Laserfiche WebLink
Daniel Arnold, Esq. April 6, 2011 <br />Denver Water Page 7 of 12 <br />A comment is made that if open coreholes and fractures exist, the water level in the mine <br />pool would not have reached its current level. This comment is not correct. Mine pool <br />water from open coreholes and fractures is conveyed to the alluvium in a diffusive, non - <br />point manner. The actual flow rate may be low, and as estimated in the EPP Review, it <br />only takes 2 gpm of mine pool water to result in the recent uranium concentrations that <br />have been observed in the site monitoring wells. Contrary to what the comment states, the <br />mine pool can continue to rise above the elevation of coreholes and fractures if the mine <br />pool inflow rate is greater than the diffusive flow from the mine pool. <br />A discussion of uranium to molybdenum concentration ratios is provided and historic <br />ratios are tabulated for MW -7. The ratios are highly variably over time because MW -7 is <br />influenced by dilution from the creek during high- stage. Therefore, conclusions made by <br />comparing ratios in the alluvial aquifer to ratios in the mine pool continue to be suspect. <br />A comment was presented to define what background uranium concentrations in <br />groundwater may have been before mining. It claims that during operations, water <br />entering the mine at the 19 Level, and below, averaged 2.09 mg/L, and may be <br />representative of background water quality in the ore deposit. This water was clearly <br />affected by mining operations by exposing rock surfaces to oxygen and not representative <br />of background conditions. Further, the source of the water is not known and could be <br />from overlying oxidized rock that would significantly increase the soluble uranium <br />concentration in the lower levels of the mine. This condition is actually stated in a later <br />comment: "During operation, drips and seeps that were investigated in the upper <br />workings for the mine closure hydrology study percolated downward into the lower levels <br />and were captured and pumped at the 7 Level or 19 Level." As noted in the in EPP, some <br />of these oxidized upper workings had uranium concentrations as high as 150 mg/L <br />(ILLRS), which would have affected the water quality at the lower levels. <br />Additional data for uranium concentrations in the mine pool are provided, which shows a <br />continued decreasing trend in concentration. This is an encouraging trend and likely <br />influenced by the increased reducing conditions in the mine pool. It is of note, however, <br />that there are other constituents that are increasing in the mine pool (e.g. manganese) or <br />are holding steady (e.g., sulfate, total dissolved solids, Radium -226, and molybdenum). <br />Reducing conditions should result in a decrease in the concentration of sulfate, absent any <br />new input of sulfate from the dissolution of minerals within the mine pool. Radium -226 <br />concentrations may increase in response to reducing conditions, due to dissolution of <br />radium sulfate solids. <br />A comment states that "ARCADIS /MP does not provide support for the statement that <br />molybdenum is more susceptible to the onset of reducing conditions and will precipitate <br />first, before uranium." Whetstone provides a discussion that uranium is expected to <br />precipitate first, in response to reducing conditions, based upon saturation index <br />calculations using PHREEQC. The use of PHREEQC for the accurate evaluation of <br />geochemical conditions and expected mineral precipitates is predicated upon the <br />inclusion of appropriate thermodynamic data for the chemical species that may form in <br />