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Daniel Arnold, Esq. January 25, 2011 <br />Denver Water Page 4 of 21 <br />by an assessment of the supporting information and data that forms the basis of the <br />findings. Known features of the mine and monitoring locations are shown on Figure 1. <br />3.1 Waste Rock Piles <br />EPP Finding: Reclamation of the east and west rock piles has been successfully <br />completed, and no impacts to water quality in Ralston Creek are detectable for the rock <br />piles. <br />Data contained in the EPP indicate that the rock piles are impacting alluvial groundwater, <br />which migrates downgradient to the central mine area where it is observed to interact <br />with and degrade the water quality of Ralston Creek (creek). Therefore, this finding is <br />misleading. The pathway for the rock piles to impact the creek is by infiltration of <br />leachate to the underlying alluvial groundwater and then eventual discharge to the creek. <br />Alluvial groundwater may not be discharging to the creek along the rock piles, and <br />therefore the point of impacts to water quality in the creek is not along the rock piles. <br />Leachate from the rock piles can migrate a considerable distance down valley in the <br />alluvial groundwater system and discharge to the creek along the central mine, which is <br />known to be a groundwater discharge zone (i.e., the creek is gaining water). <br />The following illustrates the degree of groundwater impact associated with the rock piles <br />by comparing sampling results from alluvial well MW -00, located upgradient of the <br />waste rock piles, and alluvial well MW -0 near the downstream end of the rock piles <br />(Figure 1). The difference in groundwater quality between the two alluvial wells is a <br />measure of the impacts from the rock piles. Neither well is within the mapped <br />Schwartzwalder trend and should therefore not be affected by natural uranium <br />mineralization within the trend. A graph of historic dissolved uranium concentrations in <br />the two alluvial wells using data contained in the EPP is shown on Figure 2. As <br />illustrated on Figure 2, uranium concentrations in MW -00 are consistently low having a <br />median value of 0.003 milligrams per liter (mg /L), with the exception of three of the 67 <br />uranium values that are suspect and likely to be outliers. In contrast, the uranium <br />concentrations in MW -0 are consistently higher having a median concentration of 0.167 <br />mg /L based on 64 data points. Uranium values in MW -0 have always been above the <br />federal drinking water maximum contaminant level (MCL) of 0.03 mg /L and there is <br />evidence of an increasing trend in recent years. As a result, alluvial groundwater with <br />uranium concentrations in the range of 0.2 to 0.6 mg /L (Figure 2) is currently entering <br />the central mine area from the rock piles. Concentrations of other metals also increase <br />between the two monitoring wells. For example, the median concentrations of <br />molybdenum increase by an order of magnitude from 0.002 to 0.02 mg/L between the <br />two alluvial wells. Specific conductance, which measures water's ability to conduct <br />electrical current and increases with increasing dissolved ion content, increases from 275 <br />to 382 micromhos on average between the upgradient and downgradient wells. These <br />examples show that the rock piles are leaching uranium and other metals to the alluvial <br />groundwater, and given the observed down - valley flow of alluvial groundwater and <br />