5585.0 5577.4 48.7 41.1 TW4-9 5532.1 5583.8 5576.5 51.7 44.3 TW4-10 5526.0 5578.2 5573.2 52.2 47.1 TW4-16 5481.2 5563.4 5558.0 82.2 76.8 TW4-18 5501.6 5585.4 5577.1 83.8 75.5 average:63.7 57.0 TW4-1 5514.6 5551.8 5550.7 37.2 36.1 TW4-2 5517.8 5556.2 5557.1 38.4 39.3 TW4-6 5512.3 5524.4 5538.7 12.1 26.4 TW4-7 5522.4 5560.2 5552.7 37.8 30.3 TW4-8 5517.4 5553.7 5555.0 36.3 37.6 TW4-11 5535.9 5549.6 5563.4 13.7 27.5 average:29.3 32.8 Notes: ft = feet ft amsl = feet above mean sea level H:\718000\71801\CACME2016\Table_B1_B3.xls: Table B3 TABLE B.4 Comparison of Transmissivity and Saturated Thickness Estimates Observation *Average 2003 *Average 2015 % 2003 T 2015 T % Well Saturated Saturated Difference Estimate Estimate Difference Thickness (ft) Thickness (ft)(ft2/day)(ft2/day) TW4-5 62 48 -23 87 46 -47 TW4-9 63 49 -22 71 51 -28 TW4-10 64 51 -20 46 47 2 TW4-16 79 67 -15 18 9 -50 TW4-18 80 65 -19 74 66 -11 Average 70 56 -20 59 44 -27 Notes: *average saturated thickness = average of TW4-19 and observation well saturated thicknesses T = transmissivity in feet squared per day H:\718000\71801\CACME2016\CACME_WHIP_Fits_chl.xls: Table B4 APPENDIX C NATURAL CHLOROFORM DEGRADATION Appendix C - Chloroform Mass Removal Via Natural In-Situ Degradation https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/AppC/AppC 2024CACME.docx March 28, 2024 C-1 APPENDIX C NATURAL CHLOROFORM DEGRADATION In-situ breakdown of chloroform via biologically mediated and abiotic means is expected to occur within the perched zone chloroform plume at the White Mesa site. The possible degradation mechanisms include: • reductive dechlorination (abiotic degradation) • anaerobic reductive dechlorination (anaerobic biodegradation) • cometabolic processes (aerobic biodegradation) Reductive dechlorination of chloroform involves successive replacement of chloroform atoms by hydrogen. This process occurs relatively rapidly under anaerobic conditions in the presence of naturally occurring anaerobic bacteria, but can also occur, albeit slowly, without the aid of bacteria. Degradation of chloroform can also occur under aerobic conditions by cometabolic processes. Cometabolism involves incidental biodegradation of one compound while another compound is used as a food source by the naturally occurring bacteria. Within the perched groundwater zone, naturally occurring organic carbon might be used as such a food source, allowing chloroform to be cometabolized. Based on rates provided in HydroGeoLogic, 1999, anaerobic reductive dechlorination could reduce detected concentrations by more than three orders of magnitude within a few years, provided conditions were favorable. However, this mechanism is likely to be minimal, because the relative persistence of the perched groundwater chloroform and nitrate plumes, and of nitrate associated with the chloroform plume, indicates that the perched groundwater is generally aerobic. Neither nitrate nor chloroform would be persistent under strictly anaerobic conditions as both would be expected to degrade relatively rapidly. However, the detection in perched groundwater of low concentrations of methylene chloride (dichloromethane or DCM) and chloromethane (CM), both daughter products of anaerobic reductive dechlorination, is consistent with locally anaerobic conditions. Abiotic reductive dechlorination is likely to be quite slow based on studies by Jeffers, et al, 1989, and Mabey and Mill, 1978, with expected half lives for chloroform of 1,850 to 3,650 years at neutral pH. Degradation would be more rapid at higher pH, with expected half lives of 25 to 37 years at pH 9. However, perched water at the site is generally near neutral, so the lower rates (higher half lives) would be likely for the perched zone. Appendix C - Chloroform Mass Removal Via Natural In-Situ Degradation https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/AppC/AppC 2024CACME.docx March 28, 2024 C-2 Cometabolic degradation can occur relatively rapidly if sufficient organic carbon is present in the perched zone that could serve as a food source for an indigenous methanotrophic population. Under ideal conditions, this process would be expected to proceed at rates higher than anaerobic rates. However, in natural groundwater, this mechanism is not expected to be dominant. Concentrations of DCM and CM, the daughter products of reductive chloroform dechlorination, can be used to estimate the actual degradation rates of chloroform in the perched groundwater. As discussed above, both have been detected at the site in low concentrations (typically a few µg/L). Chloroform degrades via reductive dechlorination to DCM, then CM. DCM is commonly used in analytical laboratories and its detection in some cases may have resulted from laboratory contamination. However, assuming that detections are representative of site conditions, chloroform degradation rates can be estimated by assuming the following: • the detected DCM is a degradation product of chloroform, • the rate of DCM depletion is fast compared to chloroform (because, unlike chloroform, DCM can degrade relatively rapidly under aerobic conditions), and • the concentrations of detected DCM