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Home My WebLink AboutDRC-2023-066990 - 0901a06881226150June 21, 2023 Sent VIA OVERNIGHT DELIVERY Mr. Doug Hansen Director U V 1,. , d:,jc; ~ ard Rad1at1c 1 co~trol JUN 2 3 2 23 Division of Waste Management and Radiation Control Utah Department of Environmental Quality 195 North 1950 West Salt Lake City, UT 84116 Energy Fuels Resources (USA) Inc. 225 Union Blvd. Suite 600 Lakewood, CO, US, 80228 303 974 2140 \\ \\ w .ener2v fu I . m Re: Transmittal of Nitrate Corrective Action Comprehensive Monitoring Evaluation ("CACME") UDEQ Docket No. UGW12-04 White Mesa Uranium Mill Dear Mr. Hansen : Enclosed are two copies of the Energy Fuels Resources USA Inc. ("£FRI") Corrective Action Comprehensive Monitoring Evaluation ("CACME") report for nitrate in perched groundwater at the White Mesa Uranium Mill (the "Mill") located near Blanding, Utah. This report represents a 5-year review of the Phase II Corrective Action and is being submitted as specified in the 2017 CACM£ Report dated December 11 , 2017. If you should have any questions regarding this submittal please contact me at 303-389-4134. Yours very truly, ENERGY FUELS RESOURCES (USA) INC. Kathy W einel Director, Regulatory Compliance CC: David Frydenlund Garrin Palmer Logan Shumway Scott Bakken John Uhrie Jordan App Stewart Smith (HGC) DRC-2023-066990 HYDRO GEO CHEM, INC. Environmental Science & Technology NITRATE CORRECTIVE ACTION COMPREHENSIVE MONITORING EVALUATION (CACME) REPORT WHITE MESA URANIUM MILL NEAR BLANDING, UTAH June 21, 2023 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 7180000.00-07.0 i Nitrate Corrective Action Comprehensive Monitoring Evaluation (CACME) Report Https://Hgcinc.Sharepoint.Com/VOL4/718000/71807/NCACME2022/Report/NCACME2022_Final.Docx June 21, 2023 TABLE OF CONTENTS 1. INTRODUCTION .............................................................................................................. 1 2. BACKGROUND AND OVERVIEW ................................................................................ 5 2.1 Historical Perspective ............................................................................................. 7 2.2 Perched Groundwater Occurrence, Pumping, and Impact of Wildlife Ponds ........ 7 2.3 Chloroform Pumping Wells Within and Adjacent to the Nitrate Plume .............. 11 2.4 Summary of Results and Conclusions .................................................................. 12 3. SUMMARY OF PHASE II AND PHASE III MONITORING AND PUMPING ........... 15 3.1 Elements of Quarterly Reports .............................................................................. 15 3.2 Specific Actions Taken During Phase II and Phase III......................................... 16 3.3 Key Findings ......................................................................................................... 17 3.3.1 Quarterly Monitoring ................................................................................ 17 3.3.2 2017 CACME ........................................................................................... 19 3.3.3 Revised Phase III Planning Document ..................................................... 19 4. EVALUATION OF THE EFFECTIVENESS OF PUMPING AND NATURAL ATTENUATION .......................................................................................... 21 4.1 Data Trends ........................................................................................................... 21 4.2 Natural Attenuation ............................................................................................... 25 4.2.1 Nitrate Degradation by Pyrite ................................................................... 26 4.2.2 Other Relevant Studies Regarding Nitrate Reduction by Pyrite ............... 29 4.2.3 Comparison to Oostrum Site ..................................................................... 30 4.3 Hydraulic Capture and Recalculation of ‘Background’ Flow .............................. 31 4.4 Impacts of Perched Groundwater Flow, Pumping and Natural Attenuation on the Nitrate Plume ............................................................ 35 4.5 Rate of Plume Remediation .................................................................................. 36 4.5.1 Method 1 ................................................................................................... 38 4.5.2 Method 2 ................................................................................................... 39 4.5.3 Method 3 ................................................................................................... 41 4.5.4 Method 4 ................................................................................................... 42 4.5.5 Summary ................................................................................................... 42 4.5.6 Comparison With Mass Removed by Pumping ........................................ 43 4.6 Projected Timeline to Return Groundwater Nitrate Concentrations to the Groundwater Quality Standards ............................................................................ 44 5. CONCLUSIONS AND RECOMMENDATIONS ........................................................... 47 5.1 Conclusions ........................................................................................................... 47 5.2 Recommended Changes to Phase III .................................................................... 51 6. REFERENCES ................................................................................................................. 53 ii Nitrate Corrective Action Comprehensive Monitoring Evaluation (CACME) Report Https://Hgcinc.Sharepoint.Com/VOL4/718000/71807/NCACME2022/Report/NCACME2022_Final.Docx June 21, 2023 TABLE OF CONTENTS (Continued) TABLES 1 Quarterly Nitrate Plume Area, Mass Pumped, Residual Mass, and Average Concentrations During Phase II (and including Q2 2010 and Q4 2012 data) 2 Slug Test Results (Using KGS Solution and Automatically Logged Data) 3 Summary of Nitrate Degradation Rates 4 Pyrite Contents in Samples from White Mesa Mill and Oostrum, Netherlands Site 5 Pre-pumping Saturated Thicknesses 6 Pre-pumping Hydraulic Gradients and Flow Calculations 7 Nitrate Mass Pumped During Phase II and Phase III 8 Summary of ‘Background’ Flow Estimates FIGURES 1A White Mesa Site Plan Showing Locations of Perched Wells and Piezometers 1B White Mesa Site Plan Showing 2nd Quarter, 2022 Perched Water Levels and Nitrate, Chloride and Chloroform Plumes 2 Kriged 2nd Quarter, 2022 Nitrate (mg/L), Nitrate + Nitrite as N, White Mesa Site 3 Kriged 2nd Quarter, 2022 Chloride (mg/L), White Mesa Site 4 Change in Nitrate Plume Boundary, Q2 2010 to Q2 2022, Showing Q3 2017 Kriged Perched Water Levels (detail map) 5A Nitrate and Chloride Concentrations (Beginning With Q2 2010 Baseline) in MW-30 and MW-31 5B Nitrate to Chloride Ratios (Beginning With Q2 2010 Baseline) in MW-30 and MW-31 6 Residual Nitrate Plume Mass Estimates and Trend 7A Kriged 3rd Quarter, 2015 Nitrate (mg/L), Nitrate + Nitrite as N, White Mesa Site 7B Kriged 4th Quarter, 2016 Nitrate (mg/L), Nitrate + Nitrite as N, White Mesa Site 8 Nitrate Concentrations (Beginning With Q2 2010 Baseline) in Wells East of Plume 9 Nitrate and Chloride Concentrations (Beginning With Q2 2010 Baseline) in Wells West of Plume 10A Nitrate to Chloride Ratios (Beginning with Q2 2010 Baseline) in Wells Originally West of Plume (note that TWN-7 is now within plume) 10B Average Plume Nitrate to Chloride Ratios Based on Wells Consistently Within Nitrate Plume and Nitrate to Chloride Ratios in TWN-7 (Beginning with Q2 2010 Baseline) 11A Change in Nitrate and Chloride Plume Boundaries Between Q2 2010 and Q2 2022, and Showing Q2 2022 Water Levels (detail map) 11B Change in Nitrate and Chloride Plume Boundaries Between Q2 2010 and Q2 2022, and Showing Q2 2010 Water Levels (detail map) iii Nitrate Corrective Action Comprehensive Monitoring Evaluation (CACME) Report Https://Hgcinc.Sharepoint.Com/VOL4/718000/71807/NCACME2022/Report/NCACME2022_Final.Docx June 21, 2023 TABLE OF CONTENTS (Continued) FIGURES (Continued) 12A Water Levels in Wells (Beginning With Q2 2010 Baseline) Originally Within Plume 12B Saturated Thicknesses in Wells (Beginning With Q2 2010 Baseline) Originally Within Plume 12C Water Level Elevations (Beginning With Q2 2010 Baseline) in TWN-3 and TWN-7 13 Change in Saturated Thickness, Q2 2010 to Q2 2022, White Mesa Site (detail map) 14 Percent Change in Saturated Thickness, Q2 2010 to Q2 2022, White Mesa Site (detail map) 15 Nitrate Concentrations (Beginning With Q2 2010 Baseline) in Wells Originally Within Plume 16 Nitrate Concentrations (Beginning With Q2 2010 Baseline) in Wells Consistently Within Plume 17 Q2 2010 ‘Baseline’ Nitrate Plume Mass, White Mesa Site (detail map) 18 Q2 2022 Nitrate Plume Mass White Mesa Site (detail map) 19 Average Plume Nitrate Concentrations Based on Concentrations in Wells Within Plume 20 Average Plume Nitrate Concentrations Based on Gridded (Kriged) Nitrate Concentrations 21 Total Estimated Pumping Capture and Average (Q3 21 through Q2, 22) and Q2 2022 Nitrate Plume Boundaries (detail map) 22 Approximate Area Between Second Quarter, 2022 Nitrate and Chloride Plumes Used in ‘Method 3’ Nitrate Degradation Calculations APPENDICES A Second Quarter, 2010 Well Location, Nitrate, and Chloride Concentration Maps (Figures A.1-A.3) B Evaluation of Reduced Productivity at TW4-19 and TW4-24 and Calculation of New Background Flow Through the Nitrate Plume (Attachment N of EFRI2015d) iv Nitrate Corrective Action Comprehensive Monitoring Evaluation (CACME) Report Https://Hgcinc.Sharepoint.Com/VOL4/718000/71807/NCACME2022/Report/NCACME2022_Final.Docx June 21, 2023 1 Nitrate Corrective Action Comprehensive Monitoring Evaluation (CACME) Report Https://Hgcinc.Sharepoint.Com/VOL4/718000/71807/NCACME2022/Report/NCACME2022_Final.Docx June 21, 2023 1. INTRODUCTION This Corrective Action Comprehensive Monitoring Evaluation (CACME) report for the perched groundwater nitrate plume at the White Mesa Uranium Mill (the Mill or the site) located near Blanding, Utah represents the second 5-year review of nitrate corrective action activities, and focuses on Phase III Corrective Action. Required elements of the Phase III Corrective Action are provided in the Revised Phase III Planning Document (HGC, 2018b). The first CACME, submitted on December 11, 2017 (HGC, 2017), represented the first 5-year review of the Phase II Corrective Action as specified in the final Stipulation and Consent Order (SCO) Docket No. UGW12-04. The SCO was approved on December 12, 2012 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, 2012]. The May 12, 2012 Corrective Action Plan (CAP) for Nitrate (HGC, 2012a) is an appendix to the SCO. As required under the SCO, the first CACME was to include: 1. An estimate of the rate of nitrate plume remediation (percent mass reduction and concentration reduction per year) and projected timeline to return groundwater nitrate concentrations to the Groundwater Quality Standards using Phase II alone, including any adjustments to the reclamation surety estimate; 2. Identification of any changes to Phase II to improve effectiveness and accelerate the remediation timeline, and; 3. A Phase III Planning Document. As discussed in the SCO, unless it has been determined to the satisfaction of the DIRECTOR that Phase II has returned or will return nitrate concentrations to the Utah Groundwater Quality Standard within five (5) years, then preparation of a Phase III planning document including a transport assessment, a hazard assessment, and an exposure assessment along with a corrective action assessment including an evaluation of best available remedial technologies as described in the May 12, 2012 CAP Section 7.3. In addition, the report was to bear the seal of a Professional Engineer or Professional Geologist, pursuant to UAC R3l7-6-6.15.D.3. The first CACME (the ‘2017 CACME’) met the above requirements of the SCO and discussed quarterly data collected beginning with the implementation of Phase II during the first quarter of 2013. As required, a Phase III Planning Document was included (Section 6 of the 2017 CACME). 2 Nitrate Corrective Action Comprehensive Monitoring Evaluation (CACME) Report Https://Hgcinc.Sharepoint.Com/VOL4/718000/71807/NCACME2022/Report/NCACME2022_Final.Docx June 21, 2023 At the request of DWMRC, a Revised Phase III Planning Document was prepared and submitted on December 13, 2018 (HGC, 2018b). Both the original and Revised Phase III Planning Documents recommended continuation of Phase II nitrate pumping, monitoring and reporting activities augmented by natural attenuation to address the nitrate plume. The Revised Phase III Planning Document relied on numerical flow and transport modeling of the expected fate of the nitrate plume under conservative ‘worst case’ assumptions to provide additional support for the conclusions reached in the original document. In addition, The Revised Phase III Planning Document proposed additional monitoring wells to be installed as part of the construction of proposed Tailings Management System (TMS) cells 5A and 5B. Cells 5A and 5B were to be constructed along the downgradient (southern) margin of existing TMS cells 4A and 4B. The numerical flow and transport simulations included in the Revised Phase III Planning Document were based on conservative ‘worst case’ conservative assumptions that: 1. Disregarded the natural degradation of nitrate within the plume via pyrite oxidation which resulted in the Phase III Planning Document conservatively overestimating simulated plume migration; 2. Disregarded the stability of the southern (downgradient) margin of the nitrate plume over the previous nine years, which suggested that pumping and natural attenuation processes were preventing plume expansion to the south (Appendix B); 3. Disregarded nitrate mass removal by pumping and natural dilution of nitrate concentrations via recharge by precipitation, which resulted in the Phase III Planning Document conservatively overestimating simulated plume migration; 4. Substantially overestimated hydraulic conductivities (by as much as two orders of magnitude) and hydraulic gradients (by nearly a factor of two) downgradient of the TMS, which resulted in the Phase III Planning Document substantially overestimating simulated plume migration rates; and 5. Underestimated dispersivities which resulted in the Phase III Planning Document conservatively underestimating hydrodynamic dispersion and overestimating simulated plume migration. The results of the simulations showed that, even in the absence of pumping or natural degradation via pyrite oxidation, the nitrate plume was expected to fully attenuate via hydrodynamic dispersion alone before reaching any property boundary, including the western boundary, which is closest to the plume. Although the modeling was designed to evaluate long-term changes in the nitrate plume, the current nitrate distribution is generally consistent with model predictions near the southern boundary of the plume. 3 Nitrate Corrective Action Comprehensive Monitoring Evaluation (CACME) Report Https://Hgcinc.Sharepoint.Com/VOL4/718000/71807/NCACME2022/Report/NCACME2022_Final.Docx June 21, 2023 Specific recommended actions detailed in the Revised Phase III Planning Document included: 1. Continued Phase II pumping, monitoring and reporting activities; 2. Reliance on natural attenuation processes that include natural degradation of nitrate by pyrite, hydrodynamic dispersion, and dilution by naturally infiltrating precipitation; and 3. Installation of new piezometer DR-26 and wells MW-46 and MW-47 downgradient of the existing TMS (Figure 14 of the Revised Phase III Planning Document); these installations will augment wells MW-41 through MW-45 (since renamed MW-42 through MW-46) which (although not completed at this time) are proposed for monitoring planned new cells 5A and 5B, and will provide additional data regarding groundwater conditions far down- to cross-gradient of the nitrate plume. As a result, Phase III pumping currently underway represents a continuation of Phase II pumping; and Phase III monitoring and reporting activities are essentially the same as those implemented under Phase II. Because flow and transport modeling using conservative ‘worst case’ assumptions indicated that active remediation by pumping is not needed to achieve full attenuation of the nitrate plume before reaching a property boundary, the purpose of Phase III pumping is primarily to reduce the time needed for full attenuation, after which time all nitrate concentrations associated with the plume are expected to be below the Groundwater Corrective Action Concentration Limit (GCAL) of 10 mg/L. As with the 2017 CACME, the present report relies on data collected from the fourth quarter of 2012, which reflects the quarter just prior to the initiation of Phase II pumping, as well as the ‘baseline’ data collected during the second quarter of 2010. The ‘baseline’ second quarter 2010 (rather than the fourth quarter 2012) data are used as a pre-pumping reference for evaluation of the performance of the corrective action as specified per Phase II of the CAP. 4 Nitrate Corrective Action Comprehensive Monitoring Evaluation (CACME) Report Https://Hgcinc.Sharepoint.Com/VOL4/718000/71807/NCACME2022/Report/NCACME2022_Final.Docx June 21, 2023 5 Nitrate Corrective Action Comprehensive Monitoring Evaluation (CACME) Report Https://Hgcinc.Sharepoint.Com/VOL4/718000/71807/NCACME2022/Report/NCACME2022_Final.Docx June 21, 2023 2. BACKGROUND AND OVERVIEW Perched groundwater is the shallowest groundwater encountered at the site and is the focus of all groundwater monitoring and corrective action activities. Figure 1A is a site plan showing the locations of perched groundwater monitoring wells, piezometers, and nitrate and chloroform pumping wells. Wells TW4-22, TW4-24, TW4-25, and TWN-2 are nitrate 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 are chloroform pumping wells. TW4-20, a former chloroform pumping well, was abandoned in 2020 due to casing collapse. Figure 1B is a site plan that shows kriged second quarter, 2022 perched groundwater elevations, and the locations of the three perched groundwater plumes: the commingled nitrate and chloride plumes; and the chloroform plume, the northwestern extremity of which commingles with the nitrate and chloride plumes. Specifically Figure 1B displays the kriged, second quarter, 2022 boundaries of these plumes. All three plumes originate from source areas located up-gradient to cross-gradient with respect to the site TMS. The nitrate plume (which is the focus of this report) is defined by nitrate as nitrogen (N) concentrations that equal or exceed 10 milligrams per liter (mg/L); the chloride plume by chloride concentrations that equal or exceed 100 mg/L; and the chloroform plume by chloroform concentrations that equal or exceed 70 micrograms per liter (µg/L). The nitrate and chloroform plume boundaries are based on the State of Utah Groundwater Quality Standards for these substances whereas the chloride plume is defined by a threshold concentration that appears to exceed the background chloride concentrations within the perched groundwater (INTERA, 2009b). The nitrate plume as defined in the CAP is confined to the region of the perched zone containing nitrate concentrations exceeding 10 mg/L located south of TWN-18 and north of MW-11. At the time of preparation of the CAP, the highest nitrate concentrations were historically detected at TWN-2, within the northern (upgradient) portion of the plume (Figure 1B), and within the footprint of the historical pond (subsequently referred to in this document as the ‘historic’ pond). Areas of detectable nitrate that are not continuous with the above defined area exist to the northeast (near former nitrate program wells TWN-9 and TWN-17 [now abandoned as per the CAP]), and to the east-southeast associated with the chloroform plume. Areas to the northeast are not a target of the CAP, and nitrate associated with the chloroform plume is addressed by ongoing chloroform pumping. 6 Nitrate Corrective Action Comprehensive Monitoring Evaluation (CACME) Report Https://Hgcinc.Sharepoint.Com/VOL4/718000/71807/NCACME2022/Report/NCACME2022_Final.Docx June 21, 2023 As discussed above, the nitrate, chloride and chloroform plumes commingle; however only the northwest portion of the chloroform plume commingles with the nitrate and chloride plumes, and the suspected sources of the chloroform plume, two former sanitary leach fields that received laboratory wastes prior to Mill operation (HGC, 2007; HGC, 2016), are located to the east (cross- gradient) of the chloride and nitrate plumes. The historic pond (Figure 1B), formerly located upgradient of the TMS within the upgradient extremity of the nitrate and chloride plumes, is likely a major contributing source of nitrate and chloride to the nitrate and chloride plumes, although potential additional former sources have not been ruled out. Currently, there are no known remaining active or unaddressed sources. The historic pond was active as far back as the 1920s, as many as 60 years prior to the establishment of the White Mesa Mill. Aerial and satellite photos taken over the years and dating back to the 1950s indicate that the historic pond was one of the major agricultural/livestock ponds in the area and typically contained water. However, records or information have not been obtained to evidence the actual specific uses of the pond over the years. That the historic pond was a likely major contributor of both nitrate and chloride to the commingled nitrate and chloride plumes is consistent with the overlapping of the upgradient extremities of the nitrate and chloride plumes with the footprint of the historic pond as shown in Figure 1B. As will be discussed below in Section 4.1, simultaneous increases in nitrate and chloride at TWN-7 (which was historically downgradient of the historic pond but far cross- to upgradient of the Millsite and TMS) supports a historic pond source for both elevated nitrate and chloride. That TWN-7 was historically downgradient of the historic pond is demonstrated by the relatively elevated, pre- pumping water levels at TWN-2 and TWN-3 (within and at the margin of the former pond footprint, respectively) and the relatively low water level at TWN-7. In the second quarter of 2010, the water levels at TWN-2 (5612.8 ft amsl) and TWN-3 (5603.2 ft amsl) were 53.2 ft and 43.6 ft higher than the water level at TWN-7 (5559.6 ft amsl) even though TWN-2 and TWN-3 are less than 800 feet (ft) from TWN-7. Although the chloroform and nitrate plumes had different sources, the sanitary leach field sources to the chloroform plume contributed nitrate which exceeds 10 mg/L in areas east-southeast of the nitrate plume. However, the nitrate associated with the chloroform plume is separated from the nitrate plume by wells having nitrate that is either not-detected or at concentrations less than 10 mg/L. Figures 2 and 3 are maps showing second quarter, 2022 nitrate and chloride concentrations, respectively. Second quarter, 2022 nitrate concentrations range from non-detect to approximately 47 mg/L and chloride concentrations range from approximately 6 to 1,200 mg/L. Appendix A provides figures showing second quarter 2010 (‘baseline’) nitrate and chloride concentrations. 7 Nitrate Corrective Action Comprehensive Monitoring Evaluation (CACME) Report Https://Hgcinc.Sharepoint.Com/VOL4/718000/71807/NCACME2022/Report/NCACME2022_Final.Docx June 21, 2023 Second quarter, 2010 nitrate concentrations range from non-detect to approximately 69 mg/L and chloride concentrations range from approximately 6 to 639 mg/L. The maximum nitrate concentration detected within the nitrate plume was 111 mg/L during the fourth quarter of 2016 and the maximum chloride concentration detected within the chloride plume was 1,260 mg/L during the first quarter of 2013. 2.1 Historical Perspective A detailed history of the nitrate and chloride plume investigation is provided in the CAP (HGC, 2012a). Nitrate within the area shown in Figure 1B was first detected in wells TW4-19, TW4-22, TW4-24, and TW4-25 that were installed as part of the investigation of the chloroform plume initially discovered at perched well MW-4 in 1999. Pumping of chloroform-laden perched water began in 2003 (HGC, 2007; HGC, 2016) and continues to the present time. Investigation of nitrate exceeding 10 mg/L in the perched water included installation of 19 temporary TWN-series wells shown in Figures 1A and 1B (many now abandoned as per the CAP) and numerous shallow borings as part of a source investigation. EFRI identified and prioritized potential sources of the nitrate in (INTERA, 2009a) and in (INTERA, 2011). Based on the investigations, EFRI and the Executive Secretary agreed that the corrective actions were to involve three Phases. Phase I involved source control in the vicinity of the Mill’s ammonium sulfate tanks, the one remaining potential source of nitrate contamination. Phase II involves near term active remediation of the nitrate contamination by pumping contaminated water into the Mill’s TMS for disposal, combined with monitored natural attenuation. Phase III, if necessary, was to be at the discretion of EFRI and would involve a long term solution for the nitrate contamination, in the event that the continuation of Phase II was not considered adequate or appropriate. Phase I has been completed; and as indicated in the Revised Phase III Planning Document, Phase III, currently implemented, includes the continuation of Phase II pumping, monitoring and reporting activities. 