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,1 <br />i1 <br />simulated using a geochemical mixing model. The pH simulations are described <br /> in Section 7.3.5. <br />' 7.3.2 Descriation 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 advection, 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 /> A Rd of 1simulates anon-reactive solute that is not attenuated and travels at the <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 /> areal source method was used in these analyses. <br /> The assumptions inherent to the MYGRT (Tetra Tech, Inc. 1989) code include: <br /> l <br /> • MYGRT only simulates groundwater flow and so <br />ute transport in the <br /> saturated zone; unsaturated (vadose) zone transport is not considered; ` <br />, <br />~ The groundwater seepage velocity remains constant over the distance of ~ <br /> a given simulation; <br />' ' <br /> s Law. The coefficients remain constant <br />Dispersion is represented by Fick <br />~ <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 /> <br />17 <br />