1999-10-04_REVISION - M1977208 (2)

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expression of the alluvium is quite clear compared to the outcrops and subcrops of the sedimentary rocks. Item No. 2: Modeling of the CKD Disaosal Site as an Oaen vs. Closed Disaosal Site. The model simulations were indeed done under the assumption of a closed disposal site. The rationale for this was that this would be the worst case scenario, the CKD disposal site would be full and the long term, post closure conditions would have to be modeled to determine long term impacts, if any. Even though Harry Posey has stated that conservative assumptions were used in this analysis, Southdown will commit to interim capping of the disposal site. This management practice is in addition to the other management practices described in the report referenced previously. The "worst case" leachate volume above which potential off-site water quality impacts could begin to occur is 350,000 cubic feet per year. This volume of water would have to pass through the CKD mass and flow out of the bottom of the disposal site in order to even approach off-site impacts. To prevent this situation form happening, Southdown will place at least 2-feet of compacted shale and clay over the CKD when the disposal area reaches 7 acres in areal extent and 20 feet in thickness. The following section of this letter discusses the results of a additional model simulations that demonstrate that this interim capping practice will never allow the leachate volume to approach the threshold volume of 350,000 cubic feet per year. Two additional HELP model runs were made to simulate the above described interim capping of the disposed CKD. An initial run was made to simulate an open, partially-filled CKD disposal site. This initial run was simulated as a 7 acre disposal site with approximately 20 feet of CKD capped by an interim 2-foot thick compacted shale/clay cover. Since the disposal site was simulated as an open site, precipitation run-off was not allowed to occur. The default soil type used to simulate the disposed CKD (material texture 10) was the same as that used in the previously submitted HELP model. The compacted shale/clay cover was simulated as a compacted clay (material texture 25) with an effective saturated hydraulic conductivity of 3.60E-06 centimeters per second (cm/sec). Default evapotranspiration and precipitation data for Denver, Colorado were used in the simulation. The model results of the open, partially-filled CKD disposal site indicates that the flux of leachate that will be generated at the base of the disposed CKD is about 0.3 inches per year. For this 7 acre disposal site, the annual leachate volume predicted is about 7,592 cubic feet per year. A comparision of the leachate flux generated under the interim capping scenario (0.3 inches per year) with the