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simulated using a geochemical mixing model. The pH simulations are described <br />in Section 7.3.5. <br />7.3.2 Description of the Model <br />' The hypothetical leachate release to alluvium and Dakota Sandstone <br /> groundwater was simulated using MYGRT (Version 2.0). MYGRT was developed <br /> by Tetra Tech, Inc. (1989) to simulate the migration of inorganic or organic <br />' constituents in groundwater downgradient of a constituent source. The model is <br /> based on the advection-dispersion-retardation-decay equation. Processes <br /> simulated for inorganic constituents include advecfion, dispersion, and linear <br /> sorption (retardation). Both reactive and non-reactive constituents can be <br /> simulated by varying the retardation factor (Rd). Retardation is a measure of the <br /> partitioning of reactive solutes between the aqueous phase and the solid phase. <br /> - i e so u e a is no a enua a an rave s a e <br /> same rate as groundwater. Initial constituent concentrations can be modeled by <br /> specifying the solute concentrations at the downgradient edge of the source or as <br /> an areal source. If the constituent concentration is specified as an areal source, <br /> the model calculates the solute concentration in the aquifer by mixing the <br /> upgradient aquifer water with leachate based on its flux and composition. The <br />' area! source method was used in these analyses. <br /> The assumptions inherent to the MYGRT (Tetra Tech, inc. 1989) code include: <br />~ <br />• MYGRT only simulates groundwater flow and solute transport in the <br />saturated zone; unsaturated (vadose) zone transport is not considered; <br />• The groundwater seepage velocity remains constant over the distance of <br />a given simulation; <br />• Dispersion is represented by Fick's Law. The coefficients remain constant <br />for a given simulation; <br />• Sorption is treated as linear equilibrium partitioning between aqueous and <br />solid phases: Sorption was not simulated in these analyses; <br />• Transport of a single solute species for a given simulation is adequate. <br />Interactions between species are not considered. <br />7.3.3 Data Sources <br />The shortest distance to the property line was chosen as a conservative <br />assumption. The geologic units between the C-Pit and the hypothetical receptor <br />wells were assumed to be saturated and to exhibit uniform porous medium <br />characteristics. <br />17 <br />