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DG°~aG~~' leachate that might be expected from a wide variety of landfill designs. The model is applicable to open, partially-closed, and fully-closed sites. The CKD disposal site was simulated as a closed disposal site. The closed CKD disposal site is representative of the site follows p acem t of the final cover material. The area of the disposal site modeled was 20 acres v~ oot highwall at the deepest portion of the C-Pit. Default and synthetic climatologic and default soil characteristics data were used to perform the HELP analyses. climatologic data for Denver, Colorado were used in the analyses. The HELP modeling results are summarized in Table 7.1. HELP model output is included in Appendix D. An initial HELP model run was made to determine the flux of leachate from the base of the disposed CKD. The model was designed to account for the approximately 600,000 cubic yards of CKD expected to be disposed at the site over the next 20 years. The final cover material was assumed to be 2 feet thick with a poor grass cover. The final slope of the cover material was assumed to be 2 percent. The HELP model results indicate that the closed CKD disposal site will generate a leachate flux rate of approximately 0.74 inches per year at the base of the CKD. Fora 20 acres disposal site, the annual leachate volume generated is about 53, 898 cubic feet. An additional HELP model run was made to estimate the amount of leachate that might be transported vertically beneath the CKD disposal site to the top of the Dakota Sandstone. The underlying Niobrara Formation was simulated as a clay soil with low permeability to represent the interbedded shales present within the Niobrara. The shales are the primary controls on vertical groundwater flow beneath C-Pit. The results of this HELP model run indicate that the predicted leachate flux rate to the Dakota San s~3 tone is -about 0.005 inches per year or an annual leachate volume of 366 cubic feet per year. 7.3 Simulation of Constituent Fate and Transport 7.3.1 Introduction Although no viable groundwater pathways appear to exist between the CKD disposal site and adjacent aquifers, the potential impact of a hypothetical CKD leachate release on groundwater quality in the alluvium along St. Vrain Creek and the Dakota Sandstone was simulated. Constituent transport to the alluvium was assumed to occur through the weathered Niobrara Formation that occurs in the shallow subsurface. Groundwater flow was assumed to be to the east- northeast, towards St. Vrain Creek. The impact of a hypothetical leachate release on the alluvium was evaluated by assuming that a hypothetical recepotor well is completed in the alluvium along the eastern boundary of the plant about 3,000 feet east of C-Pit. 15