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GeoScience Services <br />2263 Xingston Road <br />Grand Janctron, CO 81503 <br />(970) 314-3356 <br />1992). This data indicates that the background water quality for perched water in the Salt <br />Wash does not meet drinking water standazds. <br />Please state all other input parameters and assumptions that are implicit in the <br />model <br />All of the boundary conditions for the model are presented in the site reports. The model <br />is run under steady state conditions assuming consistent flow conditions during the <br />simulations. Representative moisture characteristic curves for the soiUrock types are <br />used to describe unsaturated flow in the vadose zone. Water content values for the <br />unsaturated zone are calculated by the model based on the moisture characteristic curves <br />and the rate of rechazge. <br />Finally, the revised geologic cross sections attached to this letter report do not exaggerate <br />the vertical scale as did the previous cross sections presented in the mine evaluation <br />reports. At this revised scale, it is clear that the predicted plume for the SM-18 mine is <br />located at a substantial distance from the hillside and does not present the potential for <br />lateral migration and subsequent discharge of potential plumes along the hillside. <br />Further, the dip of the bedrock is away from the cliff face plus rechazge along the cliff <br />induces a hydraulic gradient away from the cliff face. <br />In summary, modeling predictions using several conservative assumptions indicate that <br />the waste piles from uranium mining operations at the SM-18, JD-6, JD-8, and JD-9 <br />mines pose minimal impact to the environment. TMs statement is based on the following <br />facts: <br />1. Model predictions show that after 1000 years of transport, no species of concern <br />reach the underlying water table. <br />2. Concentrations at the leading edge of the plume are significantly lower than <br />background concentrations in the underlying aquifers. <br />3. Even using the unrealistic assumption of no sorption, the leading edge of the <br />plume as defined by a concentration of one millionth of the initial concentration <br />fails to reach the groundwater. <br />4. Under realistic conditions, the rock units underlying all of the mine waste piles <br />would exhibit significant sorption characteristic that would retazd the movement <br />of species of concern. As noted in the response to comments, the Morrison <br />Formation behaves as a geochemical trap that resulted in the Uravan Mineral Belt. <br />5. Laboratory tests on the permeability of the Summerville Formation yielded a <br />hydraulic conductivity value of less than 9.7 E-10 cm/s, effectively creating an <br />aquiclude that would prevent downward migration of water to potential aquifers <br />below the Summerville. For comparison purposes, concrete has a hydraulic <br />conductivity of lE-8 cm/s. <br />