are in pseudo steady state. The last assumption implies that the DCM degrades about as fast as it is produced. A range of first order aerobic degradation rate constants for DCM are provided in Aronson, et al, 1999. These range from 0.0036/day to 0.533/day, with a recommended rate of 0.0546/day. These rates are relatively large and imply fast rates of DCM degradation under aerobic conditions. The rates imply that degradation of DCM would be much faster (by orders of magnitude) than would be expected for chloroform which is expected to degrade very slowly under aerobic conditions. As discussed in HGC (2007) DCM was detected in perched zone wells TW4-11, TW4-15, TW4- 16, and TW4-20 in the first quarter of 2007 at concentrations ranging from 1.1 to 6.5 µg/L, and the same four wells in the fourth quarter of 2006, at concentrations ranging from 1.3 to 9.2 µg/L. Chloroform was also detected at these same four wells during the same two quarters. Because DCM was detected in the same four wells during both quarters, these detections were likely representative of site conditions, and not random laboratory analytical error. Furthermore, the similarity in DCM concentrations over these two quarters suggests a pseudo steady state condition. The average DCM concentration at these four wells over the two quarters was 3.8 µg/L. The amount of chloroform degradation implied by these DCM concentrations can be estimated by using the expected rate of DCM degradation (0.0546/day) and assuming, on a molar basis, that the amount of chloroform degraded is equal to the amount of DCM degraded. The expected amount of DCM degraded per day can be calculated using the following first order rate equation: Appendix C - Chloroform Mass Removal Via Natural In-Situ Degradation https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/AppC/AppC 2024CACME.docx March 28, 2024 C-3 tk C Cn -= 0 l Where: C = the observed concentration C0 = the concentration at time zero (initial concentration) k = the rate constant (1/day) t = the elapsed time (days) Assuming that t = 1 day, C0 = 3.8 µg/L, k = 0.0546/day, and solving for C, C = 3.60 µg/L The implied change in DCM concentration per day is 3.8 µg/L - 3.6 µg/L, or 0.20 µg/L. On a molar basis, this implies that 0.28 µg/L chloroform was degraded to replace the 0.20 µg/L DCM that was degraded in the same day. During the first quarter of 2007 and the fourth quarter of 2006, the chloroform concentrations at TW4-11, TW4-15, and TW4-16 ranged from 9 µg/L to 11,000 µg/L and averaged 2,929 µg/L. A reduction of between one and two orders of magnitude would be needed to bring this chloroform concentration to the action level of 70 µg/L. To calculate the rate of chloroform degradation implied by the daily amount of 0.28 µg/L chloroform degraded as calculated above, the same first order rate equation can be used: tk C Cn -= 0 l Using 2,929 µg/L for C0, assuming C = 2,929 – 0.28 = 2928.72 µg/L, rearranging and solving for k, yields -0.00010/day. This rate is more than an order of magnitude lower than the lowest anaerobic rate of -0.004/day reported for chloroform in HydroGeoLogic, 1999. The calculated chloroform degradation rate of -0.00010/day can then be used in the first order rate equation after solving for t to calculate the time needed to reduce chloroform concentrations by one and two orders of magnitude (C/C0 = 0.1, and C/C0 = 0.01, respectively). By rearranging and solving for t , day nt /00010.0 )1.0( - =l = And 23,025 days or 63 years for a one order of magnitude reduction, Appendix C - Chloroform Mass Removal Via Natural In-Situ Degradation https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/AppC/AppC 2024CACME.docx March 28, 2024 C-4 day nt /00010.0 )01.0( - =l = To reduce the highest concentration ever detected at the site (61,000 µg/L at TW4-20) to the action level would require nearly three orders of magnitude reduction in concentration. Performing a similar calculation where C/C0 = 0.001 yields day nt /00010.0 )001.0( - =l = Using a similar methodology, detectable and reportable chloroform and DCM concentrations from the first quarter of 2013 through the third quarter of 2023 were used to calculate degradation rates and estimate the time needed for a three order of magnitude reduction in chloroform concentrations. These calculations update those presented in HGC (2022) and are based on concentrations of chloroform and DCM in wells within and marginal to the plume that had reportable detections of both chloroform and DCM in at least one sample. Data from wells MW- 26, TW4-14, TW4-19, TW4-20, TW4-22, TW4-24, TW4-37 and TW4-39 met these criteria. The largest data set was derived from MW-26 (sampled both quarterly and monthly), which had detectable and reportable chloroform and DCM concentrations in 121 samples. TW4-20 met these criteria 10 consecutive times from the first quarter of 2013 through the second quarter of 2015; once during the first quarter of 2016; twice during the first and second quarters of 2018; and once during the second quarter of 2020. TW4-37 met these criteria 5 consecutive times from the second quarter of 2015 through the second quarter of 2016; and during