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Daniel Arnold, Esq. January 25, 2011 Denver Water Page 8 of 21 concentrations, resulting in an overall larger volume of contaminated water when the sumps are pumping. Additional evidence for this hydraulic connection with the creek was observed in October and November 2010 when trenches for new sumps were excavated. According to Cotter representatives during the mine site tour on November 5, 2010, the excavation work was targeted for the fall when water levels and flow in the creek are typically low. Trenches were initially dry only to fill with water overnight as the flow in the creek increased from releases of water upstream of the mine. Data Adequacy: Data used to evaluate the hydraulic connection between the alluvial aquifer and the creek should include detailed stream flow measurements along the central mine area. The magnitude of stream flow gain or loss would estimate the magnitude of hydraulic connection. No detailed stream flow measurements are contained in the EPP. Alluvial groundwater levels from the monitoring wells would also assist in evaluating whether the alluvial aquifer is discharging to the creek or if the creek is losing water to the aquifer. Water levels are standard measurements collected when wells are sampled, but the EPP does not contain water levels for monitoring wells. EPP Finding: Alluvial groundwater had elevated concentrations before the pumps in the mine were shut off and the mine was allowed to fill. The available data do not support this finding. Figure 4 presents a graph of historic uranium concentrations from the EPP for wells MW -6 and MW -7; the two wells are in the central mine area. As illustrated on the graph, concentrations for both wells were lower in the late 1990s than recent concentrations. Concentrations increased in 2002 and 2003 and also again in past two years, and values are much higher than before the pumps in the mine were shut down in May 2000. The sumps in the alluvial aquifer were shut off in 2002, which the EPP claims is the reason for some of the increases in uranium concentrations. However, Sump 1 continued to pump and recirculate untreated water back to Sump 4 until 2009; thus, pumping from the sumps at some level has always occurred until Sump 1 was shut down in 2009. Data Adequacy: Historic groundwater quality data in the EPP prior to the cessation of pumping in the mine in May 2000 are limited to 1998 and 1999. Several wells were installed nearly a decade earlier in 1989 including MW -3A, MW -6, and MW -7. It is not clear why the historic groundwater quality data prior to 1998 for these wells is not contained in the EPP and would be valuable to determine if groundwater concentrations were elevated before the mine was allowed to flood. EPP Finding: The shutdown of pumping at Sump 1 in 2009 is responsible for the increased uranium concentrations in the alluvial groundwater observed in the last year.