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Home My WebLink AboutDRC-2007-001301 - 0901a06880361963ORC-2010-001301 PRELIMINARY CORRECTIVE ACTION PLAN WHITE MESA URANIUM MILL NEAR BLANDING, UTAH Prepared for: DENISON MINES (USA) CORP. Independence Plaza, Suite 950 1050 Seventeenth Street Denver, Colorado 80265 ' f: V^^" ^OCT^ 7':'^' Prepared by: HYDRO GEO CHEM, INC. 51 W. Wetmore Road, Suite 101 Tucson, Arizona 85705 (520) 293-1500 Ausust 20. 2007 HYDRO GEO CHEM, INC. Environmental Science & Technology 7: 1 '\ f ;'• •i- •- \ PRELIMINARY CORRECTIVE ACTION PLAN WHITE MESA URANIUM MILL NEAR BLANDING, UTAH Prepared for: DENISON MINES (USA) CORP. Independence Plaza, Suite 950 1050 Seventeenth Street Denver, Colorado 80265 Prepared, Reviewed, and Approved by: Stewart JTSmith, UT P.G. No. 5336166-2250 Associate Hydrogeologist ' » ;• Stewart J. Sniith *. C 9 •• ; 2 i -* '. 533*186-2250 .•* •? August 20, 2007 TABLE OF CONTENTS 1. INTRODUCTION, OVERVIEW, AND SCOPE 1 2. OBJECTFVES 3 3. BACKGROUND 5 3.1 Site Hydrogeology 5 3.1.1 Geologic Setting , 5 3.1.2 Hydrogeologic Setting 7 3.1.3 Perched Zone Hydrogeology 8 3.1.3.1 Lithologic and Hydraulic Properties .• 8 3.1.3.1.1 Dakota 9 3.1.3.1.2 Burro Canyon 10 3.1.3.2 Perched Groundwater Flow 13 3.1.3.3 Saturated Thickness 15 3.1.3.4 Perched Groundwater Travel Times 15 3.2 Chloroform Occurrence 17 3.2.1 Source Areas '. 18 3.2.2 Chloroform Concentration Trends 19 3.2.3 Chloroform Mass Removal Rates 20 4. CHARACTERIZATION OF STUDY AREA 23 4.1 Extent 23 4.2 Hydrogeology 24 5. CORRECTIVE ACTION CONCENTRATION LIMITS , 27 6. CORRECTIVE ACTION PLAN - CONSTRUCTION AND OPERATION 29 6.1 Phase 1 30 6.1.1 Groundwater Pumping System 32 6.1.2 Water Level Monitoring 33 6.1.3 Water Quality Monitoring 34 6.1.4 Reporting 34 6.2 Phase 2 35 6.3 Achievement of Concentration Limits 36 Preliminary Correciive Action Plan G;\718000\7I801\CAP\Corrective Action Plan.doc Augu.st 20. 2007 7. ASSESSMENT OF CORRECTIVE ACTION AND PROTECTION OF PUBLIC HEALTH AND THE ENVIRONMENT 37 7.1 Stabihty of Plume Boundary (Phase 1) 37 7.2 Concentration Trends within the Plume (Phase 1) 38 7.3 Chloroform Mass Removal Rates Resulting from Pumping (Phase 1) 39 7.4 Stability of the Proportion of the Chloroform Plume under Hydraulic Capture (Phase 1) 40 7.5 Phase 2 40 7.6 Permanent Effect of Corrective Action 41 7.7 In-Place Contaminant Control 41 8. IMPACTS OF OFFSFFE ACTrVFTIES 43 9. PROPOSED PLUME CORRECTIVE ACTION ACTIVITIES 45 9.1 Phase 1 45 9.1.1 Groundwater Pumping 45 9.1.2 Water Level Monitoring 46 9.1.3 Water Quality Monitoring 46 9.1.4 Estimation of Capture Zones 46 9.1.5 Estimation of Pumped Chloroform Mass 46 9.1.6 Reporting 47 9.1.7 Additional Measures 47 9.2 Phase 2 48 10. REFERENCES 49 11. LIMFTATIONS STATEMENT 51 TABLES 1 Hydraulic Test Analysis Results, Single Well Tests 2 Estimated Perched Zone Hydraulic Properties Based on Analysis of Observation Wells near MW-4 and TW4-19 3 Comparison of 2nd Quarter 2005 and 1st Quarter 2007 Chloroform Concentrations 4 Comparison of 2nd Quarter 2006 and 1st Quarter 2007 Chloroform Concentrations 5 Comparison of Average Chloroform Concentrations between 1st Quarter 2007 and 2"''Quarter 2006, and Between l" Quarter 2006 and 2"^* Quarter 2005 Preliminary Corrective Action Plan G:\718000\71801\CAP\Corrective Action Plan,doc Augusl 20. 2007 TABLE OF CONTENTS (Continued) FIGURES 1 Site Plan and Perched Well Locations, White Mesa Site 2 1'' Quarter 2007 Chloroform Plume and Perched Well Locations, White Mesa Site 3 Kriged Brushy Basin Contact Elevations, White Mesa Site 4 1" Quarter 2007 Chloroform Plume Showing Area Responding to the First 7 Months of Long Term Pumping, White Mesa Site 5 Perched Water Levels August 1990 6 Perched Water Levels August 1994 7 Perched Water Levels September 2002 8 Kriged 1" Quarter 2007 Water Levels, White Mesa Site 9 Portion of USGS Black Mesa 7.5' Sheet Showing Approximate Locations of Tailing Cells in Relation to Nearby Canyons and Ruin Spring 10 Perched Zone Saturated Thickness 1" Quarter 2007, White Mesa Site 11 Depth to Perched Water 1" Quarter 2007, White Mesa Site 12 Kriged 1'' Quarter 2007 Chloroform (ng/L), White Mesa Site 13 Locations of Abandoned Scale House and Former Office Leach Fields in Relation to 1^' Quarter 2007 Chloroform Plume (detail map of northeastern portion of White Mesa Site). 14 1" Quarter 2007 Chloroform Plume Showing Estimated Capture Zones, White Mesa Site 15 Comparison of Kriged l" Quarter 2007 and l" Quarter 2006 Chloroform Plurnes, White Mesa Site APPENDICES A Perched Monitoring Well Hydrographs B Chloroform Investigation Well Chloroform Concentration Graphs C Chloroform Mass Removal via Natural In-Situ Desradation 1 111 Preliminary Corrective Acfion Plan G:\718000\71801\CAP\Corrective Action Plan.doc August 20, 2007 1. INTRODUCTION, OVERVIEW, AND SCOPE This document presents a Corrective Action Plan (CAP) to address chlorofomi contamination in a shallow perched groundwater zone beneath the White Mesa Uranium Mill Site (the site), located near Blanding, Utah. Figure 1 is a map of the site showing the locations of perched zone monitoring wells. The area of the perched groundwater zone affected by chloroform concentrations exceeding 70 \.igfL is shown in Figure 2. The sources of the chloroform have been identified as two sanitary leach fields, abandoned more than 25 years ago. that accepted sanitary wastes and laboratory wastes containing ciiloroform. The chloroform was first discovered at perched well MW-4 in 1999. Since that time, 25 temporary perched zone wells have been installed to study and delineate the chloroform. An interim remedial action, consisting of pumping perched water containing high concentrations of chloroform from areas where the perched zone has a relatively high productivity, was initiated in 2003. The elements of this document mclude the following items: • CAP objectives • A description of the site hydrogeology • The nature and extent of chlorofonn in the perched zone and relation to source areas • Ongoing interim remedial actions • Proposed corrective remedial actions and concentration limits • Proposed corrective action contingencies Preliminary Corrective Action Plan G:\718000\71801\CAP\Correciive Action Plan.doc Augusl 20. 2007 2. OBJECTIVES The objectives of the CAP include the following: 1) Minimize or prevent further downgradient migration of the chloroform plume by a combination of pumping and reliance on natural attenuation, 2) Prevent chloroform concentrations exceeding the action level from migrating y south or southwest of the tailings cells, ^ 3) Monitor to track changes in concentrations within the plume and to establish whether the plume boundaries are expanding, contracting, or stable, 4) Provide contingency plans to address potential continued expansion of the plume and the need for additional monitoring and/or pumping points, and 5) Ultimately reduce chloroform concentrations at all monitoring locations tc the action level or below. To achieve these objectives, the CAP proposes a pha.sed approach. The first phase consists of a combination of "active" and "passive'" strategies, ^he active strategy consists of removing chloroforn] mass as rapidly as practical by puinping areas that have (on a relative basis) both liigh chloroform concentrations, and high productivity. Continued moniionrjg within and outside the plume is considered part of the active strategy. The passive strategy consists of relying on natural attenuation processes to remove chlorofonn mass and reduce concentrations. Reductions in concentrations would be achieved by physical processes such as volatiliz-^tion, hydrodynamic dispersion, and abiotic degradation, and through natural biological degradation of chloroform. These are essentially the same processes that have been relied upon in the interim action. I Preliminary Correciive Action Plan G:\718000\71801\CAP\Corrective Action Plan.doc August 20. 2007 Natural attenuation is expected to reduce chloroform concentrations within the entire plume. However, within upgradient portions of the plume that occur in higher permeabihty materials that are amenable to pumping, direct mass removal via pumping will be the primary means to reduce concentrations. In downgradient portions of the plume where permeabilities are low, chloroform migration rates are low, and mass removal by pumping is not practical because achievable pumping rates would be very low, natural attenuation will be the. primary means to reduce concentrations. The second phase relies on natural attenuation (without pumping) to reduce chloroform concentrations at all monitoring locations to action levels once concentrations during Phase 1 are judged to be sufficientiy low that Phase 2 will be effective. Corrective action strategies will be discussed in detail in Section 6. Preliminary Correciive Aclion Plan G :\718000\71801 \CAP\Correcti ve Action Plan ,doc August 20, 2007 3. BACKGROUND The White Mesa Uranium Mill (the "Mill" or the "site") is located in southeastem Utah, approximately 5 miles south of the town of Blanding. It is situated on White Mesa, a flat area bounded on the east by Corral Canyon, to the west by Westwater Creek, and to the south by Cottonwood Canyon. The site consists of a uranium processing mill, and four engineered lined tailings disposal cells. 3.1 Site Hydrogeology Titan, 1994 provides a detailed description of site hydrogeology based on information available at tiiat time. A brief summar}' of site hydrogeology that is based on Titan, 1994, and that includes the results of more recent site investigations, is provided below. 3.1.1 Geologic Setting The site is located within the Blanding Basin of the Colorado Plateau physiographic province. Typical of large portions of the Colorado Plateau province, the rocks underlying the site are relatively undeformed. The average elevation of the site is approximately 5.600 feei (ft) above mean sea level (amsl). The site is underlain by unconsolidated alluvium and indurated sedimentary rocks consisting primarily of sandstone and shale. The indurated rocks are relatively flat lying with Preliminary Corrective .Action Plan G;\7I8000"\71801\CAP\Corrective Action P)an,doc Augusl 20, 2007 dips generally less than 3°. The alluvial materials consist mostly of aeolian silts and fine-grained aeolian sands with a thickness varying from a few feet to as much as 25 to 30 ft across the site. In places, the alluvium is underlain by fine grained materials that have been interpreted as erosional remnants of the Mancos Shale. The alluvium (and Mancos, where present) is underlain by the Dakota Sandstone and Buno Canyon Formation, which are sandstones having a total thickness ranging from approximately 100 to 140 ft. Beneath the Burro Canyon Formation lies the Morrison Formation, consisting, in descending order, of the Bmshy Basin Member, the Westwater Canyon Member, the Recapture Member, and the Salt Wash Member. The Brushy Basin and Recapture Members of the Morrison Formation, classified as shales, are very fine-grained and have a very low permeability. The Bmshy Basin Member is primarily composed of bentonitic mudstones, siltstones, and claystones. The Westwater Canyon and Salt Wash Members also have a low average vertical permeability due to the presence of interbedded shales. Beneath the Monison Formation he the Summerville Formation, an argillaceous sandstone with interbedded shales, and the Entrada Sandstone. Beneath the Entrada lies the Navajo Sandstone. The Navajo and Entrada Sandstones constitute the primary aquifer in the area of the site. The Entrada and Navajo Sandstones are separated from the Burro Canyon Formation by approximately 