the fourth quarter of 2021. TW4- 22 met these criteria during the first quarter of 2014; and for four consecutive quarters from the fourth quarter of 2022 through the third quarter of 2023. TW4-14 met these criteria during each quarter of 2016. TW4-39 met these criteria during the first through third quarters of 2017. Finally TW4-19 met these criteria once during the third quarter of 2021; and TW4-24 met these criteria once during the first quarter of 2014. The largest and second largest DCM concentrations of 52.4 µg/L and 43.3 µg/L were detected at MW-26. In addition to both chloroform and DCM detections at the above wells, reportable detections of both chloroform and CM occurred at least once in the same sample at the following 19 wells: MW- 4, MW-26, TW4-1, TW4-2, TW4-4, TW4-5, TW4-7, TW4-10, TW4-11, TW4-19, TW4-20, TW4- 21, TW4-22, TW4-24, TW4-26, TW4-37, TW4-39, TW4-40 and TW4-41. The largest and second 46,052 days or 126 years for a two orders of magnitude reduction. 69,077 days or 189 years for a three orders of magnitude reduction. Appendix C - Chloroform Mass Removal Via Natural In-Situ Degradation https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/AppC/AppC 2024CACME.docx March 28, 2024 C-5 largest CM concentrations of 55.2 µg/L and 40 µg/L were detected at TW4-20 and TW4-22, respectively. Rates of chloroform degradation were calculated from the chloroform and DCM data using the same approach described above for the fourth quarter 2006 and first quarter 2007 data, except that calculations were performed on an individual well basis. Because it had the largest number of DCM detections, and because the average quarterly chloroform concentration at MW-26 since the end of 2012 (approximately 1,650 µg/L) is similar to the average chloroform concentration within the plume (approximately 1,360 µg/L based on gridded concentration data and approximately 2,745 µg/L based on the average concentrations at wells within the plume), calculations using MW-26 data are likely to be the most reliable and representative. However, to be conservative, calculations using the data from all seven of the above wells having both chloroform and DCM detections are provided. Calculated degradation rates for each well are summarized in Table C.1. Using the rates calculated for each well, the geometric average first order degradation rate was -.000101/day (-1.01 x 10- 4/day) To reduce the highest 2023 concentration of 14,000 µg/L to the GCAL of 70 µg/L would take approximately 52,459 days or 144 years. If the rate of -3.17 x 10-4/day calculated using MW- 26 data were used, the time to reduce the highest 2023 concentration of 14,000 µg/L to the GCAL of 70 µg/L would take approximately 16,714 days or 46 years. The above calculations assume that reductions in chloroform concentrations occur only through biological means, and do not account for mass removal by pumping nor additional natural attenuation mechanisms that include dilution, hydrodynamic dispersion, volatilization, and abiotic degradation. The calculations are therefore conservative in that they likely underestimate the actual rates of reductions in chloroform concentrations and overestimate plume remediation times in the absence of pumping, and significantly overestimate plume remediation times considering the beneficial impacts of pumping. Appendix C - Chloroform Mass Removal Via Natural In-Situ Degradation https://hgcinc.sharepoint.com/VOL4/718000/71801/CACME2024/Final Report/AppC/AppC 2024CACME.docx March 28, 2024 C-6 REFERENCES Aronson, et al. 1999. Aerobic Biodegradation of Organic Chemicals in Environmental Media: A Summary of Field and Laboratory Studies. Environmental science Center, Syracuse Research Corporation, North Syracuse, NY. Submitted to U S Environmental Protection Agency. Hydro Geo Chem, Inc. 2007. Preliminary Corrective Action Plan. White Mesa Uranium Mill Site Near Blanding, Utah. August 20, 2007. Hydro Geo Chem, Inc. 2022. Corrective Action Comprehensive Monitoring Evaluation (CACME) Report, White Mesa Uranium Mill Near Blanding, Utah. March 30, 2022. HydroGeoLogic, Inc. 1999. Anaerobic Degradation Rates of Organic Chemicals in Groundwater: A Summary of Field and Laboratory Studies. Submitted to U. S. Environmental Protection Agency Office of Solid Waste. Jeffers, et al.1989. Homogeneous Hydrolysis Rate Constants for Selected Chlorinated Methanes, Ethanes, Ethenes, and Propanes. Environ. Sci. Technol., Vol 23, No. 8, pp 965-969. Mabey, W., and T. Mill. 1989. Critical Review of Hydrolysis of Organic Compounds in Water Under Environmental Conditions. Journal of Physical and Chemical Reference Data, Vol 7, No. 2, pp 383-415. TABLE Table C.1 Estimated First Order Rate Constants number of average CH3CL average MC -k Well sampling events concentration (µg/L) concentration (µg/L) (1/day) MW-26 121 2017 8.61 3.17E-04 TW4-14 4 5.7 2.19 2.91E-02 TW4-19 1 4450 1.25 2.25E-05 TW4-20 14 18229 3.08 1.21E-05 TW4-22 5 4084 1.95 3.67E-05 TW4-24 1 78.5 1.18 1.12E-03 TW4-37 6 19550 1.95 7.46E-06 TW4-39 3 7340 1.39 1.41E-05 geomean 1.01E-04 Notes: CH3CL = chloroform MC = methylene chloride k = first order rate constant µg/L = micrograms per liter