2011-01-25_REVISION - M1977300 - Laserfiche WebLink

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Daniel Arnold, Esq. January 25, 2011 Denver Water Page 5 of 21 known interaction with the creek, the impacted groundwater is impacting water quality in the creek. Data Adequacy: Additional data are necessary to properly characterize the rock piles and their relationship with alluvial groundwater and the creek. There are no monitoring wells within or beneath the rock piles. Monitoring wells in the rock piles would document the phreatic surface and determine if the waste rock is saturated and wells beneath the piles would provide information on the waste rock leachate impacts to groundwater. Groundwater water levels from wells below the rock piles would assess the hydraulic connection between the alluvium and the creek. Additionally, stream flow along the piles should be measured at several locations during low- and high -flow conditions to evaluate if the creek is gaining or losing flow. 3.2 Valley Fill Material EPP Finding: The waste rock fill material that was placed within the valley is acting as a source of uranium to the alluvial groundwater and loading to the creek. The available data indicate that leaching of the fill may be a source of uranium to the alluvial groundwater. However, the EPP additionally claims that the fill is the sole source and review of the available data does not support this claim. Hot spot areas of elevated gamma radiation have been mapped for the fill with the majority of the fill having low gamma radiation levels. As discussed later in the Alluvial Groundwater, Mine Pool, and Surface Water sections of this Technical Memorandum, the source of uranium in groundwater and in the creek is substantially more pervasive to be sourced only by hot spots of impacted fill. Data Adequacy: Soil sampling and soil leachate analyses are needed to determine if the fill is a source of uranium. According to Cotter representatives, soil sampling was performed in summer 2010 to characterize the fill and the analytical data should now be available. EPP Finding: Groundwater levels rise in the spring intersecting more oxidized salts in the fill thereby liberating uranium. Rising groundwater levels is unlikely to be a major release mechanism for uranium in the fill as the following example illustrates. The uranium concentration in MW -6 on June 30, 2008, which was a time of high water levels, was 0.92 mg/L. A few months later on September 30, 2008, which was a time of low water levels, the uranium concentration in the same well was actually higher at 1.14 mg/L. Therefore, the description of the uranium mobilization mechanism from the fill is not valid all the time. Another observation relative to the alluvial water table contacting fill is that the fill was placed on the ground surface of the valley floor and the water table is below the native ground surface. The only area where the water table could potentially intersect fill is