1,000 to 1,100 feet of materials having a low average vertical permeability. Groundwater within this system is under artesian pressure in the vicinity of the site, is of generally good quality, and is used as a secondairy source of water at the site. Preliminar)' Correciive Action Plan G:\718000\71801\CAP\Corrective Action Plan,doc August 20, 2007 3.1.2 Hvdrogeologic Setting The site is located within a region that has a dry to arid continental climate, with average annual precipitation of less than 11.8 inches, and average annual evapotranspiration of approximately 61.5 inches. Recharge to aquifers occurs primarily along the mountain fronts (for example, the Henry, Abajo, and La Sal Mountains), and along the flanks of folds such as Comb Ridge Monocline. Although the water quality and productivity of the Navajo/Entrada aquifer are generally good, the depth of the aquifer (approximately 1,200 feet below land surface [ft bis]) makes access difficult. The Navajo/Entrada aquifer is capable of yielding significant quantities of water to wells (hundreds of gallons per minute [gpm]). Water in wells completed across these units at the site rises approximately 800 feet above the base of the overlying Summerville Formation. Perched groundwater in the Dakota Sandstone and Burro Canyon Fonnation is used on a hmited basis to the north (upgradient) of the site because it is more easily accessible. Water quality of the Dakota Sandstone and Burro Canyon Formation is generally poor due to high total dissolved solids (TDS) and is used primarily for stock watering and irrigation. The saturated thickness of the perched water zone generally increases to the north of the site, increasing the yield of the perched zone to wells installed north of the site. Preliminary Corrective Aclion Plan G:\718000\71801\CAP\Correcfive Action Plan.doc Augusl 20. 2007 3.1.3 Perched Zone Hydrogeologv Perched groundwater beneath the site occurs primarily within the Burro Canyon Formation, but locally rises into the Dakota northeast of the tailings cells where saturated thicknesses are greater. Perched groundwater at the site has a generally low quality due to high total dissolved solids (TDS) in the range of approximately 1,200 to 5,000 miUigrams per liter (mg/L), and is used primarily for stock watering and imgation in the areas upgradient (north) of the site. Perched water is supported within the Burro Canyon Formation by the underlying, fine-grained Bmshy Basin Member of the Morrison Formation. Figure 3 is a contour map showing the approximate elevation of the contact of the Buno Canyon Formation with the Bmshy Basin Member, which essentially forms the base of the perched water zone at the site. Contact elevations are based on perched monitoring well lithologic logs and surveyed land surface elevations. As indicated, the contacl generally dips to the south/southwest beneath the site. Groundwater within the perched zone generally flows south to southwest beneath the site. Beneath the tailings cells at the site, perched water flow is generally southwest to south-southwest. East of the tailings cells, perched water flow is more southerly. 3.1.3.1 Lithologic and Hydraulic Properties Although the Dakota Sandstone and Buno Canyon Formations are often described as a single unit due to their similarity, previous investigators at the site have distinguished between them. The Dakota Sandstone is a relatively-hard to hard, generally fine-to-medium grained g Preliminary Corrective Action Plan G:\718000\71801\CAP\Corrective Action Plan,doc Augusl 20, 2007 sandstone cemented by kaolinite clays. The Dakota Sandstone locally contains discontinuous interbeds of siltstone, shale, and conglomeratic materials. Porosity is primarily intergranulai'. The underlying Burro Canyon Fomiation hosts most ofthe perched groundwater at the site, The Buno Canyon Formation is similar to the Dakota Sandstone but is generally more poorly sorted, contains more conglomeratic materials, and becomes argillaceous near its contact with the underlying Bmshy Basin Member. The permeability of the Dakota Sandstone and Buno Canyon Formation at the site is generahy low. No significant joints or fractures within the Dakota Sandstone or Buno Canyon Formation have been documented in any wells or borings installed across the site. This was the conclusion of Knight Piesold, 1998, and HGC, 2001, and is consistent with findings provided in HGC, 2005. Any fractures observed in cores collected from site borings are typically cemented, showing no open space. 3.1.3.1.1 Dakota Based on samples collected during installation of wells MW-16 (no longer used) and MW-17, located immediately downgradient of the tailings cells at the site, porosities of the Dakota Sandstone range from 13.4% to 26%, averaging 20%, and water saturations range from 3.7% to 27.2%, averaging 13.5%. The average volumetric water content is approximately 3%. The permeability of the Dakota Sandstone based on packer tests in borings installed at the site ranges from 2.71 x 10'^ centimeters per second (cm/s) to 9.12 x 10""* cm/s, with a geometric average of 3.89 x 10"^ cm/s (Titan, 1994). Preliminary Corrective Action Plan G:\718000'\71801\CAP\CorTective Action Plan,doc August 20. 2007 3.1.3.1.2 Buno Canyon The average porosity of the Buno Canyon Formation is similar to that of the Dakota Sandstone. Based on samples cohected from the Buno Canyon Formation at MW-16 (no longer used), located immediately downgradient of the tailings cells at the site. Titan, 1994. reported that porosity ranges from 2% to 29.1%, averaging 18.3%, and water saturations of unsaturated materials range from 0.6% to 77.2%, averaging 23.4%. Titan, 1994, reported that the hydraulic conductivity of the Buno Canyon Formation ranges from 1.9 x 10"^ to 1.6 x 10 ~' cm/s, with a geometric mean of 1.1 x 10cm/s, based on the results of 12 pumping/recovery tests performed in monitoring wells and 30 packer tests performed in borings prior to 1994. Hydrauhc testing of wells MW-01, MW-03, MW-05, MW-17, MW-18, MW-19, MW- 20, and MW-22 during the week of July 8, 2002, and newly installed wells MW-23, MW-25, MW-27, MW-28, MW-29, MW-30, MW-31, MW-32, TW4-20, TW4-21, and TW4-22 during June, 2005, yielded average perched zone permeabilities ranging from approximately 2 x 10"'' cm/s to 5 X 10"^ cm/s, similar to the range reported by previous investigators at the site (Hydro Geo Chem, hic [HGC], 2002; HGC, 2005). Downgradient (south to southwest) of the tailings cells, average perched zone permeabilities based on tests at MW-3, MW-5, MW-17, MW-20, MW-22, and MW-25 ranged from approximately 4x10''' to 1x10"^ cm/s Permeabihty estimates from these tests were based on pumping/recovery and slug tests analyzed using several different methodologies. Permeability estimates from these tests are summarized in Table 1. 25 temporary perched zone monitoring wells have been installed at the site to investigate elevated concentrations of chloroform initially discovered at well MW-4 in 1999. The Preliminary Corrective Action Plan ^ ^ G:\718000\71801\CAP\Corrective Action Plan.doc August 20. 2007 occunence of chloroform in the perched zone will be discussed in Sections 3.3 and 4. Some of the coarser grained and conglomeratic zones encountered within the perched zone during installation of these wells are believed to be continuous with or at least associated with a relatively thin, relatively continuous zone of higher permeability (International Uraniurn [USA] Corporation [lUSA] and HGC, 2001). The higher permeability zone defined by these wells is generally located east to northeast of the tailings cells at the site, and is hydraulically cross-gradient or upgradient of the tailings cells with respect to perched groundwater flow. Based on analyses of pumping tests at MW-4 and drilling logs from nearby temporary wells, the permeabihty of this relatively thin coarser-grained zone was estimated to be as high as 2.5 x 10"^ cm/s or 7 ft/day. Relatively high average permeabilities estimated at MW-11, located on the southeastem margin of the downgradient edge of tailings cell #3, and at MW-14, located on the downgradient edge of tailings ceh #4, of 1.4 x 10"^ cm/s and 7.5 x 10''^ cm/s, respectively (UMETCO, 1993), may indicate that this zone extends beneath the southeastem margin of the cells. However, this zone of higher permeability within the perched water zone does not appear to exist downgradient (south-southwest) of the tailings cells. At depths beneath the perched water table, the zone is not evident in lithologic logs of the southernmost temporary wells TW4-4 and TW4-6 (located east [cross-gradient] of cell #3), nor is it evident in wehs MW-3, MW-5, MW-12, MW-15, MW-16 (no longer used), MW-17, MW-20, MW-21, or MW-22, located south to southwest (downgradient) of the tailings cells, based on the lithologic logs or hydraulic testing of the wells. The apparent absence of the zone south of TW4-4 and south-southwest of the tailings cells indicates that it "pinches out" (HGC, 2005). Preliminary Corrective Action Plan ^ ^ G:\718000\71801\CAP\Corrective Action Plan.doc August 20, 2007 To test the potential existence and continuity of this higher permeability zone, and to refine hydraulic parameter estimates, long term pumping of MW-4 and TW4-19 began in April 2003. MW-26 (TW4-15) was added to the pumping network in August 2003, and TW4-20 was added in August, 2005. These wehs were selected for pumping because they were 1) located in areas of the perched zone having relatively high transmissivity, and could therefore sustain relatively high pumping rates, and 2) because the wehs were also located in perched water having relatively high chloroform concentrations, which resulted in significant rates of chloroform mass removal. As such, the pumping has constituted an interim action to mitigate chloroform in the perched zone (HGC, 2004). Analysis of drawdown data collected from wells that responded measurably to pumping between the start of pumping (April 2003) and November 2003, indicated average permeabilities ranging from 4 x 10''' to 5 x IO""* cm/s in the area east to northeast of the tailings cells, assuming the perched zone is unconfined (HGC, 2004). Table 2 summarizes the results of the testing. Figure 4 shows the approximate area where detectable drawdowns were measured during the 7 months of pumping. This area is interpreted to coincide roughly with the zone of higher permeability. Wells located outside this zone that did not respond measurably to pumping are interpreted to be completed in lower permeability materials. 12 Preliminary Corrective Action Plan G:\718000\71801 \CAP\Corrective Action Plan.doc August 20, 2007 3.1.3.2 Perched Groundwater Flow Perched groundwater flow at the site has historically been to the south/southwest. Figures 5 through 8 are perched groundwater elevation contour maps for the years 1990, 1994, 2002. and the first quarter of 2007, respectively. The 1990. 1994, and 2002 maps were hand contoured because of sparse data. As groundwater elevations indicate, the perched groundwater gradient changes from generally southwesterly in the westem portion of the site, to generally southerly in the eastern portion of the site. The most significant changes between the 2002 and 2007 water levels result from pumping of wells MW-4, TW4-19, TW4-20, and MW-26 (T^^''4- 15). These wehs are pumped to reduce chloroform mass in the perched zone east and northeast of the tailings cehs. (Chloroform occunence in the perched zone will be discussed in Sections 3.3 and 4.) In general, perched groundwater elevations have not changed significantly over most of the site since monitoring began, except in the vicinity of the wildlife ponds and the pumping wells. For example, relatively large increases in water levels occuned between 1994 and 2002 at MW-4 and MW-19, located in the east and northeast portions of the site, as shown by comparing Figures 6 and 7. These water level increases in the northeastem and eastern portions of the site are likely the result of seepage from wildlife ponds located near the piezometers shown in Figures 7 and 8, which were installed in 2001 for the purpose of investigating these changes. Increasing water levels affect many of the chloroform investigation wells as shown in the hydrographs provided in Appendix A (from Denison Mines (USA) Corp [DUSA], 2007). 