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infiltration of water into and movement of water through the backfilled CKD, 2) <br />the chemical composition of the CKD, and 3) the geochemical reactions between <br />the CKD and the water. <br /> <br /> <br />~I <br /> <br /> <br /> <br /> <br /> <br /> <br /> <br />To determine the potential impact that the backfilled CKD may pose to <br />groundwater, a hypothetical leachate release to groundwater was simulated. <br />Because the backfilled CKD is not expected to be saturated, the leachate formed <br />will migrate predominantly downward and enter the groundwater system beneath <br />C-Pit. <br />7.2 Simulation of the Potential Quantity of Leachate <br />The potential volume of leachate that may be formed at the CKD disposal site <br />was evaluated using the Hydrologic Evaluation of Landfill Performance (HELP) <br />mo~c`eT-version ~ 7 (Schroeder, et.al., 1994). The leachate volumes predicted <br />by theHELP model are used as the constituent release rates in the contaminant <br />transport simulations described in Section 7.3. <br />The HELP (Version 3, developed by U.S. Army Corps of Engineers) model is a <br />quasi-two dimensional deterministic water budget model of water movement <br />across, into, through, and out of landfills. The model accepts climatologic, soil, <br />and design data, and uses a solution technique that accounts for the effects of <br />surface storage, runoff, infiltration, percolation, evapotranspiration, soil moisture <br />storage, and lateral drainage. The program facilitates rapid estimation of the <br />amounts of runoff, drainage, infiltration, and leachate that might be expected <br />from a wide variety of landfill designs. The model is applicable to open, partially- <br />closed, and fully-closed sites. <br />The CKD disposal site was simulated as a closed disposal site. The closed CKD <br />disposal site is representative of the site following placement of the final cover <br />material. The area of the disposal site modeled was 20 acres with a 75 foot <br />highwall at the deepest portion of the C-Pit. Default and synthetic climatologic <br />and default soil characteristics data were used to perform the HELP analyses. <br />climatologic data for Denver, Colorado were used in the analyses. The HELP <br />modeling results are summarized in Table 7.1. HELP model output is included in <br />Appendix D. <br /> An initial HELP model run was made to determine the flux of leachate from the <br /> base of the disposed CKD. The model was designed to account for the <br /> approximately 600,000 cubic yards of CKD expected to be disposed at the site <br /> over the next 20 years. The final cover material was assumed to be 2 feet thick <br /> with a poor grass cover. The final slope of the cover material was assumed to be <br /> 2 percent. This disposal scenario is conservative as it minimizes the amount of <br /> evapotranspiration which allows more water to infiltrate the disposed CKD. The <br /> HELP model results indicate that the closed CKD disposal site will generate a <br /> leachate flux rate of approximately 0.74 inches per year at the base of the CKD. <br />' Fora 20 acre disposal site, the annual leachate volume generated is about 53, <br /> 898 cubic feet. <br /> <br />~5 <br />