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Environmental Protection Plan, Schwartzwalder Mine 15-4 <br />• during high stream flow conditions, but during low flow conditions uranium concentrations are not <br />expected to fall below about 0.2 mg/L(31) without mitigation of the alluvium and fill. <br />Uranium concentrations in Ralston Creek and alluvial groundwater at the site generally exceed the <br />currently applicable water quality standard of 0.03 mg/L (the same numeric standard applies to both <br />groundwater and surface water). Uranium water quality standards in Colorado have changed in recent <br />years. Prior to 2008 there was no groundwater standard for uranium. In 2008 the 0.03 mg/L uranium <br />groundwater standard was implemented. The same numeric limit for uranium in Reach 17b of Ralston <br />Creek became effective in 2010. Prior to 2010, the applicable standard in Ralston Creek was 40 pCi/L <br />(approximately equivalent to 0.06 mg/L). The 0.03 mg/L limit for uranium in surface and ground water is a <br />health-based drinking water standard, developed based on conservative modeling assumptions to limit <br />probabilistic health risks from long term human consumption (over a period of 30 years). Because these <br />are health-based standards, it is reasonable to expect that regulatory application will be consistent with the <br />intent of the standards, taking into account potential exposure pathways to humans and realistic future land <br />use receptor scenarios. <br />There are other water quality standards as well. For example the EPA standard for uranium discharge from <br />open-pit and underground uranium mines is 2.0 mg/L (Section 11(a)(ii)). With respect to aquatic <br />organisms, the acute toxicity standard for uranium is 4.12 mg/L and the chronic toxicity standard is 2.57 <br />mg/L, calculated based on the average hardness of 163 mg/L (Section 11(a)). <br />(b) Description of Mitigation Options <br />(i) Short-term Pump-and-Treat, 50-gpm Ion Exchange Treatment System <br />• A 50-gpm treament system operated at Sump 1 from July 2 to July 30, 2010 (Figure 15-1). The system <br />treated up to 50 gpm continuously and achieved 99% reduction of uranium from collected groundwater. <br />Flow rates ranged from approximately 25 gpm to 50 gpm, with an average of approximately 35 gpm. The <br />50-gpm pumping rate was based on historical pumping rates at Sump 1 and equipment availability. <br /> <br />31 The observed concentrations during low flow (Fall 2003). <br />4109C.100731 Whetstone Associates