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Daniel Arnold, Esq. January 25, 2011 <br />Denver Water Page 16 of 21 <br />3.6 Surface Water <br />EPP Finding: Uranium concentrations in the creek peaked in 2002 after turning off <br />the sumps, resulting in a first flush of reactive surfaces in the fill that is responsible for <br />the increased uranium concentrations. <br />This finding is not correct. Uranium concentrations in the creek have been higher than in <br />2002. An example of this is the September 2009 concentration of 2.78 mg /L at the SW- <br />BPL, which is at the downstream property boundary (Figure 3). It is of note that the <br />concentration in the creek at that time was the same as the concentration the alluvial <br />groundwater at MW -6, which is further evidence of the strong connection of groundwater <br />and creek. This observation also illustrates that at certain times of the year, the <br />streamflow in the creek may be entirely comprised of groundwater base flow and future <br />remedial measures need to take this into account. As previously described, the cause of <br />the 2002 peak in uranium concentrations in the creek was likely due to the beginning of <br />recirculation of untreated water from Sump 1 to Sump 4 that could flow into the creek. <br />High uranium concentrations continue to be measured in the creek and significantly <br />exceed concentrations in 2002. Recent samples collected at the SW -BOS location, which <br />is downstream of the former Ore Sorter (Figure 1), contained uranium at 5.49 and 8.11 <br />mg /L during the September 8 and 23, 2010 sampling events, respectively. <br />Data Adequacy: There are no groundwater levels or water levels in the sumps provided <br />in the EPP to show how levels changed in response to shutting down the sump pumping <br />that could potentially flush the fill. <br />EPP Finding: Surface Water at the downstream property boundary SW -BPL is stated <br />as exceeding the drinking water MCL of 0.03 mg /L only 29 percent of the time, or 66 of <br />the 226 samples that have been collected from January 1990 through March 2010. <br />This finding is misleading and likely to be incorrect considering recent increases in <br />uranium concentrations in the creek. Since May 2000 when the underground mine pumps <br />were shut off, the SW -BPL location has been sampled 103 times and the uranium <br />concentration has exceeded the MCL in 64 of these samples, or 62 percent of the time. <br />This is based on Figure 11 -8 of the EPP. The frequency of exceeding the MCL is <br />increasing as shown on Figure 3. If what is stated in this finding is correct, it means that <br />the MCL was exceeded only 2 additional times prior to May 2000, which is unlikely <br />given the known hydraulic connection between the alluvial aquifer with elevated uranium <br />concentrations and the creek. <br />Data Adequacy: Historic exceedances of the MCL cannot be confirmed because data <br />prior to 1998 are not contained in the EPP. <br />3.7 Potential Mitigation Options <br />