13 Preliminary Corrective Aclion Plan G:\7l8000\7i80l\CAP\Correcrive Action Plan.doc Auausi 20. 2007 The increases in water levels in the northeastem portion of the site have resulted in locally steepening groundwater gradients over portions of the site. Conversely, pumping of wells MW-4, TW4-19, TW4-20, and MW-26 (TW4-15) has depressed the perched water table locally and reduced average hydraulic gradients to the south and southwest of these wells. Perched zone hydraulic gradients cunently range from a maximum of approximately 0.05 ft/ft east of tailings cell #2 to approximately 0.01 ft/ft downgradient of cell #3, between cell #3 and MW-20. Perched water discharges in springs and seeps along Westwater Creek Canyon and Cottonwood Canyon to the west-southwest of the site, and along Conal Canyon to the east of the site, where the Buno Canyon Formation outcrops. The discharge point located most directiy downgradient of the tailings cells is Ruin Spring. This feature is located approximately 10,000 feet south-southwest of the taihngs cells at the site and is depicted on the USGS 7.5-minute quad sheet for Black Mesa (Figure 9). The average hydraulic gradient between the downgradient edge of tailings cell #3 and Ruin Spring is approximately 0.12 ft/ft assuming the following: 1) The elevation of Ruin Spring, based on the USGS topographic map for Black Mesa, is approximately 5,390 ft amsl. 2) The distance between the downgradient edge of tailings cell #3 and Ruin Spring is approximately 10,000 ft. 3) The average groundwater elevation at the downgradient edge of tailings cell #3 is approximately 5,510 ft amsl. 14 Preliminary Corrective Action Plan G:\718000\71801\CAP\Corrective Aclion Plan.doc August 20, 2007 3.1.3.3 Saturated Thickness The saturated thickness of the perched zone as of the first quarter of 2007 ranges from approximately 93 ft in the northeastem portion of the site to less than 5 ft in the southwest portion of the site (Figure 10), and depths to water range from approximately 14 ft in the northeastem portion of the site (adjacent to the wildlife ponds) to approximately 114 ft at the southwest margin of tailings cell #3 (Figure 11). The relatively large saturated thicknesses in the northeastem portion of the site are likely related to seepage from wildlife ponds located near the piezometers shown in Figure 10. Although sustainable yields of as much as 4 gpm have been achieved in wells intercepting the larger saturated thicknesses and higher permeability zones in the northeast portion of the site, perched zone well yields are typically low (<0.5 gpm) due to the generally low permeability of the perched zone. Sufficient productivity can generally be obtained only in areas where the saturated thickness is greater, which is the primary reason that the perched zone has been used on a hmited basis as a water supply to the north (upgradient) of the site, but not downgradient of the site. 3.1.3.4 Perched Groundwater Travel Times Average rates of movement of a conservative solute in perched groundwater (equivalent to interstitial or pore velocity) have been calculated for the area of the perched zone downgradient of the tailings cells, and beneath and immediately upgradient of the taihngs cells (HGC, 2005 and HGC, 2007). Preliminary Corrective Action Plan ^ ^ G:\718000'\71801\CAP\Correciive Aclion Plan.doc August 20, 2007 The calculated rate of movement downgradient of the tailings cells was based on an effective porosity of 0.18, an average hydraulic gradient of 0.012 ft/ft, and geometric averages of permeabilities estimated from hydraulic tests at wells located south and southwest of the cells. The geometric averages were based on slug tests performed at MW-3, MW-5, MW-17, MW-20, MW-22, and MW-25 (HGC, 2002; HGC, 2005), and pump tests performed by Peel Environmental (UMETCO, 1993) at MW-11, MW-12, MW-14, and MW-15. Two averages were calculated; one using permeabilities estimated from HGC slug test data analyzed using the Bouwer-Rice solution (Bouwer and Rice, 1976) and the other using permeabihties estimated from the same data using the KGS solution (Hyder, 1994). Included in each average were the results of the pump tests reported in UMETCO, 1993, for MW-11, MW-12, MW-14, and MW- 15. The geometric averages thus calculated were 2.3 x 10"^ and 4.3 x 10'^ cm/s. Assuming the average permeability ranges from 2.3 x 10''' to 4.3 x 10"^ cm/s (0.064 ft/day to 0.120 ft/day), the calculated average rate of movement ranges from 0.0043 ft/day to 0.0080 ft/day (or 1.6 ft/year to 2.9 ft/year). Beneath and immediately upgradient of the tailings cells, using hydrauhc gradients in the vicinity of each well, the estimated permeability at each well, and an effective porosity of 0.18, the estimated pore velocities ranged from 49.5 ft/year at TW4-21, to 0.010 ft/year at MW-23 (HGC, 2005), and have a geometric average of approximately 4.5 ft/year. Wells with relatively high calculated pore velocities, such as TW4-21, likely penetrate the relatively thin, coarser- grained, higher permeabihty zone discussed in Section 3.1.3.1.2, that is interpreted to "pinch Preliminary Correciive Action Plan ^ ^ G:\718000\7l801\CAP\Corrective Aclion Plan,doc Augusl 20, 2007 out" to the south and southwest, and does not appear to be present south of TW4-4 or south or southwest of the tailings cells. 3.2 Chloroform Occurrence Chloroform was detected in monitoring well MW-4 during groundwater split sampling conducted on May 11, 1999. The results of the sampling and analyses indicated chloroform concentrations of 4,520 and 4,700 |ig/L. As a result of the chloroform detection, the Utah Department of Environmental Quality (UDEQ) issued a Notice of Violation and groundwater Conective Action Order (the "Order") dated August 23, 1999. Subsequent investigation has included the installation of 25 temporary perched zone monitoring wells to delineate and monitor the chloroform (Figure 1). Chlorofomi concentrations in the perched zone as of the first quarter of 2007 are shown in Figure 12. Chloroform concentrations in the perched zone have ranged from non-detect to a maximum of 61,000 ft.g/L at weh TW4-20 in the second quarter of 2006. The chloroform concentration at TW4-20 has fluctuated, and was 4,400 \ig/L as of the first quarter of 2007. Chloroform concentrations at the most downgradient well, TW4-6, were non-detect for approximately 5 years, became detectable in the second quarter of 2005, and have been slowly increasing but have not exceeded 70 f,ig/L as of the furst quarter of 2007. Compounds associated with the chloroform include methylene chloride (DCM), chloromethane (CM), and nitrate. DCM and CM have been detected sporadically in chloroform 17 Preliminary Corrective Aclion Plan G:\718000\71801\CAP\Corrective Action Plan,doc Auausi 20. 2007 investigation wells at low concentrations (typically a few [igfL). Nitrate concentrations in chloroform investigation wells range from non-detect to the low mg/L range (typically < 10 mg/L, but have exceeded 10 mg/L at some locations near source areas). 3.2.1 Source Areas Investigation of potential source areas for the chloroform included a soil gas survey conducted in September 1999 (HGC, 1999). Detectable chloroform concentrations were measured in two suspected source areas; 1) the abandoned scale house leach field located approximately 1,100 ft north (upgradient) of MW-4, and 2) the former office leach field, located immediately southeast of the office building and immediately north-east of tailings cell #2 (Figure 13). These leach fields, abandoned more than 25 years ago, were known to have accepted sanitary wastes as well as laboratory wastes containing chloroform at quantities sufficient to have resulted in the measured groundwater concentrations. Discussions of the results of the soil gas survey and the identification of the abandoned scale house leach field as the most likely source of the chloroform detected at MW-4 are provided in lUSA and HGC, 2000. The former office leach field is considered the most likely source of the chloroform detected immediately northeast and upgradient of tailings cell #2. In general, the leach-field origin of the chloroform is supported by the following factors: 1) The leach fields are upgradient of the chloroform contamination, 2) Based on records of chloroform used in the laboratory, sufficient chloroform was disposed in the leach fields to result in the measured groundwater concentrations. 18 Preliminary Corrective Action Plan G :\718000\71801 \CAP\Correcti ve Action Plan.doc Augusl 20, 2007 3) Elevated nitrate is associated with the chloroform, and 4) The leach fields were designed to infiltrate water rapidly, which would reduce travel times to the perched water through the vadose zone. An additional conclusion based on the low soil gas chloroform concentrations detected was that a significant residual vadose zone source does not exist in either source area (HGC, 1999). The association of nitrate with the chloroform is discussed in lUSA and HGC, 2001. 3.2.2 Chloroform Concentration Trends Appendix B contains graphs of chloroform concentrations over time at the chloroform investigation wells (from DUSA, 2007). The chloroform investigation wells include MW-4, MW-26 (TW4-15), MW-32 (TW4-17), and the TW4-series wehs. As indicated, within the last few years, chloroform concentrations at most of the wells have been decreasing. Concentrations at some of the wells, for example TW4-20, have fluctuated substantially, even though concentrations at this well have been on a general downward trend during the last few quarters. Historically, the highest chloroform concentrations have been detected near MW-4 and TW4-19. The highest detected concentration was 61,000 \ig/L at TW4-20 in the second quarter of 2006. As of the first quarter of 2007, the chloroform concentration at TW4-20 was 4,400 fxg/L. TVv'4- 20 is located immediately downgradient of TW4-19 and the former office leach field source area. The highest detected concentration near MW-4 was 6,300 fig/L during the second quarter of 2001. As of the first quarter of 2007, the chloroform concentration at MW4 was 2,900 \ig/L. MW-4 is located downgradient of the abandoned scale house leach field source area. M\\'-4, 19 Preliminary Corrective Action Plan G:\718000\71801\CAP\Corrective Aclion Plan,doc Augusl 20, 2007 MW-26 (TW4-15), TW4-19, and TW4-20 are all pumping chloroform laden water as part of the interim remedial action for the site. The general reduction in chloroform concentrations within the plume over the last 2 years is illustrated in Tables 3, 4, and 5. Table 3 compares chloroform concentrations from the first quarter, 2007 with concentrations from the second quarter of 2005. Table 4 compares concentrations from the first quarter, 2007 with concentrations from the second quarter, 2006. Table 5 compares average concentrations over the four quarters from second quarter, 2006 to first quarter, 2007 with average concentrations over the four quarters from second quarter, 2005 to first quarter, 2006. Only wells with consistently detectable concentrations are included in these Tables. Between the second quarter, 2005 and first quarter, 2007 (Table 3), 11 wells decreased in concentration, 4 increased, and 1 remained the same. Between the second quarter, 2006, and the first quarter, 2007, 11 wells decreased in concentration, and 5 increased. Using the averages (Table 5), 10 wells decreased in concentration, and 6 increased. These comparisons indicate that despite short term fluctuations, chloroform concentrations within most of the plume area are decreasing. This decrease is attributed to mass removal by pumping and natural attenuation. 