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DQaf~~ The hypothetical receptor well constituent concentrations predicted in the Dakota simulations may therefore be overestimated by as much as 100 times. Leachate seepage was assumed to occur over the area of the C-Pit, which is estimated to be about 20,000 acres. MYGRT simulations were conducted for selenium, thallium, TDS, and gross beta. These constituents were the only ones in the synthetic CKD leachate that exceeded the Colorado water quality standards. Gross beta was simulated as potassium-40. Potassium-40, a naturally occuring radionuclide in shales, was determined to be the predominant beta emitter in the CKD by an analysis conducted by the EPA in 1992. The concentrations of the leachate constituents used in the solute transport analyses are shown in Table 7.3. These concentrations were those determined by laboratory analysis of the synthetic CKD leachate. Transport of these constituents was simulated be assuming that they are not attenuated. A Rd value of 1 was used in the analyses. An Rd of 1 simulates anon-reactive solute that is not attenuated and travels at the same rate as groundwater. Simulating the constituents as non-reactive provides the most conservative assumption for constituent transport. 7.3.4 Results of the MYGRT Simulations The results of the solute transport simulations are summarized in Table 7.4. The results are shown as a relative increase in constituent concentration for the alluvium and the Dakota Sandstone. Table 7.4 shows the predicted concentrations for each constituent for the range of hydraulic gradients simulated. The solute transport analyses predict that a hypothetical leachate release will not impact groundwater quality, under the scenarios evaluated, in either the St. Vrain Creek alluvium or the Dakota Sandstone at a hypothetical receptor well located about 3,000 feet east of the C-Pit. The background concentration for these constituents in the alluvium and the Dakota Sandstone are shown on the table for comparison. Solute attenuation or decay was not simulated in these analyses so that a conservative simulation was maintained. However, some of the constituents present in the CKD leachate may undergo attenuation or decay during transport. Metal cations, such as thallium or selenium, are reactive constituents that may be readily attenuated during transport if sorptive solid phases such as clay minerals or iron and manganese hydroxides are present in the aquifer matrix. Because of the low concentrations of metals anticipated in a potential leachate release, the presence of sorptive materials in the aquifer matrix would rapidly attenuate most cations released to groundwater. 7.3.5 Mixing Simulation of pH in the Alluvium and Dakota Sandstone Evaluation of the impact of a hypothetical CKD leachate release on alluvium and Dakota Sandstone groundwater pH was performed using PHREEQC. PHREEQC 18