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I, An additional HELP model run was made to estimate the amount of leachate that might be transported vertically beneath the CKD disposal site to the top of the Dakota Sandstone. The underlying Niobrara Formation was simulated as a clay soil with low permeability (1 X 10-6 cm/s) to represent the interbedded shales present within the Niobrara. The shales are the primary controls on vertical groundwater flow beneath C-Pit. The results of this HELP model run indicate that the predicted leachate flux rate to the Dakota Sandstone is about 0.005 inches per year or an annual leachate volume of 366 cubic feet per year. 7.3 Simulation of Constituent Fate and Transport 7.3.1 Introduction oug no via a group water pathway appears to exist between the CKD disposal site and adjacent aquifers, the potential impact of a hypothetical CKD leachate release on groundwater quality in the alluvium along St. Vrain Creek and the Dakota Sandstone was simulated. Constituent transport to the alluvium was assumed to occur through the weathered Niobrara Formation that occurs in the shallow subsurface. Groundwater flow was assumed to be to the east- ' northeast, towards St. Vrain Creek. The impact of a hypothetical leachate release on the alluvium was evaluated by assuming that a hypothetical recepotor well is completed in the alluvium along the eastern boundary of the plant about 3,000 feet east of C-Pit . ' The impact of a hypothetical leachate release on the deep Dakota Sandstone aquifer beneath the disposal site was also evaluated. A hypothetical leachate release was assumed to occur at the base of the CKD and is transported ' vertically downward to the Dakota Sandstone. This model assumption is very conservative, as the rock formations lying between the base of the CKD and the top of the Dakota Sandstone consist of a large percentage of low permeability shales which would greatly impede vertical groundwater flow. The impact of the hypothetical leachate release on Dakota Sandstone groundwater quality was evaluated by assuming that a hypothetical receptor well is completed in the Dakota Sandstone along the eastern boundary of the plant, about 3,000 feet east of the C-Pit. Groundwater flow in the Dakota Sandstone was assumed to be towards the east. Constituent transport in both aquifers was simulated using aquasi-analytical groundwater flow and solute transport model. Groundwater flow in the alluvium and Dakota Sandstone was assumed to occur as porous flow. Conservative aquifer parameters and assumptions were used to simulate the hypothetical CKD leachate release and subsequent constituent transport to the hypothetical receptor wells completed at the plant boundary. Solute transport simulations were performed for selenium, thallium, total dissolved solids (TDS), and gross beta (simulated as potassium-40). These constituents were selected because ' they were the only constituents present in the leachate that exceeded water quality standards. pH also exceeded the water quality standard and was ~s