3.2.3 Chloroform Mass Removal Rates The interim action, which has included pumping of MW-4, MW-26 (TW4-15), TW4-19, and TW4-20, bas resulted in substantial removal of chloroform mass from the perched zone. 20 Preliminary Corrective Action Pian G:\718000\71801\CAP\Corrective Aclion Plan.doc Augusl 20. 2007 Chloroform mass removal rates and the cumulative mass removed can be estimated using the cumulative pumped volumes for each well and the average chloroform concentrations over the pumping period. Based on DUSA, 2007, during the first quarter of 2007, the approximate total volumes of water pumped were 81,230 gallons from MW-4; 54,400 gahons from MW-26; 605,400 gallons from TW4-19; and 163,520 gallons from TW4-20. Since the start of pumping, the lotal approximate volumes of water pumped were 1,307,110 gallons from MW-4; 930,510 gallons from MW-26; 6,768,986 gallons from TW4-19; and 642,290 gallons from TW4-20. Using first quarter, 2007, chloroform concentrations, and the first quarter pum.ped volumes, chloroform mass removal rales were approximately 0.15 lbs/day (pounds per day), and the lolal chloroform removed wilhin the quarter was approximately 13.6 lbs or 1.1 gallons. Since pumping began, using the total pumped volumes and average chloroform concentrations of 3,370 ng/L for MW-4, 1,660 ng/L for MW-26, 2,660 ng/L for TW4-19, and 16,240 \Lgfl. for TW4-20, an estimated 283 lbs, or 23 gallons of chloroform have been removed by pumping from the perched zone. Average chloroform concentrations used in the above calculations are the averages of all chlorofonn analytical resulls for each well during each well's pumping period. The lotal amount of chloroform estimated to remain in the plume is approximately 650 lbs or 52 gallons. This estimate is based on the first quarter, 2007 saturated thicknesses (Figure 10), and the average chloroform concentrations from the second quarter of 2006 to the first quarter of 2007. Average chloroform concentrations were used because of the large fluctuations in concentrations measured at pumping well TW4-20. The lotal amounl esiimated to have been removed by pumping is approximately 44 % of the estimated amount remaining. Assuming that 21 Preliminary Correciive Action Plan G:\718000\71801 \CAP\Corrective Aclion Plan.doc Aucusi 20, 2007 no natural attenuation of chloroform has occuned, the total amount lhat entered the perched zone can be approximated as the sum of the esiimated amounts pumped and remaining, or approximately 75 gallons. The tolal removed by pumping would then be approximately 30 % of the initial amount. The actual percentage of the inilial amount removed by pumping may be somewhat less than 30 % because natural attenuation of chloroform, in particular biodegradation of chloroform as discussed in Appendix C, has likely been a significant mass removal mechanism. Accounting for natural attenuation would increase the estimate of the initial chloroform mass in the perched zone. Regardless of the mass reduction contributed by nalural attenuation, the amount of chloroform removed by pumping has been significant. 22 Preliminary Corrective Action Plan G:\718000\7180I\CAP\Corrective Action Plan.doc August 20, 2007 4. CHARACTERIZATION OF STUDY AREA The study area encompasses an area in the northeastem portion of the site where the chloroform plume has been detected and bounded by a series of chloroform investigaiion wells. These include wehs MW-4 and MW-26 (TW4-15), and temporary weUs TW4-1 through TW4- 25. Characterization of the study area has been based on dala collected from MW-4, MW-4A, MW-26 (TW4-15), and TW4-1 through TW4-22. Wells TW4-23, TW4-24, and TW4-25 were installed in May, 2007 to refine the boundaries of the plume and will require recovery time before represenlalive waler level and analytical data can be collected. The extent and hydrogeology of the study area is discussed below. Sources of the chloroform plume, and trends in waler levels and chloroform concentrations within the study area are discussed in Section 3.2. 4.1 Extent The study area includes the region of the perched zone containing chlorofomi concentrations exceeding 70 ng/L, and the immediately sunounding area. The area containing chloroform exceeding 70 ng/L, as ofthe first quarter of 2007, is shown in Figure 12. This area is located east and northeast (cross- and up-gradient) of the tailings cells. As discussed in section 3, the highest chloroform concentrations have historically been delecled near MW-4 and TVv^4-19. in areas downgradient of the source areas. The chlorofonn plume, as defined by the 70 ng/L concentration boundary, is bounded to the south by TW4-6 and MW-32, and to the east by TW4-3, TW4-5, TW4-8, TW4-9. T\\^4-13, 23 Preliminary Correciive Aclion Plan G;\718000\71801\CAP\Corrective Aclion Plan,doc Augusl 20, 2007 TW4-14, and TW4-18. The southern half of the plume is bounded lo the west by MW-32 and TW4-16. The northem half of the plume is bounded lo the south and soulhwesi by MW-31 and is somewhat poorly bounded to the north by MW-27 and to the west by MW-28. Wells TW4-23, TW4-24, and TW4-25 (Figure 1) were instaUed in May, 2007, lo refine the limits of the plume lo the south, west, and north, respectively. Some time will be required for recovery of these wells before analytical resulls can be considered representative. 4.2 Hydrogeology A description of the hydrogeology of the site in the vicinity of the chloroform plume is provided in Section 3.2. In general, the chloroform plume is associated with a region of relatively high perched zone permeabilities. This region is generally defined by wells that responded measurably to pumping at MW-4, MW-26 (TW4-15), and TW4-19 during the first 7 months of the interim remedial aclion (long-lerm pumping lesl) (Figure 4). Most of the detecied chloroform is considered to be within higher permeability malerials because most of the detections have been in wells lhal responded to the long term pumping. To the south, in the downgradient portion of the plume, the chloroform is considered lo have migrated into lower permeabihty materials near TW4-4 and TW4-6. The chloroform delecled al TW4-11 is also considered to be localed within lower permeability materials. TW4-4, TW4-6. and TW4-11 are all wells that did not respond measurably during the first 7 months of long term pumping. Perched groundwater flow in the area of the chloroform plume ranges from south westerly in the westem portion of the study area lo southerly in the eastem portion of the area. 24 Preliminary Corrective Aclion Plan G:\718000'\71801\CAP\Corrective Action Plan,doc Augusl 20, 2007 Saturated thicknesses in the study area are generally higher than in areas to the south and southwest. As shown in Figure 10 they range from a maximum of approximately 84 ft at TW4- 18 lo approximately 22 fl al TW4-6. TW4-6 is the most downgradient temporary chloroform investigation well. At TW4-14 (east of TW4-6), the perched zone apparentiy pinches out, with saturated thicknesses of only a few feet. In general, saturated thicknesses increase toward the northeast, where the wildlife ponds are located, and are locally affected by pumping al MW-4, MW-26 (TW4-15), TW4-19, and TW4-20. Average permeabilities within the general area of the chloroform plume based on analysis of drawdowns from wells that responded to the first 7 months of pumping (HGC, 2004) ranged from approximately 4 x 10"^ to 5 x 10'"^ cm/s, and have a geometric average of 1 x 10"''' cm/s, assuming unconfined conditions, as discussed in Section 3. Estimated average permeabilities near TW4-19 were approximately five limes higher than those near MW-4 (HGC, 2004). Estimated storage coefficients indicated that on average the perched zone in this area is unconfined but approaches conditions intermediate between confined and unconfined, and may be confined locally. The combination of relatively high permeability and relatively lai-ge saturated thickness in the northem and central portions of the area make the productivity of the perched zone high in these areas. Sustainable pumping rates of as much as about 4 gpm allow relatively high chloroform mass removal rates at the existing pumping wells. The range in esiimated perched zone permeabilities in the vicinity of the chloroform plume, which are representative of the higher permeabihty zone, are one to two orders of magnitude greater than estimates for areas downgradient of the chloroform plume and the 25 Preliminary Corrective Action Plan G:\718000\71801\CAP\Corrective Aclion Plan,doc Augusl 20, 2007 tailings cells. This reduction in permeability to the south and southwest is interpreted as a "pinching oul" of a coarser-grained, higher permeability zone identified during installation of many of the temporary wells (HGC, 2005). The pinching oul of this zone is important in limiting the rate of downgradient migration of chloroform, in stabilizing the plume boundaries, and in allowing natural attenuation to be more effective in limiting plume migration. The combination of relatively high permeability and relatively large saturated thickness in the upgradient portions of the plume that make the productivity of the perched zone high and allow relatively high chloroform mass removal rates, is absent al downgradient wells such as TW4-4 and TW4-6. The combination of relatively low permeability and small saturated thickness near these downgradient wells makes pumping al these wells impractical. 26 Preliminary Corrective Action Plan G:\718000\71801\CAP\Corrective Action Plan,doc August 20. 2007 5. CORRECTIVE ACTION CONCENTRATION LIMITS The conective action concentration limit for chlorofomi is 10\igfL.. This concentration is considered to bound the outer extent of the plume and is the ultimate target for reducing chloroform concentrations within the plume. As discussed in Section 9, once the chloroform concentrations in all chloroform moniloring wehs are 70\igfL or less, concunence with UDEQ will be sought that the plume is remediated and the conective action complete. 