2.2 Perched Groundwater Occurrence, Pumping, and Impact of Wildlife Ponds An extensive description of the site hydrogeology, which focuses on the perched groundwater zone, is provided in HGC (2022). As noted above, perched groundwater is the shallowest groundwater encountered beneath the site and is the primary focus of all groundwater monitoring and corrective action (nitrate and chloroform pumping) activities. 8 Nitrate Corrective Action Comprehensive Monitoring Evaluation (CACME) Report Https://Hgcinc.Sharepoint.Com/VOL4/718000/71807/NCACME2022/Report/NCACME2022_Final.Docx June 21, 2023 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 (Kirby, 2008; United States Nuclear Regulatory Commission, 1979). The generally low permeability of the perched zone limits well yields. Although sustainable yields of a few gallons per minute (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, 2011) many of the wells went dry during surging and bailing and required several sessions on subsequent days to remove the proper volumes of water. Perched groundwater flow within the Burro Canyon Formation has historically been to the south/southwest. Local depression of the perched water table occurs near 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 near nitrate pumping wells TW4-22, TW4-24, TW4-25, and TWN-2 (Figure 1B). Chloroform pumping wells are pumped to reduce chloroform mass within the chloroform plume east and northeast of the TMS, and nitrate pumping wells are pumped to reduce nitrate mass within the nitrate plume as per Phase II of the CAP; and per Phase III as described in the Revised Phase III Planning Document. Specifically, as per Phase II of the CAP; and per Phase III as described in the Revised Phase III Planning Document; nitrate pumping is designed to sufficiently contain and hydraulically control the nitrate plume to prevent physical expansion of the plume. Nitrate pumping is not designed to hydraulically contain the entire plume, but to remove mass as rapidly as is practical from areas of the plume having both relatively high concentrations and relatively high productivity, and to hydraulically contain a large enough proportion of the plume to prevent expansion. Downgradient areas of the plume not under direct hydraulic control rely on natural attenuation assisted by upgradient pumping that reduces nitrate mass flow to these areas. Natural attenuation mechanisms include concentration reduction via natural dilution and hydrodynamic dispersion and nitrate mass removal (reduction) via oxidation of naturally-occurring pyrite and/or organic material in the Burro Canyon Formation (HGC, 2012b; HGC, 2022). Natural reduction of nitrate was not discussed in the CAP as a potential mass removal mechanism because pyrite (which is likely to be 9 Nitrate Corrective Action Comprehensive Monitoring Evaluation (CACME) Report Https://Hgcinc.Sharepoint.Com/VOL4/718000/71807/NCACME2022/Report/NCACME2022_Final.Docx June 21, 2023 the primary reductant) within the perched zone had not been quantified at the time of preparation of the CAP. Natural attenuation will be discussed in detail in Section 4. Perched water discharges in springs and seeps along Westwater Canyon and Cottonwood Canyon to the west-southwest of the site, and along Corral Canyon to the east of the site (Figure 1B), where the Burro Canyon Formation outcrops. The closest discharge points downgradient of the TMS are Westwater Seep (approximately 2,800 feet downgradient) and Ruin Spring (approximately 9,400 feet downgradient [HGC, 2010]). Westwater Seep is also the closest discharge point for the western portion of the nitrate plume, and Ruin Spring the closest discharge point for the eastern portion of the nitrate plume (HGC, 2022). The nitrate plume has been impacted by past water delivery to wildlife ponds located east-northeast of the nitrate plume (Figure 1B). Perched groundwater mounds that resulted from seepage from these unlined ponds have been decaying since water delivery ceased in March, 2012. A perched groundwater mound also existed in the vicinity of TWN-2 just north of the Millsite (Figure 1B); and still persists in the vicinity of TWN-3 (located north-northeast of TWN-2). Both TWN-2 and TWN-3 are located within the northern (upgradient) extremity of the nitrate plume. The perched groundwater mound in this area is likely a residual mound resulting from low permeability conditions; the location of TWN-2 within the footprint of the historic pond; and the location of TWN-3 just outside the footprint of the historic pond (Figure 1B). Although the historic pond no longer exists and does not contain standing water, the remaining topographic depression associated with the pond likely resulted in enhanced infiltration of precipitation before re-grading of the land surface in that area circa 1980. Slightly enhanced infiltration of precipitation since the re-grading (due to the flatness of the area) and relatively low permeability conditions at TWN-2 and TWN-3 likely allowed the mound to persist. Although nitrate pumping well TWN-2 eventually depressed this mound in the immediate vicinity of TWN-2, the decay of the mound has been relatively slow due to the relatively low permeability which not only restricts the productivity of TWN-2 but has allowed the mound to persist at TWN-3. Past seepage from the northern wildlife ponds was a source of dilution that helped to limit nitrate and chloride concentrations within the nitrate, chloride, and chloroform plumes. At the same time, the groundwater mounds associated with these ponds increased hydraulic gradients and contributed to downgradient migration of all three plumes. After water delivery to the ponds ceased in March, 2012, and the associated groundwater mounds began to decay, nitrate and chloride concentrations within the nitrate and chloride plumes, and chloroform and nitrate concentrations within the chloroform plume, were expected to increase, at least temporarily. 10 Nitrate Corrective Action Comprehensive Monitoring Evaluation (CACME) Report Https://Hgcinc.Sharepoint.Com/VOL4/718000/71807/NCACME2022/Report/NCACME2022_Final.Docx June 21, 2023 However, localized increases in concentrations of constituents such as nitrate and chloride within and near the nitrate plume may occur even when the nitrate plume is under control based on the requirements specified in the CAP. Ongoing mechanisms that can be expected to increase the concentrations of nitrate and chloride locally as a result of reduced wildlife pond recharge include but are not limited to: 1. Reduced dilution - the mixing of low constituent concentration pond recharge into existing perched groundwater will be reduced over time. 2. Reduced saturated thicknesses – dewatering of higher permeability zones receiving primarily low constituent concentration pond water will result in wells intercepting the zones receiving a smaller proportion of the low constituent concentration water. The impacts associated with cessation of water delivery to the northern ponds were expected to propagate downgradient (south and southwest) over time. Wells close to the ponds were generally expected to be impacted sooner than wells farther downgradient of the ponds. Therefore, constituent concentrations were generally expected to increase in downgradient wells close to the ponds before increases were detected in wells farther downgradient of the ponds. Although such increases were anticipated to result from reduced dilution, the magnitude and timing of the increases were anticipated to be and have been difficult to predict due to the complex permeability distribution at the site and factors such as pumping and the rate of decay of the groundwater mound. Because of these complicating factors, some wells completed in higher permeability materials were expected to be impacted sooner than other wells completed in lower permeability materials even though the wells completed in lower permeability materials were closer to the ponds. In general, nitrate concentrations within and adjacent to the nitrate plume appear to have been impacted to a lesser extent than chloroform and nitrate concentrations within and in the vicinity of the chloroform plume. This behavior is reasonable considering that the nitrate plume is less directly downgradient of and presumably less hydraulically connected (via higher permeability materials) to the wildlife ponds. However, as shown in Figure 4 and Table 1, the area of the nitrate plume has increased since 2010, primarily due to westward expansion of the kriged plume boundary toward TWN-7 (now within the plume) and MW-28. Most of this expansion occurred post-2017. Conversely, the plume boundary has contracted away from MW-27 and nitrate pumping well TW4-25. The relative stability of the plume area (Table 1) from 2010 through 2017 likely resulted from a combination of competing factors. Nitrate mass removal by pumping and naturally occurring nitrate degradation, which tend to reduce concentrations within the plume and shrink the plume boundaries, were counteracted by reduced dilution from the wildlife ponds, which tends to increase 11 Nitrate Corrective Action Comprehensive Monitoring Evaluation (CACME) Report Https://Hgcinc.Sharepoint.Com/VOL4/718000/71807/NCACME2022/Report/NCACME2022_Final.Docx June 21, 2023 concentrations within the plume and expand the plume boundaries. The interaction of these two mechanisms resulted in a plume that appeared to be in dynamic equilibrium with respect to area. As discussed above, since 2017, the kriged nitrate plume boundary has expanded to the west, toward MW-28 and TWN-7. Some of this expansion can be attributed to reduced dilution. Both MW-28 and TWN-7 were located generally downgradient of the plume. However, as water levels at TWN-7 have risen, and water levels at TWN-2 and TWN-3 have dropped, TWN-7 has become increasingly cross-gradient rather than downgradient of the northern extremity of the plume. Regardless, with respect to perched groundwater flow, TWN-7 has remained far cross- to upgradient of the Millsite and TMS. Although the plume boundary has not expanded to encompass MW-28 (which has a second quarter, 2022 concentration of approximately 5 mg/L, or half the GCAL of 10 mg/L), the plume expanded to encompass TWN-7 for the first time in the second quarter of 2018. In addition, slight downgradient expansion of the kriged plume boundary to the south, towards MW-11, has occurred. Since the first quarter of 2021, nitrate concentrations at MW-11 have increased from < 1mg/L to more than 2 mg/L. Chloride concentrations at MW-11 are also increasing, indicating that nitrate increases are the result of migration of the commingled nitrate and chloride plumes. However, any future increases in nitrate at MW-11 are expected to be limited by pyrite oxidation, which affects nitrate but not chloride. 2.3 Chloroform Pumping Wells Within and Adjacent to the Nitrate Plume Nitrate pumping wells TW4-22, TW4-24, TW4-25, and TWN-2, the original nitrate pumping wells specified in Phase II of the CAP, began pumping in the first quarter of 2013 and have continued to remove nitrate mass under both Phase II and Phase III. However, because of the overlap of the northwestern portion of the chloroform plume with the nitrate plume (Figure 1B), significant nitrate mass is removed from chloroform pumping wells that are within or adjacent to the nitrate plume. Chloroform pumping well TW4-19 and former pumping well TW4-20 (Figures 1A and 1B), which had been pumping several years prior to the initiation of nitrate pumping, were occasionally within the nitrate plume due to the quarter to quarter fluctuations in the position of the eastern plume boundary. The nitrate mass removal rates from these wells also fluctuated depending on their positions relative to the plume. Even during quarters when these wells were not within the nitrate plume (concentrations in the wells were less than 10 mg/L), the concentrations in these wells were typically large enough that they contributed significant nitrate mass removal. Since the abandonment of TW4-20, TW4-19 continues to contribute to nitrate mass removal. In addition, chloroform pumping wells TW4-21 and TW4-37, which became operational in 2015, are typically and consistently within the nitrate plume, respectively, and consistently 12 Nitrate Corrective Action Comprehensive Monitoring Evaluation (CACME) Report Https://Hgcinc.Sharepoint.Com/VOL4/718000/71807/NCACME2022/Report/NCACME2022_Final.Docx June 21, 2023 remove relatively large masses of nitrate from the plume. Furthermore, as discussed in HGC (2022), the abandonment of TW4-20 had little to no measurable impact on pumping, mass removal rates, and capture in the vicinity of TW4-20, as increases in pumping at TW4-19 subsequent to TW4-20 failure more than compensated for the loss of pumping at TW4-20. Although TW4-21 and TW4-37, which became operational in 2015, are not considered nitrate pumping wells because they were installed as part of the chloroform program to increase chloroform mass removal rates, they nevertheless in practice represent an enhancement to and expansion of the nitrate pumping system. 2.4 Summary of Results and Conclusions The following results and conclusions are based on information and calculations detailed in Sections 3 through 5 below as well as information and conclusions presented in the Revised Phase III Planning Document: 1. As discussed in Sections 3.3 and 4 below, between the second quarter of 2010 and the second quarter of 2022, the mass of nitrate contained within the plume has been reduced by approximately 16% to 27%. Mass reductions result from direct removal by pumping, reductions in saturated thicknesses, and as a result of natural attenuation. Natural attenuation processes include hydrodynamic dispersion, dilution by natural recharge, and reduction of nitrate by naturally-occurring pyrite in the perched zone. 2. There is sufficient pyrite in the perched zone within the path of the plume to completely attenuate the plume through natural reduction of nitrate alone. 3. As indicated in the Revised Phase III Planning Document, numerical flow and transport modeling using conservative ‘worst case’ assumptions indicates that active remediation by pumping is not needed to achieve full attenuation of the nitrate plume before reaching a property boundary, and that the plume will fully attenuate via hydrodynamic dispersion alone, without pumping, dilution by recharge or degradation by pyrite. Although the modeling was designed to evaluate long-term changes in the nitrate plume, the current nitrate distribution is generally consistent with model predictions near the southern boundary of the plume. The purpose of Phase III pumping is primarily to reduce the time needed for full attenuation, at which time all nitrate concentrations associated with the plume will be below 10 mg/L. 4. Based on pumping and estimated natural attenuation rates determined to date, the mass of the plume will be reduced by approximately 517 to 792 lbs per year, and nitrate concentrations within the plume are expected to be reduced to negligible values (less than 10 mg/L) within approximately 40 to 62 years. In the absence of pumping, between approximately 71 and 191 years would be required. Because nitrate mass removal by pumping is likely to drop off in the future due to reduced nitrate concentrations and reduced saturated thicknesses (which will limit achievable pumping rates), the expected time to 13 Nitrate Corrective Action Comprehensive Monitoring Evaluation (CACME) Report Https://Hgcinc.Sharepoint.Com/VOL4/718000/71807/NCACME2022/Report/NCACME2022_Final.Docx June 21, 2023 reduce nitrate concentrations to negligible values (less than 10 mg/L), assuming pumping continues, will be between approximately 40 and less than 200 years. As the estimated time for impacted water to reach the nearest discharge point (Westwater seep or Ruin Spring) is greater than 2,895 years, there is no concern at this time that the continuation of Phase III will not result in remediation of the plume well before it can reach any exposure to the public or wildlife. 5. As discussed in Section 5.2 below, no changes to Phase III to improve effectiveness and accelerate the restoration timeline have been identified or are recommended, other than the planned installation of two new wells and a piezometer as part of the completion of proposed new tailings cells 5A and 5B, as discussed in the Revised Phase III Planning Document. 14 Nitrate Corrective Action Comprehensive Monitoring Evaluation (CACME) Report Https://Hgcinc.Sharepoint.Com/VOL4/718000/71807/NCACME2022/Report/NCACME2022_Final.Docx June 21, 2023 15 Nitrate Corrective Action Comprehensive Monitoring Evaluation (CACME) Report Https://Hgcinc.Sharepoint.Com/VOL4/718000/71807/NCACME2022/Report/NCACME2022_Final.Docx June 21, 2023 3. SUMMARY OF PHASE II AND PHASE III MONITORING AND PUMPING The following subsections discuss elements included in the quarterly Nitrate Monitoring reports, and summarize and interpret key findings and results. As per the CAP, since the start of Phase II pumping in the first quarter of 2013, and subsequent Phase III pumping, thirty-nine quarterly Nitrate Monitoring reports were submitted (Energy Fuels Resources (USA) Inc [EFRI], 2013b; EFRI, 2013c; EFRI, 2013d; 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, 2017a; 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; and EFRI, 2022d;). Actions taken under Phase II and Phase III of the CAP are consistent with objectives specified in the CAP to: • Minimize or prevent further downgradient migration of the perched nitrate plume by a combination of pumping and reliance on natural attenuation; • Prevent nitrate concentrations exceeding the action level (10 mg/L) from migrating to any potential point of exposure; • Monitor to track changes in concentrations within the plume and to establish whether the plume boundaries are expanding, contracting, or stable; • Provide contingency plans to address potential continued expansion of the plume and the need for additional monitoring and/or pumping points; and • Ultimately reduce nitrate concentrations at all monitoring locations to the action level (10 mg/L) or below. 3.1 Elements of Quarterly Reports The elements of the quarterly Nitrate Monitoring reports that have been submitted since the first quarter of 2013 are consistent with the requirements specified as per Phase II of the CAP as well as requirements under Phase III. These elements include the following: • description of the nitrate program monitoring; • quality assurance and data validation; • data interpretation; • description of long-term pumping operation; 16 Nitrate Corrective Action Comprehensive Monitoring Evaluation (CACME) Report Https://Hgcinc.Sharepoint.Com/VOL4/718000/71807/NCACME2022/Report/NCACME2022_Final.Docx June 21, 2023 • description of any corrective action taken; • conclusions and recommendations; and • electronic analytical data files. Data interpretation each quarter included generation and discussion of perched water elevation and nitrate and chloride concentration contour maps, and, beginning with the fourth quarter of 2013, a map estimating capture zones resulting from both nitrate and chloroform pumping. Data interpretation also included discussions of changes in perched water levels, nitrate concentrations, plume boundaries, and capture between current and previous quarters. Graphs of perched water levels and nitrate and chloride concentrations were provided. 3.2 Specific Actions Taken During Phase II and Phase III Some of the specific work performed under Phase II of the nitrate CAP and under Phase III as described in the Revised Phase III Planning Document includes: • Computation of quarterly nitrate plume residual mass estimates and trend analysis; • Initiation (in the first quarter of 2013) and continued pumping of nitrate pumping wells TW4-22, TW4-24, TW4-25, and TWN-2; • All required quarterly sampling, monitoring, quality control, pumping, and reporting activities; • Evaluation of the relative importance of data from each particular well in calculating residual nitrate mass estimates (EFRI, 2015a); • Evaluation of hydraulic capture based on kriged quarterly water levels and based on comparison of pumping and calculated ‘background’ flow through the plume; • Re-calculation of ‘background’ flow through the plume based on reduced hydraulic gradients, saturated thicknesses, and average hydraulic conductivities resulting from decay of the perched groundwater mound (EFRI, 2015d; and the present report); • Preliminary evaluation of reduced productivity at TW4-24 (and TW4-19) [EFRI, 2015d]; • Accounting for the beneficial impact of the addition of chloroform pumping wells TW4- 21 and TW4-37 on nitrate mass removal and plume control; • Preparation of the 2017 Nitrate CACME; and • Preparation of the Revised Phase III Planning Document. 17 Nitrate Corrective Action Comprehensive Monitoring Evaluation (CACME) Report Https://Hgcinc.Sharepoint.Com/VOL4/718000/71807/NCACME2022/Report/NCACME2022_Final.Docx June 21, 2023 3.3 Key Findings Key findings based on quarterly monitoring; and based on analyses provided in the 2017 CACME and the Revised Phase III Planning Document are provided below in Sections 3.3.1, 3.3.2 and 3.3.3. 3.3.1 Quarterly Monitoring The quarterly Nitrate Monitoring reports provide the information outlined in Section 3.1. Through analysis and interpretation of the quarterly data a number of findings with regard to the nitrate plume were presented and discussed in each report. Some of the key findings detailed in the quarterly reports submitted beginning with the first quarter of 2013 (EFRI, 2013b) include: • The nitrate plume is completely bounded by the existing monitoring network (Figure 2); • Based on concentration criteria presented in Phase II of the CAP, the nitrate plume is under control; • The kriged nitrate plume boundary has expanded to the west toward TWN-7 and MW-28; conversely, the plume boundary has contracted away from MW-27 and nitrate pumping well TW4-25. MW-28 has not been incorporated into the plume; and TWN-7, located far cross- to upgradient of the Millsite and TMS, was first incorporated into the plume during the second quarter of 2018; • The downgradient (southern) plume boundary is relatively stable but is slowly migrating towards MW-11. The downgradient boundary remains as defined in the CAP as located between MW-30 and MW-31(within the toe of the plume) and MW-5 and MW-11 (downgradient of the plume) [Figure 1B and Figure 2]; • Chloride concentrations in the toe of the plume (at MW-30 and MW-31) are increasing while nitrate concentrations are stable (Figure 5A) causing a decrease in nitrate to chloride ratios (Figure 5B); • Increasing chloride and stable nitrate within the downgradient toe of the plume are consistent with pyrite oxidation by nitrate (nitrate reduction by pyrite) as discussed in HGC (2022); • Based on the quarterly nitrate plume residual mass estimates, the mass of nitrate within the plume is trending downward (Figure 6 and Table 1); • As discussed in EFRI (2015a), data from wells TWN-2, TW4-22, and TW4-24 were the most important in computing the quarterly mass estimates (up through the fourth quarter of 2014); • Reduced wildlife pond recharge is expected to reduce dilution, to increase nitrate (and chloride) concentrations within the plume, and to increase concentrations of nitrate associated with the chloroform plume to the east. Reduced dilution has caused fluctuations 18 Nitrate Corrective Action Comprehensive Monitoring Evaluation (CACME) Report Https://Hgcinc.Sharepoint.Com/VOL4/718000/71807/NCACME2022/Report/NCACME2022_Final.Docx June 21, 2023 in the eastern nitrate plume boundary (including temporary excursions of the boundary to include TW4-18 as shown in Figures 7A and 7B); • The rate of perched groundwater flow within the plume (calculated near the approximate center of the plume in the vicinity of TW4-22 and TW4-24) is decreasing as a result of the decay of the groundwater mounds associated with the northern wildlife ponds; • The rate of perched groundwater flow within the plume (to the south-southwest near TW4- 22 and TW4-24) had decreased from a pre-pumping calculated range of approximately 1.31 to 2.79 gpm to approximately 0.79 to 1.67 gpm by the second quarter of 2015; has further decreased to approximately 0.63 to 1.34 gpm as of the second quarter of 2022 (Section 4.3); and is expected to continue to decrease; • The decline in the rate of perched groundwater flow within the plume reduces the pumping rates needed to maintain control of the plume; • Reduced productivity at TW4-24 results in part from reductions in saturated thickness and is mitigated by the reduced rate of flow through the plume and by the addition of chloroform pumping wells TW4-21 and TW4-37; and • Nitrate pumping at TW4-22 and TW4-24 caused cross-gradient expansion of the chloroform plume to the west. As will be discussed in Section 5.2, this impact of nitrate pumping on chloroform migration illustrates the expected negative impact should nitrate pumping at a more downgradient location (for example MW-30 and MW-31) be implemented. As noted above, quarterly estimates of residual nitrate mass within the plume are trending downward. Changes in the quarterly mass estimates are expected to result from several factors, primarily 1) nitrate mass removed directly by pumping; 2) natural attenuation of nitrate; 3) re- distribution of nitrate within the plume; and 4) changes in saturated thicknesses. In addition, because the sum of sampling and analytical error is typically about 20%, and because the mass estimates are based on quarterly nitrate concentrations in groundwater samples collected from wells within and marginal to the plume, changes in the mass estimates from quarter to quarter of up to 20% could result from typical sampling and analytical error alone. Although there is ‘noise’ in the quarter to quarter mass estimates, the long-term trend has remained downward. Comparing the second quarter, 2010 baseline mass estimate of 43,700 lb with the second quarter, 2022 mass estimate of approximately 31,933 lb suggests that the plume mass has decreased approximately 11,767 lb (nearly 27%). Based on the Figure 6 trendline, the plume mass has decreased approximately 7,200 lb (approximately 16%). 