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2) CC&V must commit to place a minimum of three feet of inert low infiltration soil as part of the <br /> reclamation plan for ECOSA. Please revise the reclamation plan for ECOSA, adding three feet of <br /> inert low infiltration cover as part of the final reclamation plan. <br /> CC&V Response <br /> CC&V respectfully declines to adopt the proposed mitigation, for the following reasons. <br /> 1. The potential for flow along a pathway that might deliver water from ECOSA to Grassy Creek in <br /> has been eliminated by the submitted design of the ECOSA.Infiltration of ECOSA seepage will <br /> occur at all locations under the footprint with high reliability.To ensure that lateral flow cannot <br /> occur in locations where ECOSA is underlain by Precambrian rock, collector drains are designed <br /> 1 into the foundation of ECOSA to conduct any such flow to the diatreme.Thus mitigation is built <br /> in to the ECOSA design, and additional mitigation proposed by DRMS is unnecessarily <br /> duplicative. <br /> `-1`b 2. The proposed 3 feet inert low infiltration layer will not be more effective at limiting flow than <br /> the proposed cover design.The currently-proposed surface cover for the ECOSA is 6 inches of <br /> clayey soil salvaged from the ECOSA footprint, placed on a 2.5:1 slope, and revegetated. <br /> Infiltration through soil is controlled by the balance between available water at the surface and <br /> water uptake and evapotranspiration of water penetrating the surface by the vegetation <br /> growing on the cover.Thus the proposed cover design will limit infiltration to the natural rate of <br /> 6" per year8.The same will be true of the thicker cover proposed by DRMS: infiltration and <br /> seepage through the cover will be controlled at 6" per year by the vegetative demandof the <br /> reclaimed surface of the cover, regardless of the thickness and hydraulic conductivity of the <br /> underlying material9. This is demonstrated by the natural infiltration on the ECOSA footprint: <br /> approximately 6" of infiltration occurs through a vegetated layer of up to 30 feet of the same <br /> "low infiltration soil" that would be used for any ECOSA cover.The only circumstance where a <br /> cover material would control infiltration is when the saturatedhydraulic conductivity is <br /> substantially lower than 5x10-7 cm/sec,which is required to prevent infiltration of 6" per year <br /> under gravity drainage when the cover is saturated. This hydraulic conductivity cannot be <br /> achieved or maintained with locally-available materialplaced on an overburden storage facility <br /> because it cannot be effectively compacted on the loose, generally sloping surface of the OSA. <br /> Nor can the hydraulic conductivity be maintained on the upper surface of the OSA in the long <br /> term due to settlement disturbance, desiccation, freeze-thaw movements, and root <br /> penetration. Accordingly,there are no circumstances whereby the 36" cover will be more <br /> effective in limiting flow than the proposed 6" cover. <br /> tka (WO tAk_ ,S(p‘Pkg,A- -KAI\C <br /> 6..vv\-04 • , ) <br /> • <br /> k,_A/vuiA , <br /> 8Cressson Project Hydrogeochemical Evaluation, MLE2 Permit,Volume IV("Hydrogeochemical Report"),Section <br /> 2.4.2. <br /> 9 See for example, Maxey and Eakin, 1949,where montane infiltration is a function of precipitation only. <br /> Adrian Brown Consultants, Inc. Page 4 of 8 <br />