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foot (fr/ft) and 0.005 ft/ft, was assumed for the Dakota sandstone aquifer. These gradients correspond to head changes in [he Dakota sandstone aquifer of 30 ft and 15 ft over the 3,000 ft distance between the C-Pit and the hypothetical receptor well along the site boundary. The saturated thickness of the Dakota sandstone was estimated to be about 65 ft based on the screened intervals of two wells drilled on the Hall property about 3,000 ft southeast of C-Pit. The effective porosity for all saturated formations was assumed to be 0.25. This value is typical of weathered soils, stream alluvium, and sandstones. Longitudinal and transverse dispersion coefficients, constituent transport parameters, were estimated using an approach suggested by Gelhar and others (1985). Conservative dispersion coefficients were used in the simulations to limit the spreading of the plume and result in earlier constituent arrival times at the hypothetical receptor well. The rate of seepage of ponded C-Pit water to the Dakota sandstone through the unweathered shales underlying the C-Pit was estimated at about I fUy using Darcy's Law (Attachment A). The thickness of the shales was estimated to be about 650 ft based on the well log for the Hall property well. For the 7.5 acre C-Pit pond, this corresponds to an annual C-Pit discharge volume of approximately 3.34 x 105 cubic feet (ft'). This discharge volume is considered conservative since it represents about 7 percent of the current volume of water (4.90 x 106 ft') ponded in the C- Pit. In the model, C-Pit seepage was conservatively assumed to be released instantaneously to the Dakota sandstone for a period of three years. The seepage rate and volume calculations are provided in Attachment A. C-Pit water constituent concentrations were obtained from the available analytical data collected by Cemex or the Colorado Division of Minerals and Geology (DMG) during 2004. Background constituent concentrations in the Dakota sandstone aquifer were obtained from the analyses previously conducted by Banks and Gesso (1999). The highest constituent concentrations observed to date in ponded C-Pit water were selected for seepage from the C-Pit to the underlying aquifer. C-Pit water constitutent concentrations used in the transport simulations include chloride (750 milligrams per liter [mg/L]), selenium (0.35 milligrams per liter [mg/L]), pH (12.5), sulfate (1,800 mg/L), thallium (0.0069 mg/L), and TDS (5,550 mg/L). These constituents were assumed to be non-reactive (i.e., no sorption or decay), thus, a Rd value of 1 was used in the simulations. This results in a conservative transport analysis since the constituents are transported at the same rate as groundwater flow. 4 Results of the MYGRT Simulations The results of the MYGRT simulations are shown on Figures 1 through 6 for two model cases. Results are shown for a base case and for case "A." The base case simulations were conducted at a hydraulic gradient of 0.01 (30 ft change in head over 3,000 ft) while case A was conducted at a October 29, 2004 4 Revision 0