19 Nitrate Corrective Action Comprehensive Monitoring Evaluation (CACME) Report Https://Hgcinc.Sharepoint.Com/VOL4/718000/71807/NCACME2022/Report/NCACME2022_Final.Docx June 21, 2023 3.3.2 2017 CACME Quarterly monitoring from the first quarter of 2013 through the third quarter of 2017 was summarized in the 2017 CACME. An analysis of rates of plume remediation was also presented. Estimates of the rates of plume remediation were based on estimates of natural degradation of nitrate by pyrite oxidation and on direct mass removal by pumping. Between the first quarter of 2013 and third quarter of 2017, the average nitrate mass removed by pumping wells within and marginal to the plume was approximately 401 lb/yr. The averages of the estimated rates of natural nitrate reduction ranged from approximately 172 lb/yr to 200 lb/yr depending on the proportion of the plume to which the rate is assumed to be applicable. Thus the estimated total rate of mass reduction ranged from approximately 573 lb/yr to 601 lb/yr. Projecting these mass removal rates into the future, and assuming a zero order rate of natural reduction of nitrate and a third quarter 2017 nitrate plume residual mass of approximately 32,940 lb implied that between approximately 54 and 57 years would be required to reduce all the nitrate within the plume to a negligible value; and between approximately 164 and 192 years would be required via natural degradation alone. Because nitrate mass removal by pumping was assumed likely to drop off in the future due to reduced nitrate concentrations and reduced saturated thicknesses (thus limiting achievable pumping rates), the actual time, assuming pumping continued, was estimated to be more than 54 and less than 192 years (from the third quarter of 2017). These estimates will be updated in Sections 4.5 and 4.6. 3.3.3 Revised Phase III Planning Document As discussed in Section 1, the Revised Phase III Planning Document relied on numerical flow and transport modeling of the expected fate of the nitrate plume under conservative ‘worst case’ assumptions to provide additional support for the conclusions reached in the Phase III Planning Document included in the 2017 CACME. In addition, The Revised Phase III Planning Document proposed additional monitoring wells to be installed as part of the construction of proposed TMS cells 5A and 5B. Cells 5A and 5B were to be constructed along the downgradient (southern) margin of existing TMS cells 4A and 4B. The numerical flow and transport simulations included in the Revised Phase III Planning Document were based on ‘worst case’ conservative assumptions that: 1. Disregarded the natural degradation of nitrate within the plume via pyrite oxidation, which caused overestimation of simulated plume migration; 20 Nitrate Corrective Action Comprehensive Monitoring Evaluation (CACME) Report Https://Hgcinc.Sharepoint.Com/VOL4/718000/71807/NCACME2022/Report/NCACME2022_Final.Docx June 21, 2023 2. Disregarded the stability of the southern (downgradient) margin of the nitrate plume over the previous nine years, which suggested that pumping and natural attenuation processes were preventing plume expansion to the south (Appendix B); 3. Disregarded nitrate mass removal by pumping and natural dilution of nitrate concentrations via recharge by precipitation, which caused overestimation of simulated plume migration; 4. Substantially overestimated hydraulic conductivities (by as much as two orders of magnitude) and hydraulic gradients (by nearly a factor of two) downgradient of the TMS, which caused substantial overestimation of simulated plume migration rates; and 5. Underestimated dispersivities, which caused underestimation of hydrodynamic dispersion and overestimation of simulated plume migration. Although the modeling was designed to evaluate long-term changes in the nitrate plume, the current nitrate distribution is generally consistent with model predictions near the southern boundary of the plume. The results of the simulations showed that, even in the absence of pumping or natural degradation via pyrite oxidation, the nitrate plume was expected to fully attenuate via hydrodynamic dispersion alone before reaching any property boundary, including the western boundary, which is closest to the plume. Therefore, continued Phase III pumping can be considered a means to shorten remediation times rather than an action necessary to prevent offsite plume migration. 21 Nitrate Corrective Action Comprehensive Monitoring Evaluation (CACME) Report Https://Hgcinc.Sharepoint.Com/VOL4/718000/71807/NCACME2022/Report/NCACME2022_Final.Docx June 21, 2023 4. EVALUATION OF THE EFFECTIVENESS OF PUMPING AND NATURAL ATTENUATION As discussed in Section 2 and the 2017 CACME, the relative stability of the nitrate plume through 2017 was attributable to a combination of competing factors. Nitrate mass removal by pumping and naturally occurring nitrate degradation, which tend to reduce concentrations within the plume and shrink the plume boundaries, were counteracted by reduced dilution from the wildlife ponds, which tends to increase concentrations within the plume and expand the plume boundaries. The interaction of these two mechanisms resulted in a plume that appeared to be in dynamic equilibrium with respect to area. As discussed in Section 2.2, since 2017, the kriged nitrate plume boundaries have expanded primarily to the west, towards MW-28 and TWN-7. Both MW-28 and TWN-7 were located generally downgradient of the plume. However, as water levels at TWN-7 have risen, and water levels at TWN-2 and TWN-3 have dropped, TWN-7 has become increasingly cross-gradient (rather than downgradient) of the northern extremity of the plume. Regardless, with respect to perched groundwater flow, TWN-7 has remained far cross- to upgradient of the Millsite and TMS. Although the plume has not expanded to encompass MW-28 (which has a second quarter, 2022 concentration of approximately 5 mg/L, or half the GCAL of 10 mg/L), the plume expanded to encompass TWN-7 for the first time in the second quarter of 2018. In addition, slight downgradient expansion of the kriged plume boundary to the south, towards MW-11, has occurred. Since the first quarter of 2021, concentrations at MW-11 have increased from < 1mg/L to more than 2 mg/L. Chloride concentrations at MW-11 are also increasing, indicating that nitrate increases are the result of migration of the commingled nitrate and chloride plumes. However, any future increases in nitrate at MW-11 are expected to be limited by pyrite oxidation, which affects nitrate but not chloride. Data and mechanisms that support this general plume behavior, and that have resulted in plume control based on concentration criteria presented as per Phase II of the CAP and per Phase III, are provided and discussed in the following Sections. 4.1 Data Trends As discussed in Section 2.2 and the 2017 CACME, nitrate plume boundaries and area were relatively stable from the second quarter of 2010 (which defines the ‘baseline’ data as specified in the CAP) through 2017. However, a comparison of second quarter 2010 and second quarter 2022 plume boundaries (Figure 4) indicates that the kriged nitrate plume boundaries and area have 22 Nitrate Corrective Action Comprehensive Monitoring Evaluation (CACME) Report Https://Hgcinc.Sharepoint.Com/VOL4/718000/71807/NCACME2022/Report/NCACME2022_Final.Docx June 21, 2023 expanded, primarily toward the west; and slightly towards the south. Most of this expansion has occurred since 2017. Conversely, the plume boundary has contracted away from MW-27 and nitrate pumping well TW4-25. Although the northernmost portion of the plume has expanded to encompass TWN-7; and the kriged plume boundary has expanded towards MW-28; the contraction of the plume to the south of TWN-2 (away from MW-27 and TW4-25) has caused the kriging function to separate the northern 1/4 and southern 3/4 of the plume. This separation is attributable primarily to TWN-2 pumping. As discussed in Section 3, the kriged downgradient (southern) plume boundary has been relatively stable but has expanded slightly toward MW-11. Fluctuations of the eastern nitrate plume boundary have also occurred. These fluctuations result in part from fluctuations in nitrate concentrations associated with the chloroform plume. Nitrate within the chloroform plume to the east of the nitrate plume is expected to be impacted more strongly by reduced dilution from the wildlife ponds. Fluctuations in the eastern nitrate plume boundary are also likely related to chloroform pumping immediately east of the nitrate plume. Figure 8 provides time-series plots of nitrate concentrations in wells typically east of the nitrate plume. MW-25 and MW-32 (not shown) are consistently either non-detect for nitrate or have nitrate detections of less than 1 mg/L. Nitrate wells TWN-1 and TWN-4 are consistently below 10 mg/L. Chloroform program wells TW4-16 and MW-26 (pumping) are also consistently below 10 mg/L. Chloroform wells TW4-18, TW4-19 (pumping), and TW4-20 (former pumping well) periodically exceed(ed) 10 mg/L. During quarters when all three exceeded 10 mg/L, or when TW4- 18, TW4-19, and TW4-21 exceeded 10 mg/L, the kriged nitrate plume boundary extended a ‘spur’ to the east to incorporate TW4-18 (for example as shown in Figure 7A). This has occurred twice since the first quarter of 2013. In the fourth quarter of 2016, an eastward trending ‘spur’ incorporated TW4-39 and TW4-10 (EFRI, 2017a) as shown in Figure 7B. Apparent expansion of the western nitrate plume boundary, which is cross- to downgradient with respect to perched groundwater flow, is attributable to concentration increases at MW-28 from a few tenths of a mg/L to approximately 5 mg/L (as of second quarter 2022), and concentration increases at TWN-7 from less than 1 mg/L to approximately 15 mg/L (Figure 9), bringing TWN- 7 into the plume for the first time during the second quarter of 2018. Overall, nitrate and chloride concentrations at MW-27 have been relatively stable while increases in both nitrate and chloride have occurred at TWN-7 and MW-28 (Figure 9). Although concentrations are relatively low, as shown in Figure 10A, the nitrate to chloride ratio is decreasing to stable at TWN-7; increasing to stable at MW-27; and generally increasing to stable at MW-28. As shown in Figure 10B, the generally downward trend in nitrate to chloride ratios at TWN-7 is similar to the generally 23 Nitrate Corrective Action Comprehensive Monitoring Evaluation (CACME) Report Https://Hgcinc.Sharepoint.Com/VOL4/718000/71807/NCACME2022/Report/NCACME2022_Final.Docx June 21, 2023 downward trend of the average plume nitrate to chloride ratio based on data from the seven wells consistently located within the nitrate plume (wells MW-30, MW-31, TW4-22, TW4-24, TW4- 37, TWN-2 and TWN-3). Figures 11A and 11B compare second quarter 2010 with third quarter 2022 nitrate and chloride plume boundaries. Figure 11A displays second quarter, 2022 perched water levels and Figure 11B second quarter 2010 ‘baseline’ perched water levels. The directions of perched groundwater flow implied by the second quarter 2010 and second quarter 2022 water level distributions are similar, except that hydraulic gradients were generally steeper in 2010 and were more westerly- ( and even northwesterly-) directed in the northernmost portion of the plume. As shown in Figure 11B, although in 2010, TWN-7 was far up- to cross-gradient of the Millsite and TMS, TWN-7 was almost directly downgradient of TWN-3 and of the historic pond. The nearly simultaneous increases in nitrate and chloride at TWN-7 shown in Figure 9 are strong evidence that the historic pond was a source of both nitrate and chloride to the commingled nitrate and chloride plumes. As shown in Figure 11B the upgradient extremities of the nitrate and chloride plumes overlap with the footprint of the historic pond; and the pre-pumping hydraulic gradient was directed from the vicinities of TWN-2 and TWN-3, located within the upgradient extremities of the nitrate and chloride plumes, toward TWN-7. If the historic pond were a source only of nitrate, then only nitrate would be expected to increase at TWN-7 rather than both nitrate and chloride. The relatively slow migration of nitrate and chloride to TWN-7 is consistent with the low permeability at TWN-7 (approximately 3.6 x 10-7 centimeters per second [cm/s] or 1 x 10-3 feet per day [ft/day] as per Table 1 of HGC, 2022). The low permeability in this area presumably delayed migration of nitrate and chloride from the historic pond to TWN-7. As indicated in Figures 11A and 11B, while the upgradient portion of the chloride plume has shrunk, the plume has expanded downgradient (to the south-southwest) and cross- to downgradient (to the east and west). Eastward expansion may result in part from chloroform pumping at MW- 26, TW4-37 and TW4-39 and from reduced dilution from wildlife pond seepage. Shrinkage upgradient and expansion downgradient are consistent with upgradient pumping and continued, but slow, migration of chloride in the direction(s) of groundwater flow implied by the kriged perched water elevations. Although advection is presumed to be the primary mechanism for plume expansion, some cross- and downgradient expansion of both the chloride and nitrate plumes is expected to result from hydrodynamic dispersion. 24 Nitrate Corrective Action Comprehensive Monitoring Evaluation (CACME) Report Https://Hgcinc.Sharepoint.Com/VOL4/718000/71807/NCACME2022/Report/NCACME2022_Final.Docx June 21, 2023 Perched water levels within the majority of the nitrate plume have been decreasing as a result of pumping and reduced wildlife pond recharge. Consequently, saturated thicknesses within most of the plume have been generally decreasing. Figures 12A through 14 illustrate changes in water levels and saturated thicknesses within the plume. Figures 12A and 12B show changes in water levels and saturated thicknesses at wells originally within the plume; and Figure 12C shows water levels at TWN-7, first incorporated into the plume during the second quarter of 2018. Figures 13 and 14 are plan maps showing the changes in saturated thicknesses and percent changes in saturated thicknesses within the second quarter, 2022 plume boundary, respectively. Although water levels and saturated thicknesses have decreased over most of the area of the plume, increases at some relatively isolated locations have occurred (such as near TWN-7). Figure 12C shows that, while water levels at TWN-7 have increased, water levels at TWN-3 have decreased. The overall reduction in saturated thicknesses reduces the volume of water within the plume and consequently reduces the mass of nitrate within the plume. The reduction in saturated thickness within the northern (upgradient) portion of the plume near TWN-2 has exceeded 50%; and near TW4-22, close to the center of mass of the plume, has exceeded 25% (Figure 14). The volume of groundwater within the second quarter, 2010 plume was approximately 8.92 x 105 cubic meters (m3) or 3.15 x 107 cubic feet (ft3) and the volume within the second quarter, 2022 plume approximately 6.50 x 105 m3 (2.40 x 107 ft3). This change represents a decline of approximately 2.12 x 105 m3 (7.5 x 106 ft3), or 24%. As discussed in Section 3, and as shown in Figure 6, the nitrate mass within the plume since the second quarter of 2010 has been reduced by nearly 27% based on the difference between second quarter, 2010 and second quarter, 2022 mass estimates, and by approximately 16% based on the Figure 6 trendline. These estimates bracket the approximate 24% reduction in plume volume since the second quarter of 2010. Nitrate concentrations in the majority of wells consistently within the plume are stable to declining; however, until the second quarter of 2021, concentrations at TW4-22 were generally increasing; and, since the fourth quarter of 2013, concentrations at TWN-2 have been generally decreasing (Figures 15 and 16). These trends in concentration combined with changes in saturated thicknesses within the plume have resulted in a significant change in nitrate mass distribution within the plume. Figures 17 and 18 provide the nitrate mass distributions in the second quarter of 2010 and second quarter of 2022, respectively. As shown, since the second quarter of 2010, the center of mass has migrated from the vicinity of TWN-2 to the vicinities of TW4-22 and TW4-24. 25 Nitrate Corrective Action Comprehensive Monitoring Evaluation (CACME) Report Https://Hgcinc.Sharepoint.Com/VOL4/718000/71807/NCACME2022/Report/NCACME2022_Final.Docx June 21, 2023 Overall, internal changes have occurred (as anticipated in the CAP); and the average nitrate concentrations within the plume have fluctuated but have generally declined since 2013, as shown in Table 1 and Figure 19. Table 1 also shows the number of wells within the plume each quarter and the nitrate mass removed each quarter by pumping wells located within and marginal to the plume. Table 2 compares nitrate mass removed each quarter by all pumping wells; wells within and marginal to the plume; and wells only within the plume. Since the first quarter of 2013, the total nitrate mass removed by all pumping wells is approximately 3,663 lb; by wells within and marginal to the plume approximately 3,329 lb; and by only wells within the plume approximately 3,083 lb. As shown in Tables 1 and 2, although pumped nitrate mass fluctuates from quarter to quarter, the rate of mass removal by pumping peaked in the third quarter of 2013 and has generally declined since then, in part due to reduced concentrations in pumping wells, and in part due to reduced productivities resulting from decreased saturated thicknesses. Figures 19 and 20 show changes in average nitrate concentrations within the plume based on average concentrations in wells within the plume and based on average gridded (kriged) nitrate concentrations within the plume. The averages based on gridded concentrations, which more accurately reflect the concentration distributions along the plume margins, are more stable and lower than the averages based on concentrations at individual wells located within the plume. Figure 19 shows that average concentrations based on wells within the plume have decreased since 2013 (as discussed above); but, as shown in Figure 20, have decreased only slightly based on gridded (kriged) concentration data. This difference results from the influence of wells outside the plume on gridded concentrations, for example, the influence of concentration increases at MW-28 (from < 1 mg/L to approximately 5 mg/L). 4.2 Natural Attenuation As discussed in the CAP, natural attenuation mechanisms that are expected to impact the nitrate plume and reduce nitrate concentrations include dilution and hydrodynamic dispersion. Based on the results of numerical flow and transport modeling presented in the Revised Phase III Planning Document, even under ‘worst case’ assumptions, hydrodynamic dispersion alone is likely sufficient to reduce all nitrate concentrations within the plume to less than the GCAL of 10 mg/L before reaching a property boundary. Although the modeling was designed to evaluate long-term changes in the nitrate plume, the current nitrate distribution is generally consistent with model predictions near the southern boundary of the plume. However, an additional mechanism that was not envisioned at the time of preparation of the CAP is nitrate reduction by naturally occurring pyrite and/or organic carbon in the perched zone. Natural reduction of nitrate by pyrite is discussed below in Sections 4.2.1 through 4.2.3. 26 Nitrate Corrective Action Comprehensive Monitoring Evaluation (CACME) Report Https://Hgcinc.Sharepoint.Com/VOL4/718000/71807/NCACME2022/Report/NCACME2022_Final.Docx June 21, 2023 4.2.1 Nitrate Degradation by Pyrite As discussed in HGC (2012b) and HGC (2014; 2018a; 2022), nitrate can be reduced in the presence of organic material or pyrite; both have been noted within the perched zone in drilling logs at the site. Specifically, pyrite has been noted in drilling logs, subsamples of drill cuttings submitted for laboratory analysis, or both, at many wells located within and adjacent to the nitrate plume. These include wells MW-23, MW-24, MW-25, MW-26, MW-27, MW-28, MW-29, MW- 30, MW-31, MW-32, TW4-16, TW4-3, TW4-5, TW4-9, TW4-10, TW4-21, TW4-22, TW4-25, TWN-2, TWN-3, and TWN-18. Detailed logs are not available for wells MW-5 or MW-11, located immediately downgradient of the plume, nor for MW-14 and MW-15, located farther downgradient, so the presence or absence of pyrite at these locations is unknown. However, pyrite was noted in logs for wells MW-34 through MW-37, also located farther downgradient of the plume, to the west and southwest of MW-5, MW-11, MW-14 and MW-15. The following discussion regarding nitrate reduction by pyrite is taken primarily from HGC (2014; 2018a; and 2022). As discussed in HGC (2012b), nitrate will degrade in the presence of pyrite. Nitrate will also degrade, and more readily, in the presence of organic matter. Both pyrite and organic material in the form of carbonaceous matter have been logged in drill cuttings from the perched zone. As discussed in (Korom, 1992), the thermodynamically favored electron donor for reduction of nitrate in groundwater is typically organic matter. This process under neutral conditions is represented via the following generalized reaction (e.g. van Beek, 1999; Rivett et al., 2008; Tesoriero and Puckett, 2011; Zhang, 2012): 2 3 2 3 2 3 2 5 4 2 4 2CH O NO N HCO H CO H O - -+ = + + + (Reaction 1) In acidic (pH<6.4) aquifer conditions, reduction of nitrate by organic matter can be generalized by the following pathway: 2 3 2 2 3 2 5 4 4 2 5 2CH O NO H N H CO H O - ++ + = + + (Reaction 2) In both cases, five moles of organic matter are required to reduce four moles of nitrate. Under acidic conditions the alkalinity generated by denitrification by organic matter consumes acid. In the absence of dissolved oxygen, pyrite can also be oxidized by nitrate. Denitrification by pyrite may occur via two primary reaction pathways. The pathway most commonly applied in 27 Nitrate Corrective Action Comprehensive Monitoring Evaluation (CACME) Report Https://Hgcinc.Sharepoint.Com/VOL4/718000/71807/NCACME2022/Report/NCACME2022_Final.Docx June 21, 2023 geochemical studies (Kolle et al., 1983, 1985; Postma et al., 1991; Korom, 1992; Robertson et al., 1996; Pauwels et al., 1998; Hartog et al., 2001, 2004; Spiteri et al., 2008) is a bacteria-mediated reaction that yields ferrous iron, sulfate, water, and nitrogen gas as follows: 2 2 2 3 2 4 2 5 14 4 7 10 5 2FeS NO H N SO Fe H O - + - ++ + = + + + (Reaction 3) By Reaction 3, five moles of pyrite reduce 14 moles of nitrate, consuming four moles of acid. Reaction 3 is considered applicable when pyrite concentrations exceed nitrate concentrations (van Beek,1999). Where nitrate concentrations exceed pyrite concentrations, Reaction 4 is a more likely mechanism (Kolle et al., 1987; van Beek, 1999; Schlippers and Jorgensen, 2002): 2 2 3 2 2 4 3 2 6 4 3 4 2 ( ) 2FeS NO H O N SO Fe OH H -- ++ + = + + + (Reaction 4) By Reaction 4, two moles of pyrite reduce six moles of nitrate, yielding iron hydroxide, sulfate, acid, and nitrogen gas. Therefore, when nitrate concentrations exceed pyrite concentrations (Reaction 4), denitrification by pyrite is more efficient than when pyrite is in excess (Reaction 3). Additionally, Reaction 4 produces acid, while Reaction 3 consumes acid, indicating that the impact of denitrification by pyrite on aquifer geochemistry is controlled by the relative abundance of pyrite and nitrate. Reaction 4 is an overall reaction that combines Reaction 3 and a second step whereby ferrous iron is oxidized by nitrate. This second step is more likely to occur when excess nitrate is present and available to oxidize ferrous iron (Kolle et al., 1987; Rivett et al., 2008; Zhang 2012). Stoichiometric calculations were used to determine the weight percent of perched zone pyrite that would