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Daniel Arnold, Esq. April 6, 2011 Denver Water Page 6 of 12 a decade and this resulted in a "first flush" of the reactive surfaces. Several issues come out of this discussion: • The water level data were not contained in the EPP. Water levels are a fundamental part of data collection and omission of water levels in the EPP is a significant oversight. It is noted that water levels will be submitted in any future updates of the EPP. • The EPP states that sump pumping (re- cycling of water from Sump 1 to Sump 4) continued after 2002, and the new information claims that sump pumping stopped in June 2002. This is an error that has significant implications to the interpretation of groundwater quality data. For example, if there has been no sump pumping since June 2002, then the dramatic increase in uranium concentrations observed in site wells MW -6 and MW -7 since 2006 (see Figure 4 of the EPP Review) is significant. Essentially, no water management operations occurred (sump pumping, re- cycling, water treatment discharges) at the site that could be responsible for the increase in concentrations with one exception - the rising mine pool. The mine pool reached the elevation of these wells around the 2006 timeframe and this is compelling evidence that the mine pool water has been bleeding out into the alluvium since 2006. • As stated in a previous comment, the alluvial groundwater is not diluted by water from the creek. However, the new hydrographs indicate that this is not true. For example, the hydrograph of MW -9 after the sumps were shut down in 2002 shows a water level response that is very similar to a typical hydrograph of the creek. Water levels in the well rise in the spring due to the increased stage in the creek, and then fall from the late summer through winter as the stage in the creek falls. This indicates a naturally strong hydraulic connection with the creek where the creek supplies water to the alluvial aquifer (and dilutes concentrations) during high stage. This inflow of creek water will increase as sump pumping resumes, and will confound future interpretations as to whether concentrations are actually decreasing or just decreasing due to dilution from creek water. A discussion is presented relative to the discharging corehole and a graph of the mine pool elevation response to the discharge is provided. Over an 8 -day period in early October 2010, the mine pool dropped 3 feet as the corehole continued to discharge. The trend of the decrease is linear, suggesting that it would have continued to drop without any sign of tapering. Cotter has stated that the discharge rate from the corehole was about 20 gpm. Therefore, it is likely that the mine pool could be pumped at a relative low rate to lower the mine pool elevation to below the elevation of the creek. Table 8 -14 of the EPP contains estimated inflow rates during refilling of the mine and confirms this, as the inflow rate at the 500 Level was estimated to be only 18.7 gpm. This low of an inflow means that the mine could be pumped down at a rate that could be accommodated by the existing water treatment facility, ensuring that the mine pool doesn't affect groundwater or surface water quality.