27 Preliminary Correciive Action Plan G:\718000'\71801\CAP\Corrective Action Plan,doc Augusl 20, 2007 6. CORRECTIVE ACTION PLAN - CONSTRUCTION AND OPERATION The conective action for the sile is proposed lo occur in two phases. The first phase (Phase 1) wih essentially be a continuation of the cunently implemented interim action for the sile, with specified contingencies. Phase 1 relies on both pumping and natural attenuation lo remove chloroform mass, reduce chloroform concentrations within the plume, and minimize or prevent plume migration. Included in Phase 1 are continued moniloring wilhin and outside the plume lo verify plume boundaries (as defined by a concentration of 70 ng/L), estimate changes in hydraulic capture, and track changes in chloroform concentrations within the plume. The second phase (Phase 2) will rely only on nalural attenuation to reduce residual chloroform concentrations wilhin the plume to 70ng/L or less, and monitoring to verify plume boundaries and track changes in concentrations within the residual plume. Once residual concentrations have dropped to 70ng/I- or less at all monitored locations, concurrence with UDEQ will be sought that the conective action is complete. Both Phase 1 and Phase 2 have contingencies to be implemented if needed based on moniloring as discussed in Section 7, The termination of Phase 1 and implementation of Phase 2 will be with the concunence of UDEQ and will be based on dala collected as part of the routine monitoring during Phase 1. and quantitative calculations that may include the use of numerical models. These calculations will consider residual chloroform concentrations, natural attenuation rates, and expected chloroform migration rates in the absence of pumping. An important goal of Phase 2 is to manage Phase 1 and to implement Phase 2 such thar chloroform concentrations exceeding the action level wih not migrate into the area south or 29 Preliminary Correciive Aclion Plan G:\718000\71801\CAP\Correciive Aclion Plan,doc Ausust 20, 2007 southwest of the tailings impoundments wilhin 200 years. As discussed in Appendix C, migration of the chlorofonn plume inlo this area is nol expected lo occur. However, the decision lo terminate Phase 1 and implement Phase 2 will be based on Phase 1 monitoring data and quantitative calculations that indicate this goal is attainable 6.1 Phase 1 Phase 1 consists of two active components and one passive component. The active components are: 1) Removal of chloroform mass from the perched zone as rapidly as is practical by pumping from wells localed in areas having both high chloroform concentrations and high productivities, 2) Perched zone waler level and chloroform moniloring to assess changes in chloroform concentrations within the plume, verify the location of the plume boundary over time, and estimate hydraulic capture zones. Pumped waler will be disposed in the tailings cells as 1 I.e.(2) byproduct. The passive component consists of relying on natural attenuation to remove chloroform mass and reduce concentrations. Physical mechanisms lhal will reduce chlorofonn concentrations include processes that remove chloroform mass such as volatilization from the water table and abiotic reductive dechlorination, and processes such as hydrodynamic dispersion that rely on mixing with recharge and waters outside the plume. In addiiion to these physical mechanisms, biologically mediated decomposition of chloroform is also expected to reduce chlorofonn mass and concentrations within the plume. A discussion of expected rates of chloroform degradation by biological and abiotic means is provided in Appendix C. 30 Preliminary Corrective Action Plan G:\718000\71801\CAP\Corrective Aclion Plan,doc Auausi 20, 2007 In addiiion lo these mechanisms which reduce chloroform concentrations wilhin the plume, retardation of the plume by sorption onto nalural organic carbon in the subsurface will act to slow the rale of downgradient migration. Sorption onto organic carbon and mass loss by volatilization will act to retard the migration of chlorofonn with respect to more conservative constituents such as nitrate which does not sorb and is nol volatile. In general. Phase 1 is a continuation of the existing and ongoing interim remedial action al the site. Monitoring of the interim action, which began in 2003, has included estimation of capture zones for pumping wells MW-4, MW-26 (TW4-15), TW4-19, and TW4-20 based on quarterly waler level contour maps generated as part of the quarterly chloroform monitoring reports submitted by DUSA lo UDEQ. The latest report (DUSA, 2007) covered the first quart.er of 2007. Figure 14 is a map showing the plume boundary, the esiimated combined capture zones of MW-26 (TW4-15), TW4-19, and TW4-20, and the estimated capture zone for MW-4 for the first quarter, 2007 (from DUSA, 2007). As shown, hydraulic capture of approximately 2/3 tc 3/4 of the plume has been achieved. A portion of the southern half of the plume is cunently outside the esiimated capture zone. Although the extent of the capture zone is expected lo increase over lime, including expansion to the south, it is unlikely that complete hydraulic capture of the plume is achievable with the cunent pumping scheme. However, pumping in the southern (downgradient) extremity of the plume is impractical due to low permeability and low saturated thickness (HGC, 2005). Because low permeabihty condilions lo the south, and flattening hydraulic gradients resulting from upgradient pumping will reduce rales of downgradient 31 Preliminary Correciive Action Plan G:\718000\71801\CAP\Correciive Action Plan,doc Auaust 20, 2007 migration, natural attenuation will likely be effective in treating that portion of the plume that will remain outside hydraulic capture (Appendix C). As a result of the above factors (reduction in hydraulic gradients, transition to lower permeability conditions lo the south, nalural attenuation), and the reduction in chloroform concentrations in upgradient areas resulting from pumping, the plume is expected to stabilize. For example. Figure 15 compares the extents of the chloroform plume in the first quarters of 2006 and 2007. Over this period, the plume seems to be relatively stable, having expanded slightly in some areas and contracted slightiy in others. Although the plume appears lo have stabilized, continued monitoring is needed to verify this condilion. If continued monitoring indicates the plume has not stabilized, then contingencies wih be implemented. Data collected during Phase 1 moniloring will be used lo calculate chloroform mass removal rates by pumping, estimate mass removal by natural in-situ degradation, and estimate migration rales once pumping ceases. Numerical and/or analytical models will be used as needed lo assist in evaluating the data and estimating nalural in-situ degradation. 6.1.1 Groundwater Pumping System The Phase 1 conective action groundwater pumping system wih be the same as cunently used for the interim remedial action. Wells MW-4, MW-26 (TW4-15), TW4-19, and TW4-20 are the cunent pumping wehs. Each well is equipped with a Gmndfos submersible pump. To prevent damage lo the pumps, each operates on a cycle that allows pumping only when sufficient water 32 Preliminary Corrective Action Plan G:\718000\71801 \CAP\CorTective Action Plan,doc August 20, 2007 is present in the well. The capaciiy of each pump is greater lhan the sustainable pumping rale for each well. Therefore, the average amounl of water pumped from each well is, in general, the maximum practical. These wells were selected for pumping because they are located in areas of the perched zone having both high chloroform concentrations and relatively high permeabilities that allow relatively high rates of mass removal lhal are nol possible within low permeability, low yield downgradient areas. Water pumped from each well is considered ll.e.(2) byproduct and is routed by discharge lines lo the tailings cells for disposal. The discharge line near each wellhead is equipped wilh an in-line flow meter and totalizer. Readings from each totalizer are used to report quarterly pumped volumes and average pumping rales. Operation of the wellfield will be as described in the Operations and Maintenance Plan, Chloroform Pumping System, White Mesa Mill, Blanding, Utah, which includes provisions for daily inspections: The contingencies described in Section 7 will be implemented should chloroform mass removal rales drop significantly due to losses in well productivity. 6.1.2 Water Level Monitoring Continuation of the monthly water level monitoring for all non-pumping chloroform investigation wells, and weekly moniloring for pumping wells, is proposed for Phase 1, The chloroform investigation wehs are MW-4, MW-26 (TW4-15), MW-31, and all TW4-series wells. Depths to water will be measured using an electric water level meter in the same wa)' they 33 Preliminary Corrective Action Plan G:\718000'\71801\CAP\Corrective Action Plan,doc Augusl 20, 2007 are cunentiy collected. Hydraulic capture zones wih be estimated from water level contour maps generated quarterly from the waler level data. The contingencies described in Seclion 7 will be implemented should the proportion of the remaining chloroform plume that is under hydraulic capture shrink significantly. 6.1.3 Water Qualitv Monitoring Continuation of the quarterly water quality monitoring for ah chloroform investigation wells is proposed for Phase 1. Sampling and analytical procedures will be the same as cunently employed for the chloroform monitoring as described in the quarterly chloroform moniloring reports submitted by DUSA to UDEQ. Each well will be sampled for the following constituents: • Chloroform • Chloromethane • Carbon tetrachloride • Methylene chloride • Chloride • Nitrogen, Nitrate -i- Nitrile as N Should concentrations within the plume begin to generally increase (disregarding short- term fluctuations), or the plume boundaries begin to expand, the contingencies discussed in Section 7 will be implemented. 6.1.4 Reporting Conective aclion reporting is proposed to occur semi-annually, using a formal and content similar lo the quarterly chloroform moniloring reports submitted by DUSA to UDEQ 34 Preliminary Correciive Action Plan G:\718000\71801\CAP\Corrective Action Plan.doc Augusl 20. 2007 (see DUSA, 2007). The first semi-annual report submitted each year will cover the third and fourth quarters of the previous year, and the second semi-annual report submitted each year will cover the first and second quarters of lhal year. The semi-annual reports will, in addition to the elements of the cunent quarterly reports, contain the following: 1) calculation of quarterly chloroform mass removed by pumping 2) comparison of the cunent areal extent of the chloroform plume from the latest quarter wilh the latest quarter of the previous reporting period 3) discussion of any contingencies to be implemented 6.2 Phase 2 Phase 2 will consist of 1) reliance on natural attenuation lo reduce remaining chlorofonn within the plume to action levels and 2) continued monitoring. Phase 2 will be implemented wilh the concunence of UDEQ once concentrations have been judged to have been reduced sufficiently that pumping can cease, and nalural attenuation wih be sufficient to remediate the remaining chloroform. At a minimum, factors that will be considered include 1) expected rates of nalural in-situ degradation of chloroform, 2) hydrodynamic dispersion, mixing, and volatilization, 3) retardation by sorption and 4) perched zone hydrauhc condilions. These factors will all be considered in estimating the rale of migration of the residual plume and the time required for all concentrations to be reduced to the action levels. Numerical and/or analytical models will be used as needed in the evaluation. 35 Preliminary Corrective Action Plan G:\71800O\71801 \CAP\Correclive Aclion Plan.doc August 20. 2007 6.3 Achievement of Concentration Limits The CAP, as described in sections 6.1 and 6.2 above, is designed lo meet the chloroform concentration limil of 10\ig/L. Alternate Standards are not believed to be necessary al this time. 36 Preliminary Correciive Action Plan G:\718000\71801 \CAP\Corrective Action Plan.doc August 20, 2007 7. ASSESSMENT OF CORRECTIVE ACTION AND PROTECTION OF PUBLIC HEALTH AND THE EP^IRONMENT The effectiveness of Phase 1 of the conective aclion will be assessed based on the following criteria: 1) stability of plume boundaries 2) concentraiion trends within the plume 3) chloroform mass removal rales resulting from pumping, and 4) stability of capture zones with respect to the proportion of the chloroform plume wilhin hydraulic capture The effectiveness of Phase 2 of the conective action will be assessed based only on above criteria 1 and 2. The following sections describe the contingencies to be implemented based on effectiveness assessed using the above criteria. 