be required to reduce the ‘baseline’ estimate of 43,700 lbs of nitrate via reaction mechanisms 3 and 4 (assuming each was the only denitrification reaction occurring). This represents a conservative calculation because the total estimated nitrate mass within the plume as of the second quarter of 2022 is only 31,933 lb. The ‘baseline’ second quarter, 2010 estimate of 43,700 lbs of nitrate corresponds to 19,822 kg and 319,684 moles. Although organic matter is noted in lithologic logs, the organic matter content of the perched zone has not been quantified so calculations regarding nitrate degradation by reactions 1 and 2 are not presented, even though significant nitrate reduction via these mechanisms is likely to occur. Nitrate can either migrate towards Ruin Spring to the south-southwest or to Westwater Seep to the west. Assuming the entire nitrate plume migrated south towards Ruin Spring, the volume of the 28 Nitrate Corrective Action Comprehensive Monitoring Evaluation (CACME) Report Https://Hgcinc.Sharepoint.Com/VOL4/718000/71807/NCACME2022/Report/NCACME2022_Final.Docx June 21, 2023 perched zone through which the nitrate plume would migrate was assumed to be on average 20 feet thick, 1,200 feet wide (conservatively narrow considering the larger width of the plume as of the second quarter, 2022), and 10,000 feet long, representing a total saturated formation volume of 2.4 x 108 ft3 or 6.8 x 109 liters. Assuming the entire nitrate plume migrated west toward Westwater Seep, the volume of the perched zone through which the nitrate plume would migrate was assumed to be on average 18 feet thick, 2,800 feet wide, and 4,950 feet long, representing a total saturated formation volume of 2.5 x 108 ft3 or 7 x 109 liters. To be conservative, the following calculations are based on the smaller volume of 6.8 x 109 liters. Using these estimates, reaction 3 would require 114,173 moles of pyrite to consume 43,700 lbs of nitrate, and would consume 91,338 moles of acid (1.34 x 10-5 moles H+ per liter of formation). Reaction 4 would require 106,561 moles of pyrite to degrade the nitrate, producing 106,561 moles of acid or 1.57 x 10-5 moles H+ per liter of formation. Assuming a conservatively large porosity of 0.2 for the perched zone (HGC, 2012b), the total volume of water is 1.36 x 109 liters; and assuming a solids density of 2.6 kg L-1, yields a total solid mass of 1.4 x 1010 kg. Using this solid mass, Reactions 3 and 4 would require pyrite formation weight percents of 0.000098% (9.8 x 10-5 %) and 0.000091% (9.1 x 10-5 %), respectively, to degrade 43,700 lbs of nitrate. These calculated pyrite weight percents are orders of magnitude less than conservative estimates of pyrite content based on samples analyzed during the pyrite investigation (HGC, 2012c), which ranged from 0.0056% to 0.08% (5.6 x 10-3 % to 8 x 10-2 %). These results suggest that the available pyrite content in the path of the nitrate plume is two to three orders of magnitude greater than needed to degrade the total ‘baseline’ mass (43,700 lbs) of nitrate. These calculations are conservative in that they assume the degradation of the entire mass of nitrate and not just the mass needed to reduce concentrations below 10 mg/L. Although there is sufficient pyrite in the path of the nitrate plume to degrade it, as discussed in the Revised Phase III Planning Document, hydrodynamic dispersion alone is sufficient to reduce nitrate concentrations within the plume to less than the GCAL of 10 mg/L before reaching a property boundary. Furthermore, as will be discussed in Section 4.6, even in the absence of pumping or concentration reduction due to hydrodynamic dispersion, natural nitrate degradation via pyrite oxidation is likely to degrade the nitrate mass to a negligible value within approximately 200 years or less which is well within the thousands of years needed for nitrate to potentially be 29 Nitrate Corrective Action Comprehensive Monitoring Evaluation (CACME) Report Https://Hgcinc.Sharepoint.Com/VOL4/718000/71807/NCACME2022/Report/NCACME2022_Final.Docx June 21, 2023 transported to a property boundary or discharge point based on travel time calculations presented in HGC (2022). Whether or not pyrite oxidation by nitrate at the site is generating or consuming acid depends largely on whether oxidation of ferrous iron by nitrate is occurring (i.e. whether pyrite denitrification is occurring by Reaction 3 or Reaction 4; whether nitrate exists in excess). The preferred mechanism for denitrification by pyrite is likely to vary spatially. If pyrite is assumed to be relatively evenly distributed throughout the formation, while nitrate occurs in a discrete plume, Reaction 3 may dominate on the plume edges while Reaction 4 may dominate the core of the plume. 4.2.2 Other Relevant Studies Regarding Nitrate Reduction by Pyrite As discussed in HGC (2022) nitrate degradation by pyrite is a well-known mechanism discussed extensively in the literature. USEPA (2007) recognizes the importance of pyrite-bearing aquifers in reducing or eliminating nitrate contamination, stating that “pyrite-bearing aquifers represent important hydrological compartments due to their capacity to eliminate nitrate.” Other relevant excerpts from available literature are provided below: • Jioyang (2014) indicates that pyrite is suitable for nitrate remediation with a nitrate removal rate constant of 0.95/day. • Krieger (2014) indicates that “the major electron donors for denitrification are organic carbon (OC), pyrite (FeS2) and ferrous iron silicate minerals. In the […] tracer tests, increases in sulfate indicated that the oxidation of pyrite explained a significant [proportion] of the denitrification.” • Zhang (2012) indicates that “Pyrite oxidation leads to sulfate production and trace metal release to groundwater. This process can have a major impact on local and regional water quality.” • Zhang (2012) also indicates that “denitrification with pyrite can be the dominant pathway of nitrate removal from groundwater, even when organic matter is present.” • Zhang (2009) concludes that “nitrate removal from the groundwater below cultivated fields correlates with sulfate production, and the release of dissolved Fe2+ and pyrite-associated trace metals (e.g. As, Ni, Co and Zn). These results, and the presence of pyrite in the sediment matrix within the nitrate removal zone, indicate that denitrification coupled to pyrite oxidation is a major process in the aquifer.” • Tesoriero (2011) indicates that “A review of published rates suggests that denitrification tends to occur more quickly when linked with sulfide oxidation than with carbon oxidation.” 30 Nitrate Corrective Action Comprehensive Monitoring Evaluation (CACME) Report Https://Hgcinc.Sharepoint.Com/VOL4/718000/71807/NCACME2022/Report/NCACME2022_Final.Docx June 21, 2023 • Bosch (2011) states “Here, we provide evidence for the capability of Thiobacillus denitrificans to anaerobically oxidize a putatively nanosized pyrite particle fraction with nitrate as electron acceptor. Nanosized pyrite was readily oxidized to ferric iron and sulfate with a rate of 10.1 μM h-1. The mass balance of pyrite oxidation and nitrate reduction revealed a closed recovery of the electrons. This substantiates a further ‘missing lithotrophy’ in the global cycles of sulfur and iron and emphasizes the high reactivity of nanominerals in the environment.” • Aguerri (2010) identified areas within the Osona region of Spain where, based on hydrogeological and multi-isotopic methods, nitrate degradation via pyrite oxidation was occurring. • Torrento (2010) indicates that “Nitrate reduction was satisfactorily accomplished in experiments with pyrite as the sole electron donor, in presence of the autotrophic denitrifying bacterium Thiobacillus denitrificans and at nitrate concentrations comparable to those observed in contaminated groundwater. The experimental results corroborated field studies in which the reaction occurred in aquifers.” • Jorgensen (2009) concludes that microbes can control groundwater nitrate concentrations by denitrification “using primarily pyrite as electron donor at the oxic-anoxic boundary in sandy aquifers.” Note the potentially important impacts on water quality resulting from the trace metal and sulfate release from pyrite oxidation as discussed in Zhang (2009; 2012). In addition, as discussed above, depending on the particular reaction pathway, acid may also be released causing a decrease in pH resulting in mobilization of additional naturally-occurring metals. 4.2.3 Comparison to Oostrum Site As discussed in HGC (2022) Bosch and Meckenstock (2012) discuss degradation of nitrate via pyrite oxidation in field and laboratory studies and provide calculated rates. These rates are summarized in Table 3. Of particular interest are the rates calculated for the Oostrum, Netherlands site, an agricultural area which overlies a pyritic sandy aquifer. The Oostrum site is discussed in detail in Zhang (2009) and Zhang (2012). Similarities between the Oostrum and Mill sites include: • Sandy materials containing pyrite host groundwater; • Locally anaerobic conditions are present (inferred at Mill from detectable chloroform daughter product concentrations and persistence of pyrite); • Similar pyrite concentrations (from <0.1 to approximately 0.8 wt% at both sites; and similar average concentrations as shown in Tables 3 and 4); 31 Nitrate Corrective Action Comprehensive Monitoring Evaluation (CACME) Report Https://Hgcinc.Sharepoint.Com/VOL4/718000/71807/NCACME2022/Report/NCACME2022_Final.Docx June 21, 2023 • Calculated nitrate (as nitrogen) degradation rates at the Mill that are similar to, but lower than the rate calculated for the Oostrum site (approximately 5.4 x 10-4 lb/ft3-yr at Oostrum; and approximately 5.4 x 10-6 to 6.35 x 10-6 lb/ft3 yr at the Mill [from HGC, 2017 and HGC, 2022] as shown in Table 4). The rate reported for the Oostrum site, which has pyrite concentrations that are similar to those measured at the Mill, is one to two orders of magnitude higher than the rates calculated for the Mill, suggesting that the rates calculated for the Mill are conservatively low and may underestimate actual rates. Regardless, as discussed in the Revised Phase III Planning Document, even in the absence of any nitrate reduction by pyrite or mass removal via pumping, numerical flow and transport modeling using conservative assumptions indicates that hydrodynamic dispersion alone will reduce all nitrate concentrations within the plume to less than the 10 mg/L GCAL before reaching a property boundary. 4.3 Hydraulic Capture and Recalculation of ‘Background’ Flow The specific methodology for calculating the quarterly nitrate capture zones is substantially the same as that used since the fourth quarter of 2005 to calculate the capture zones for the chloroform program, as agreed to by the DWMRC and EFRI. The procedure for calculating nitrate capture zones is as follows: 1. Calculate water level contours by gridding the water level data on approximately 50-foot centers using the ordinary linear kriging method in SurferTM. Default kriging parameters are used that include a linear variogram, an isotropic data search, and all the available water level data for the quarter, including relevant seep and spring elevations. 2. Calculate the capture zones by hand from the kriged water level contours following the rules for flow nets: a. From each pumping well, reverse track the stream tubes that bound the capture zone of each well, b. Maintain perpendicularity between each stream tube and the kriged water level contours. The eventual goal of pumping, as specified as per Phase II of the CAP, and consistent with Phase III goals, is to capture the entire nitrate plume upgradient of TW4-22 and TW4-24. Hydraulic capture within the nitrate plume remains difficult to assess based on kriged quarterly water levels because of the divergent flow field resulting from the remaining perched groundwater mound to the northeast and the remaining groundwater mound near TWN-3; although water level decreases 32 Nitrate Corrective Action Comprehensive Monitoring Evaluation (CACME) Report Https://Hgcinc.Sharepoint.Com/VOL4/718000/71807/NCACME2022/Report/NCACME2022_Final.Docx June 21, 2023 at TWN-2, and increases in water levels at TWN-7, have helped to better define the capture zone associated with TWN-2. Figure 21 displays the total capture associated with nitrate and chloroform pumping systems in the vicinity of the nitrate plume from the third quarter of 2021 through the second quarter of 2022. The proportion of nitrate mass under capture during the second quarter of 2022 is approximately 20%. The relatively low proportion of the total mass under pumping capture is due primarily to the substantial reduction in concentrations since 2010 within the northern extremity of the plume (near TWN-2) and near TW4-25 (which dropped below 10 mg/L in the first quarter of 2013). However, due to low permeability conditions and transient groundwater flow conditions (resulting from reduced wildlife pond recharge), capture zones associated with nitrate pumping are likely continuing to develop. Furthermore, capture upgradient of TW4-22 and TW4-24 is likely adequate based on total nitrate plume pumping rates that are within or exceed the calculated range of ‘background’ flow through the plume. Pre-pumping ‘background’ flow through the nitrate plume near TW4-22 and TW4-24 was initially estimated using Darcy’s Law to lie within a range of approximately 1.31 gpm to 2.79 gpm (EFRI, 2014a). Calculations were based on an average hydraulic conductivity range of 0.15 feet per day (ft/day) to 0.32 ft/day (depending on the calculation method), a pre-pumping hydraulic gradient of 0.025 feet per foot (ft/ft), a plume width of 1,200 feet, and a saturated thickness (at TW4-22 and TW4-24) of 56 feet. The hydraulic conductivity range was estimated by averaging the results obtained from slug test data that were collected automatically by data loggers from wells within the plume and analyzed using the KGS unconfined slug test solution available in AqtesolvTM (HGC, 2005; HGC, 2009a; HGC, 2009b) These results are summarized in Table 5. Data from the fourth quarter, 2012 were used to estimate the pre-pumping hydraulic gradient, and saturated thickness. These data are summarized in Tables 6 and 7. The average hydraulic conductivity was estimated to lie within a range of 0.15 ft/day to 0.32 ft/day. Averages were calculated four ways. As shown in Table 3, arithmetic and geometric averages for wells MW-30, MW-31, TW4-22, TW4-24, TW4-25, TWN-2, and TWN-3 were calculated as 0.22 and 0.15 ft/day, respectively. Arithmetic and geometric averages for a subset of these wells (MW- 30, MW-31, TW4-22, and TW4-24) were calculated as 0.32 and 0.31 ft/day, respectively. The lowest value, 0.15 ft/day, represented the geometric average of the hydraulic conductivity estimates for all the plume wells. The highest value, 0.32 ft/day, represented the arithmetic average for the four plume wells having the highest hydraulic conductivity estimates (MW-30, MW-31, TW4-22, and TW4-24). Using the arithmetic average hydraulic conductivity of a subset of plume wells having the highest conductivities, although considered less representative of actual 33 Nitrate Corrective Action Comprehensive Monitoring Evaluation (CACME) Report Https://Hgcinc.Sharepoint.Com/VOL4/718000/71807/NCACME2022/Report/NCACME2022_Final.Docx June 21, 2023 conditions than using the geometric average conductivity of all of the plume wells, ensured that the upper estimate of ‘background’ flow (2.79 gpm) was conservatively large. Pre-pumping hydraulic gradients (Table 7) were estimated at two locations; between TW4-25 and MW-31 (estimated as 0.023 ft/ft), and between TWN-2 and MW-30 (estimated as 0.027 ft/ft). These results were averaged to yield the value used in the calculation (0.025 ft/ft). The pre- pumping saturated thickness of 56 feet was an average of pre-pumping saturated thicknesses at TW4-22 and TW4-24. The hydraulic gradient and saturated thickness used in the calculations are assumed to represent a steady state ‘background’ condition. However, assumption of a steady state ‘background’ is inconsistent with the March 2012 cessation of water delivery to the northern wildlife ponds, located upgradient of the nitrate plume. Hydraulic gradients and saturated thicknesses within the plume are declining as a result of two factors: reduced recharge from the ponds, and the effects of nitrate pumping. Separating the impacts of nitrate pumping from the impacts of reduced recharge from the ponds is problematic. Should pumping cease and ‘background’ conditions be allowed to re-establish, however, smaller hydraulic gradients and saturated thicknesses would be expected due to reduced wildlife pond recharge, which would lower estimates of ‘background’ flow. Changes related to reduced wildlife pond recharge have also resulted in reduced well productivity. Generally reduced productivities of nitrate pumping well TW4-24 and chloroform pumping well TW4-19 since the third quarter of 2014 are at least partly the result of reduced wildlife pond recharge as discussed in EFRI (2015d). ‘Background’ flows through the nitrate plume since the initial estimates were made have continued to decline independent of pumping as a result of reduced hydraulic gradients and saturated thicknesses within upgradient portions of the plume due to reduced wildlife pond recharge. As a result, the initial ‘background’ flow range of 1.31 gpm to 2.79 gpm calculated using the hydraulic gradient of 0.025 ft/ft and saturated thickness of 56 feet became increasingly larger than the actual flow was likely to be and was recalculated in the third quarter of 2015 (using second quarter, 2015 data), as presented in Attachment N (Tab N) of EFRI (2015d). The analysis of reduced productivity that was provided concluded that pumping from the nitrate plume was adequate even considering the reduced productivity of TW4-24. The recalculation of background flow and the well productivity analysis, as presented in Attachment N of EFRI (2015d) is provided in Appendix B. As presented in Appendix B, using the updated saturated thickness, hydraulic gradient, and hydraulic conductivity data, the original pre-pumping ‘background’ flow range of 1.31 gpm to 2.79 gpm was recalculated to range from 0.79 gpm to 1.67 gpm. This calculation was considered 34 Nitrate Corrective Action Comprehensive Monitoring Evaluation (CACME) Report Https://Hgcinc.Sharepoint.Com/VOL4/718000/71807/NCACME2022/Report/NCACME2022_Final.Docx June 21, 2023 conservative because the high end of the range assumed an arithmetic average hydraulic conductivity of a subset of plume wells having the highest conductivities. As of the second quarter of 2022, pumping from TW4-22, TW4-24, TW4-25, TW4-37, and TWN-2 of approximately 2.5 gpm exceeds the high end of the recalculated ‘background’ flow range by approximately 0.83 gpm, or a factor of approximately 1.5. Because hydraulic gradients and saturated thicknesses within the plume have continued to decline since the pre-pumping ‘background’ flow was recalculated based on second quarter 2015 data, the actual ‘background’ flow range as of the second quarter of 2022 is smaller than 0.79 gpm to 1.67 gpm. Since the recalculation of pre-pumping ‘background’ flow based on second quarter 2015 data, average saturated thickness within the plume has decreased by nearly 6 ft; and the average saturated thickness near the center of the plume has decreased from approximately 52 ft (based on data from TW4-22, TW4-24 and TW4-37) to approximately 39 ft (based on data from TW4-22, TW4-24, TW4-37 and MW-28). Note that MW-28 is included in the latter calculation because the kriged western boundary of the plume has expanded toward MW-28 since 2015. This approximately 13 ft decrease in saturated thickness near the center of the plume represents a reduction of approximately 25%. Based on data from non-pumping wells TWN-1 and MW-25 (located just outside the eastern margin of the plume), since the second quarter of 2015, the average hydraulic gradient within the plume has decreased from approximately 0.018 ft/ft to 0.017 ft/ft, a reduction of nearly 6%. The decreased saturated thicknesses near the center of the plume combined with decreased hydraulic gradients indicate that ‘background’ flow through the plume since the second quarter of 2015 has decreased, even though the width of the central portion of the plume has increased from approximately 1,425 ft to 1,600 ft, or about 12%. Accounting for the decreases in hydraulic gradient and saturated thickness; and the increase in width; ‘background’ flow through the plume since the second quarter of 2015 has decreased by approximately 20%, from a range of 0.79 to 1.67 gpm to a range of 0.63 to 1.34 gpm (assuming no change in minimum and maximum hydraulic conductivity estimates). Using this new range, the second quarter, 2022 nitrate pumping of approximately 2.5 gpm exceeds the high end of the newly recalculated ‘background’ flow range by approximately 1.2 gpm, or a factor of approximately 1.9. Changes in ‘background’ flow estimates are summarized in Table 8. Overall, hydraulic capture in combination with natural attenuation appears adequate at the present time based on the relative stability of the southern boundary of the nitrate plume and generally 35 Nitrate Corrective Action Comprehensive Monitoring Evaluation (CACME) Report Https://Hgcinc.Sharepoint.Com/VOL4/718000/71807/NCACME2022/Report/NCACME2022_Final.Docx June 21, 2023 decreasing concentrations at the majority of wells within the plume (since 2013). Plume expansion to the west has caused incorporation of TWN-7 but not MW-28; and the plume has not expanded to the south to MW-5 or MW-11, even though the kriged southern boundary of the nitrate (and commingled chloride) plume is slowly migrating towards MW-11. The kriged chloride plume boundary has expanded more rapidly downgradient (to the west and south) than has the kriged nitrate plume boundary. More rapid expansion of the chloride plume is generally expected as chloride is not subject to degradation by pyrite (or any other known mechanism at the site) in the same way as nitrate. Eastward migration of the kriged eastern (generally cross-gradient) chloride plume boundary likely results from hydrodynamic dispersion, chloroform pumping to the east of the plume, and reduced dilution from wildlife pond seepage. 4.4 Impacts of Perched Groundwater Flow, Pumping and Natural Attenuation on the Nitrate Plume Perched groundwater flow to the south-southwest within and in the vicinities of the commingled nitrate and chloride plumes causes constituents within the plumes to migrate to the south- southwest. Pumping in upgradient areas of the plumes and reduced wildlife pond recharge (caused by cessation of water delivery to the ponds in March, 2012) act to reduce hydraulic gradients and slow downgradient migration of the plumes. In addition, nitrate and chloroform pumping both remove nitrate and chloride mass from the nitrate and chloride plumes and from areas east of the plumes, acting to reduce concentrations of these constituents in the groundwater. Natural attenuation also acts to reduce nitrate and chloride concentrations within the nitrate and chloride plumes and within areas east of the plumes. Natural attenuation mechanisms include dilution and hydrodynamic dispersion (which impacts both nitrate and chloride concentrations) and nitrate reduction by naturally occurring pyrite and/or organic matter in the perched groundwater zone (which impacts only nitrate concentrations). The combined impacts of perched groundwater flow, pumping and natural attenuation on the nitrate plume since the first quarter of 2013 include the following: 1. Pumping and natural attenuation have maintained control of the nitrate plume. Average nitrate concentrations within the plume have decreased since 2013; and the southern plume boundary remains between MW-30/MW-31 and MW-5/MW-11; 2. The relative stability of the kriged southern nitrate plume boundary; the stability of nitrate concentrations in the toe of the plume (at MW-30 and MW-31); and increasing chloride in the toe of the plume imply that: 36 Nitrate Corrective Action Comprehensive Monitoring Evaluation (CACME) Report Https://Hgcinc.Sharepoint.Com/VOL4/718000/71807/NCACME2022/Report/NCACME2022_Final.Docx June 21, 2023 a. The commingled chloride plume is continuing to migrate downgradient to the south-southwest which is expected because this portion of the plume is beyond the hydraulic capture of the pumping wells (as anticipated and as discussed in the CAP); b. Nitrate is being degraded in the toe of the plume (at MW-30 and MW-31); otherwise concentrations would be increasing along with the chloride concentrations (the nitrate plume would be expanding to the south-southwest at about the same rate as the chloride plume); and nitrate to chloride concentration ratios would be stable rather than decreasing; and c. The nitrate degradation is consistent with nitrate reduction by naturally occurring pyrite and/or organic matter in the perched zone. 3. Increasing nitrate and chloride at TWN-7 and MW-28 (although second quarter, 2022 nitrate concentrations at MW-28 are only about 5 mg/L) are consistent with the apparent expansion of the western kriged nitrate plume boundary and continuing downgradient migration of nitrate and chloride. 