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<br />expression of the alluvium is quite clear compared to the outcrops and subcrops <br />of the sedimentary rocks. <br />Item No. 2: Modelino of the CKD Disposal Site as an Open vs. Closed Disposal <br />Site. <br />The model simulations were indeed done under the assumption of a closed <br />disposal site. The rationale for this was that this would be the worst case <br />scenario, the CKD disposal site would be full and the long term, post closure <br />conditions would have to be modeled to determine long term impacts, if any. <br />Even though Harry Posey has stated that conservative assumptions were used in <br />this analysis, Southdown will commit to interim capping of the disposal site. This <br />' management practice is in addition to the other management practices described <br />in the report referenced previously. <br />~ ~`e ~~ ~ The "worst case" leachate volume above which potential off-site water quality <br />~.~ ~ impacts could begin to occur is 350,000 cubic feet per year. This volume of <br />` ~ ~~' water would have to pass through the CKD mass and flow out of the bottom of <br />~ the disposal site in order to even approach off-site impacts. To prevent this <br />ituation form happening, Southdown will place at least 2-feet of compacted <br />shale and clay over the CKD when the disposal area reaches 7 acres in areal <br />ent and 20 feet in thickness. 6 . ~,,,~-.~. c F <br />The following section of this letter discusses the results of a additional model <br />simulations that demonstrate that this interim capping practice will never allow <br />the leachate volume to approach the threshold volume of 350,000 cubic feet per <br />year. <br />Two additional HELP model runs were made to simulate the above described <br />interim capping of the disposed CKD. An initial run was made to simulate an <br />open, partially-filled CKD disposal site. This initial run was simulated as a 7 acre <br />disposal site with approximately 20 feet of CKD capped by an interim 2~ont thick <br />c9mpacted shale/clay cover. Since the disposal site was simulated as an open <br />site, precipitation run-off was not allowed to occur. The default soil type used to <br />simulate the disposed CKD (material texture 10) was the same as that used in <br />he previously submitted HELP model. The compacted shale/clay cover was <br />simulated as a compacted clay (material texture 25) with an effective saturated <br />hydraulic conductivity of 3.60E-06 centimeters per second (cm/sec). Default <br />evapotranspiration and precipitation data for Denver, Colorado were used in the° <br />simulation. ~ ~ 3 `='^/~~ I <br />~{V~IM N(/VL <br />The model results of the open, partially-filled CKD disposal site indicates that the <br />flux of leachate that will be generated at the base of the disposed CKD is about <br />0.3 inches per year. For this 7 acre disposal site, the annual leachate volume <br />predicted is about 7,592 cubic feet per year. A comparision of the leachate flux <br />generated under the interim capping scenario (0.3 inches per year) with the <br />