7.1 Stability of Plume Boundary (Phase 1) The stabihty of the plume boundary, based on Phase 1 CAP monitoring activities discussed in Sections 6 and 8, will be used to determine the following: Whether additional perched zone moniloring wells will be installed, and The need to reevaluate the Phase 1 strategy. Under condilions where the plume boundaries remain stable or contract, no additional downgradient moniloring wells wih be installed, and no reevaluation of Phase 1 will be needed. Under conditions where the plume migrates beyond existing downgradient moniloring wells, and 37 Preliminary Correciive Aclion Plan G:\718000'\7180l\CAP\Corrective Action Plan,doc August 20, 2007 with the concunence of UDEQ, one round of addilional downgradient wells will be installed. The addilional downgradient wells will be installed within one year. If the plume migrates beyond these wells, then Phase 1 will be reevaluated. Analytical or numerical models will be used if needed in the reevaluation to develop a response. The reevaluation process will be completed wilhin one year. Anticipated responses to this condilion would likely include adding existing or new wells to the pumping network to slow the migration rates and/or lo bring more of the plume under hydrauhc capture, and installation of additional downgradient moniloring wells as needed. 7.2 Concentration Trends within the Plume (Phase 1) Concentration trends within the plume will be used to determine the need for reevaluation of Phase 1. Concentration trends will be based on analytical dala collected through Phase 1 GAP monitoring. Under condilions where concentrations within the plume continue their cunent generally downwai-d trend (disregarding short term fluctuations), no reevalualion will be required. Should concentrations within the plume begin to generally increase (disregarding short term fluctuations), then reevaluation of Phase 1 will be required. Analytical or numerical models will be used in the reevaluation if needed lo develop a response. The reevaluation process wih be completed wilhin one year. Anticipated responses to this condilion would likely include adding 38 Preliminary Corrective Action Plan G:\718000\71801\CAP\Corrective Action Plan.doc Auaust 20. 2007 I I existing or new wells lo the pumping network, or olher measures designed to achieve a more rapid rate of mass reduction. 7.3 Chloroform Mass Removal Rates Resulting from Pumping (Phase 1) Under conditions where chloroform mass removal rates by pumping drop substantially as a resull of reduced concentrations within the plume, no aclion will be taken. Under conditions where chloroform mass removal rates by pumping drop substantially as a result of lost well productivities, then an evaluation of the lost productivity wih be undertaken. If the lost productivity is detennined to be a well efficiency problem, the inefficient wells will be re- developed or replaced wilhin one year. Should the lost productivity be determined lo be due to a general reduction in saturated thickness, analytical or numerical models wih be used to evaluate the potential effectiveness of adding existing or new wells to the pumping network to improve overall productivity. If the analysis indicates that overall productivity will not improve significantly by adding wehs, then no action wih be taken. A loss in productivity due lo a general decrease in saturated thickness wih be offset by the benefits of the reduced saturated thickness. First, this condition would indicate lhat removal of a substantial amount of chloroform laden water had already laken place. Second, the reduced saturated thickness wilhin the chloroform plume would reduce average hydraulic gradients and reduce the potential for downgradient migration. These factors wih be considered in any reevaluation that may be performed. 39 Preliminary Correciive Aclion Plan G:\718000\71801\CAP\Correciive Action Plan.doc August 20. 2007 I 7.4 Stability of the Proportion of the Chloroform Plume under Hydraulic Capture (Phase 1) Under condilions where the proportion of the remaining chloroform plume lhat lies wilhin hydraulic capture shrinks substantially, an evaluation of the factors resulting in this condition will be undertaken. If the condition is delermined lo resuh from lost productivity of the pumping wells due to well efficiency problems, the inefficient wells will be re-developed or replaced wilhin one year. Should the loss in capture be determined to resuh from other condilions, then Phase 1 will be reevaluated. Analytical or numerical models will be used in the reevaluation if needed to develop a response. The reevaluation process will be completed wilhin one year. Anticipated responses to this condilion would likely include adding existing or new wells to the pumping network to bring a larger proportion ofthe plume within hydraulic capture. 7.5 Phase 2 As part of Phase 2. water levels and chloroform concentrations wilhin the residual plume will be monilored in the same fashion as in Phase 1. except that pumping related monitoring will not occur. Because no pumping will occur, the plume may migrate to some extent depending on the relative rates of natural attenuation and perched groundwater flow, and any additional downgradient monitoring wells needed lo define the limils of the plume will be installed. Installation of additional downgradient wells to define the plume boundary would be wilhin one year of concentrations in the existing downgradient wells exceeding the action level and wilh the 40 Preliminary Corrective Aclion Plan G:\718000'\71801\CAP\Correclive Action Plan.doc Augusl 20, 2007 I concunence of UDEQ. Should concentrations within the plume begin to generally increase (disregarding short term fluctuations), or should the plume migrate near the southern edge of the taihngs cells, then Phase 2 will be reevaluated. Analytical or numerical models will be used in the reevaluation as needed lo develop a response. The reevaluation process will be completed wilhin one year. Anticipated responses to these conditions would likely include a resumption of pumping or olher measures taken to slow the rale of chloroform migration and increase the rate of mass reduction within the residual plume. 7.6 Permanent Effect of Corrective Action Phase 1, Phase 2, and the contingencies outlined above (Sections 7.1 through 7.5) are designed to proiect the public health and the environment by containing the chlorofonn plume within the site property boundary and reducing chloroform concentrations wilhin the plume to the concentration limit of 70ng/L. As concentrations will then continue lo be reduced by natural attenuation, the corrective action will have a permanent effect. 7.7 In-Place Contaminant Control As discussed in Section 6, the conective aclion relies on active and passive strategies to meet CAP objectives. The passive strategy includes in-place conlaminant conlrol by removing chloroform via in-situ natural biodegradation. A significant portion of the chloroform wilhin the plume is anticipated to be treated in place via nalural biodegradation as discussed in Appendix C. 41 Preliminary Correciive Action Plan G:\718000'\71801 \CAP\Correciive Action Plan,doc August 20, 2007 8. IMPACTS OF OFFSITE ACTFVITIES As discussed in Section 6, chloroform will be treated in place by natural attenuation and removed from the perched zone by pumping. Because all pumped water is considered 1 l.e.(2) byproduct and will be disposed onsite in the tailings cells, there will be no offsite impacts resulting from CAP implementation. 43 Preliminary Corrective Aclion Plan G:\718000\71801\CAP\Corrective Action Plan,doc Augusl 20, 2007 9. PROPOSED PLUME CORRECTIVE ACTION ACTIVITIES Phase 1 and Phase 2 conective aclion activities and contingencies are discussed in detail in Sections 6 and 7. These activities are summarized in Sections 8.1 and 8.2 below. 9.1 Phase 1 Phase 1 corrective action activities include continued pumping of wells MW-4. MW-26 (TW4-15), TW4-19, and TW4-20. moniloring and maintenance of the pumping system, water level monitoring, monitoring for chloroform and olher constituents, estimation of hydraulic capture, implementation of contingencies as needed, and reporting. 9.1.1 Groundwater Pumping Wehs MW-4, MW-26 (TW4-15), TW4-19, and TW4-20 will continue lo be pumped at the mdximum practical rales. Pumped waler will be disposed in the tahings cells as Jl.e.(2) byproduct. The wellfield will be operated and maintained in accordance wilh the Operations and Maintenance Plan, Chloroform Pumping System, White Mesa Mill, Blanding, Utah. Moniloring will include pumping rates and volumes for each well. 45 Preliminary Correciive Aclion Plan G;\718000"\71801\CAP\Conrective Aclion Plan.doc Augusl 20, 2007 9.1.2 Waler Level Monitoring Waler levels will be monilored al ah chloroform investigation wehs (MW-4, MW-26, MW-32, and TW4-series wells) at the same frequency and using the same methods as cunently employed. This includes weekly monitoring of pumping wells and monthly monitoring of non- pumped wells. Water level contour maps of the data will be generated quarterly. 9.1.3 Water Quahty Monitoring All chloroform investigation wells will be sampled quarterly using the same meihods as cunentiy employed. Samples wih be analyzed for chloroform, chloromethane, methylene chloride, carbon tetrachloride, chloride, and nitrogen, nitrate and nitrite as N (Seclion 6.1.3). 9.1.4 Estimation of Capture Zones Hydraulic capture zones will be generated from the quarterly waier level contour maps in the same manner as they are cunently generated. 9.1.5 Estimation of Pumped Chloroform Mass Quarterly estimates of chloroform mass removed by pumping will be made based on cumulative pumped volumes at each pumped well and chloroform concentrations at each pumped well. 46 Preliminary Correciive Action Plan G:\718000\71801\CAP\Corrective Aclion Plan,doc Auaust 20. 2007 9.1.6 Reporting Semi-annual reports will be prepared that contain the elements of the cunent quarterly chloroform moniloring reports submitted by DUSA lo UDEQ (see DUSA, 2007) in addiiion to the following: 1) quarterly chloroform mass removed by pumping 2) comparison of the areal extent of the chloroform plume from the latest quarter with the latest quarter of the previous reporting period 3) discussion of any contingencies implemented or to be implemented The first semi-annual report submitted each year will cover the third and fourth quarters of the previous year, and the second semi-annual report submitted each year will cover the first and second quarters of lhat year. 9.1.7 Additional Measures Based on Phase 1 moniloring, and the criteria discussed in Section 7, contingencies lhat include potential installation of additional wehs, well rehabilitation or replacement, expansion of the pumping well network, and reevalualion of the Phase 1 strategy will be implemented as needed. Factors that could trigger the implementation of contingencies include 1) expansion of the plume boundaries, 2) generally increasing chloroform concentrations wilhin the plume, 3) reductions in chloroform mass removal rates due lo losses in pumping well productivities, and 4) decreases in the proportion of the plume under hydraulic capture. 47 Preliminary Corrective Aclion Plan G:\718O0O\71801 \CAP\Corrective Action Plan.doc Auaust 20. 