4. Decreasing nitrate concentrations at TWN-2 (since 2013) and contraction of the plume immediately to the south of TWN-2 are attributable to mass removal by pumping and redistribution of nitrate within the plume. Redistribution (as anticipated in the CAP) appears to have caused the mass center to migrate from the area of TWN-2 to the area of TW4-22 and TW4-24. It is likely that TWN-2 was located within the upgradient portion of this mass center in the second quarter of 2010 and that TW4-22 and TW4-24 are now within this mass center. Redistribution is consistent with changes in saturated thickness and continuing downgradient migration of nitrate within the plume (to the south-southwest) enhanced by pumping. 5. Decreasing saturated thicknesses within the majority of the plume have resulted in a decreasing plume volume which contributes to a decreasing trend in the quarterly residual plume mass estimates. 6. Hydrodynamic dispersion, mass removal by pumping and naturally occurring nitrate degradation, which tend to reduce concentrations in wells within the plume and shrink the plume boundaries, are partially counteracted by reduced dilution from the wildlife ponds, which tends to increase concentrations in wells within the plume and expand the plume boundaries. 7. The interaction of the above mechanisms apparently resulted in a plume that appeared to be in dynamic equilibrium with respect to area through about 2017. Subsequently, while plume mass and average concentrations generally declined, the plume area generally increased until the third quarter of 2020; then began to trend generally downward (Table 1). 4.5 Rate of Plume Remediation As discussed in Section 4.2, both pumping and nitrate reduction by naturally occurring pyrite and/or organic material in the perched zone act to reduce mass within the nitrate plume. Dilution 37 Nitrate Corrective Action Comprehensive Monitoring Evaluation (CACME) Report Https://Hgcinc.Sharepoint.Com/VOL4/718000/71807/NCACME2022/Report/NCACME2022_Final.Docx June 21, 2023 and hydrodynamic dispersion act to reduce nitrate concentrations within the plume even in the absence of nitrate mass removal from the groundwater. As discussed in the 2017 Nitrate CACME, the plume appeared to be in a state of dynamic equilibrium from 2010 through about 2017. The relative stability of average nitrate concentrations within the nitrate plume and the relative stability of the plume area likely resulted from a combination of competing factors. Nitrate mass removal by pumping and naturally occurring nitrate degradation, which tend to reduce concentrations within the plume and shrink the plume boundaries, were counteracted by reduced dilution from the wildlife ponds, which tends to increase concentrations within the plume and expand the plume boundaries. The interaction of these two mechanisms resulted in a plume that appeared to be in dynamic equilibrium with respect to area. Since 2017, the area of the plume has generally increased. However, since 2013, while the plume area has been generally stable to increasing, average nitrate concentrations within the plume have generally decreased, attributable to nitrate mass removed via pumping and by reduction of nitrate by naturally occurring pyrite and/or organic material (as discussed in Section 4.2). The relative stability of the downgradient edge of the plume (at MW-30 and MW-31) is attributable in part to degradation of nitrate. As discussed in Section 4.4, nitrate is likely being degraded in the toe of the plume, otherwise concentrations would be increasing along with the chloride concentrations, and nitrate to chloride ratios would be stable rather than decreasing. Preliminary estimates of nitrate degradation rates were provided in the 2017 CACME using three methodologies. Methods focused on generally downgradient portions of the plume that are less likely to be impacted by pumping and changes in wildlife pond recharge. Each method assumed negligible dilution and dispersion, and a steady rate of flow through the plume. The first two methods were: 1. Based on changes in nitrate concentrations between the center of the plume (at TW4-22 and TW4-24), and the toe of the plume (at downgradient wells MW-30 and MW-31); and 2. Based on changes in nitrate to chloride ratios in the toe of the plume at MW-30 and MW-31. The third method focused on the margins of the nitrate plume, within areas between the nitrate and chloride plumes, and was based on the assumption that the plume boundaries would be more similar were it not for nitrate degradation. These three methods have been updated in the present report based on the additional data collected from the third quarter of 2017 through the second quarter of 2022. 38 Nitrate Corrective Action Comprehensive Monitoring Evaluation (CACME) Report Https://Hgcinc.Sharepoint.Com/VOL4/718000/71807/NCACME2022/Report/NCACME2022_Final.Docx June 21, 2023 An additional method (Method 4) is included in the present report. Method 4 is based on changes in average nitrate to chloride ratios within the plume using gridded (kriged) nitrate and chloride concentration data. Changes between the first quarter of 2013 and the second quarter of 2022 are considered. 4.5.1 Method 1 Nitrate concentrations decrease between the center of the plume (near TW4-22 and TW4-24) and the toe of the plume (near MW-30 and MW-31). Based on average nitrate concentrations since the first quarter of 2013 at TW4-22 (approximately 59.7 mg/L); TW4-24 (approximately 34.6 mg/L), MW-30 (approximately 17.6 mg/L); and MW-31 (approximately 19.4 mg/L), the change in average concentration between the center and toe of the plume is approximately 29 mg/L. Assuming that these changes primarily result from nitrate degradation (rather than dilution and dispersion), the rate of nitrate degradation within the southern half of the plume can be estimated based on the following assumptions: 1. The volume of water within the southern half of the nitrate plume (south of TW4-22 and TW4-24) is approximately 1.35 x 107 ft3 based on the second quarter 2022 plume boundary and saturated thicknesses; 2. Flow though the southern half of the plume area is a steady 1.5 gpm or 105,401 ft3/yr (the average of the midpoints of the pre-pumping background flow range of 1.31 to 2.79 gpm, and the newly recalculated [Section 4.3] range of 0.63 to 1.34 gpm); 3. Water enters the southern half of the plume at an average nitrate concentration of 47.2 mg/L (average of concentrations at TW4-22 and TW4-24) and leaves at an average concentration of 18.5 mg/L (average of concentrations at MW-30 and MW-31), yielding a change in concentration of approximately 29 mg/L; 4. The nitrate degradation rate is zero order. Based on these assumptions, the change in concentration of 29 mg/L implies a change in mass flow of approximately 191 lb/yr; and the nitrate degradation rate per unit volume of groundwater within the 1.35 x 107 ft3 volume of the southern half of the plume is approximately 1.41 x 10-5 pounds per cubic foot of groundwater per year (lb/ft3 yr). Assuming that this rate is applicable within the entire nitrate plume, which has a second quarter, 2022 groundwater volume of approximately 2.4 x 107 ft3, yields a total nitrate degradation rate within the plume of approximately 340 lb/yr. This estimate will be impacted to some extent by pumping at TW4-22 and TW4-24. 39 Nitrate Corrective Action Comprehensive Monitoring Evaluation (CACME) Report Https://Hgcinc.Sharepoint.Com/VOL4/718000/71807/NCACME2022/Report/NCACME2022_Final.Docx June 21, 2023 Therefore, these same calculations were also performed based on the second quarter, 2010 baseline data, which are unimpacted by pumping and reduced wildlife pond recharge. These calculations assumed a pre-pumping flow through the plume of 2.1 gpm (the approximate midpoint of the pre- pumping background flow range of 1.31 to 2.79 gpm). The average concentration at TW4-22 and TW4-24 was approximately 24.6 mg/L and at MW-30 and MW-31, approximately 19.1 mg/L, implying a change in concentration from the middle to the toe of the plume of approximately 5.5 mg/L. The change in concentration of 5.5 mg/L implies a change in mass flow of approximately 49 lb/yr. Based on these assumptions, the nitrate degradation rate per unit volume of groundwater within the second quarter, 2010 volume of the southern half of the plume (1.0 x 107 ft3) is approximately 4.9 x 10-6 lb/ft3 yr. Assuming that this rate is applicable within the entire second quarter, 2010 nitrate plume, which had groundwater volume of approximately 3.16 x 107 ft3, yields a total nitrate degradation rate within the plume of approximately 155 lb/yr. The average of the calculated second quarter, 2010 and second quarter, 2022 rates for the southern half of the plume is approximately 120 lb/yr, and for the entire plume, approximately 248 lb/yr. 4.5.2 Method 2 Nitrate to chloride concentration ratios have been declining at MW-30 and MW-31 as chloride concentrations increase and nitrate concentrations remain relatively stable. Between the second quarter of 2010 (baseline) and second quarter of 2022, chloride concentrations at MW-30 have increased from approximately 97 mg/L to 173 mg/L (a change of approximately 76 mg/L or 78%) and chloride concentrations at MW-31 have increased from approximately 128 mg/L to 372 mg/L (a change of approximately 244 mg/L or 191%). Therefore the average increase in chloride in the toe of the plume since the second quarter of 2010 is approximately 160 mg/L (the average of changes at MW-30 and MW-31) or 142%. Over this time period the nitrate to chloride concentration ratio has decreased from approximately 0.16 to 0.098 at MW-30 and from approximately 0.18 to 0.048 at MW-31. Presumably, if there were no degradation of nitrate, the nitrate to chloride ratios would not change, since both nitrate and chloride are expected to migrate at about the same rate as the groundwater (neither is significantly retarded by adsorption onto perched zone materials). Assuming that these changes result from a process that degrades nitrate but not chloride (such as nitrate reduction by 40 Nitrate Corrective Action Comprehensive Monitoring Evaluation (CACME) Report Https://Hgcinc.Sharepoint.Com/VOL4/718000/71807/NCACME2022/Report/NCACME2022_Final.Docx June 21, 2023 naturally occurring pyrite/and or organic material in the perched zone), the rate of nitrate degradation can be estimated based on the following assumptions: 1. The average rate of flow is approximately 1.5 gpm or 105,401 ft3/yr (the average of the midpoints of the pre-pumping background flow range of 1.31 to 2.79 gpm, and the newly recalculated [Section 4.3] range of 0.63 to 1.34 gpm); 2. The degradation has occurred within the volume of water that has passed MW-30 and MW- 31 since the second quarter of 2010 (approximately 1.26 x 106 ft3); 3. The nitrate concentration increases in the toe of the plume that would be expected if there were no degradation can be calculated from the nitrate to chloride concentration ratios; 4. Variations in the nitrate to chloride concentration ratios are assumed to result only from degradation of nitrate; 5. The increases in nitrate concentrations that would be expected if there were no degradation can be used to calculate a nitrate degradation rate; and 6. The nitrate degradation rate is zero order. To maintain a constant nitrate to chloride ratio at the toe of the plume since the second quarter of 2010 would require the nitrate concentration (average of MW-30 and MW-31 concentrations) to increase by approximately 27 mg/L (142%), from approximately 19 mg/L to approximately 46 mg/L. Because the change of approximately 27 mg/L is relatively linear, the average change over this time period is approximately 14 mg/L, implying a total additional mass of nitrate of approximately 1,100 lb within the volume of groundwater (approximately 1.26 x 106 ft3) passing through the toe of the plume since the second quarter of 2010. The average rate of nitrate increase within that volume on a mass per unit volume basis would be approximately 7.28 x 10-5 lb/ft3 yr. Because nitrate concentrations did not increase, the rate of 7.28 x 10-5 lb/ft3 yr is the approximate average rate of implied nitrate degradation within the volume passing through the toe of the plume, or 92 lb/yr. Assuming that this rate is applicable within the entire plume (having a groundwater volume of approximately 2.4 x 107 ft3) yields a nitrate degradation rate of approximately 1,750 lb/yr. However, this rate does not appear reasonable when applied to the entire plume (as 1,750 lb/yr over the 12 years since the second quarter of 2010 would exceed the total estimated mass reduction of between 7,200 lb and 11,767 lb based on quarterly plume mass estimates as discussed in Section 3.3). 41 Nitrate Corrective Action Comprehensive Monitoring Evaluation (CACME) Report Https://Hgcinc.Sharepoint.Com/VOL4/718000/71807/NCACME2022/Report/NCACME2022_Final.Docx June 21, 2023 4.5.3 Method 3 Method 3 considers the area between the nitrate and chloride plumes within primarily the downgradient 3/4 of the nitrate plume and assumes that the boundaries of the nitrate and chloride plumes would be roughly coincident if not for the degradation of nitrate. Currently (as of the second quarter of 2022), the chloride plume extends to the west, east, and south of the nitrate plume; Figure 22 shows the approximate areas between the two plume boundaries (the area ‘marginal’ to the nitrate plume). The groundwater volume within this marginal area is approximately 1.44 x 107 ft3. The nitrate mass within the marginal area is approximately 4,980 lb. Presumably, if not for nitrate degradation, the mass within this marginal volume would be larger. The nitrate mass that would exist within this marginal volume in the absence of degradation can be estimated based on the average nitrate to chloride ratio within the nitrate plume. The average nitrate to chloride ratio was calculated from wells consistently within the plume using second quarter, 2010 (baseline) data and quarterly data collected between the fourth quarter of 2012 and the second quarter of 2022. TWN-2 and TWN-3 were excluded from the calculation because they are within the head of the plume and are presumably the most influenced by changes in wildlife pond seepage. Using data from wells MW-30, MW-31, TW4-22, TW4-24, and TW4- 37, the average nitrate to chloride ratio within the plume is approximately 0.088. Calculating the nitrate mass within the marginal volume based on the second quarter, 2022 chloride concentrations and the nitrate to chloride ratio of 0.088 yields a nitrate mass of approximately 10,028 lb, which is 5,048 lb larger than the calculated mass of approximately 4,980 lb based on second quarter 2022 nitrate data. The 5,048 lb difference is assumed to be the result of natural degradation since the historic pond (the presumed major source) became active circa 1925. The average rate of degradation within the marginal volume is approximately 52 lb/yr, or 3.61 x 10-6 lb/ft3 yr. Assuming this rate is applicable within the nitrate plume, which has a second quarter, 2022 volume of approximately 2.4 x 107 ft3, yields a total average nitrate degradation rate within the plume of approximately 87 lb/yr. If this same methodology is applied to the second quarter, 2010 (baseline) data, an average degradation rate of approximately 69 lb/yr is calculated for the marginal area between the second quarter, 2010 nitrate and chloride plumes (having a volume of approximately 1.14 x 107 ft3), yielding a rate per unit volume of approximately 6.1 x 10-6 lb/yr ft3. Assuming this rate is applicable within the nitrate plume (having a volume of approximately 3.15 x 107 ft3), yields a rate within the plume of approximately 192 lb/yr. 42 Nitrate Corrective Action Comprehensive Monitoring Evaluation (CACME) Report Https://Hgcinc.Sharepoint.Com/VOL4/718000/71807/NCACME2022/Report/NCACME2022_Final.Docx June 21, 2023 The average of the calculated second quarter, 2010 and second quarter, 2022 rates for the marginal areas of the plume is approximately 61 lb/yr, and within the entire plume, approximately 140 lb/yr. 4.5.4 Method 4 Method 4 considers the changes in nitrate to chloride ratios within the entire plume based on the differences in average nitrate to chloride ratios using gridded (kriged) nitrate and chloride concentration data. Changes between nitrate to chloride ratios in the first quarter of 2013 (initial) and the second quarter of 2022 (final) are considered in the analysis. The average nitrate to chloride ratios for the first quarter of 2013 and second quarter of 2022 were computed by 1) ‘clipping’ the chloride concentration grids for the two quarters to the boundaries of the nitrate plume for the respective quarters; 2) dividing the nitrate concentration grids by the respective ‘clipped’ chloride concentration grids yielding nitrate to chloride concentration grids covering only the areas of the respective nitrate plumes; then 3) computing the average nitrate to chloride grid value for the two quarters. Using gridded data, the average nitrate to chloride ratio for the first quarter of 2013 is approximately 0.11; and the average nitrate to chloride ratio for the second quarter of 2022 is approximately 0.087, a reduction of approximately 21%. This change implies a corresponding reduction in nitrate mass within the plume of 21%, or approximately 8,680 lb, assuming a first quarter, 2013 mass of 41,350 lb (Table 1). Between the first quarter of 2013 and the second quarter of 2022, the total mass reduction of 8,680 lb implies an average annual mass reduction rate of approximately 940 lbs/yr. The estimate of 940 lbs/yr lies between that calculated using Methods 1 through 3. The mass change of 8,680 lb is similar to the approximately 9,420 lb mass reduction between the first quarter of 2013 and the second quarter of 2022 based on quarterly mass estimates shown in Table 1. If the 940 lb/yr rate is applied beginning with the second quarter of 2010, the total implied mass reduction would be approximately 11,280 lb as of the second quarter of 2022. This estimate of 11,280 lb lies within the mass reduction estimate range of 7,200 lb to 11,767 lb (based on quarterly plume mass estimates) as discussed in Section 3.3. 4.5.5 Summary As discussed above, methods 1 and 2 focus on downgradient areas within the nitrate plume; method 3 on the primarily downgradient margins of the nitrate plume; and method 4 on the entire plume. Nitrate degradation rates calculated using methods 1 and 2 are therefore more representative of downgradient areas within the plume; rates calculated using method 3 more 43 Nitrate Corrective Action Comprehensive Monitoring Evaluation (CACME) Report Https://Hgcinc.Sharepoint.Com/VOL4/718000/71807/NCACME2022/Report/NCACME2022_Final.Docx June 21, 2023 representative of primarily downgradient areas at the plume margins; and rates calculated using method 4 more representative of the entire plume. To estimate the total degradation rates implied by these calculations it is appropriate to add the rates calculated from methods 1 and 2 to the rates calculated from method 3. The total degradation rate based on method 1 and method 3 calculations would be 181 lb/yr (the sum of 120 lb/yr and 61 lb/yr); and based on method 2 and method 3 calculations, 153 lb/yr (the sum of 92 lb/yr and 61 lb/yr). The average of these total degradation rates is approximately 167 lb/yr. Rates calculated using methods 1 through 3 were also divided by the volumes of water for which they were known to be representative, yielding rates per unit volume. Assuming that these rates were applicable within the entire plume, they were then multiplied by the entire plume volumes to yield a total rate within the plume. These rates were approximately 248 lb/yr (method 1); 1,750 lb/yr (method 2); and 140 lb/yr (method 3). The rate for method 4, which applied to the entire plume, was 940 lb/yr. As discussed in Section 4.5.4, the method 2 rate of 1,750 lb/yr (for the entire plume) is not considered reasonable. Excluding the method 2 value, the average of the method 1, method 3 and method 4 rates is approximately 442 lb/yr. 4.5.6 Comparison With Mass Removed by Pumping Table 2 shows the mass of nitrate removed by pumping wells during Phase II. A total of approximately 3,663 lbs has been removed by all (both chloroform and nitrate) pumping wells; a total of approximately 3,329 lbs has been removed by pumping wells within and marginal to the nitrate plume; and a total of approximately 3,083 lbs has been removed by pumping wells only within the plume. Beginning with the first quarter of 2013, the average nitrate mass removed only by pumping wells within the plume is approximately 325 lb/yr. and from pumping wells within and marginal to the plume approximately 350 lb/yr. The estimated rates of natural degradation (excluding method 2 results) average approximately 167 lb/yr assuming the calculated rate is applicable to only the area of the plume or plume margin from which the calculation was derived, and 442 lb/yr assuming the rates are applicable within the entire plume. The latter rate of 442 lb/yr is more than double the rate of 200 lb/yr presented in the 2017 CACME primarily due to the influence of the new method 4 calculation. Regardless, the rates removed by pumping are on the same order of the calculated natural degradation rates. 44 Nitrate Corrective Action Comprehensive Monitoring Evaluation (CACME) Report Https://Hgcinc.Sharepoint.Com/VOL4/718000/71807/NCACME2022/Report/NCACME2022_Final.Docx June 21, 2023 4.6 Projected Timeline to Return Groundwater Nitrate Concentrations to the Groundwater Quality Standards As discussed in the Revised Phase III Planning Document, based on numerical flow and transport modeling using conservative, ‘worst-case’ assumptions, all nitrate concentrations within the plume are projected to drop below the GCAL of 10 mg/L prior to reaching a property boundary. The plume is expected to fully attenuate as a result of hydrodynamic dispersion alone, assuming no dilution, mass removal by pumping or degradation by pyrite. The time over which this would occur varies depending on the hydraulic conductivity, hydraulic gradient, and dispersivity assumed in a particular simulation. For simulations assuming conservatively large hydraulic conductivities (orders of magnitude larger than averages based on measured values) and hydraulic gradients (nearly double measured values), the time for full attenuation could be as short as 30 to 35 years. Methods not relying on numerical flow and transport modeling can also be used to assess remediation times. Such methods include projecting concentration trends and plume mass reduction estimates into the future; or calculating nitrate degradation rates by pyrite and projecting these rates, alone or in combination, with projections of future mass removal by pumping, to estimate times to achieve full attenuation of the plume. These methods are discussed below. First, because nitrate concentrations at many wells within the plume are not yet decreasing, projecting a timeline to return groundwater nitrate concentrations to groundwater quality standards is problematic based on concentration trends to date. As discussed above, nitrate concentrations within the plume are impacted by the competing mechanisms of reduced dilution from reduced wildlife pond seepage (which tends to increase concentrations) and mass removal by pumping and natural attenuation of nitrate (which tend to reduce concentrations). However, as the impact of reduced dilution on nitrate concentrations diminishes, nitrate concentrations at all wells within the plume are expected to begin trending downward. Second, remediation times can be estimated without relying on concentration trends or numerical flow and transport modeling by projecting nitrate mass removal rates (calculated based on pumping and natural degradation) into the future, and thus estimate the time needed to reduce the current (second quarter, 2022) residual mass within the plume to a negligible value. Presumably, if the residual nitrate mass was reduced to a negligible value, concentrations would also be reduced to negligible values. The time to reduce plume mass to a negligible value could be estimated by projecting the trendline calculated from the quarterly residual mass estimates; however, the downward trend of this trendline appears to be largely due to the reduction in saturated thicknesses within the plume. Because projecting this trendline would more or less depend on the time needed 45 Nitrate Corrective Action Comprehensive Monitoring Evaluation (CACME) Report Https://Hgcinc.Sharepoint.Com/VOL4/718000/71807/NCACME2022/Report/NCACME2022_Final.Docx June 21, 2023 to reduce saturated thicknesses to negligible values (physically unlikely) this method is likely to prove unreliable in assessing plume remediation times. Alternatively, the time to reduce the mass within the plume to a negligible value can be estimated by projecting calculated nitrate mass removal rates based on estimates of the natural nitrate mass reduction rate and the measured mass removal rate via pumping. As discussed in Section 4.5, since the first quarter of 2013, the average nitrate mass removed by pumping wells within and marginal to the plume is approximately 350 lb/yr. The averages of the estimated rates of natural nitrate reduction range from approximately 167 lb/yr to 442 lb/yr depending on the proportion of the plume to which the rate is assumed to be applicable. Thus the estimated total rate of mass reduction ranges from approximately 517 lb/yr to 792 lb/yr. Projecting these mass removal rates into the future, and assuming a zero order rate of natural reduction of nitrate and a current nitrate plume residual mass of approximately 31,933 lb, (as calculated for the second quarter of 2022), implies that between approximately 40 and 62 years would be required to reduce all the nitrate within the plume to a negligible value, and between approximately 72 and 191 years would be required via natural degradation alone. Because nitrate mass removal by pumping is likely to drop off in the future due to reduced nitrate concentrations and reduced saturated thicknesses (which will limit achievable pumping rates), the actual time, assuming pumping continues, will be more than 40 and less than approximately 200 years. However, because it is only necessary to reduce nitrate concentrations within the plume below 10 mg/L, and it will not be necessary to essentially remove all the nitrate mass to achieve this condition, the actual time to remediate the plume will be smaller than as calculated above. In addition, expected continuing reductions in plume volume will also reduce remediation times. Furthermore, natural attenuation processes that include dilution and hydrodynamic dispersion will reduce nitrate concentrations within the plume and contribute to an additional reduction of the remediation time. As discussed above, numerical flow and transport modeling using conservative ‘worst case’ assumptions indicated that hydrodynamic dispersion alone, without pumping, dilution by recharge or degradation by pyrite, is likely sufficient to reduce all nitrate concentrations within the plume to less than the GCAL of 10 mg/L before reaching a property boundary. However, once concentrations in all wells within the plume begin to decline, improved projections of the time required to reduce all concentrations to less than 10 mg/L will be possible. If projections are made using the nitrate degradation estimates presented above, under worst-case conditions of no pumping, and no attenuation via dilution or hydrodynamic dispersion, natural degradation of nitrate will reduce mass within the plume to a negligible value within no more than approximately 200 years. Because thousands of years would be required for nitrate within the 46 Nitrate Corrective Action Comprehensive Monitoring Evaluation (CACME) Report Https://Hgcinc.Sharepoint.Com/VOL4/718000/71807/NCACME2022/Report/NCACME2022_Final.Docx June 21, 2023 plume to migrate to a discharge point (either Westwater Seep or Ruin Spring) based on calculations presented in HGC (2022), there is more than sufficient time to reduce mass within the plume to a negligible value before a discharge point is reached. Specifically, the estimated travel time from MW-23, located on the western margin of the TMS (Figure 1B), to the nearest discharge point Westwater Seep, is approximately 2,895 years (HGC, 2022). As the nitrate plume (located upgradient of MW-23) is almost as distant from MW-23 as is MW-23 from Westwater Seep, the total travel time from the nitrate plume to Westwater Seep would be substantially greater than 2,895 years, yielding more than ample time for natural nitrate degradation to fully attenuate the plume before reaching a property boundary. 