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Western Slope Carbon, Inc. Page 15 December 15, 1981 ' 2. Recharge to the site groundwater basin is through direct precipitation only. ' 3. After initial saturation, the site native soils have a very low permeability which results in a high runoff ' coefficient and a small percentage of infiltration. Thus, saturation of the coal process waste by direct precipitation appears ' to be the only reasonable method for leachate generation. Infiltration into the waste can be significantly reduced or almost completely eliminated by limiting the amount of exposed waste surface to the minimum necessary for efficient placement, spreading, and compaction. An interim placement of ' native soil cover with its high runoff and extremely low infiltration characteristics over the remaining portion of the wastes would essentially ' eliminate infiltration. In this manner, saturation of the waste could be prevented and leachate would not be generated. When the Pit No. 6 area has been filled to the final planned grade, application of the full four feet ' of native soil cover would, in our opinion, reduce the potential for leachate generation due to infiltration to an extremely low possibility. t In the unlikely event that leachate were generated, quantities would likely ' be very small due to the low permeability of the cover material. If desired, the piezometer installed in Boring 5 could be periodically sampled to monitor ' possible leachate generation. Should leachate be observed, remedial measures, such as installation of a cut-off wall around the downslope portion of the fill, could be provided. 1 CLOSURE t The conclusions and recommendations in this report apply to the proposed construction as described and result from engineering and geologic analyses based on our interpretation of geotechnical conditions. ' We request the opportunity to review the final Coal Waste Disposal Operation and Reclamation Plan for compliance with the intent of our recommendations.