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Daniel Arnold, Esq. January 25, 2011 <br />Denver Water Page 6 of 21 <br />along the trace of the original creek bed. Otherwise, the fill material should be above the <br />alluvial water table. Moreover, most of the fill was put in place in the 1950s and 1960s to <br />provide areas for construction of the mine facilities. Decades have now passed since the <br />fill was in place allowing adequate opportunity for the reactive surfaces of the fill to flush <br />with the seasonal rise and fall of the water table and flushing from precipitation. Given <br />this considerable length of time, most of the reactive surfaces should have been flushed <br />by now. Reactions associated with dissolution from mineral surfaces generally involve <br />two steps: 1) reaction between water and the mineral to form a surface chemical species, <br />and 2) detachment of the surface chemical species and release into the solution phase <br />(Stumm, 1992). The first reaction is generally fast (instantaneous) and the second <br />reaction can be slower but is enhanced by the chemical characteristics of the water. <br />Introduction of oxygenated water from decades of precipitation and exposure to alluvial <br />groundwater will have resulted in the oxidation of uranium in the fill, and natural <br />alkalinity will have promoted the dissolution of the oxidized uranium. The mineral form <br />of uranium in the fill is likely uraninite (UO2). In the dynamic environment of the <br />alluvium, uraninite in the fill will oxidize to more soluble forms of uranium such as <br />uranophane (Ca(H3O)2(UO2)2(SiO4)2). The dissolution of these oxidation products have <br />been shown to be strongly enhanced in the presence of bicarbonate (alkalinity) (Perez et <br />al., 2000). Bicarbonate concentrations in the alluvial groundwater have historically been <br />—100 to 150 mg/L, adequate to promote uranium dissolution and flush reactive surfaces <br />of sorbed and precipitated forms of uranium. Uranium concentrations exhibit increasing <br />trends recently in both the alluvial groundwater and in the creek, pointing to additional <br />sources of uranium than only the fill. <br />Data Adequacy: There are no groundwater level data in the EPP to show the degree of <br />rise of water levels in the alluvial material in the spring. Nor are there any wells in the fill <br />material to show that the water table indeed rises to a level to intersect the fill. <br />Information on the mineral forms of uranium in the fill material and on fill material <br />surfaces is also lacking; laboratory leach tests can provide information to assess the <br />extent to which reactive surfaces remain active and contribute uranium from the fill <br />material. <br />EPP Finding: Increased precipitation in 2009 is believed to be partially responsible for <br />the increase in alluvial groundwater concentrations. <br />According to the precipitation gage at Ralston Reservoir, the average annual precipitation <br />is 18.43 inches. The precipitation in 2009 was 20.06 inches, which is 9 percent greater <br />than normal. However, the annual precipitation in 2004, a year during which groundwater <br />concentrations experienced stable values and some of the lowest values in the last 8 <br />years, was 22.90 inches or 24 percent greater than average. An example of these lower <br />concentrations in 2004 is noted in well MW -6 (Figure 3). If leaching of the valley fill <br />material is believed to be a primary source of uranium then there should have been a <br />corresponding increase in uranium concentrations in the alluvial groundwater in 2004, <br />but there was not. Furthermore, in 2008 there was a modest increase in uranium <br />concentrations in alluvial wells over values in 2004, but the annual precipitation was only <br />