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.: , ~ III illliliiillll 111 sTATL~F COLORADO <br />MINED LAND RECLAMATION DIVISION <br />Depanment of Natural Resources <br />7313 Sherman St.. Room 215 <br />Denver, CO 80203 <br />303 866-3567 <br />Fn%~ 303 8328706 <br />OF ~~r0 <br />~r'ic~~A' <br />. ~.; `. <br />/e 76 ~ <br />Roy Romer. <br />Governor <br />M~cnael B. Long, <br />DrviSion Director <br />DATE: April 21, 1992 <br />T0: Bruce Humphries <br />FROM: Allen Sorenson ~~ <br />/ <br />RE: Potential Seepage Through <br />San Louis Project <br />the Tailings Impoundment Liner at the <br />Potential leakage through the VLDPE liner could occur through flaws in the <br />liner, and by permeation through the liner. Unsaturated flow from the base of <br />the tailings impoundment to the water table will occur when the specific <br />retention of the sub-soils (estimated by SRK to be ten percent) is exceeded. <br />The imperative question is whether the seepage from the tailings impoundment <br />will be of a quantity that will raise the moisture content of the sub-soils <br />beyond the moisture content at which gravity flow through the soil will occur. <br />One factor upon which the rate of seepage depends is the head applied to the <br />liner. The sub-drainage layer above the liner is designed to minimize the <br />head on the liner. I have asked Rob Dory of SRK to supply the Division with a <br />demonstration of what the maximum head will be, based on the permeability of <br />the drainage layer and the amount of water entering 1t. <br />I have performed calculations for the rate of seepage (see attached sheet) for <br />both one and two feet of applied head. The amount of seepage due to <br />permeation through the liner under a two-foot head is .0061 cu.ft./yr. <br />per sq.ft. of liner. With 7,300,000 sq.ft. of liner at build out, this <br />results in 44,530 cu.ft./yr. seepage. For one foot applied head, the rate of <br />seepage due to permeation through the liner is .0031 cu.ft./yr. per sq.ft. of <br />liner or 22,630 cu.ft./yr. for the impoundment. <br />The amount of seepage due to flaws in the liner, based on one .1 sq.cm. hole <br />per acre, is insignificant compared to seepage due to permeation. SVZK has <br />caiculated an amount of seepage due to liner flaws, assuming one-foot head, to <br />be 63.4 cu.ft./yr. I have asked Rob Dory of SRK to submit these calculations <br />for review. These calculations become very complicated, involving some <br />complex differential equations, so I have as yet not attempted them myself. <br />These models should involve hems-spherical or conical zones of flow above and <br />below the liner flaw; this is what should be looked for in the SRK submittals. <br />