47 Nitrate Corrective Action Comprehensive Monitoring Evaluation (CACME) Report Https://Hgcinc.Sharepoint.Com/VOL4/718000/71807/NCACME2022/Report/NCACME2022_Final.Docx June 21, 2023 5. CONCLUSIONS AND RECOMMENDATIONS The following Sections detail the conclusions derived from data collected since the initiation of Phase II and Phase III pumping and recommendations regarding the continuation of Phase III. As will be discussed in Section 5.2, no changes to Phase III are recommended. 5.1 Conclusions Since the initiation of Phase II pumping during the first quarter of 2013, control of the nitrate plume has been maintained in accordance with the Phase II concentration criteria presented in the CAP. Phase II and Phase III pumping not only removes nitrate mass from the nitrate plume, but removes chloride mass from the commingled chloride plume, thus contributing to the reduction of mass within both plumes. Nitrate concentrations in the toe of the plume at MW-30 and MW-31 are stable; the nitrate plume has not expanded downgradient to MW-5 or MW-11; and quarterly residual mass estimates are trending downward. The plume area has generally increased since about the end of 2017 due to expansion of the kriged plume boundary toward the west; however average concentrations within the plume have generally decreased since the end of 2013; and, although larger now than at the end of 2017, the plume area has trended generally downward since the third quarter of 2020. In addition, the plume has contracted immediately to the south of TWN-2; this contraction is largely attributable to TWN-2 pumping. Comparing the second quarter, 2010 baseline mass estimate of 43,700 lb with the second quarter, 2022 mass estimate of approximately 31,933 lb (Table 1) suggests that the plume mass has decreased approximately 11,767 lb or nearly 27%. Based on the Figure 6 trendline, the plume mass has decreased by approximately 7,200 lb or 16%. The relative stability of the plume area through about the end of 2017 likely resulted from a combination of competing factors. Nitrate mass removal by pumping and naturally occurring nitrate degradation, which tend to reduce concentrations within the plume and shrink the plume boundaries, were counteracted by reduced dilution from the wildlife ponds, which tends to increase concentrations within the plume and expand the plume boundaries. The interaction of these two mechanisms (through about 2017) resulted in a plume that appeared to be in dynamic equilibrium with respect to area. However, these competing factors likely continue to impact the post-2017 plume, and account for generally decreasing mass and average concentrations within the plume over a time period during which the plume area has generally increased. 48 Nitrate Corrective Action Comprehensive Monitoring Evaluation (CACME) Report Https://Hgcinc.Sharepoint.Com/VOL4/718000/71807/NCACME2022/Report/NCACME2022_Final.Docx June 21, 2023 Nitrate and chloroform pumping continue to remove both nitrate and chloride mass from the commingled nitrate and chloride plumes and from areas east of the plumes, acting to reduce concentrations of these constituents in the groundwater. Natural attenuation also acts to reduce nitrate and chloride concentrations within the nitrate and chloride plumes and within areas east of the plumes. Natural attenuation mechanisms include dilution by recharge and hydrodynamic dispersion (which impact both nitrate and chloride concentrations) and nitrate mass removal via reduction by naturally occurring pyrite and/or organic material in the perched groundwater zone (which impacts only nitrate concentrations). Dilution, hydrodynamic dispersion, and nitrate mass removal via reduction by pyrite and/or organic material all act to reduce nitrate concentrations. Specifically, the combined impacts of perched groundwater flow, pumping and natural attenuation on the nitrate plume since the first quarter of 2013 include the following: 1. Pumping and natural attenuation have maintained control of the nitrate plume. Average nitrate concentrations within the plume have decreased since 2013; and the southern boundary of the plume remains between MW-30/MW-31 and MW-5/MW-11; 2. The relative stability of the kriged southern nitrate plume boundary; the stability of nitrate concentrations in the toe of the plume (at MW-30 and MW-31); and increasing chloride in the toe of the plume imply that: a. The commingled chloride plume is continuing to migrate downgradient to the south-southwest which is expected because this portion of the plume is beyond the hydraulic capture of the pumping wells (as anticipated and as discussed in the CAP); b. Nitrate is being degraded in the toe of the plume (at MW-30 and MW-31); otherwise concentrations would be increasing along with the chloride concentrations (the nitrate plume would be expanding to the south-southwest at about the same rate as the chloride plume); and nitrate to chloride concentration ratios would be stable rather than decreasing; and c. The nitrate degradation is consistent with nitrate reduction by naturally occurring pyrite and/or organic matter in the perched zone. 3. Increasing nitrate and chloride at TWN-7 and MW-28 (although second quarter, 2022 nitrate concentrations at MW-28 are only about 5 mg/L) are consistent with the apparent expansion of the western kriged nitrate plume boundary and continuing downgradient migration of nitrate and chloride. 4. Decreasing nitrate concentrations at TWN-2 (since 2013) and contraction of the plume immediately to the south of TWN-2 are attributable to mass removal by pumping and redistribution of nitrate within the plume. Redistribution (as anticipated in the CAP) appears to have caused the mass center to migrate from the area of TWN-2 to the area of TW4-22 and TW4-24. It is likely that TWN-2 was located within the upgradient portion of this mass center in the second quarter of 2010 and that TW4-22 and TW4-24 are now 49 Nitrate Corrective Action Comprehensive Monitoring Evaluation (CACME) Report Https://Hgcinc.Sharepoint.Com/VOL4/718000/71807/NCACME2022/Report/NCACME2022_Final.Docx June 21, 2023 within this mass center. Redistribution is consistent with changes in saturated thickness and continuing downgradient migration of nitrate within the plume (to the south-southwest) enhanced by pumping. 5. Decreasing saturated thicknesses within the majority of the plume have resulted in a decreasing plume volume which contributes to a decreasing trend in the quarterly residual plume mass estimates. 6. Hydrodynamic dispersion, mass removal by pumping and naturally occurring nitrate degradation, which tend to reduce concentrations in wells within the plume and shrink the plume boundaries, are partially counteracted by reduced dilution from the wildlife ponds, which tends to increase concentrations in wells within the plume and expand the plume boundaries. 7. The interaction of the above mechanisms apparently resulted in a plume that appeared to be in dynamic equilibrium with respect to area through about 2017. Subsequently, while plume mass and average concentrations generally declined, the plume area generally increased until the third quarter of 2020; then began to trend generally downward (Table 1). Although since 2013 average nitrate concentrations within the plume have generally declined, and concentrations at most wells within the plume are decreasing, concentrations at the remaining wells are stable or increasing. Overall, however, the diminishing impacts of reduced dilution by wildlife pond seepage, and continuing mass removal by pumping and reduction by naturally occurring pyrite and/or organic matter in the perched zone, are expected to eventually cause nitrate concentrations at more wells to decline. Under current conditions, nitrate concentration trends cannot be used to estimate a remediation time (at which all nitrate concentrations within the plume were reduced below 10 mg/L); however the time to reduce the plume mass to a negligible value can be estimated by projecting a calculated nitrate mass removal rate. Presumably, if the nitrate residual mass were reduced to a negligible value, concentrations would also be reduced to negligible values. The calculated mass removal rate would be based on an estimate of the nitrate mass removal via natural reduction and the measured mass removal via pumping. As discussed in Section 4.4, since the first quarter of 2013, the average nitrate mass removed only by pumping wells within and marginal to the plume is approximately 350 lb/yr. Averages of the estimated nitrate reduction rates resulting from calculations of natural nitrate reduction range from approximately 167 lb/yr to 442 lb/yr depending on the proportion of the plume to which the rate is assumed to be applicable. Thus the estimated total rate of mass reduction ranges from approximately 517 lb/yr to 792 lb/yr assuming pumping continues. Projecting these mass removal rates into the future, and assuming a zero order rate of natural reduction and a current nitrate plume residual mass of approximately 31,933 lb, (as calculated for the second quarter of 2022), implies that between 40 and 62 years would be required to reduce the 50 Nitrate Corrective Action Comprehensive Monitoring Evaluation (CACME) Report Https://Hgcinc.Sharepoint.Com/VOL4/718000/71807/NCACME2022/Report/NCACME2022_Final.Docx June 21, 2023 mass within the plume to a negligible value, and between approximately 72 and 191 years would be required via natural reduction alone. Because nitrate mass removal by pumping is likely to drop off in the future due to reduced nitrate concentrations and reduced saturated thicknesses (which will limit achievable pumping rates), the actual time, assuming pumping continues, will be more than 40 and less than approximately 200 years. However, because it is only necessary to reduce nitrate concentrations within the plume below 10 mg/L, and it will not be necessary to essentially remove all the nitrate mass to achieve this condition, the actual time to remediate the plume will be smaller than as calculated above. In addition, natural attenuation processes that include dilution by recharge and hydrodynamic dispersion will reduce nitrate concentrations within the plume and contribute to an additional reduction of the remediation time. As discussed in the Revised Phase III Planning Document, conservative ‘worst-case’ numerical flow and transport modeling indicates that hydrodynamic dispersion alone, without dilution, pumping or natural degradation of nitrate, is likely sufficient to reduce all concentrations within the plume to the GCAL of 10 mg/L before reaching a property boundary. Although the modeling was designed to evaluate long-term changes in the nitrate plume, the current nitrate distribution is generally consistent with model predictions near the southern boundary of the plume. Furthermore, expected continuing reductions in nitrate plume volume will also reduce remediation times. Disregarding hydrodynamic dispersion, in the event that pumping was substantially reduced or should cease, even in the near term, the nitrate plume would continue to diminish through natural attenuation processes that include nitrate mass removal via reduction by pyrite. As discussed in HGC (2014; 2018a; and 2022), the mass of naturally occurring pyrite in the perched zone within the anticipated downgradient path of the nitrate plume is two to three orders of magnitude larger than needed to degrade all of the nitrate to non-detectable levels before reaching a site property boundary or a discharge point. The estimated times to reduce the nitrate plume mass to a negligible value indicate that even under worst-case conditions of no pumping, and assuming no hydrodynamic dispersion or dilution, natural degradation of nitrate by pyrite is likely to reduce mass within the plume to a negligible value within less than 200 years. Because thousands of years would be required for nitrate within the plume to migrate to a discharge point (either Westwater Seep or Ruin Spring) based on calculations presented in HGC (2014; 2018a; and 2022), there is more than sufficient time to reduce mass within the plume to a negligible value before a discharge point is reached. 51 Nitrate Corrective Action Comprehensive Monitoring Evaluation (CACME) Report Https://Hgcinc.Sharepoint.Com/VOL4/718000/71807/NCACME2022/Report/NCACME2022_Final.Docx June 21, 2023 5.2 Recommended Changes to Phase III Phase III pumping is considered effective in maintaining the relative stability of the nitrate plume and in reducing plume attenuation times. Because flow and transport modeling using conservative ‘worst case’ assumptions (as discussed in the Revised Phase III Planning Document) indicates that active remediation by pumping is not needed to achieve full attenuation of the nitrate plume before reaching a property boundary, the purpose of Phase III pumping is primarily to reduce the time needed for full attenuation, after which time all nitrate concentrations associated with the plume will be below the GCAL of 10 mg/L. Pumping appears adequate at the present time based on estimates of background flow through the plume as discussed in Section 4.3. Estimates of nitrate mass removal by naturally occurring pyrite and/or organic matter in the perched zone suggest that nitrate mass removal by pumping is of the same order as these estimates and that pumping should continue. Although not technically part of the nitrate pumping system, the addition of chloroform pumping wells TW4-21 and TW4-37, which are typically and consistently within the nitrate plume, respectively, constitutes an enhancement to the nitrate pumping system even though these wells are pumped primarily to reduce chloroform mass. At the present time, no changes to the pumping system are recommended. In particular, potential pumping at the toe of the nitrate plume is considered undesirable. Nitrate pumping at TW4-22 and TW4-24 caused chloroform from the vicinity of TW4-20 to migrate to the west, thus expanding the chloroform plume. This measured expansion supports the likelihood that pumping in the toe of the nitrate plume, at MW-30 and MW-31, would induce undesirable downgradient migration of chloroform. Presumably, if the nitrate residual mass were reduced to a negligible value, concentrations would also be reduced to negligible values. Assuming that mass removal by pumping continues at the same average rate since initiation of Phase II, and using preliminary estimates of natural nitrate degradation by pyrite, between 40 and 62 years would be required to reduce the plume mass to a negligible value. In the absence of pumping, relying entirely on estimates of natural degradation by pyrite alone, between approximately 72 and approximately 191 years are estimated to be required. Because nitrate mass removal by pumping is likely to drop off in the future due to reduced nitrate concentrations and reduced saturated thicknesses (which will limit achievable pumping rates), the actual time, assuming pumping continues, will be more than 40 and less than approximately 200 years. However, because it is only necessary to reduce nitrate concentrations within the plume below 10 mg/L, and it will not be necessary to essentially remove all the nitrate mass via pyrite oxidation to 52 Nitrate Corrective Action Comprehensive Monitoring Evaluation (CACME) Report Https://Hgcinc.Sharepoint.Com/VOL4/718000/71807/NCACME2022/Report/NCACME2022_Final.Docx June 21, 2023 achieve this condition, the actual time to remediate the plume will be smaller than as estimated above. In addition, natural attenuation processes that include dilution by recharge and hydrodynamic dispersion will reduce nitrate concentrations within the plume and contribute to an additional reduction of the remediation time. As discussed above, conservative ‘worst-case’ numerical flow and transport modeling indicates that hydrodynamic dispersion alone, without dilution or mass removal by pumping or pyrite degradation, is likely sufficient to completely attenuate the nitrate plume before reaching a property boundary. Although the Phase III modeling was designed to evaluate long-term changes in the nitrate plume, the current nitrate distribution is generally consistent with model predictions near the southern boundary of the plume. Furthermore, expected continuing reductions in plume volume will also reduce remediation times. As a result, even if the plume migrated to MW-5 or MW-11, it would be expected to fully attenuate before reaching a property boundary, eliminating any potential risk of exposure. Although (as discussed in HGC, 2014; 2018; and 2022) there is more than sufficient pyrite in the perched zone to degrade all of the nitrate mass before reaching a property boundary or discharge point; and, as discussed in the Revised Phase III Planning Document, conservative numerical flow and transport modeling indicates that hydrodynamic dispersion alone will fully attenuate the plume before reaching a property boundary or discharge point, essentially eliminating any potential risk of exposure; continuation of Phase III pumping is recommended for now, as it significantly contributes to nitrate mass reduction and to the reduction in the volume of the plume. In addition, pumping helps to reduce hydraulic gradients, thereby slowing the rate of downgradient plume migration. 53 Nitrate Corrective Action Comprehensive Monitoring Evaluation (CACME) Report Https://Hgcinc.Sharepoint.Com/VOL4/718000/71807/NCACME2022/Report/NCACME2022_Final.Docx June 21, 2023 6. REFERENCES Aguerri, 2010. Denitrification With Pyrite for Bioremediation of Contaminated Groundwater. PHD Thesis. University of Barcelona. Bosch, Julian; Keun-Young Lee; Guntram Jordan; Kyoun-Woong Kim; and Rainer U. Meckenstock 2011. Anaeorobic, Nitrate dependent Oxidation of Pyrite Nanoparticles by Thiobacillus Denitrificans. Environmental Science and Technology, vol. 46, pp 2095-2101. Bosch, Julian and Rainer U. Meckenstock 2012. Rates and Potential Mechanism of Anaerobic Nitrate- Dependent Microbial Pyrite Oxidation. Biochemical Society Transactions, vol. 4, part 6, pp 1280- 1283. Energy Fuels Resources (USA) Inc. (EFRI). 2013b. White Mesa Uranium Mill Nitrate Monitoring Report, State of Utah Stipulated Consent Agreement, January, 2009. UDEQ Docket No. UGW09- 03, 1st Quarter (January through March) 2013. June 1, 2013. EFRI. 2013c. White Mesa Uranium Mill Nitrate Monitoring Report, State of Utah Stipulated Consent Agreement, January, 2009. UDEQ Docket No. UGW09-03,2nd Quarter (April through June) 2013. September 1, 2013. EFRI. 2013d. White Mesa Uranium Mill Nitrate Monitoring Report, State of Utah Stipulated Consent Agreement, January, 2009. UDEQ Docket No. UGW09-03, 3rd Quarter (July through September) 2013. December 1, 2013. EFRI. 2014a. White Mesa Uranium Mill Nitrate Monitoring Report, State of Utah Stipulated Consent Agreement, January, 2009. UDEQ Docket No. UGW09-03, 4th Quarter (October through December) 2013. March 1, 2014. EFRI. 2014b. White Mesa Uranium Mill Nitrate Monitoring Report, State of Utah Stipulated Consent Agreement, January, 2009. UDEQ Docket No. UGW09-03, 1st Quarter (January through March) 2014. June 1, 2014. EFRI. 2014c. White Mesa Uranium Mill Nitrate Monitoring Report, State of Utah Stipulated Consent Agreement, January, 2009. UDEQ Docket No. UGW09-03, 2nd Quarter (April through June) 2014. September 1, 2014. EFRI. 2014d. White Mesa Uranium Mill Nitrate Monitoring Report, State of Utah Stipulated Consent Agreement, January, 2009. UDEQ Docket No. UGW09-03, 3rd Quarter (July through September) 2014. December 1, 2014. EFRI. 2015a. White Mesa Uranium Mill Nitrate Monitoring Report, State of Utah Stipulated Consent Agreement, January, 2009. UDEQ Docket No. UGW09-03, 4th Quarter (October through December) 2014. March 1, 2015. EFRI. 2015b. White Mesa Uranium Mill Nitrate Monitoring Report, State of Utah Stipulated Consent Agreement, December 2014. UDEQ Docket No. UGW12-04, 1st Quarter (January through March) 2015. June 1, 2015. EFRI. 2015c. White Mesa Uranium Mill Nitrate Monitoring Report, State of Utah Stipulated Consent Agreement, December 2014. UDEQ Docket No. UGW12-04, 2nd Quarter (April through June) 2015. September 1, 2015. 