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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
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