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alluvium was not evaluated since there does not appear to be a hydraulic connection between the ponded water in the C-Pit and the alluvium. Geologically, C-Pit is excavated in the Fort Hays Limestone Member of the Upper Cretaceous Niobrara Formation. The Niobrara Formation is underlain by a thick sequence (about 500 feet [ft]) of interbedded, low permeability claystones, siltstones, shales, and limestones of the Carlile Shale, Greenhorn Limestone, Graneros Shale, and Mowry Shale formations (Madole et al., 1998). The Dakota Group, comprised of sandstones, siltstones, and shales, underlies the Mowry Shale. These formations generally dip between 6 and 15 degrees to the east (Madole et al., 1998). Groundwater present in sandstones of the Dakota Group is locally used for water supply. The thickness of the claystones, siltstones, shales overlying the Dakota sandstone beneath the site was estimated to be about 650 ft based on the well log for the Hall property well (Permit MH-35268). Constituent transport to the Dakota sandstone aquifer was conservatively assumed to occur as one-dimensional vertical leakage of C-Pit water through the underlying low permeability, unweathered claystones, siltstones, shales, and limestones of the undivided Carlile Shale, Greenhorn Limestone, Graneros Shale, and Mowry Shale formations. The seepage was assumed to be instantaneously released to the Dakota sandstone. Groundwater quality impacts were evaluated using MYGRT (Tetra Tech, 1989), atwo-dimensional quasi-analytical groundwater flow and solute transport model. The potential groundwater quality impacts on the Dakota sandstone were evaluated for a well completed at a depth of about 800 ft in the Dakota sandstone along the eastern boundary of the Cemex plant, about 3,000 ft downgradient of the C-Pit. 2 Model Used The potential groundwater quality impacts were simulated using MYGRT Version 2.0 (Tetra Tech, 1989). MYGRT was developed to simulate the migration of inorganic and organic constituents in groundwater based on the advection-dispersion-retardation-decay equation. Processes simulated for inorganic constituents include advection, dispersion, and linear sorption (retardation). Both reactive and non-reactive constituents can be simulated by varying the retardation factor (Rd). Initial constituent concentrations were modeled by specifying the solute concentrations as an areal source and allowing the model to calculate aquifer constituent concentrations by mixing upgradient aquifer groundwater with leachate based on its flux and composition. Assumptions inherent to the MYGRT code include: • MYGRT only simulates groundwater flow and solute transport in the satwated zone; • Groundwater seepage velocity remains constant over the distance of a given simulation; October 29, 2004 2 Revision 0