54 Nitrate Corrective Action Comprehensive Monitoring Evaluation (CACME) Report Https://Hgcinc.Sharepoint.Com/VOL4/718000/71807/NCACME2022/Report/NCACME2022_Final.Docx June 21, 2023 EFRI. 2015d. White Mesa Uranium Mill Nitrate Monitoring Report, State of Utah Stipulated Consent Agreement, December 2014. UDEQ Docket No. UGW12-04, 3rd Quarter (July through September) 2015. December 1, 2015. EFRI. 2016a. White Mesa Uranium Mill Nitrate Monitoring Report, State of Utah Stipulated Consent Agreement, December 2014. UDEQ Docket No. UGW12-04, 4th Quarter (October through December) 2015. March 1, 2016. EFRI. 2016b. White Mesa Uranium Mill Nitrate Monitoring Report, State of Utah Stipulated Consent Agreement, December 2014. UDEQ Docket No. UGW12-04, 1st Quarter (January through March) 2016. June 1, 2016. EFRI. 2016c. White Mesa Uranium Mill Nitrate Monitoring Report, State of Utah Stipulated Consent Agreement, December 2014. UDEQ Docket No. UGW12-04, 2nd Quarter (April through June) 2016. September 1, 2016. EFRI. 2016d. White Mesa Uranium Mill Nitrate Monitoring Report, State of Utah Stipulated Consent Agreement, December 2014. UDEQ Docket No. UGW12-04, 3rd Quarter (July through September) 2016. December 1, 2016. EFRI. 2017a. White Mesa Uranium Mill Nitrate Monitoring Report, State of Utah Stipulated Consent Agreement, December 2014. UDEQ Docket No. UGW12-04, 4th Quarter (October through December) 2016. March 1, 2017. EFRI. 2017b. White Mesa Uranium Mill Nitrate Monitoring Report, State of Utah Stipulated Consent Agreement, December 2014. UDEQ Docket No. UGW12-04, 1st Quarter (January through March) 2017. June 1, 2017. EFRI. 2017c. White Mesa Uranium Mill Nitrate Monitoring Report, State of Utah Stipulated Consent Agreement, December 2014. UDEQ Docket No. UGW12-04, 2nd Quarter (April through June) 2017. September 1, 2017. EFRI. 2017d. White Mesa Uranium Mill Nitrate Monitoring Report, State of Utah Stipulated Consent Agreement, December 2014. UDEQ Docket No. UGW12-04, 3rd Quarter (July through September) 2017. December 1, 2017. EFRI. 2018a. White Mesa Uranium Mill Nitrate Monitoring Report, State of Utah Stipulated Consent Agreement, December 2014. UDEQ Docket No. UGW12-04, 4th Quarter (October through December) 2017. March 1, 2018. EFRI. 2018b. White Mesa Uranium Mill Nitrate Monitoring Report, State of Utah Stipulated Consent Agreement, December 2014. UDEQ Docket No. UGW12-04, 1st Quarter (January through March) 2018. June 1, 2018. EFRI. 2018c. White Mesa Uranium Mill Nitrate Monitoring Report, State of Utah Stipulated Consent Agreement, December 2014. UDEQ Docket No. UGW12-04, 2nd Quarter (April through June) 2018. September 1, 2018. EFRI. 2018d. White Mesa Uranium Mill Nitrate Monitoring Report, State of Utah Stipulated Consent Agreement, December 2014. UDEQ Docket No. UGW12-04, 3rd Quarter (July through September) 2018. December 1, 2018. EFRI. 2019a. White Mesa Uranium Mill Nitrate Monitoring Report, State of Utah Stipulated Consent Agreement, December 2014. UDEQ Docket No. UGW12-04, 4th Quarter (October through December) 2018. March 1, 2019. 55 Nitrate Corrective Action Comprehensive Monitoring Evaluation (CACME) Report Https://Hgcinc.Sharepoint.Com/VOL4/718000/71807/NCACME2022/Report/NCACME2022_Final.Docx June 21, 2023 EFRI. 2019b. White Mesa Uranium Mill Nitrate Monitoring Report, State of Utah Stipulated Consent Agreement, December 2014. UDEQ Docket No. UGW12-04, 1st Quarter (January through March) 2019. June 1, 2019. EFRI. 2019c. White Mesa Uranium Mill Nitrate Monitoring Report, State of Utah Stipulated Consent Agreement, December 2014. UDEQ Docket No. UGW12-04, 2nd Quarter (April through June) 2019. September 1, 2019. EFRI. 2019d. White Mesa Uranium Mill Nitrate Monitoring Report, State of Utah Stipulated Consent Agreement, December 2014. UDEQ Docket No. UGW12-04, 3rd Quarter (July through September) 2019. December 1, 2019. EFRI. 2020a. White Mesa Uranium Mill Nitrate Monitoring Report, State of Utah Stipulated Consent Agreement, December 2014. UDEQ Docket No. UGW12-04, 4th Quarter (October through December) 2019. March 1, 2020. EFRI. 2020b. White Mesa Uranium Mill Nitrate Monitoring Report, State of Utah Stipulated Consent Agreement, December 2014. UDEQ Docket No. UGW12-04, 1st Quarter (January through March) 2020. June 1, 2020. EFRI. 2020c. White Mesa Uranium Mill Nitrate Monitoring Report, State of Utah Stipulated Consent Agreement, December 2014. UDEQ Docket No. UGW12-04, 2nd Quarter (April through June) 2020. September 1, 2020. EFRI. 2020d. White Mesa Uranium Mill Nitrate Monitoring Report, State of Utah Stipulated Consent Agreement, December 2014. UDEQ Docket No. UGW12-04, 3rd Quarter (July through September) 2020. December 1, 2020. EFRI. 2021a. White Mesa Uranium Mill Nitrate Monitoring Report, State of Utah Stipulated Consent Agreement, December 2014. UDEQ Docket No. UGW12-04, 4th Quarter (October through December) 2020. March 1, 2021. EFRI. 2021b. White Mesa Uranium Mill Nitrate Monitoring Report, State of Utah Stipulated Consent Agreement, December 2014. UDEQ Docket No. UGW12-04, 1st Quarter (January through March) 2021. June 1, 2021. EFRI. 2021c. White Mesa Uranium Mill Nitrate Monitoring Report, State of Utah Stipulated Consent Agreement, December 2014. UDEQ Docket No. UGW12-04, 2nd Quarter (April through June) 2021. September 1, 2021. EFRI. 2021d. White Mesa Uranium Mill Nitrate Monitoring Report, State of Utah Stipulated Consent Agreement, December 2014. UDEQ Docket No. UGW12-04, 3rd Quarter (July through September) 2021. December 1, 2021. EFRI. 2022a. White Mesa Uranium Mill Nitrate Monitoring Report, State of Utah Stipulated Consent Agreement, December 2014. UDEQ Docket No. UGW12-04, 4th Quarter (October through December) 2021. March 1, 2022. EFRI. 2022b. White Mesa Uranium Mill Nitrate Monitoring Report, State of Utah Stipulated Consent Agreement, December 2014. UDEQ Docket No. UGW12-04, 1st Quarter (January through March) 2022. June 1, 2022. EFRI. 2022c. White Mesa Uranium Mill Nitrate Monitoring Report, State of Utah Stipulated Consent Agreement, December 2014. UDEQ Docket No. UGW12-04, 2nd Quarter (April through June) 2022. September 1, 2022. 56 Nitrate Corrective Action Comprehensive Monitoring Evaluation (CACME) Report Https://Hgcinc.Sharepoint.Com/VOL4/718000/71807/NCACME2022/Report/NCACME2022_Final.Docx June 21, 2023 Hartog, N., Griffionen, J., Van Bergen, P., and Van Der Weidjen, C. 2001. Determining The Reactivity of Reduced Components in Dutch Aquifer Sediments. Proceedings of a Symposium Held During the Sixth IAHS Scientific Assembly at Maastricht, the Netherlands, July 2001). Hydro Geo Chem, Inc. (HGC). 2005. Perched Monitoring Well Installation and Testing at the White Mesa Uranium Mill, April Through June 2005 (submitted August 3, 2005). HGC. 2007. Preliminary Corrective Action Plan. White Mesa Uranium Mill Site Near Blanding, Utah. August 20, 2007. HGC. 2009a. Perched Nitrate Monitoring Well Installation and Hydraulic Testing, White Mesa Uranium Mill (submitted March 10, 2009). HGC. 2009b. Letter Report to David Frydenlund, Esq, regarding installation and testing of TW4-23, TW4-24, and TW4-25 (submitted March 17, 2009). HGC. 2010. Hydrogeology of the Perched Groundwater Zone and Associated Seeps and Springs Near the White Mesa Uranium Mill Site, Blanding, Utah. HGC. 2011. Redevelopment of Existing Perched Monitoring Wells. White Mesa Uranium Mill Near Blanding, Utah. September 30, 2011. HGC. 2012a. Corrective Action Plan for Nitrate. White Mesa Uranium Mill Near Blanding, Utah. May 7, 2012. HGC. 2012b. Investigation of Pyrite in the Perched Zone. White Mesa Uranium Mill Site. Blanding, Utah. December 7, 2012. HGC 2014. Hydrogeology of the White Mesa Uranium Mill, Blanding, Utah. June 6, 2014. HGC. 2016. Corrective Action Comprehensive Monitoring Evaluation (CACME) Report, White Mesa Uranium Mill Near Blanding, Utah. March 31, 2016. HGC. 2017. Nitrate Corrective Action Comprehensive Monitoring Evaluation (CACME) Report, White Mesa Uranium Mill Near Blanding, Utah. December 11, 2017. HGC 2018a. 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, 2018b. Revised Phase III Nitrate Corrective Action Planning Document and Recommended Phase III Corrective Action, White Mesa Uranium Mill Near Blanding, Utah. December 13, 2018. HGC 2022. Hydrogeology of the White Mesa Uranium Mill, Blanding, Utah. July 13, 2022. INTERA. 2009a. Source Review Report for Nitrate and Chloride in Groundwater at the White Mesa Mill. INTERA. 2009b. Nitrate Contamination Investigation Report. White Mesa Uranium Mill Site, Blanding, Utah. December 30, 2009. INTERA. 2011. Nitrate Investigation Revised Phases 2 through 5 Work Plan Rev 2.0. White Mesa Mill Site, Blanding, Utah. August 18, 2011. Jiaoyang Pu; Chuanping Feng; Ying Liu; Rui Li; Zhe Kong; Nan Chen; Shuang Tong; Chunbo Hao; and Ye Liu 2014. Pyrite-based Autotrophic Denitrification for Remediation of Nitrate Contaminated Groundwater. Bioresource Technology, vol. 173, pp 117-123. 57 Nitrate Corrective Action Comprehensive Monitoring Evaluation (CACME) Report Https://Hgcinc.Sharepoint.Com/VOL4/718000/71807/NCACME2022/Report/NCACME2022_Final.Docx June 21, 2023 Jorgensen, Christian Juncker; Olestig Jacobsen; Bo Eberling; and Jens Aamand. 2009. Microbial Oxidation of Pyrite Coupled to Nitrate Reduction in Anoxic Groundwater Sediment. Environmental Science and Technology, vol. 43, pp 4851-4657. Kirby. 2008. Geologic and Hydrologic Characterization of the Dakota-Burro Canyon Aquifer Near Blanding, San Juan County, Utah. Utah Geological Survey Special Study 123. Kolle, W., P. Werner, O. Strebel, and J. Bottcher. 1983. Denitrification in a reducing aquifer. Vom Wasser 1983, 61, 125-147. Kolle, W., O. Strebel, and J. Bottcher. 1987. Reduced sulphur compounds in sandy aquifers and their interactions with groundwater. Proceedings of the Dresden Symposium of Groundwater Monitoring and Management, March, 1987. Korom, S.F. 1992. Natural denitrification in the saturated zone: A review. Water Resources Research, 1992, 28, 1657-1668. Krieger, Amanda M. 2014. Electron Donor Contributions to Denitrification in the Elk Valley Aquifer, North Dakota. M.S. Thesis, University of North Dakota. Pauwels, H., W. Kloppmann, J.C. Foucher, A. Martelat, and V. Fritsche. 1998. Field tracer test for denitrification in a pyrite-bearing schist aquifer. Applied Geochemistry, 1998, 13 (6), 767-778. Postma, D., C. Boesen, H. Kristiansen, and F. Larsen. 1991. Nitrate reduction in an unconfined sandy aquifer - water chemistry, reduction processes, and geochemical modeling. Water Resource Research, 1991, 27 (8), 2027-2045. Rivett, M.O., S.R. Buss, P. Morgan, J.W.N. Smith, and C.D. Bemment. 2008. Nitrate attenuation in groundwater: A review of biogeochemical controlling processes. Water Research, 2008, 42, 4215-4232. Robertson, W.D., B.M. Russel, and J.A. Cherry. 1996. Attenuation of nitrate in aquitard sediments of southern Ontario. Journal of Hydrology, 1996, 180 (1), 267-281 Schippers, A., and B.B. Jorgensen. 2002. Biogeochemistry of pyrite and iron sulfide oxidation in marine sediments. Geochimica et Cosmochimica Acta, 2002, 66 (1), 85-92. Spiteri, C., C.P. Slomp, K. Tuncay, and C. Meile. 2008. Modeling biogeochemical processes in subterranean estuaries: Effect of flow dynamics and redox conditions on submarine groundwater discharge of nutrients. Water Resources Research, 2008, 44, W02430. Tesoriero, A.J. and L.J. Puckett. 2011. O2 reduction and denitrification rates in shallow aquifers. Water Resources Research, 2011, 37, W12522. Torrento, Clara; Jordi Cama; Jordi Urmeneta; Neus Otero; and Albert Solerand 2010. Denitrification of groundwater with pyrite and Thiobacillus denitrificans. Chemical Geology, vol. 278, pp 80-91. US Environmental Protection Agency (USEPA), 2007. Monitored Natural Attenuation of Inorganic Contaminants in Ground Water. Volume 2: Assessment for Non-Radionuclides Including Arsenic, Cadmium, Chromium, Copper, Lead, Nickel, Nitrate, Perchlorate, and Selenium. United States Nuclear Regulatory Conmission. 1979. Environmental Statement Related to Operation of White Mesa Uranium Project, Energy Fuels Nuclear, Inc. May 1979. Utah Department of Environmental Quality Division of Waste Management and Radiation Control (DWMRC). 2012. Stipulation and Consent Order Docket No.UGW12-04. December 12, 2012. 58 Nitrate Corrective Action Comprehensive Monitoring Evaluation (CACME) Report Https://Hgcinc.Sharepoint.Com/VOL4/718000/71807/NCACME2022/Report/NCACME2022_Final.Docx June 21, 2023 van Beek, C.G.E.M. 1999. Redox Processes Active in Denitrification. Chapter in: Redox Fundamentals, Processes, and Applications, J. Schuring, H.D. Schulz, W.R. Fischer, J. Bottcher, and W.H.M. Duijnisveld, eds. Springer-Verlag New York, 1999. Zhang, Yan Chung; Caroline P. Slomp; Hans Peter Broers; Hilde F. Passier; and Philippe Van Cappellen 2009. Denitrification coupled to pyrite oxidation and changes in groundwater quality in a shallow sandy aquifer. Geochimica et Cosmochimica Acta, vol. 73, pp 6716-6726. Zhang, Y. 2012. Coupled biogeochemical dynamics of nitrogen and sulfur in a sandy aquifer and implications for groundwater quality. Thesis presented at Utrecht University, Netherlands, November 19, 2012. TABLES TABLE 1 Quarterly Nitrate Plume Area, Mass Pumped, Residual Mass, and Average Concentrations During Phase II and Phase III (and including Q2 2010 and Q4 2012 data) Number Plume 1Total Mass Residual 2Average Nitrate 3Average Nitrate 4Average Nitrate of Plume Area Pumped/Quarter Plume Mass Concentration Concentration Concentration Quarter Wells (m2) (lb) (lb) (mg/L) (mg/L) (mg/L) Q2 2010 8 2.77E+05 NA 43700 26.3 30.4 21.2 Q4 2012 9 2.60E+05 NA 33845 18.6 20.9 18.4 Q1 2013 7 2.74E+05 89.2 41350 32.3 35.7 22.2 Q2 2013 7 2.67E+05 85.3 34140 30.7 33.6 19.8 Q3 2013 8 2.80E+05 169.3 36930 28.1 33.8 20 Q4 2013 6 2.89E+05 154.8 41150 43 43.0 23.1 Q1 2014 7 2.64E+05 96.4 31410 28.4 31.2 19.3 Q2 2014 7 2.57E+05 96.2 30620 29 32.0 19.6 Q3 2014 6 2.29E+05 87.5 24140 27.3 27.3 17.6 Q4 2014 7 2.77E+05 102.0 34370 32.5 36.2 21 Q1 2015 7 2.87E+05 72.8 38740 30.9 34.2 21.7 Q2 2015 8 2.73E+05 61.4 33042 29.2 31.4 19.9 Q3 2015 10 2.92E+05 109.1 34880 26.8 32.3 19.6 Q4 2015 9 2.65E+05 116.1 30980 26.8 30.9 19.6 Q1 2016 10 2.76E+05 124.0 33083 27.1 32.9 19.8 Q2 2016 9 2.56E+05 91.3 28465 25.5 29.3 18.5 Q3 2016 10 2.79E+05 93.1 32230 25.7 31.3 20.4 Q4 2016 13 2.90E+05 98.7 31798 22 29.1 18.7 Q1 2017 12 3.15E+05 104.3 43787 24.6 32.4 23.3 Q2 2017 8 2.68E+05 71.6 32145 28.7 31.2 20.4 Q3 2017 10 2.74E+05 84.1 32939 25.4 30.7 20.6 Q4 2017 9 2.81E+05 97.2 31501 28.1 32.4 20 Q1 2018 8 2.89E+05 103.8 33616 27.9 29.7 21.1 Q2 2018 10 2.84E+05 72.6 31257 24.2 29.6 19.3 Q3 2018 8 2.45E+05 51.8 25568 25.2 27.2 19.3 Q4 2018 9 3.13E+05 88.8 28805 27 31.0 20 Q1 2019 8 3.33E+05 91.2 29509 28.9 31.1 19.4 Q2 2019 8 3.39E+05 89.0 31455 31.3 35.8 21 Q3 2019 10 3.22E+05 69.2 30976 27.0 31.0 20.6 Q4 2019 8 3.20E+05 67.5 29870 26.0 27.7 19.8 Q1 2020 9 3.20E+05 75.9 32740 24.1 27.6 19.6 Q2 2020 9 3.13E+05 74.5 30467 25.9 29.0 19.5 Q3 2020 10 3.46E+05 73.0 35525 25.0 30.1 20.5 Q4 2020 9 2.81E+05 77.9 25875 24.0 26.4 18 Q1 2021 9 3.37E+05 80.2 35052 27.1 30.3 20.4 Q2 2021 9 3.25E+05 78.1 34143 29.3 32.3 20.5 Q3 2021 9 3.11E+05 51.1 28932 21.6 23.8 18.4 Q4 2021 9 2.51E+05 89.0 28290 26.0 29.3 21.0 Q1 2022 8 2.88E+05 47.1 27146 24.6 25.6 18.7 Q2 2022 8 3.04E+05 43.8 31933 26.1 27.6 20.9 Notes: 1 = from wells within and along plume margin 2 = average of concentrations in wells within plume 3 = average of concentrations in wells consistently within plume 4 = average concentrations based on gridded data (weighted average) lb = pounds mg/L = milligrams per liter m2 = square meters NA = not applicable H:\718000\71807\NCACME2022\NCACME22_NandWL_data_1Q2013_2Q2022.xlsx: Table 1 (mass and C) TABLE 2 Nitrate Mass Pumped During Phase II and Phase III Quarter all wells (lb) plume wells only (lb) plume and plume margin wells (lb) Q1 2013 96 66 89 Q2 2013 92 72 85 Q3 2013 177 161 169 Q4 2013 162 131 155 Q1 2014 103 89 96 Q2 2014 102 91 96 Q3 2014 93 81 87 Q4 2014 109 92 102 Q1 2015 83 59 73 Q2 2015 69 59 61 Q3 2015 119 106 109 Q4 2015 125 114 116 Q1 2016 133 123 124 Q2 2016 100 89 91 Q3 2016 101 91 93 Q4 2016 106 98 99 Q1 2017 116 104 104 Q2 2017 80 70 72 Q3 2017 93 82 84 Q4 2017 106 94 97 Q1 2018 112 93 104 Q2 2018 84 71 73 Q3 2018 62 48 52 Q4 2018 98 87 89 Q1 2019 101 74 91 Q2 2019 102 80 89 Q3 2019 80 68 69 Q4 2019 77 60 67 Q1 2020 87 67 76 Q2 2020 86 66.5 74.5 Q3 2020 82 72.3 73 Q4 2020 89 75.4 77.9 Q1 2021 88.2 70.5 80.2 Q2 2021 91.1 71.9 78.1 Q3 2021 56.8 41.8 51.1 Q4 2021 97.3 88.3 89.0 Q1 2022 54.8 42.4 47.1 Q2 2022 51.6 34.8 43.8 Totals (lb) 3663 3083 3329 Note: lb = pounds H:\718000\71807\NCACME2022\NCACME22_NandWL_data_1Q2013_2Q2022.xlsx: Table2 (mass pumped) TABLE 3 Summary of Nitrate Degradation Rates Source Type Pyrite Species Pyrite Weight % Pyrite oxidation rate (µM/h NO3-) Pyrite oxidation rate (NO3-N lbs/ft3/yr) Torrento et al. (2010) Incubation Crystals of 25-100 µm 99.5 2.04 1.56E-02 Bosch et al. (2012) Incubation Nanoparticles of ~1 µm 100 38.56 2.95E-01 Jorgensen et al. (2009) Columns Crystals of 45-200 µm amended in sediment 1.0 0.05 3.83E-04 Torrento et al. (2010) Columns Crystals of 25-100 µm amended in sediment 99.5 4.67 3.58E-02 Zhang et al. (2009) Field study pyritic sands < 0.1 to 0.85 0.07 5.36E-04 White Mesa XRD Analysis Field study pyritic sands < 0.1 to 0.8 a5.4e-6 to 6.4e-6 Notes: µM/h NO3- = micromoles per liter nitrate per hour NO3-N lbs/ft3/yr = pounds per cubic foot per year nitarte as nitrogen a =average based on HGC (2017) H:\718000\71807\NCACME2022\NCACME2022_T3_T4_rate_summary_rev2.xlsx: Table 3 - rates TABLE 4 Pyrite Contents in Samples From White Mesa Mill and Oostrum, Netherlands Site White Mesa Uranium Mill site Oostrum, Netherlands site well depth (ft) Mill wt% pyrite (XRD) 1Mill 'equiv' wt% pyrite depth (m) depth (ft) 2Oostrum wt% pyrite MW-3A 89.5 0.1 0.3 5.1 16.73 0 MW-23 108 0 0.3 5.2 17.06 0.01 MW-24 118.5 0.8 1.2 5.4 17.72 0.01 MW-25 66.25 0 0.1 7 22.97 0.01 MW-26 91.25 0.3 0.2 9.1 29.86 0 MW-27 81.25 0.4 0.3 9.3 30.51 0.01 MW-28 88.5 0.2 0.1 15.2 49.87 0.09 MW-29 102 0 0.1 19 62.34 0.85 MW-30 66.25 0 0 21.7 71.19 0.25 MW-31 96.25 0 0 23.3 76.44 0.49 MW-32 106.25 0.5 0.5 23.3 76.44 NA 25.8 84.65 0.37 27.5 90.22 0.29 29.2 95.80 0.09 31.2 102.36 0.08 33.2 108.92 0.19 35.3 115.81 0.09 36.9 121.06 0.38 37.2 122.05 NA 39.1 128.28 0.17 average 0.21 0.28 0.28 (pyritic depths only) Notes: XRD = X-ray diffraction 1 = Based on total iron and sulfur contents 2 = Based on total sulfur content 0 = not detected (< 0.1%) m = meters ft = feet H:\718000\71807\NCACME2022\NCACME2022_T3_T4_rate_summary_rev2.xlsx: Table 4 - pyrite content TABLE 5 Slug Test Results (Using KGS Solution and Automatically Logged Data) Well K (cm/s) K (ft/day) MW-30 1.0E-04 0.28 MW-31 7.1E-05 0.20 TW4-22 1.3E-04 0.36 TW4-24 1.6E-04 0.45 TW4-25 5.8E-05 0.16 TWN-2 1.5E-05 0.042 TWN-3 8.6E-06 0.024 Average 1 0.22 Average 2 0.15 Average 3 0.32 Average 4 0.31 Notes: Average 1 = arithemetic average of all wells Average 2 = geometric average of all wells Average 3 = arithemetic average of MW-30, MW-31, TW4-22, and TW4-24 Average 4 = geometric average of MW-30, MW-31, TW4-22, and TW4-24 cm/s = centimeters per second ft/day = feet per day K = hydraulic conductivity KGS = KGS Unconfined Slug Test Solution in AqtesolveTM. H:\718000\71807\NCACME2022\NCACME2022_T5_T6_T7_T8.xls: Table 5 TABLE 6 Pre-Pumping Saturated Thicknesses Depth to Depth to Water Saturated Thickness Well Brushy Basin Fourth Quarter, 2012 Above Brushy Basin (ft) (ft) (ft) TW4-22 112 53 58 TW4-24 110 55 55 Notes: ft = feet H:\718000\71807\NCACME2022\NCACME2022_T5_T6_T7_T8.xls: Table 6 TABLE 7 Pre-Pumping Hydraulic Gradients and Flow Calculations Path Length Head Change Hydraulic Gradient (ft) (ft) (ft/ft) TW4-25 to MW-31 2060 48 0.023 TWN-2 to MW-30 2450 67 0.027 average 0.025 1 min flow (gpm)1.31 2 max flow (gpm)2.79 Notes: ft = feet ft/ft = feet per foot gpm = gallons per minute 1 assumes width = 1,200 ft; saturated thickness = 56 ft; K = 0.15 ft/day; and gradient = 0.025 ft/ft 2 assumes width = 1,200 ft; saturated thickness = 56 ft; K = 0.32 ft/day; and gradient = 0.025 ft/ft Pathline Boundaries H:\718000\71807\NCACME2022\NCACME2022_T5_T6_T7_T8.xls: Table 7 Table 8 Summary of 'Background' Flow Estimates minimum maximum (gpm) (gpm) pre-pumping (Q2, 2010)1.31 2.79 Q2 2015 re-calculation 0.79 1.67 Q2 2022 re-calculation 0.63 1.34 Notes: Q2 = second quarter gpm = gallons per minute 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 TW4-34 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 TW4-39MW-24A abandoned abandoned abandoned abandoned abandoned abandoned abandoned abandoned abandoned abnd wildlife pond wildlife pond wildlife pond TW4-43 MW-41 EXPLANATION perched monitoring well perched piezometer seep or spring WHITE MESA SITE PLAN SHOWING LOCATIONS OF PERCHED WELLS AND PIEZOMETERS 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 MW-38 perched monitoring well installed February 2018 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-41 perched monitoring well installed July, 2022 1AH:/718000/70807/ NCACME2022/maps/WL/Uwelloc0922_rev.srf HYDRO GEO CHEM, INC. EXPLANATION perched monitoring well perched piezometer seep or spring showing elevation in feet amsl MW-5 PIEZ-1 RUIN SPRING temporary perched monitoring well temporary perched nitrate monitoring well TW4-12 TWN-7 5380 MW-38 TW4-42 temporary perched nitrate monitoring well installed April, 2021 temporary perched monitoring well installed September, 2021 TW4-43 TWN-20 WHITE MESA SITE PLAN SHOWING 2nd QUARTER 2022 PERCHED WATER LEVELS AND KRIGED NITRATE, CHLORIDE AND CHLOROFORM PLUMES H:/718000/70807/ NCACME2022/maps/WL/UwlNClChl0622.srf 1B Q2 2022 chloroform plume boundary Q2 2022 chloride plume boundary Q2 2022 nitrate plume boundary approximate footprint of historical pond HYDRO GEO CHEM, INC. EXPLANATION temporary perched monitoring well showing concentration in mg/L temporary perched nitrate monitoring well showing concentration in mg/L perched piezometer (not sampled) TW4-7 TWN-1 PIEZ-1 5.9 2 perched monitoring well showing concentration in mg/L MW-32 ND MW-38 14 1.1 0.24 TW4-43 temporary perched monitoring well installed September, 2021 showing concentration in mg/L KRIGED 2nd QUARTER, 2022 NITRATE (mg/L) (NITRATE + NITRITE AS N) WHITE MESA SITE nitrate plume area HYDRO GEO CHEM, INC. KRIGED 2nd QUARTER, 2022 CHLORIDE (mg/L) WHITE MESA SITE EXPLANATION temporary perched monitoring well showing concentration in mg/L temporary perched nitrate monitoring well showing concentration in mg/L perched piezometer (not sampled) TW4-7 TWN-1 PIEZ-1 70 perched monitoring well showing concentration in mg/L MW-32 27 MW-38 40 40 42 TW4-43 temporary perched monitoring well installed September, 2021 showing concentration in mg/L 3 chloride plume area HYDRO GEO CHEM, INC. 5 5 2 0 5 5 2 5 5 5 3 0 5530 5 5 3 5 5560 5 5 6 5 5 5 7 0 5 5 7 5 5 5 8 0 5 5 8 5 EXPLANATION perched monitoring well temporary perched nitrate (TWN-series) or chloroform (TW4-series) monitoring well MW-25 TWN-3 CHANGE IN NITRATE PLUME BOUNDARY Q2 2010 TO Q2 2022 SHOWING Q2 2022 KRIGED PERCHED WATER LEVELS (detail map) temporary perched monitoring well installed April, 2019 TW4-42 Q2 2022 kriged nitrate plume boundary Q2 2010 kriged nitrate plume boundary 5520 Q2 2022 kriged perched water elevation 4 approximate footprint of historic pond H:\718000\71807\NCACME2022\NCACME22_NandWL_data_1Q2013_2Q2022.xlsx: F5a MW30 31 N Cl 0 50 100 150 200 250 300 350 400 450 Q2 10 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 co n c e n t r a t i o n ( m g / L ) dateMW-30 N MW-31 N MW-30 Cl MW-31 Cl Linear (MW-30 Cl)Linear (MW-31 Cl) NITRATE AND CHLORIDE CONCENTRATIONS (BEGINNING WITH Q2 2010 BASELINE) IN MW-30 AND MW-31 HYDRO GEO CHEM, INC.Approved FigureDateAuthorDate File Name SJS 5AF5a MW30 31 N ClSJS H:\718000\71807\NCACME2022\NCACME22_NandWL_data_1Q2013_2Q2022.xlsx: F5b MW30 31 NtoCl 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20 Q2 10 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 co n c e n t r a t i o n r a t i o date MW-30 N/Cl MW-31 N/Cl NITRATE TO CHLORIDE RATIOS (BEGINNING WITH Q2 2010 BASELINE) IN MW-30 AND MW-31 HYDRO GEO CHEM, INC.Approved FigureDateAuthorDate File Name SJS 5BF5b MW30 31 NtoClSJS H:\718000\71807\NCACME2022\NCACME22_NandWL_data_1Q2013_2Q2022.xlsx: F6 mass 0 5000 10000 15000 20000 25000 30000 35000 40000 45000 50000 re s i d u a l n i t r a t e a s N m a s s ( l b ) mass Linear (mass) RESIDUAL NITRATE PLUME MASS (INCLUDES BASELINE AND Q4 2012 MASS ESTIMATES) HYDRO GEO CHEM, INC.Approved FigureDateAuthorDate File Name SJS 6F6 MassSJS date HYDRO GEO CHEM, INC. 5 5 5 5 5 5 5 5 10 10 10 1 0 15 15 1 5 15 20 20 3 0 30 KRIGED 3rd QUARTER, 2015 NITRATE (mg/L) (NITRATE + NITRITE AS N) WHITE MESA SITE H:/718000/71807/ NCACME2022/maps/Unt0915rev2.srf EXPLANATION perched monitoring well showing concentration in mg/L temporary perched monitoring well showing concentration in mg/L temporary perched nitrate monitoring well showing concentration in mg/L perched piezometer showing concentration in mg/L temporary perched monitoring well installed May, 2014 showing concentration in mg/L MW-32 TW4-7 TWN-1 PIEZ-1 TW4-35 0.1 4.7 0.62 5 0.3 NS = not sampled; ND = not detected 10 kriged nitrate isocon and label 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; wells installed after Q3 2015 (TW4-38 through TW4-43; and TWN-20 and TWN-21) not shown TW4-37 perched pumping well installed March, 2015 showing concentration in mg/L32.4 nitrate plume area 7A HYDRO GEO CHEM, INC. 10 10 10 10 10 15 15 1 5 20 20 20 30 KRIGED 4th QUARTER, 2016 NITRATE (mg/L) (NITRATE + NITRITE AS N) WHITE MESA SITE EXPLANATION perched monitoring well showing concentration in mg/L temporary perched monitoring well showing concentration in mg/L temporary perched nitrate monitoring well showing concentration in mg/L perched piezometer showing concentration in mg/L MW-32 TW4-7 TWN-1 PIEZ-1 ND 4.3 2.0 6.4 NS = not sampled; ND = not detected; NA = not applicable 10 kriged nitrate isocon and label PIEZ-3A 8.4 May, 2016 replacement of perched piezometer Piez-03 showing concentration in mg/L TW4-38 11 temporary perched monitoring well installed October, 2016 showing concentration in mg/L H:/718000/71807/ NCACME2022/maps/Unt1216rev2.srf 7B nitrate plume area H:\718000\71807\NCACME2022\NCACME22_NandWL_data_1Q2013_2Q2022.xlsx: F8 N East wells 0 5 10 15 20 25 30 35 40 45 50 Q2 10 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 ni t r a t e a s N c o n c e n t r a t i o n ( m g / L ) dateTW4-10 TW4-16 TW4-18 TW4-19 TW4-20 TW4-39 MW-26 TWN-1 TWN-4 NITRATE CONCENTRATIONS (BEGINNING WITH Q2 2010 BASELINE) IN WELLS TYPICALLY EAST OF PLUME HYDRO GEO CHEM, INC.Approved FigureDateAuthorDate File Name SJS 8F8 N East WellsSJS H:\718000\71807\NCACME2022\NCACME22_NandWL_data_1Q2013_2Q2022.xlsx: F9 N