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Daniel Arnold, Esq. January 25, 2011 <br />Denver Water Page 3 of 21 <br />water. Cessation of the sump pumping in 2009 and the observed increase in <br />groundwater concentrations illustrate this condition as dilution from the creek water <br />also ceased. The recirculation of untreated water from Sump 1 to Sump 4 (from 2002 <br />to 2009) is likely to have increased the water table around Sump 4 and caused <br />impacted groundwater to flow into the creek. <br />▪ The connection between the mine pool and the alluvium is claimed to be weak. <br />Several lines of indirect evidence are provided in the EPP and each is inconclusive. <br />The recent discovery of the discharging borehole is evidence of connection. The <br />borehole is angled down into the mine and connects the mine pool with the alluvium. <br />Additional evidence of connection is the Illinois Fault zone that passes through the <br />mine and intersects the creek. The rising mine pool has been above the elevation of <br />this intersection since 2007, and to the extent that the mine pool elevation is above <br />other borehole collars along the valley floor, they too are able to discharge mine pool <br />water to the alluvium. <br />▪ The discharge from the mine pool into the alluvium is claimed to be extremely low. <br />Using data reported in the EPP, it was shown that only 2 gpm of discharge is required <br />to result in the uranium concentrations observed in the alluvium. <br />▪ The rate of water level rise in the mine pool is slowing. A contributing factor to this <br />tapering is that the mine pool is finally reaching the level of the angled boreholes that <br />are now discharging into the alluvium. The EPP claims that the tapering is solely due <br />to reduced inflow to the mine, but this is not entirely true considering the discovery of <br />the flowing borehole. <br />▪ Uranium concentrations in the alluvial groundwater and in the creek are increasing, <br />and the frequency of exceeding the MCL is increasing in recent years. This suggests <br />additional sources of uranium at the mine other than the fill. <br />• The current mitigation strategy of water collection and treatment is removing uranium <br />from the alluvial groundwater. Most of this water, however, is from the creek and <br />additional pumping from the sumps will likely exceed the treatment capacity. The <br />current mitigation does not address other sources including the rising mine pool and <br />leachate from the waste rock piles. <br />• Potential future mitigation options have been preliminarily evaluated including source <br />(fill) removal and disposal, permeable reactive barrier (PRB), creek isolation, and <br />constructed wetlands. The long -term effectiveness of each option to reduce <br />contaminant concentrations and protect water quality is questionable primarily <br />because each option does not address the rising mine pool or waste rock piles. The <br />proposed borehole plugging program should be implemented considering the recent <br />discovery of the borehole connecting the mine pool with the alluvial aquifer. <br />3.0 REVIEW OF EPP <br />The following presents ARCADIS/Malcolm Pirnie's review that is based on the major <br />waste sources and media of concern at the mine that include: waste rock piles, valley fill, <br />alluvial groundwater, mine pool, bedrock groundwater, and surface water. Potential <br />mitigation options are also reviewed. Major findings of the EPP are presented followed <br />