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DG°~Q~~ Eight wells completed in the alluvial aquifer have been recently sampled and analyzed for a variety of water quality parameters. These results are presented in Appendix C and they generally indicate good water quality that meets the various groundwater quality standards. There is no indication of elevated pH, which is again interpreted as evidence that the water in the alluvial aquifer is not affected by CKD disposal. Figure 1 shows the location of the Dakota well and the eight alluvial wells. 7.0 Fate and Transport of Certain Constituents of Concern in Groundwater 7.1 Introduction The potential groundwater quality impacts resulting from the disposal of CKD in C-Pit is the formation and migration of leachate from the disposed CKD to groundwater. Leachate forms as precipitation infiltration percolates through the backfilled CKD and dissolves soluble constituents present in the CKD. The amount and composition of leachate is governed by three considerations: 1) the infiltration of water into and movement of water through the backfilled GKD, 2) the chemical composition of the CKD, and 3) the geochemical reactions between the CKD and the water. Leachate from the backfilled CKD is of concern because of the possibility that heavy metals, such as selenium, present in the CKD may be dissolved, enter the groundwater system and contaminate present or future groundwater resources. 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 C-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. The leachate volumes predicted by theHELP model are used as the contaminant source infiltration rates in the contaminant transport simulations described in Section 7.3. The HELP 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 ~a