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~1 infiltration of water into and movement of water through the backfilled CKD, 2) the chemical composition of the CKD, and 3) the geochemical reactions between the CKD and the water. To determine the potential impact that the backfilled CKD may pose to groundwater, a hypothetical leachate release to groundwater was simulated. Because the backfilled CKD is not expected to be saturated, the leachate formed will migrate predominantly downward and enter the groundwater system beneath G-Pit. ' 7.2 Simulation of the Potential Quantity of Leachate The potential volume of leachate that may be formed at the CKD disposal site was evaluated using the Hydrologic Evaluation of Landfill Performance (HELP) model, version 3.07 (Schroeder, et.al., 1994). The leachate volumes predicted by theHELP model are used as the constituent release rates in the contaminant transport simulations described in Section 7.3. The HELP (Version 3, developed by U.S. Army Corps of Engineers) model is a quasi-two dimensional deterministic water budget model of water movement across, into, through, and out of landfills. The model accepts climatologic, soil, and design data, and uses a solution technique that accounts for the effects of surface storage, runoff, infiltration, percolation, evapotranspiration, soil moisture storage, and lateral drainage. The program facilitates rapid estimation of the amounts of runoff, drainage, infiltration, and 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 following placement of the final cover material. The area of the disposal site modeled was 20 acres with a 75 foot 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 ~i00,000 subi6 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. This disposal scenario is conservative as it minimizes the amount of ' evapotranspiration which allows more water to infiltrate the disposed CKD. 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 acre disposal site, the annual leachate volume generated is about 53, 898 cubic feet. ' 15