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Page 5 December 19, 2013 <br />Response: <br />Based on analytical data collected in support of the CDPS permit for the 001 Outfall, sulfate <br />concentrations in LW -001 range from 1,140 to 1,510 mg/L, with a mean of 1,255 mg/L. Sulfate <br />concentrations in Well LWCW -IA range from 1,200 to 1,950 with a mean of 1,575 mg/L. Sulfate <br />concentrations at Well LW -22 were measured at 1,400 mg/L from a single sample. Data from Hester <br />(1983) show that sulfate ranges from 5 to 9,500 mg/L, with a mean of 1,027 mg/L in 992 samples of <br />wells and springs in the vicinity of the mine. These data indicate that sulfate concentrations reported for <br />LW -001 are not anomalously higher than the concentrations reported for other wells in the area; in fact, <br />the concentrations reported for LW- 001are very similar to the levels reported for other wells in the <br />vicinity of the site. <br />6 Molybdenum, fluoride and boron are all observed at levels higher than regulatory limits. Is it possible <br />that the increased levels in LW -001 could impact the ambient groundwater conditions of Dry Gulch? <br />Response: <br />The minimum, maximum, and average boron concentrations reported for the LW -001 mine water are <br />3,700, 5,200 and 4,411 µg/L, respectively. The Colorado agricultural water quality standard for boron <br />is 750µg/L; there is no domestic water supply standard for boron. The minimum, maximum, and <br />average fluoride (dissolved) concentrations reported for LW -001 are 1,800, 3,000, and 2,382 pg/L, <br />respectively. The Colorado Domestic Water Supply standard for fluoride is 4,000 µg/L, and the <br />agricultural standard is 2,000 pg/L. The LW -001 mine water data show a minimum concentration of <br />molybdenum (potentially dissolved) of 230 pg/L, a maximum of 350 pg/L, and an average of 285 <br />pg/L. The Colorado domestic water supply standard for molybdenum is 210 pg/L. <br />Ambient groundwater chemistry has most recently been evaluated at Well LWCW -IA, located in Dry <br />Gulch approximately 300 feet downslope of the Lower Dump. Well LWCW -IA was installed in 1999 <br />as a point of compliance well for the mine site. Dissolved boron concentrations at Well LWCW -IA <br />range from 6,550 to 11,000 µg/L with an average of 9,134 µg/L. Dissolved fluoride concentrations at <br />Well LWCW -lA range from 410 to 3,100 µg/L with an average of 1,278 µg/L. Only the October 2013 <br />sampling event at LWCW -lA included an analysis for molybdenum, at which time a concentration of <br />570 µg/L was reported for the sample. <br />As shown, the concentrations of boron in the groundwater of LWCW -IA are greater than <br />concentrations in the LW -001 mine water indicating the infiltration of the mine water should not <br />negatively impact boron concentrations in ambient groundwater downgradient of the mine. The range <br />of fluoride concentrations in the LW -001 mine water and groundwater at LWCW -lA are similar with <br />the LWCW -IA groundwater showing lower minimum and average concentrations. However, the <br />maximum fluoride concentration reported for LW -001 (3,000 µg/L) is less than the maximum fluoride <br />concentration reported for LWCW -lA (3,100 µg/L), and the overall range of fluoride results for Well <br />LWCW -IA is slightly greater than the range for the LW -001 mine water. Based on these findings, the <br />infiltration of LW -001 mine water is not expected to greatly impact ambient concentrations at well <br />LWCW -IA. The molybdenum result of 570 µg/L for Well LWCW -IA is approximately two times <br />greater than the concentration reported for LW -001 mine, indicating that infiltration of the LW -001 <br />mine water will likely not negatively impact ambient molybdenum concentrations downgradient of the <br />mine site. <br />Naturally high boron and fluoride concentrations are commonly reported for groundwater samples <br />collected within the Parachute Creek Member of the Green River Formation in the Piceance Creek <br />Basin. This is well - established in numerous published documents. Locally, Hester (1983) demonstrated <br />that boron and fluoride concentrations are naturally high in groundwater within the Parachute Creek <br />TMarshaII121713.doc <br />