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DQQG~`~ 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 ww i~fl~liuvwm a a o a an s one was assumed to occur as po~rous_fl~. 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 simulated using a geochemical mixing model. The pH simulations are described in Section 7.3.5. 7.3.2 Description of the Model The hypothetical leachate release-to alluviu nd Dakota Sandstone groundwater was simulated u ~ MYGRT rsion 2.0). MYGRT was developed by Tetra Tech, Inc. (1989) to simula a the migration of inorganic or organic constituents in groundwater downgradient of a constituent source. The model is based on the advection-dispersion-retardatio -d ca equation. Processes simulated for inorganic cons ituents include advection, ispe sion, and linear sorption (retardation). Both reactive and non-reactive constituents can be simulated by varying the retardation factor). Retardation is a measure of the parti i nin o reactive solutes betty a has a the so id p se. ,4 Rd of 1 simulates a r]rm-reacti~.. to that is not attenuated and trave s e same rate as groundwater. Initial constituen concentrations can be modeled by specifying the solute concentrations at the downgradient edge of the source or as an areal source. If the constituent concentration is specified as an areal source, the model calculates the solute concentration in the aquifer by mixing the upgradient aquifer water with leachate based on its flux and composition. The areal source method was used in these analyses. The assumptions inherent to the MYGRT code include: ~s