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Daniel Arnold, Esq. January 25, 2011 <br />Denver Water Page 14 of 21 <br />the mixing calculation previously described in the Mine Pool section shows that it only <br />takes 2 gpm of mine pool water at a uranium concentration of 34 mg/L to produce the <br />uranium concentrations observed in the alluvial aquifer near the downstream end of the <br />mine. <br />EPP Finding: The water level in the mine pool is not expected to rise significantly <br />beyond current levels. <br />This finding may be accurate given the slowing of water level increase in the past several <br />years that appears to be reaching an asymptotic level around 6,580 feet. The cause of this <br />tapering is due to reduced inflow to the mine, and another factors not discussed in the <br />EPP such as the unplugged angled boreholes. This tapering of mine pool elevation is <br />likely a result of the mine pool finally reaching the level of the angled boreholes that have <br />begun to discharge into the alluvium in recent years. These angled boreholes would have <br />been collared on relatively flat areas of the valley fill below the Steve adit, which places <br />them a few l Os of feet below the current mine pool elevation. The positioning of the <br />boreholes sets up conditions for mine pool water to discharge into the alluvium and <br />tapering of the mine pool rise. It is likely that the boreholes are now discharging at a rate <br />that is equal to the inflow to the mine, hence the near steady elevation of the mine pool. <br />EPP Finding: Uranium concentrations in the mine pool are decreasing and uranium <br />values will decrease to background levels in 9 years. <br />Background uranium concentrations in the Schwartzwalder trend and adjacent bedrock <br />have not been defined. Bedrock monitoring wells MW -10 and MW -11 have been <br />installed above the mine to define background concentrations in the bedrock. Both wells <br />have become contaminated with cement grout and the sample results from the wells are <br />therefore unreliable. The estimation of 9 years to reached background levels is not <br />supportable because reliable background values have not been defined. Additionally, the <br />decreasing trend in uranium may be short lived and not representative of a long -term <br />trend. Evidence of this is molybdenum concentrations in the mine pool that have been <br />stable at 1.5 mg /L since 2007. <br />As noted in the EPP, molybdenum and uranium are sensitive to oxidation - reduction <br />potential (ORP). In water containing dissolved oxygen they will be soluble as molybdate <br />(MoO - ; with molybdenum in the +6 oxidation state) and uranyl carbonate [UO2CO3, <br />UO2(CO3)2 or calcium uranium carbonate (Ca2UO2(CO3)3)];with uranium in the +6 <br />oxidation state). As dissolved oxygen is depleted, they will be less soluble. Under these <br />conditions, molybdenum can be reduced to the +4 oxidation state and form low- solubility <br />molybdenum minerals. Likewise, uranium can be reduced to the +4 oxidation state and <br />form hydrated uranium dioxide (UO2•H2O); this mineral form of uranium has low <br />solubility under reducing conditions. Generally molybdenum is more susceptible to the <br />onset of reducing conditions and will precipitate first; as discussed molybdenum <br />concentrations have been stable at 1.5 mg /L. In addition the geochemical conditions in <br />the mine pool, specifically the slightly alkaline pH and elevated concentrations of <br />