2007 9.2 Phase 2 Phase 2 conective aclion aclivilies include quarterly moniloring for chloroform and the other analytical parameters listed in section 6.1.3, quarterly water level moniloring, and semi- annual reporting. Because the pumping system will be inactive, the semi-annual reports will not contain information related to the pumping system. Other elements of the reporting will be the same as for Phase 1. Based on Phase 2 moniloring, and the criteria discussed in Seclion 7, contingencies lhal include potential installation of additional wells, reestabhshment of pumping, and reevaluation of the Phase 2 strategy, will be implemented as needed. Once chloroform concentrations at all monitoring locations are at or below the action level of 70 ng/L, concunence with UDEQ wih be sought that the conective aclion is complete, as discussed in Section 5. I I 48 preliminary Corrective Aclion Plan G:\718000\71801\CAP\Corrective Aclion Plan,doc Auaust 20, 2007 10. REFERENCES Bouwer, H. and R.C. Rice. 1976. A slug lesl meihod for determining hydrauhc conductivity of unconfined aquifers with completely or partially penetrating wells. Water Resources Research, Vo. 12:3. Pp. 423-428. Denison Mines (USA) Corp, 2007. White Mesa Mill Chloroform Monitoring Report. State of Utah Notice of Violation and Groundwater Conective Action Order UDEQ Docket No. UGQ-20-01. 1'' Quarter (January Ihrough March), 2007. Hyder, Z., J.J. Butler, CD. McElwee, and W. Liu. 1994. Slug tests in partially penetrating wells. Waler Resources Research. Vol. 30:11. Pp. 2945-2957. Hydro Geo Chem. 1999. Letter Report Submitted to Michelle Rehmann, Intemational Uranium (USA) Corporation, Denver, Colorado. Hydro Geo Chem. 2001. Letter Report Submitted to Harold Roberts, Intemational Uranium (USA) Corporation, Denver, Colorado. Hydro Geo Chem. 2001b. Evaluation of Hydraulic Test Dala al MW-4, White Mesa Uranium Mill Sile, Blanding, Ulah. Submitted to Intemational Uranium (USA) Corporation, Denver, Colorado. Hydro Geo Chem. 2002. Hydraulic Testing al the White Mesa Uranium Mih near Blanding, Ulah, During July, 2002. Submitted to Intemational Uranium (USA) Corporation, Denver, Colorado. Hydro Geo Chem. 2004. Final Report. Long Term Pumping at MW-4, TW4-19, and TW4-15, While Mesa Uranium Mhl Near Blanding, Ulah. Submitied lo Intemational Uranium (USA) Corporation, Denver, Colorado. Hydro Geo Chem. 2005. Perched Monitoring Well Installation and Testing at the Whhe Mesa Uranium Mih, April Through June, 2005. Submitted lo Intemational Uranium (USA) Corporation, Denver, Colorado. Hydro Geo Chem. 2007. Site Hydrogeology and Estimation of Groundwater Travel Times in the Perched Zone, While Mesa Uranium Mill Site near Blanding, Utah. Submitted to Denison Mines (USA) Corp., Denver, Colorado. Intemational Uranium (USA) Corporation and Hydro Geo Chem. 2000. Investigation of Elevated Chloroform Concentrations in Perched Groundwater al the White Mesa Uranium Mill near Blanding, Utah. 49 Preliminary Corrective Aclion Plan G:\718000\71801\CAP\Corrective Aclion Plan,doc Augusl 20, 2007 hiteraaiional Uranium (USA) Corporation and Hydro Geo Chem. 2001. Update to report "Investigation of Elevated Chloroform' Concentrations in Perched Groundwater at the While Mesa Uranium Mill near Blanding, Ulah". Knighi-Piesold. 1998. Evaluation of Potential for Taihngs Ceh Discharge - While Mesa Mill. Attachment 5, Groundwater Information Report, White Mesa Uranium Mill, Blanding, Utah. Submitted to UDEQ. TFT AN. 1994. Hydrogeological Evaluation of While Mesa Uranium Mhl. Submitted to Energy Fuels Nuclear. UMETCO. 1993. Groundwater Study. White Mesa Facihties. Blanding, Utah. Prepared by , UMETCO Minerals Corporation and Peel Environmental Services. I I 50 Preliminary Correciive Aclion Plan G:\718000\7180I\CAP\Correciive Aclion Plan.doc August 20, 2007 11. LIMITATIONS STATEMENT The opinions and recommendations presented in this report are based upon the scope of services and informaiion obtained Ihrough the performance of the services, as agreed upon by HGC and the party for whom this report was originally prepared. Results of any investigations, tests, or findings presented in this report apply solely to condilions existing at the time HGC's investigative work was performed and are inherentiy based on and limited lo the available data and the extent of the investigation activities. No representation, warranty, or guarantee, express or implied, is intended or given. HGC makes no representation as to the accuracy or completeness of any infonnation provided by other parties nol under contract lo HGC to the extent that HGC relied upon that information. This report is expressly for the sole and exclusive use of the party for whom this report was originally prepared and for the particular purpose lhat it was intended. Reuse of this report, or any portion thereof, for other lhan ils intended purpose, or if modified, or if used by third parties, shah be at the sole risk of the user. 51 Preliminary Correciive Aclion Plan G:\718000\71801\CAP\Corrective Aclion Plan.doc Auaust 20, 2007 I I TABLES 10 3 in « 0) >< I— lij S = -I 5 DO < 5 I- l) u •-= CO 3 (0 •o >> X I I I I c 0) 0) 0} IB m 0 0 Tf c c c C c C c in o o o o o o o CD tf) c c c c c c c CD « IL •2 * c 2 S o o o o CN CM CN o o o o CO 00 oo in ID CO oo So*: C-J CM iri iri Tf Tf IT) in CO ^ tf) H 0) O) CO k. c s t o fo o t ) o b o b c s t o fo o t ) [ { 1 ! X ; 1 X 1 1 I X 1 1 1 X 1 CN CD r— CO 'o — 0) Tf Tf a tf) >• in in b > CN CO CN 00 CO O) O b Tf CO b CN in b or a t O q o O O O o b o b CO O) O b • b Tf O b o b o o b ! X CN O b X tf) csi >> :^:l-s a b b in b b in O in b b b in b in b in b in b t o t o .1- b b 2 o „ •o-o £ T— T— T— T— T— T— T— '— '— T— 2 o „ •o-o £ X X X X X X X X X X X X X X X X X 2 o „ •o-o £ r~-CD ro o in O) Tf CT) CD Tf CO CD Tf I o ~ od cri cS c\i CN CN CN Tf uS CN csi o £:• £- > > > 0) o O o o o C3) a> O) a> O) CD CD CD Oi O) a> a> 0) 3 3 3 3 3 3 3 3 1-Q, E Q, Q. E Q. Q. E 3 Q. tfl to tf] (/) (o 0) Ui cn l/i U) (/) 05 . . T3 •a 0) a) 0 0 _c c C C ,—, c 0) •o -o i.— i.— I.— c >: C <D c 0) c c 0 c 0 c o di] > c > .E > 8 > ,E O 3 > .£ o > .£ o J2 o CL di] d 8 a o g Q. o § dl O 3 Q. dl d5 tL CL 9° d5 o .c CO -CO X CO o CO . I CO o CO . 03 „-CO o CO -CO o CO -^ 4.* LU D X LU c LU ID I LU tz LU 0 LU 9> X LU 1= LU 0 X X LU c UJ 0 Q. 0> s = 1- _-1- D i= 3 1- ^ g 1- 3 1- .y g g 1- 3 1- .y Q. 0> s = O 0 o -g ^ CD o a: o Q: < s O a: o Q: < S in t <| < c ^ CD < o 3 0 ra o « >i C o I o ^ O 0 a c o is -o S o 0 £ ^ 44 a. 0 s = c 0 X dii CO . HI O 0 < o > .£ O § CO o ^5 ^ CD s E O •£ CO o LU O I- - o w < CD O D CO -LU 0 1- .y o ^ .E dl tf) o 1^5 o 5 CO -LU g o a: g _i • o CO LU I- D o CO o CO LU I-O < > .£ _i ' O CO O CO LU > .g dl CO O 3 CO CN o CQ o CO LU . X tf) •o "> ffl <A JS 0 ^ 2 = 0 2 o <S: ra 2 tf) t o 5 ° u 0 a. tf) >> -> o *. tf) ^ -5 -J ra o « •D -D E >. c o T o ~-O 0 a. >> I- c o iS o 0 f 1_ «,( Q. V 0 = c 0 3 > .E _i ' O CO LLI > .E _l O o CO o ico-^ CD > .£ _i •• O CO ^3 > .£ _i •• O CO 3 s o CO ^3 to 3 E > ,E _j O CO ^3 O CO > ,E I- 3 I- •a 0 > ,E _i • O o CO o ^3 i<o ^ CD :£ O n CO . LU 0 I- ,y a < ii s 3 O > ,E _i ' O CO < JJ. S 2 o « U > < O) tf) C 0) 'Z c m J£ 0) u •2 = N O £ tn o o > ^ M >< tf) ra ra .1 o 5 T3 •S ® •-O c " I i •c si 3 c _o ra 0) U) .Q o Ui b t o in CD Ui 2x - 3x ' X CO ,7 x ' co T-T* IT) CO CM cn oo CO CO CM r-J o o TD 0 c M— c o o c CO m CO CO CD o o o o CD oo Tt CO TD CD c 'c o o c 3 CO o o o o CD CD 1^ 0) c i*— c o o c 3 CM CO <J) CO O CD IO C3) Tt CO •o CD c "c o o c 3 X X Tt lO lO LO LO CJ) CO CD CD O b CO LO LO CO •a CD c c^ c o o c 3 < Tt o o CD CO CD 1- CNJ CO O O O O CD TD CD C C4— c o o c: 3 I- S CD CD X CD T- CO CX5 •r^ C\i CO CO Tt O O o o CD 00 00 o o X X CD CM CO T- CD CO CO CO ^ Tt o o o o 1^ TD (D c c o o c 3 X CO OJ LO og CD CD 1^ o o o o 00 CD Tt ^ •o c c o o c 3 Tt b o X CD CM CM I-~ T- CD CO CO CO o o o o 10 CM TD CD c "c o o c 3 X o X cvl 00 CM 00 10 CO CD 10 CO CO o o o o 0 to TD CD c c o o c 3 TD 0 C M— c o O CD I ra CL < 0 o O o o CM CN io, 2 s Q. I- re jfl UJ £ i2 -I -D o OQ >• > I ^ c o UJ ^ I- 0) c .2 re N 0) 0) « 5 fe o •g.12 ts « c re c (fl < UJ ^ o 1 ra *^ "D .2 o I g i2 » -o E O) c u ra o « O > < 2 (0 ^ ^ > s: ^-tS « = 5 SU. £ ra o > < a> tt) C 0) ~ c ra J« « u a> Sra c o N o £ (/) o u > ^ •« >. tf) re £.5 re .2 O T3 3 ^ C — a> ._ o c tt) = " s; P 0) 5i5 c o ra 0) tt) o •T O X X CO CO oo C\j CD CO CD CD t Tt O O O O CD Tt t^ T3 0) C ii— c o o c 3 00 t o X CO CO CM OM b b TD 0) c c^ c o o c 3 T3 C o o 0) CO 0) Q. 0) c O II o Q) o o 2 I I,. 03 Q. •o CD 3 CO 0) II CM 0 _0 .D O < o O o o o CO TABLE 3 Comparison of 2nd Quarter 2005 and 1st Quarter 2007 Chloroform Concentrations Well Q2 2005 Chloroform (pg/L) Q1 2007 Chloroform (iigIL) Change MW-4 3170 2300 -870 TW4-1 3080 1900 -1180 TW4-2 3750 2900 -850 TW4-4 2400 2200 -200 TW4-5 113 33 -80 TW4-6 2,5 46 43 TW4-7 2700 1100 -1600 TW4-10 62.4 500 438 TW4-11 3590 3500 -90 TW4-15 442 570 128 TW4-16 212 8,7 -203 TW4-18 29,8 9,2 -21 TW4-19 1200 1200 0 TW4-20 39000 4400 -34600 TW4-21 192 160 -32 TW4-22 340 440 100 H:\718000\71801\Corrective Action Plan\Tables,xls: Table 3 8/16/2007 I I TABLE 4 Comparison of 2nd Quarter 2006 and 1st Quarter 2007 Chloroform Concentrations Well Q2 2006 Chloroform (\iglL) Q1 2007 Chloroform (pg/L) Change MW-4 3000 2300 -700 TW4-1 2200 1900 -300 TW4-2 3200 2900 -300 TW4-4 2600 2200 -400 TW4-5 51 33 -18 TW4-6 19 46 27 TW4-7 2200 1100 -1100 TW4-10 300 500 200 TW4-11 4300 3500 -800 TW4-15 830 570 -260 TW4-16 13 8.7 -4 TW4-18 12 9.2 -3 TW4-19 1100 1200 100 TW4-20 61000 4400 -56600 TW4-21 130 160 30 TW4-22 390* 440 50 Notes: * Q1 2006 Concentration H:\718000\71801\CorrectiveAction Plan\Tables.xls: Table 4 8/16/2007 TABLE 5 Comparison of Average Chloroform Concentrations between 1st Quarter 2007 and 2nd Quarter 2006 and Between 1st Quarter 2006 and 2nd Quarter 2005 Well Average Concentrations Q2, Q3, Q4, 2005 and Ql 2006 Average Concentrations Q2, Q3, Q4, 2006 and Ql 2007 Change MW-4 3192 2738 -455 TW4-1 2770 2305 -465 TW4-2 3738 3410 -328 TW4-4 2825 2505 -320 TW4-5 81 46 -35 TW4-6 15 30 15 TW4-7 2550 2060 ^90 TW4-10 166 439 273 TW4-11 4198 3885 -313 TW4-15 876 963 88 TW4-16 88 10 -77 TW4-18 24 11 -13 TW4-19 1650 1118 -533 TW4-20 17750 20425 2675 TW4-21 119 134 15 TW4-22 335 558 223 H:\718000\71801\Corrective Action Plan\Tables,xls: Table 5 8/16/2007 FIGURES I 3anoid psoo||8/v\/ueic|uoipv9Aipajjoo /lOQUIOOOQlim ' ,3,^,^^^^ Z003/9I./8 3iva srs oiAoyddv DKl 'IM3HD OMQAH 31\S VS3I/\I 31IHM SNOIlVDOn m3M a3H0il3d QNV NVld 31IS DKl 'IM3HD OMQAH (eieLUjxojdde suojieooi) ZOOS'^^IAJ paneisui IISM 6u|joi!UOLu psgojad /tiBJoduja} /wgu gOOS'lMdv ps||Bisu! ||9M 6UU01IU0LU paijojed AjBJodnigj 9002 'llJdV ps||e»su! 