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Daniel Arnold, Esq. April 6, 2011 <br />Denver Water Page 8 of 12 <br />the system under study. In this case, the PHREEQC thermodynamic database has omitted <br />the calcium uranium carbonate complexes; these soluble forms of uranium are likely <br />present in the mine pool due to the abundance of calcium, bicarbonate alkalinity, and the <br />circumneutral pH (Bernhard, et al., 1996; Bernhard, et al., 2001). The complexation of <br />uranium stabilizes the higher oxidation state, lowering the reduction potential of the <br />soluble uranium (VI) /low solubility uranium (IV) couple. Equilibrium thermodynamic <br />calculations indicate that calcium uranium carbonate complex is a weaker electron <br />acceptor than the other U (VI) forms. The presence of calcium uranium carbonate <br />actually lowers the reduction potential more than 0.1 V lower than uranium carbonate <br />alone (Brooks et al., 2003), to approximately 40 mV. This must be considered when <br />evaluating the redox potential at which the reduction of molybdenum (VI) to <br />molybdenum (IV) (a form of molybdenum with lower solubility than molybdenum [VI]) <br />will occur, as well reduction of molybdenum (IV) to molybdenum sulfide. <br />A data adequacy issue raised in our EPP Review is the absence of water quality data prior <br />to 1998, while historic interpretations in the EPP are based on pre -1998 data. This is the <br />case for alluvial groundwater quality data that is claimed to be elevated in uranium prior <br />to when mine dewatering ceased. A comment is made that addresses this issue, stating <br />that the data tables in the EPP included only post -1998 data because these data were <br />available electronically, but that data prior to 1998 were not included because they did <br />not meet Whetstone's database QC requirement of being derived from laboratory <br />electronic data files. The pre -1998 data, however, were included in the statistical <br />summaries in the EPP as the comment mentions. If the pre -1998 data were used in <br />statistical summaries, that implies that the data are reliable and the raw data should be <br />made available. Having the pre -1998 water quality data, particularly the groundwater <br />data, would be helpful to substantiate interpretations in the EPP. <br />The capacity and effectiveness of the current water treatment facility is discussed and <br />additional information is provided on how the sump pumping system is operated. It is <br />stated that sump pumping is managed and optimized for maximum groundwater <br />interception and mass removal, targeting the groundwater discharge area and high <br />concentration areas while minimizing inflow from the creek. There is no information <br />provided on how the optimization is actually implemented, however. Information to <br />assess optimization would include comparison of groundwater levels in the <br />alluvium /sumps to the creek, and calculating the loadings from each sump. Because the <br />mine pool has impacted the alluvial groundwater, any optimization of mass would <br />include the mine pool loading to the alluvial groundwater. The optimized mass removal <br />for the site would likely show that pumping of the mine pool is needed to reduce the <br />greatest amount of mass. <br />Additional information is provided on the location of valley floor coreholes. The <br />information is helpful to determine where conduits may still exist that can discharge mine <br />pool water into the alluvium. The discharging corehole discovered in July 2010 is <br />believed to be J -10 and its location is shown on Figure 13 of Whetstone's comments, and <br />included in this Addendum as Attachment A. This location is not consistent with the <br />