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GeoScience Services <br />2263 Kingston Road <br />Crand Junction, CO 81503 <br />(970) 31 ¢3356 <br />Listed on the following pages are specific questions presented in the Colorado DMG <br />letter (Posey and Pickford, 2006) and responses to those questions. <br />Please list the presumed concentrations of regulated parameters. <br />Conservative initial concentrations for the species of concern were used in all of the <br />contaminant transport simulations. For example, at mine SM-18 higher concentrations of <br />uranium from the ore and not the waste rock (Posey, 2005) were used for initial <br />concentrations-a conservative assumption. A value of 2700 ug/I of uranium, the highest <br />value reported for any species of concern, either waste rock or ore samples, was input <br />into the model. After 1,000 yeazs of transport under no sorption conditions, the leading <br />edge of the uranium plume as defined by a concentration of 1E-06 of the initial <br />concentrations (i.e., 0.0027 ug/l) migrated a vertical distance of approximately 400 ft <br />failing to reach the water table that is another 80 ft below the leading edge of the plume. <br />Even if the uranium plume did reach the water table, background concentrations in the <br />upper most aquifer based on a well located approximately 4000 feet southeast of Uravan <br />yielded background uranium concentrations of 18 and 22 ug/1-nearly four orders of <br />magnitude higher than potential uranium leached from the ore piles. None of the other <br />species of concem (Al, As, Pb, or Se) exhibited a SPLP value as high as the ore values <br />for uranium. Since no sorption was assumed for transport of the species of concem, a <br />similar decrease of 1E-06 of the initial concentration at the leading edge of the plume <br />would be expected. Further, the other species of concern exhibit similar rafios of leached <br />concentrations to background water quality concentrations. <br />The implications of this analysis suggest that infiltrating water would actually dilute <br />groundwater concentrations of species of concern. Realistically, the impact from the <br />waste rock piles on the underlying groundwater quality would be negligible. <br />Please clarify the composition of simulated rainwater used in the SPLP tests. The <br />write-up appears to indicate that rainwater (snowmelt} pH is neutral, which is not <br />correct. <br />DMG is correct that the pH of rainwater is not neutral but slightly acidic. Due to the <br />cazbonate composition of the bedrock underlying the waste rock sites, it is expected that <br />the pH of the pore water would increase significantly. This assumption allows for the <br />transport of uranium as a complex carbonate species (i.e., UOZC03) <br />Please identify the transmissivity of the various units as used in the model, and <br />indicate whether the transmissivity was measured or assumed. <br />There is only a limited amount of hydraulic data available for the geologic formations <br />underlying the waste piles at the four mines evaluated in this study. Consequently, <br />estimates of hydraulic conductivity (permeability) based on limited site data and reported <br />values for the type of rocks underlying the mines were used in the modeling. Reference <br />