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N0llVNVndX3 n 3ynoid pS ZO/OeOIM3/UB|d UOjPV 9AIP8JJ03 /1.081.Z/0008I.Z/:H 33N3y3d3y Z002/9I./8 3iva srs aSAOMddV ONI 'iMaHO oao OMQAH 31IS VS3I/\I 31IHM S3NOZ 35dn±dVD a31VIAIIlS3 ONIMOHS 3i/«fnd i/\iyodOdonH0 zooz asidvno ^^i ONI 'iMaHO oao OMQAH I/on Uj uo|jBj)U90uoo 6u|Mons 9002 'llJdy P9||BJSU! ||9M 6u|J01!UOLU p9gOJ9d AjBJOdLUa) l/On u| uouBJiuaouoo 6U!MOL|S 9002 IPdv p9||Bjsu| IiaM 6U|JO)|UOLU pagojad (paiduJBS )ou) ja}aLuoza!d pagojad j/On u| uo!)BJiU90uoo 6u!Mogs l|9M 6UU01IUOLU p9goj9d AjBJodujg) 1)91 -(J> ON 3e-AAtN SN e l,-Z3ld D033 O APPENDIX A PERCHED MONITORING WELL HYDROGRAPHS 0) £ I- o JZ a Q "53 c o 0) 4 ^^c9^ '^<yc-. ii c^# (dW "Miq) J31BM 01 yjdaa 0) E O I E E CO c t) E k. 3 tn 01 O u IB o (dl/\l"M|q) ja»BM o» Mjdaa 0) E P im o I 0) CM I a> >« CO o Q. E CO CO 0) c £ 3 ID n (U »^ o (V D 0) E P i- O I 0) 0) 4—1 CO > V. CO o a E .21 CO V) 5 o C3 (dWMjq) jajBM Ol qidaa CD E P O I CO c E 0) Cfl O o a E CO (0 o d (dW^iq) JSJEM OJ Mjdaa Oi E P O 1 ii i- 0) CO I CO o a E o CO E u i_ 3 (0 d) o -A) fl> x^ o c (dWmiq) J31BM OJ qjdaa cu E O 0) CO to "QJ o a E CO cn cu o cc D -5^ <0' C QJ E Ei 3 in cc i i cn o c {dW«i|q) JajEM OJ qjdaa cu E P % O 0) CO 0) co o Q. E cu cu 0) 3 tn CD o IC Q jajBM OJ qjdaa cu E P O cu 0) 4—' CO 00 CO o Q. E CQ CO cu c ID E ID CC CD o IU cc Q (dl/\l"M|q) JajBM oj qjdaa (U E O ,—^ CO o Q. E CO CO cu Si c cu E cu 3 tn ec cu O a) re D *^ C^ .4 to <7o (di/\| M|q) jajBM OJ qjdaa 0) E P >-% O I E- (0 o a. E CO en tu c <D E £ 3 en tc cu re • 4= o (dlAI"M|q) jajBM oj qjdaa cu E P O 0) CO o Q. E 0) c tu £ ID 3 tn re ID O Q) re D 4 <0- <5" o d Cl c o 6 o d (dW'Miq) jajBM oj qjdaa 0) E P % O r— o ns o Q. E CD cn cu c cu £ 3 cn re 0) O re Q A ,53 t^ (5 4>" i •A? C6- o d o o o d o c o d (dW| M|q) jajBM OJ qjdaa cu E P o CO I ^> 2 o a E CO 0) E 0> i-3 to re OJ o 0) re Q (dl/\| M|q) jajBM OJ qjdaa cu E P E E c 0) £ 0) 3 tn re 0) o 0) *>' re Q la- A/ fV r> ^" fy- fy- fy^ f# fy fy- (dW'Miq) jajEM oj qjdaQ cu E P O in cu E §. E .cu IU 0) £ OJ 3 in re OJ o OJ re Q (dlAI'Miq) jajBM oj qidaa o 1 IB o a E CO cn a> c 0) £ E 3 in re 0) o OJ •rf re O f§^ §5 fy-•A? f^" rip fy-A? fy- fy-A? fy^ fy f^^ fy f# fy fy-A/ .A/ -fy fV fy^ fy- O d o d (dWMiq) jajBM OJ qjdaa OJ E cu > O CM n I o a E CO cn OJ 5 c OJ £ OJ 3 en re OJ o o re a fy-ff \^ fy- fy fy- 4' fy- fy- ff fy-O fS- fy-<f A/ n fl^ fy- fy- fy -A? fV > • o d (dW'wiq) JOIBM OJ qjdaa cu E P 1- Si o I o a E .cu 10 in cu cu c OJ £ OJ 3 tn re OJ o OJ re Q o d (dWMiq) jaiBM OJ qjdaQ cu E o sr I E §. E CO cn cu c tu E (D 3 tn ro OJ O OJ 4— 10 O (dWMiq) jajBM OJ qjdaQ IU E o o CM 4 I >. 10 E CO in 01 E 01 ^ 3 tn re 01 O B re D (dW/wiq) JSIEM OJ Mjdaa cu E P I O '—, "t- CM CB >_ o o. E CO in o c Cl E ID k-3 tn to 01 O OJ *rf re D pi fb-.So tv- •5s i f«- -p o o d (dl/\l"M|q) JajBM OJ qjdaa QJ E p o CM CM I £^ E o Q. E OJ E OJ 3 tn ra OJ o OJ ra Q S3 fB- $3 ff^ B f«- >CJ o o d o d (dWMjq) jajBM OJ qjdaa I I I I I I I I I I I I I I APPENDIX B CHLOROFORM INVESTIGATION WELL CHLOROFORM CONCENTRATION GRAPHS O) CO 0> 3 CO > o o 5 3 CO "co > E c O o c o I i- CO > 2 o im c o CN ;r-i;S*|:|^.|- 'i^^I^-C; .••.;vSf:.,v:!>i-i-j-' o o o o o o CO o o o in o o o o o o CO o o o CXJ o o o \ < % % \ I (0 "ro > "o o IT <r> =0. I I 'Ji "ro > o o o o cp I I I a) "ro > C I 03 ro > o 5 a I 3) "in > 0 o o c i7m^7. o o I I I I I I I I I I (0 "5 > p 5 CM CO as o o r •f -e' . 00 >^,^%t ^ 1 I O) (0 OJ "ro > p o cc in 3) <n ro > I -J "a cn OJ "ro > 3) to "ro > o CO Lp "a "a •fe I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I 3) a) OJ "ro > E 5 p 5 o cn I- to "ro > o I •a- I I I I I 3) 3) o o LO CM O O OJ O LO O O O LO a 'V Yi Yi 35 ro > O ] CN CM O o o o o o o CO o o o o CO o o LO o o o o CO o o o o CM 1- I I I I I I I I I I I I I I I I I I I APPENDIX C CHLOROFORM MASS REMOVAL VIA NATURAL IN-SITU DEGRADATION CHLOROFORM MASS REMOVAL VIA NATURAL IN-SITU DEGRADATION In-situ breakdown of chloroform via biologically mediated and abiotic means is expected to occur within the perched zone chloroform plume at the White Mesa site. The possible degradation mechanisms include: • reductive dechlorination (abiotic degradation) • anaerobic reductive dechlorination (anaerobic biodegradation) • cometabolic processes (aerobic biodegradation) Reductive dechlorination of chloroform involves successive replacement of chloroform atoms by hydrogen. This process occurs relatively rapidly under anaerobic conditions in the presence of naturally occurring anaerobic bacteria, but can also occur, albeit slowly, without the aid of bacteria. Degradation of chloroform can also occur under aerobic conditions by cometabolic processes. Cometabolism involves incidental biodegradation of one compound while another compound is used as a food source by the naturally occurring bacteria. Within the perched zone, naturally occurring organic carbon might be used as such a food source, allowing chloroform to be cometabolized. Based on rates provided in HydroGeoLogic, 1999, anaerobic reductive dechlorination could reduce detected concentrations by more than three orders of magnitude within a few years, provided conditions were favorable. However, this mechanism is likely to be minimal, because G:\718O0O\71801\CAP\AppxC.doc p_i August 20. 2007 the nitrate associated with the chloroform plume at the site indicates that the perched zone is aerobic. Nitrate would not be persistent under anaerobic conditions and would be expected to degrade relatively rapidly. Abiotic reductive dechlorination is likely to be quite slow based on studies by Jeffers, et al, 1989, and Mabey and Mill, 1978, with expected half lives for chloroform of 1850 to 3650 years at neutral pH. Degradation would be more rapid at higher pH, with expected half lives of 25 to 37 years at pH 9. However, perched water at the site is generally near neutral, so the lower rates (higher half lives) would be likely for the perched zone. Cometabolic degradation can occur relatively rapidly if sufficient organic carbon is present in the perched zone that could serve as a food source for an indigenous methanotrophic population. Under ideal conditions, this process would be expected to proceed at rates higher than anaerobic rates. However, in natural groundwater, this mechanism is not expected to be dominant. One method of estimating the actual degradation rates of chloroform is to look for the daughter products of reductive dechlorination, methylene chloride (DCM) and chloromethane (CM). Both have been detected at the site in low concentrations (typically a few [igfL). Chloroform degrades via reductive dechlorination to DCM, then CM. DCM is commonly used in analytical laboratories and its detection in some cases may have resulted from laboratory contamination. However, assuming that detections are representative of site conditions, chloroform degradation rates can be estimated by assuming the following: G;\718000\71801\CAP\Appx Cdoc Ausust 20, 2007 I • the detected DCM is a degradation product of chloroform, • the rate of DCM depletion is fast compared to chloroform (because, unlike chloroform, DCM can degrade relatively rapidly under aerobic conditions), and • the concentrations of detected DCM are in pseudo steady state. The last assumption implies that the DCM degrades about as fast as it is produced, A range of zero order aerobic degradation rate constants for DCM are provided in Aronson, et al, 1999. These range from 0.0036/day to 0.533/day, with a recommended rate of 0.0546/day. These rates are relatively large and imply fast rates of DCM degradation under aerobic conditions. The rates imply that degradation of DCM would be much faster (by orders of magnitude) than would be expected for chloroform which is expected to degrade very slowly under aerobic conditions. DCM was detected in perched zone wells TW4-11, TW4-15, TW4-16, and TW4-20 in the first quarter of 2007 at concentrations ranging from 1.1 to 6.5 ng/L, and the same four wells in the fourth quarter of 2006, at concentrations ranging from 1.3 to 9.2 \igfL. Because DCM was detected in the same four wells during both quarters, these detections are likely representative of site conditions, and not random laboratory analytical error. Furthermore, the similarity in DCM concentrations over the two quarters suggests a pseudo steady state condition. The average DCM concentration at these four wells over the two quarters is 3.8 fig/L. The amount of chloroform degradation implied by these DCM concentrations can be estimated by using the expected rate of DCM degradation (0.0546/day) and assuming, on a molar basis, that the amount of chloroform degraded is equal to the amount of DCM degraded. The G:\718000\71801\CAP\Appx Cdoc o Augusl 20, 2007 expected amount of DCM degraded per day can be calculated using the following first order rate equation: Where: C = the observed concentration Co = the concentration at time zero (imtial concentration) k = the rate constant (l/day) Ar = the elapsed time (days) Assuming that Ar = 1 day, Co = 3.8 \igfL, k = 0.0546/day, and solving for C, C = 3.60 \xgfL The impUed change in DCM concentration per day is 3.8 |ag/L - 3.6 ng/L, or 0.20 ng/L. On a molar basis, this implies that 0.28 ng/L chloroform was degraded to replace the 0.20 \ig/L DCM that was degraded in the same day. During the first quarter of 2007 and the third quarter of 2006, the chloroform concentrations at TW4-11, TW4-15, and TW4-16 ranged from 9 ng/L to 11,000 \igfL and averaged 2929 \igfL. A reduction of between one and two orders of magnitude would be needed to bring these chloroform concentrations to the action level of 70 [igfL. To calculate the rate of chloroform degradation imphed by the daily amount of 0.28 [ig/L chloroform degraded as calculated above, the same first order rate equation can be used: £« — = -/cAr I I I I I I G:\718000\7l801\CAP\Appx Cdoc 4 August 20, 2007 I I I I Using 2929 ng/L for Co, assuming C = 2929 - 0.28 = 2928.72 ng/L, rearranging and solving for k, k = -0.00010/day. This rate is more than an order of magnitude lower than the lowest anaerobic rate of -0.004/day reported for chloroform in HydroGeoLogic, 1999. The calculated chloroform degradation rate of -0.00010/day can then be used in the first order rate equation after solving for Ar to calculate the time needed to reduce chloroform concentrations by one and two orders of magnitude (C/CQ = 0.1, and C/CQ = 0.01, respectively). By rearranging and solving for Ar, in(0 1) Ar = = 23,025 days or 63 years for a one order of magnitude reduction, -0.00010/day And Ar = _ 45 052 days or 126 years for a two orders of magnitude -0.00010/day reduction. To reduce the highest concentration ever detected at the site (61,000 ng/L at TW4-20) to the action level would require three orders of magnitude reduction in concentration. Performing a similar calculation where C/CQ = 0.001 yields Ar =—— _ 59 077 days or 189 years for a three orders of magnitude -0.00010/day reduction. These calculations assume that reductions in chloroform concentrations occur only through biological means, and do not account for additional natural attenuation mechanisms that G:\718000\71801 \CAP\Appx Cdoc p.5 Aueust 20, 2007 I include hydrodynamic dispersion, volatilization, and abiotic degradation. When considering the I results of the above calculations in addition to 1) the other natural attenuation mechanisms that jjj will act to reduce concentrations within the plume, 2) the chloroform mass removal by pumping, and 3) the estimated perched zone travel times of a few feet per year in the areas south and ^ southwest of the chloroform plume, it is unlikely that chloroform concentrations exceeding the | action level will ever migrate south or southwest of the tailings impoundments. ^ I I I I I I I I I I I I G:\718000\7l801\CAP\Appx Cdoc 5 August 20, 2007 I REFERENCES Aronson, et al. 1999. Aerobic Biodegradation of Organic Chemicals in Environmental Media: A Summary of Field and Laboratory Studies. Environmental science Center, Syracuse Research Corporation, North Syracuse, NY. Submitted to U S Environmental Protection Agency. HydroGeoLogic, Inc. 1999. Anaerobic Degradation Rates of Organic Chemicals in Groundwater: A Summary of Field and Laboratory Studies. Submitted to U. S. Environmental Protection Agency Office of Solid Waste. Jeffers, et al.l989. Homogeneous Hydrolysis Rate Constants for Selected Chlorinated Methanes, Ethanes, Ethenes, and Propanes. Environ. Sci. Technol., Vol 23, No. 8, pp 965-969. Mabey, W., and T. Mill, 1989. Critical Review of Hydrolysis of Organic Compounds in Water Under Environmental Conditions. Journal of Physical and Chemical Reference Data, Vol 7, No. 2, pp 383-415. G:\718000\71801\CAP\AppxC.doc (-> 7 August 20, 2007