2010-04-20_REVISION - M1977300 (9)

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Environmental Protection Plan, Schwartzwalder Mine 15-3 • creek from the alluvium near the mine, thus breaking this exposure pathway. If the surface water pathway were eliminated, there would be essentially no risk to downstream users. No drinking water wells are installed within the uranium plume at the site, so at present, there is no direct pathway for human health risk from drinking alluvial groundwater. Institutional controls may be required to ensure that water wells are not installed in the affected alluvium. A detailed human health risk assessment can be conducted to further identify any other potential pathways for human exposure to uranium and molybdenum from alluvial groundwater. If no potential for exposure exists, then alternative concentration limits (ACLs) that are not based on human health drinking water standards may apply to the alluvial aquifer. (iv)Affect of Alluvial Water Quality on Ralston Creek Ralston Creek is essentially devoid of metals above the mine site, and picks up uranium and molybdenum as it flows past the mine and interacts with groundwater in the alluvium. The waste rock dumps have had no discernable effect on water quality in Ralston Creek. Uranium concentrations at surface water stations above (SW-AWD) and below (SW-A001) the waste rock dumps are consistently low (Figure 14-1). However, uranium concentrations in Ralston Creek increase as the creek flows past the mine site downstream from SW-A001, then decrease past the property boundary (Figure 11-6) as sorption and precipitation processes remove uranium from solution. Uranium concentrations in Ralston Creek spiked after the alluvial sumps were turned off 2002, and spiked a second time after Sump 1 recirculation was discontinued in 2008. In 2.002-2003, reactive surfaces on alluvium and fill had been exposed to subaerial weathering for over a decade while the sumps were in operation. The "first flush" caused uranium concentrations to spike and then decrease in later sampling • events. A similar spike occurred in 2008, several months after the cessation of Sump 1 recirculation. Although uranium concentrations have trended downward since the spike in 2008, they are not expected to fall below about 0.2 mg/L32 without mitigation. • Uranium concentrations in Ralston Creek and alluvial groundwater exceed the Colorado groundwater standard of 0.03 mg/L but are below the surface water aquatic toxicity standards. The acute toxicity standard for uranium is 4.12 mg/L and the chronic toxicity standard is 2.57 mg/L, calculated based on the average hardness of 163 mg/L (Section II(a)). The in-stream water quality standards published by the CDPHE Water Quality Control Commission (WQCC) establish an upper limit of 0.059 mg/L (40 pCi/L) for uranium at a water supply intake in Reach 17b of the Clear Creek Basin (5CCR 1002-38). No water supply intake exists above Ralston Reservoir, and flow from Ralston Creek represents a very small fraction of flow into Ralston Reservoir (Section 8(a)(i).2). Therefore, the 0.059 mg/L intake standard may not be applicable to this segment of Ralston Creek. The EPA standards for uranium discharge from open-pit and underground uranium mines give a limit of 2.0 mg/L (Section 11(a)(ii)). However, DRMS has indicated that the groundwater standard of 0.03 mg/L may be applicable. Cotter may seek higher alternate concentration limits (ACLs) based on human health risk. (b) Description of Mitigation Options (i) Strategically Designed and Constructed Wetlands Constructed wetlands are passive treatment systems that are typically self-sustaining, provide natural resource habitat, and are economical to build and operate. These systems can continue to operate with relatively little attention long after mine decommissioning. Modern constructed wetlands usually consist of 32 The observed concentrations during low flow (Fall 2003). 4109C.100419 Whetstone Associates