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I' <br />I' <br />I, <br />I <br /> <br /> <br /> <br />1 <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br />The aquifer parameters used in MYGRT are summarized in Table 7.2. Solute <br />transport simulations to the alluvium were conducted using a hydraulic <br />conductivity value of 2,000 feet per year for the weathered Niobrara Formation. <br />This value was determined by packer tests conducted in the Niobrara Formation <br />as part of an evaluation of the water storage potential of the A and C-Pits (Rocky <br />Mountain Consultants, 1988). The hydraulic conductivity of the Dakota <br />Sandstone was also assumed to be about 2,000 feet per year. A range of <br />reasonable hydraulic gradients, 0.1, 0.05, and 0.01, were used in the simulations <br />since the hydraulic gradient at the site is not known.. These values are typical of <br />similar hydrogeoiogic systems. Longitudinal and transverse dispersion <br />coefficients were estimated using an approach suggested by Gelhar and others <br />(1985). The downgradient flow direction was assumed to be due east towards <br />the St. Vrain Creek. The saturated thickness of the alluvium and weathered <br />Niobrara was assumed to be about 30-feet. The saturated thickness of the <br />the weathered Niobrara and the Dakota Sandstone was assumed to be 0.25. <br />This value is considered typical of weathered soils and sandstone units. <br />The hypothetical leachate release rate for the alluvium transport simulation was <br />estimated to be about 0.07 feet per year. The leachate release rate was <br />estimated using the HELP model described in Section 7.2. A leachate release <br />rate of 0.03 feet per year was estimated for the Dakota Sandstone transport <br />simulation. The leachate flux rate used in the Dakota Sandstone simulation is <br />about 80 times higher than the leachate flux rate (0.0004 feet per year) <br />calculated by the HELP model because it is the lowest value that can be <br />simulated using MYGRT. The hypothetical receptor well constituent <br />concentrations predicted in the Dakota simulations may therefore be <br />overestimated by as much as 80 times. Leachate seepage was assumed to <br />occur over the area of the C-Pit, which is estimated to be about 20 acres. <br />MYGRT simulations were conducted for selenium, thallium, TDS, and gross beta. <br />These constituents were the only ones in the synthetic CKD leachate that <br />exceeded the Colorado water quality standards. Gross beta was simulated as <br />potassium-40. Potassium-40, a naturally occuring radionuclide in shales, was <br />determined to be the predominant beta emitter in the CKD by an analysis <br />conducted by the EPA in 1992. The concentrations of the leachate constituents <br />used in the solute transport analyses are shown in Table 7.3. These <br />concentrations were determined by laboratory analysis of the synthetic CKD <br />leachate. Transport of these constituents was simulated by assuming that they <br />are not attenuated. A Rd value of 1 was used in the analyses. An Rd of 1 <br />simulates anon-reactive solute that is not attenuated and travels at the same <br />rate as groundwater. Simulating the constituents as non-reactive provides the <br />most conservative assumption for constituent transport. <br />7.3.4 Results of the MYGRT Simulations <br />The results of the solute transport simulations are summarized in Table 7.4. The <br />results are shown as a relative increase in constituent concentration for the <br />~a <br />