Cl west wells 0.1 1 10 100 1000 Q2 10 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 co n c e n t r a t i o n ( m g / L ) dateMW-27 N MW-28 N TWN-7 N MW-27 Cl MW-28 Cl TWN-7 Cl NITRATE AND CHLORIDE CONCENTRATIONS (BEGINNING WITH Q2 2010 BASELINE) IN WELLS ORIGINALLY WEST OF PLUME (note that TWN-7 is now within plume) HYDRO GEO CHEM, INC.Approved FigureDateAuthorDate File Name SJS 9F9 N&Cl West WellsSJS H:\718000\71807\NCACME2022\NCACME22_NandWL_data_1Q2013_2Q2022.xlsx: F10a NtoCl W wells 0.00 0.05 0.10 0.15 0.20 0.25 0.30 Q2 10 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 co n c e n t r a t i o n r a t i o dateMW-27 N/Cl MW-28 N/Cl TWN-7 N/Cl NITRATE TO CHLORIDE RATIOS (BEGINNING WITH Q2 2010 BASELINE) IN WELLS ORIGINALLY WEST OF PLUME (note that TWN-7 is now within plume) HYDRO GEO CHEM, INC.Approved FigureDateAuthorDate File Name SJS 10AF10 NtoCL W WellsSJS https://hgcinc.sharepoint.com/VOL4/718000/71807/NCACME2022/NCACME22_NandWL_data_1Q2013_2Q2022_r1.xlsx: f10B NtoCl all, TWN-7 0.00 0.05 0.10 0.15 0.20 0.25 Q2 10 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 co n c e n t r a t i o n r a t i o ( N / C l ) date TWN-7 N/Cl Plume wells average N/Cl Linear (TWN-7 N/Cl) Linear (Plume wells average N/Cl) AVERAGE PLUME NITRATE TO CHLORIDE RATIOS BASED ON WELLS CONSISTENTLY WITHIN NITRATE PLUME AND NITRATE TO CHLORIDE RATIOS IN TWN-7 (BEGINNING WITH Q2 2010 BASELINE) HYDRO GEO CHEM, INC.Approved FigureDateAuthorDate File Name SJS F10B N/Cl PLM &TWN-7 r1 10BSJS HYDRO GEO CHEM, INC. 5 5 2 0 5 5 2 5 5 5 3 0 5530 5 5 3 5 5560 5 5 6 5 5 5 7 0 5 5 7 5 5 5 8 0 5 5 8 5 EXPLANATION perched monitoring well temporary perched nitrate (TWN-series) or chloroform (TW4-series) monitoring well MW-25 TWN-3 CHANGE IN NITRATE AND CHLORIDE PLUME BOUNDARIES Q2 2010 TO Q2 2022 SHOWING Q2 2022 KRIGED PERCHED WATER LEVELS (detail map) temporary perched nitrate monitoring well installed April, 2021 TWN-20 Q2 2022 kriged nitrate plume boundary Q2 2010 kriged nitrate plume boundary 5 5 2 0 Q2 2022 kriged perched water elevation 11 A Q2 2010 kriged chloride plume boundary Q2 2022 kriged chloride plume boundary approximate footprint of historic pond HYDRO GEO CHEM, INC. 5 5 1 5 5 5 2 0 5 5 2 5 5 5 3 0 5 5 3 5 5 5 4 0 5 5 4 0 5 5 4 5 5 5 5 5 5555 5 5 6 0 5570 5575 5 5 8 0 5 5 8 5 5 5 9 0 5595 5600 EXPLANATION perched monitoring well temporary perched nitrate (TWN-series) or chloroform (TW4-series) monitoring well MW-25 TWN-3 CHANGE IN NITRATE AND CHLORIDE PLUME BOUNDARIES Q2 2010 TO Q2 2022 SHOWING Q2 2010 KRIGED PERCHED WATER LEVELS (detail map) temporary perched nitrate monitoring well installed April, 2021 TWN-20 Q2 2022 kriged nitrate plume boundary Q2 2010 kriged nitrate plume boundary 5 5 2 0 Q2 2010 kriged perched water elevation 11 B Q2 2010 kriged chloride plume boundary Q2 2022 kriged chloride plume boundary approximate footprint of historic pond H:\718000\71807\NCACME2022\NCACME22_NandWL_data_1Q2013_2Q2022.xlsx: F12a WL pl wells 2 5520 5530 5540 5550 5560 5570 5580 5590 5600 5610 5620 Q2 10 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 wa t e r l e v e l e l v a t i o n ( f e e t a m s l ) date TW4-21 TW4-22 TW4-24 TW4-25 TWN-2 TWN-3 MW-30 MW-31 WATER LEVELS IN WELLS (BEGINNING WITH Q2 2010 BASELINE) ORIGINALLY WITHIN PLUME HYDRO GEO CHEM, INC.Approved FigureDateAuthorDate File Name SJS 12AF12A WL pl wells 2SJS H:\718000\71807\NCACME2022\NCACME22_NandWL_data_1Q2013_2Q2022.xlsx: F12b sat pl wells 2 10 20 30 40 50 60 70 80 90 Q2 10 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 sa t u r a t e d t h i c k n e s s ( f e e t ) date TW4-21 TW4-22 TW4-24 TW4-25 TWN-2 TWN-3 MW-30 MW-31 SATURATED THICKNESS IN WELLS (BEGINNING WITH Q2 2010 BASELINE) ORIGINALLY WITHIN PLUME HYDRO GEO CHEM, INC.Approved FigureDateAuthorDate File Name SJS 12BF12B sat pl wells 2SJS 5540 5550 5560 5570 5580 5590 5600 5610 5620 Q2 10 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 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 ) date TWN-7 TWN-3 WATER LEVEL ELEVATIONS (BEGINNING WITH Q2 2010 BASELINE) IN TWN-3 and TWN-7 HYDRO GEO CHEM, INC.Approved FigureDateAuthorDate File Name SJS 12CF12c WL TWN3&TWN7 SJS HYDRO GEO CHEM, INC. EXPLANATION perched monitoring well temporary perched nitrate (TWN-series) or chloroform (TW4-series) monitoring well MW-25 TWN-3 temporary perched nitrate monitoring well installed April, 2021 TWN-20 5 5 2 0 Q2 2022 kriged perched water elevation 13 Q2 2022 kriged nitrate plume boundary change in saturated thickness (feet) -45 -35 -25 -20 -15 -10 -5 0 1.25 2.5 CHANGE IN SATURATED THICKNESS Q2 2010 TO Q2 2022 WHITE MESA SITE (detail map) HYDRO GEO CHEM, INC. EXPLANATION perched monitoring well temporary perched nitrate (TWN-series) or chloroform (TW4-series) monitoring well MW-25 TWN-3 temporary perched nitrate monitoring well installed April, 2021 TWN-20 5 5 2 0 Q2 2022 kriged perched water elevation 14 Q2 2022 kriged nitrate plume boundary -60 -50 -40 -30 -25 -20 -15 -10 -5 0 5 10 25 PERCENT CHANGE IN SATURATED THICKNESS Q2 2010 TO Q2 2022 WHITE MESA SITE (detail map) percent change in saturated thickness H:\718000\71807\NCACME2022\NCACME22_NandWL_data_1Q2013_2Q2022.xlsx: F15 N orig plume wells 2 0 20 40 60 80 100 120 Q2 10 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 ni t r a t e a s N c o n c e n t r a t i o n ( m g / L ) date TW4-21 TW4-22 TW4-24 TW4-25 TWN-2 TWN-3 MW-30 MW-31 NITRATE CONCENTRATIONS (BEGINNING WITH Q2 2010 BASELINE) IN WELLS ORIGINALLY WITHIN PLUME HYDRO GEO CHEM, INC.Approved FigureDateAuthorDate File Name SJS 15F15 N orig plume wells 2SJS H:\718000\71807\NCACME2022\NCACME22_NandWL_data_1Q2013_2Q2022.xlsx: F16 N consistent plm wells 0 20 40 60 80 100 120 Q2 10 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 ni t r a t e a s N c o n c e n t r a t i o n ( m g / L ) date TW4-22 TW4-24 TW4-37 TWN-2 TWN-3 MW-30 MW-31 NITRATE CONCENTRATIONS (BEGINNING WITH Q2 2010 BASELINE) IN WELLS CONSISTENTLY WITHIN PLUME HYDRO GEO CHEM, INC.Approved FigureDateAuthorDate File Name SJS 16F16 N consist plm wellsSJS HYDRO GEO CHEM, INC. 5 5 1 5 5 5 2 0 5 5 2 5 5 5 3 0 5 5 3 5 5 5 4 0 5 5 4 0 5 5 4 5 5 5 5 5 5555 5 5 6 0 5570 5575 5 5 8 0 5 5 8 5 5 5 9 0 55 9 5 5600 EXPLANATION perched monitoring well temporary perched nitrate (TWN-series) or chloroform (TW4-series) monitoring well MW-25 TWN-3 temporary perched nitrate monitoring well installed April, 2021 TWN-20 5 5 2 0 Q2 2010 kriged perched water elevation 17 Q2 2022 kriged nitrate plume boundary Q2 2010 NITRATE PLUME MASS WHITE MESA SITE (detail map) 1 10 20 30 40 50 60 70 80 90 100 grid cell nitrate mass (lb) grid cell HYDRO GEO CHEM, INC. 5 5 2 0 5 5 2 5 5 5 3 0 5530 5 5 3 5 5560 5 5 6 5 5 5 7 0 5 5 7 5 5 5 8 0 5 5 8 5 EXPLANATION perched monitoring well temporary perched nitrate (TWN-series) or chloroform (TW4-series) monitoring well MW-25 TWN-3 temporary perched nitrate monitoring well installed April, 2021 TWN-20 5 5 2 0 Q2 2022 kriged perched water elevation 18 Q2 2022 kriged nitrate plume boundary Q2 2022 NITRATE PLUME MASS WHITE MESA SITE (detail map) 1 10 20 30 40 50 60 70 80 90 grid cell nitrate mass (lb) grid cell H:\718000\71807\NCACME2022\NCACME22_NandWL_data_1Q2013_2Q2022.xlsx: F 19 10 15 20 25 30 35 40 45 ni t r a t e a s N c o n c e n t r a t i o n ( m g / L ) date based on all wells within plume quarter by quarter based only on the 7 wells consistently within plume AVERAGE PLUME NITRATE CONCENTRATIONS BASED ON CONCENTRATIONS IN WELLS WITHIN THE PLUME HYDRO GEO CHEM, INC.Approved FigureDateAuthorDate File Name SJS 19F19 avg N plume wellsSJS H:\718000\71807\NCACME2022\NCACME22_NandWL_data_1Q2013_2Q2022.xlsx: F 20 10 15 20 25 30 35 40 45 ni t r a t e a s N c o n c e n t r a t i o n ( m g / L ) date based on average plume grid cell concentration AVERAGE PLUME NITRATE CONCENTRATIONS BASED ON GRIDING (KRIGING) HYDRO GEO CHEM, INC.Approved FigureDateAuthorDate File Name SJS 20F20 avg N gridded concSJS HYDRO GEO CHEM, INC. EXPLANATION perched monitoring well temporary perched nitrate (TWN-series) or chloroform (TW4-series) monitoring well MW-25 TWN-3 temporary perched nitrate monitoring well installed April, 2021 TWN-20 Q2 2022 kriged nitrate plume boundary 21 TOTAL ESTIMATED PUMPING CAPTURE AND AVERAGE (Q3 21 THROUGH Q2 22) AND Q2 2022 NITRATE PLUME BOUNDARIES (detail map) calculated average kriged nitrate plume footprint from Q3 2021 through Q2 2022 Q2 2022 total estimated capture Q1 2022 total estimated capture Q4 2021 total estimated capture Q3 2021 total estimated capture HYDRO GEO CHEM, INC. EXPLANATION temporary perched monitoring well temporary perched nitrate monitoring well perched piezometer (not sampled) TW4-7 TWN-1 PIEZ-1 perched monitoring wellMW-32 MW-38 TW4-43 temporary perched monitoring well installed September, 2021 marginal area APPROXIMATE AREA BETWEEN SECOND QUARTER 2022 NITRATE AND CHLORIDE PLUMES USED IN 'METHOD 3' NITRATE DEGRADATION CALCULATIONS (detail map) 22 APPENDIX A SECOND QUARTER, 2010 WELL LOCATION, NITRATE, AND CHLORIDE CONCENTRATION MAPS (FIGURES A.1-A.3) 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-16 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 PIEZ-1 PIEZ-2 PIEZ-3 PIEZ-4 PIEZ-5 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 TW4-26 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 SITE PLAN AND PERCHED WELL LOCATIONS WHITE MESA SITE TW4-19 EXPLANATION wildlife pond SJS temporary perched nitrate monitoring well TWN-1 temporary perched monitoring well installed May, 2010 TW4-26 A.1 H:/718000/71807 NCACME/report/AppA/FA1_welloc.srf HYDRO GEO CHEM, INC.APPROVED DATE REFERENCE FIGURE 3332 3000 SCALE IN FEET 0 ND ND 0.3 ND ND ND ND 0.2 0.9 ND 2.6 8.4 3.1 0.2 0.1 ND 5.8 0.2 ND 16 23 6.8 6.7 3 7.6 7.1 2.5 1.5 1.6 6.9 11 5.2 2.9 1.1 4.7 ND 94.4 5.6 12 19 7.9 7.2 0.6 1.6 NS NS 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-26 MW-32 PIEZ-1 PIEZ-2 PIEZ-3 PIEZ-4 PIEZ-5 3.9 5.1 NDMW-04 ND 30 16 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 0.6 69 26 1 0.3 1.4 1.2 0.1 7.7 1 1.3 0.1 2.9 1 1.3 11 1.8 6.2 MW-4 PIEZ-3 5.1 1.6 perched monitoring well showing concentration in mg/L perched piezometer showing concentration in mg/L 7.6 temporary perched monitoring well showing concentration in mg/L KRIGED 2nd QUARTER, 2010 NITROGEN (mg/L) (NITRATE + NITRITE AS N) WHITE MESA SITE H:/718000/71807 NCACME/report/AppA/FA2_nit0610.srf EXPLANATION NOTES: ND = not detected, NS = not sampled temporary perched monitoring well installed May, 2010 showing concentration in mg/L SJS temporary perched nitrate monitoring well showing concentration in mg/L0.6 TWN-1 7.9 A.2 HYDRO GEO CHEM, INC.APPROVED DATE REFERENCE FIGURE 3332 3000 SCALE IN FEET 0 18 7 63 52 32 64 17 35 35 52 28 57 59 7 46 31 42 108 35 97 128 40 43 24 35 28 33 33 42 40 29 52 49 58 64 32 35132 200 266 134 33 52 8 36 NS NS 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-26 MW-32 PIEZ-1 PIEZ-2 PIEZ-3 PIEZ-4 PIEZ-5 31 41 42MW-04 40 639 306 TW4-24 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 20 97 118 22 44 22 6 11 175 30 72 49 30 39 35 87 63 113 MW-4 PIEZ-3 41 36 perched monitoring well showing concentration in mg/L perched piezometer showing concentration in mg/L 35 temporary perched monitoring well showing concentration in mg/L KRIGED 2nd QUARTER, 2010 CHLORIDE (mg/L) WHITE MESA SITE EXPLANATION NOTES: ND = not detected, NS = not sampled temporary perched monitoring well installed May, 2010 showing concentration in mg/L33 SJS temporary perched nitratemonitoring well showing concentration in mg/L20 TWN-1 H:/718000/71807 NCACME/report/AppA/FA3_cl0610.srf A.3 APPENDIX B EVALUATION OF REDUCED PRODUCTIVITY AT TW4-19 AND TW4-24 AND CALCULATION OF NEW BACKGROUND FLOW THROUGH THE NITRATE PLUME (ATTACHMENT N OF EFRI2015d) B-1 Appendix B: H:\718000\71807\NCACME2022\Report\Appb\3Q15_ATTACHMENT N_Rev3.Doc ATTACHMENT N EVALATION OF REDUCED PRODUCTIVITY AT TW4-19 AND TW4-24 AND CALCULATION OF NEW ‘BACKGROUND’ FLOW THROUGH THE NITRATE PLUME 1. INTRODUCTION AND OVERVIEW This analysis considers nitrate and chloroform program data up through the second quarter of 2015. As shown in Figures N.1 and N.2, the productivities of chloroform pumping well TW4-19 and nitrate pumping well TW4-24 have dropped since the third quarter of 2014. The decreases in average pumping rates at these wells have caused reductions in pumped chloroform and nitrate masses at each well. As per the nitrate and chloroform CAPs, reductions in productivity of nitrate and chloroform 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. Although under the chloroform CAP such an evaluation is only required as part of the 2-year review process (two-year Corrective Action Comprehensive Monitoring Evaluation ["CACME"]), to be proactive, and because the chloroform and nitrate pumping systems overlap, the evaluation of both systems is commencing at the present time. 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 low chloroform concentrations. Reduced productivity at TW4-24 is mainly of concern to the nitrate program because of moderately high nitrate concentrations and potentially reduced capture effectiveness. However, potential reductions in capture effectiveness will be mitigated by decreases in saturated thicknesses, decreases in hydraulic gradients, and potentially lower average hydraulic conductivities that in combination will significantly reduce non-pumping ‘background’ flow through the nitrate plume. Reduced ‘background’ flow reduces the amount of pumping needed to maintain effective capture. The impact of reduced productivity at TW4-19 on chloroform mass removal will be mitigated by factors that include: 1) chloroform concentrations at TW4-19 are on average lower than concentrations at nearby chloroform pumping wells; and 2) the recent addition of five wells to the chloroform pumping system: four existing wells (TW4-1, TW4-2, TW4-11, and TW4-21), B-2 Appendix B: H:\718000\71807\NCACME2022\Report\Appb\3Q15_ATTACHMENT N_Rev3.Doc and one new well (TW4-37). The addition of these wells increases chloroform mass removal rates and reduces the relative importance of TW4-19. At the present time, because nitrate pumping is likely to be adequate even with reduced pumping at TW4-24, and because of the beneficial impact of adding five wells to the chloroform pumping system (which reduces the relative importance of TW4-19), it is considered too early to commit to any particular course of action other than continuing evaluation of the pumping system. 2. CALCULATION OF NEW ‘BACKGROUND’ FLOW THROUGH THE NITRATE PLUME Reduced productivity at TW4-24 is likely the result of four factors other than potential losses in well efficiency: 1) smaller saturated thickness (by approx 11%) related to reduced wildlife pond recharge; 2) smaller hydraulic gradients (by approx 26%) also related to reduced wildlife pond recharge; 3) smaller average hydraulic conductivities (by approx 9%, presumably as a result of dewatering relatively shallow zones of higher permeability); and interference between pumping wells. ‘Background’ flow through the nitrate 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-nitrate pumping hydraulic gradient within the nitrate plume was calculated based on water levels at wells TW4-25 and MW-31 and wells TWN-2 and MW-30. These calculations yielded an average hydraulic gradient of 0.025 ft/ft. This is essentially identical to the pre- nitrate pumping hydraulic gradient calculated immediately east of the plume based on pre-nitrate pumping water levels at wells TWN-1 and MW-32. The hydraulic gradient within the nitrate plume has been reduced by decay of the groundwater mound resulting from cessation of water delivery to the northern wildlife ponds and by pumping. To assess the magnitude of the decrease in hydraulic gradient due only to the decay of the groundwater mound, two methods were employed. The first used the average decrease in water levels since Q4 2012 (approximately 10 ft) at non- pumping wells TWN-1, TWN-3, TWN-4, MW-19, MW-27, Piez-2 and Piez-3. Q4 2012 was the quarter just prior to the start of nitrate pumping. Water levels at these wells are assumed to have responded primarily to cessation of water delivery to the northern wildlife ponds (Figures N.3 through N.9). The average decrease (approximately 10 ft) was then assumed to represent the decrease in water level that would have occurred at pumping well TW4-25 under non-pumping conditions. The new ‘background gradient’ for Q2 2015 was then calculated as 0.019 ft/ft based on the water level calculated for TW4-25 (5597 ft amsl -10 ft = 5587 ft amsl) and the water level at MW-31 (5548 ft amsl). The second assumed that the new ‘background’ gradient through the nitrate plume is equal to the Q2 2015 gradient between non-pumping wells TWN-1 and MW-25 (0.018 ft/ft). This is nearly identical to the gradient calculated by the first method. The new ‘background’ gradient is therefore assumed to be the average of the two methods (0.0185 ft/ft), a 26% reduction from the original (0.025 ft/ft). B-3 Appendix B: H:\718000\71807\NCACME2022\Report\Appb\3Q15_ATTACHMENT N_Rev3.Doc An assessment of the change in transmissivity (product of saturated thickness and conductivity) was performed based on changes in water levels in non-pumping 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 clearly responded to the reduction in pumping at TW4-19 and TW4-24. As shown in Figures N.10 through N.14, 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 long-term pumping test conducted in year 2003. 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 subtracting out the average downward water level trends at wells TW4-5, TW4-9, TW4-10, TW4-16, and TW4-18. This eliminated the impact of water level reductions resulting from reduced wildlife pond recharge. The data were then 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 N.15 through N.19 provide the results and the fits between measured and simulated displacements at TW4-5, TW4-9, TW4-10, TW4-16, and TW4-18. Transmissivity estimates are similar, but lower, than estimates derived from the long-term pumping test (HGC, 2004). The reduction in transmissivity is primarily related to reduced saturated thickness; however, as shown in Table N.1, compared to the year 2003 analysis, the average reduction in transmissivity is approximately 27% whereas the average reduction in saturated thickness is only 20%. This implies a reduction in average conductivity of approximately 9%. The reduction in average saturated thickness within the pumped portion of the nitrate plume based on water levels at wells TWN-2, TWN-3, TW4-22, TW4-24, and TW4-25 is approximately 11% as of Q2 2015. This calculation is affected by pumping at the majority of these wells; however, the calculated 11% reduction is about the same as the 10% reduction calculated above based on non-pumping wells impacted by reduced wildlife pond recharge. Assuming that the 9% reduction in conductivity is representative of the nitrate plume area, the reduced hydraulic gradient (-26%), reduced saturated thickness (-11%), and reduced conductivity (-9%) in combination yield a new ‘background’ flow through the nitrate plume that is approximately 40% lower than the original calculated range of 1.31 to 2.79 gpm. The new ‘background’ flow is estimated to range from 0.79 gpm to 1.67 gpm. The current (third quarter, 2015) total pumping from the nitrate plume (2.03 gpm) exceeds the high end of this range indicating that pumping is likely adequate even with reduced productivity at TW4-24. 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. B-4 Appendix B: H:\718000\71807\NCACME2022\Report\Appb\3Q15_ATTACHMENT N_Rev3.Doc 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, it is important to avoid the generation of rectangular grids of wells or triangular patterns of wells. 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 premature at this time; nitrate pumping appears adequate even with reduced productivity at TW4-24, and chloroform mass removal rates remain adequate due to the recent addition of five chloroform pumping wells. Additional data collection is considered necessary to evaluate the impacts of these additional wells on long-term pumping well productivities. 4. POTENTIAL FUTURE EVALUATION OF TW4-19 AND TW4-24 WELL EFFICIENCIES Should continued monitoring indicate that the reduced productivities at TW4-19 and TW4-24 need to be addressed, the wells will be tested for reduced efficiency. Reduced efficiency would likely be related to partial clogging of well screens. Step-rate pumping tests would be conducted as part of this evaluation. 5. REFERENCES 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. August 22, 2002. HGC, 2004. Final Report. Long Term Pumping at MW-4, TW4-15, and TW4-19. White Mesa Uranium Mill Near Blanding, Utah. May 26, 2004. Table N.1 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) (ft 2/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 (assuming confined analysis) H:\718000\aug15\Nitrate\Pvolume.xls: F1 N pump 0 50,000 100,000 150,000 200,000 250,000 300,000 Q2 13 volume Q3 13 volume Q4 13 volume Q1 14 volume Q2 14 volume Q3 14 volume Q4 14 volume Q1 15 volume Q2 15 volume 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 10/9/15 N.1N pump10/9/15SJS H:\718000\aug15\Nitrate\Pvolume.xls: F2 chl pmp 0 50,000 100,000 150,000 200,000 250,000 300,000 350,000 400,000 450,000 Q2 13 volume Q3 13 volume Q4 13 volume Q1 14 volume Q2 14 volume Q3 14 volume Q4 14 volume Q1 15 volume Q2 15 volume 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 10/9/15 N.2chl pmp10/9/15SJS H:\718000\aug15\Nitrate\DTW_TimeSeries.xls: FX TWN-1 TIME SERIES OF DEPTHS TO WATER AT TWN-1 SINCE Q1 2012 HYDRO GEO CHEM, INC.Approved FigureDateAuthorDate File Name N.3DTW_TimeSeries.xls10/8/15GEM 20 25 30 35 40 45 50 55 60 65 70 De p t h t o W a t e r ( f t ) Date H:\718000\aug15\Nitrate\DTW_TimeSeries.xls: FX TWN-3 TIME SERIES OF DEPTHS TO WATER AT TWN-3 SINCE Q1 2012 HYDRO GEO CHEM, INC.Approved FigureDateAuthorDate File Name N.4DTW_TimeSeries.xls10/8/15GEM 20 25 30 35 40 45 50 55 60 65 70 De p t h t o W a t e r ( f t ) Date H:\718000\aug15\Nitrate\DTW_TimeSeries.xls: FX TWN-4 TIME SERIES OF DEPTHS TO WATER AT TWN-4 SINCE Q1 2012 HYDRO GEO CHEM, INC.Approved FigureDateAuthorDate File Name N.5DTW_TimeSeries.xls10/8/15GEM 20 25 30 35 40 45 50 55 60 65 70 De p t h t o W a t e r ( f t ) Date H:\718000\aug15\Nitrate\DTW_TimeSeries.xls: FX MW-19 TIME SERIES OF DEPTHS TO WATER AT MW-19 SINCE Q1 2012 HYDRO GEO CHEM, INC.Approved FigureDateAuthorDate File Name N.6DTW_TimeSeries.xls10/8/15GEM 20 25 30 35 40 45 50 55 60 65 70 De p t h t o W a t e r ( f t ) Date H:\718000\aug15\Nitrate\DTW_TimeSeries.xls: FX MW-27 TIME SERIES OF DEPTHS TO WATER AT MW-27 SINCE Q1 2012 HYDRO GEO CHEM, INC.Approved FigureDateAuthorDate File Name N.7DTW_TimeSeries.xls10/8/15GEM 20 25 30 35 40 45 50 55 60 65 70 De p t h t o W a t e r ( f t ) Date H:\718000\aug15\Nitrate\DTW_TimeSeries.xls: FX PIEZ-2 TIME SERIES OF DEPTHS TO WATER AT PIEZ-2 SINCE Q1 2012 HYDRO GEO CHEM, INC.Approved FigureDateAuthorDate File Name N.8DTW_TimeSeries.xls10/8/15GEM 20 25 30 35 40 45 50 55 60 65 70 De p t h t o W a t e r ( f t ) Date H:\718000\aug15\Nitrate\DTW_TimeSeries.xls: FX PIEZ-3 TIME SERIES OF DEPTHS TO WATER AT PIEZ-3 SINCE Q1 2012 HYDRO GEO CHEM, INC.Approved FigureDateAuthorDate File Name N.9DTW_TimeSeries.xls10/8/15GEM 20 25 30 35 40 45 50 55 60 65 70 De p t h t o W a t e r ( f t ) Date H:\718000\aug15\Nitrate\DTW_TimeSeries.xls: FX TW4-5 TIME SERIES OF DEPTHS TO WATER AT TW4-5 SINCE Q1 2012 HYDRO GEO CHEM, INC.Approved FigureDateAuthorDate File Name N.10DTW_TimeSeries.xls10/9/15GEM 50 55 60 65 70 De p t h t o W a t e r ( f t ) Date H:\718000\aug15\Nitrate\DTW_TimeSeries.xls: FX TW4-9 TIME SERIES OF DEPTHS TO WATER AT TW4-9 SINCE Q1 2012 HYDRO GEO CHEM, INC.Approved FigureDateAuthorDate File Name N.11DTW_TimeSeries.xls10/9/15GEM 50 55 60 65 70 De p t h t o W a t e r ( f t ) Date H:\718000\aug15\Nitrate\DTW_TimeSeries.xls: FX TW4-10 TIME SERIES OF DEPTHS TO WATER AT TW4-10 SINCE Q1 2012 HYDRO GEO CHEM, INC.Approved FigureDateAuthorDate File Name N.12DTW_TimeSeries.xls10/9/15GEM 50 55 60 65 70 De p t h t o W a t e r ( f t ) Date H:\718000\aug15\Nitrate\DTW_TimeSeries.xls: FX TW4-16 TIME SERIES OF DEPTHS TO WATER AT TW4-16 SINCE Q1 2012 HYDRO GEO CHEM, INC.Approved FigureDateAuthorDate File Name N.13DTW_TimeSeries.xls10/9/15GEM 50 55 60 65 70 De p t h t o W a t e r ( f t ) Date H:\718000\aug15\Nitrate\DTW_TimeSeries.xls: FX TW4-18 TIME SERIES OF DEPTHS TO WATER AT TW4-18 SINCE Q1 2012 HYDRO GEO CHEM, INC.Approved FigureDateAuthorDate File Name N.14DTW_TimeSeries.xls10/9/15GEM 50 55 60 65 70 De p t h t o W a t e r ( f t ) Date H:\718000\71807\NCACME\report\AppB\WHIP_Fits_corrected.xls: Fig N15 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 N.1510/8/15GEM H:\718000\71807\NCACME\report\AppB\WHIP_Fits_corrected.xls: Fig N16 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 N.1610/8/2015GEM Results Transmissivity = 50.8 ft 2/d Storativity = 1.23E-04 H:\718000\71807\NCACME\report\AppB\WHIP_Fits_corrected.xls: Fig N17 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 N.1710/8/2015GEM H:\718000\71807\NCACME\report\AppB\WHIP_Fits_corrected.xls: Fig N18 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 N.1810/8/2015GEM H:\718000\71807\NCACME\report\AppB\WHIP_Fits_corrected.xls: Fig N19 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 N.1910/8/2015GEM