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1 <br /> San Luis Project -Phase 11 Raise I Expansion Canatrvction Statue Report <br /> could take one to two years to occur. To reach the "equilibrium" condition used in the <br />' evaluation of head (modeled using the agricultural file equation) it was anticipated that the head <br /> on the liner would exceed the long term condition which has been considered in the potential <br /> seepage impact evaluations. <br /> Utilizing a conservatively low drain layer permeability of 2 x 10~ cm/sec and a conservatively <br />hi <br />h <br />li <br />ili <br />ili <br />f <br />10~ <br />i <br /> g <br />, control <br />ng ta <br />ngs permeab <br />ty o <br />1 x <br />cmisec coupled with a dra <br />n pipe spacing of <br /> 40 ft., the maximum head on the liner between drain pipes would be 14 ft. and the average head <br />' on the liner roughly 7 ft. The period required to deposit sufficient tailings to create 14 ft of <br /> head on the liner is sufficient to allow consolidation of tailings in contact with the liner and <br /> desiccation of tailings on the beach to occur. It is highly unlikely therefore, considering the <br /> operation of the facility that a theoretical head of 14 ft could be developed on the liner. Using <br /> a drain layer permeability of 5 x 10-5 cmf sec the avenge head decreases to 1.4 ft. Using a more <br />li <br />i <br />lli <br />bili <br />f <br />0•' <br />li <br />d <br /> rea <br />st <br />c contro <br />ng tailings permea <br />I x 1 <br />ty o <br />cm/sec the average head on the <br />ner re <br />uces <br /> to 0.4 ft. <br />' <br /> Once multiple layers of variable permeability tailings are developed in fire impoundment, <br />' drainage will be controlled by the lower permeability layers. Drainage from the tailings then <br /> becomes a function of supply of solution to the drain. At this time further reduction in head on <br />' the liner will occur. <br /> As per previous potential seepage impact studies which are contained in the v2rious documents <br /> referenced in section 1.0, seepage by permeation through the intact VLDPE liner is negligible. <br /> The primary concern with regard to seepage from the facility is holes or flaws in the liner. <br />' Using the EPA approach outlined in the "Draft Leak Detection Rule," one lOmm2 flaw or hole <br /> per acre of geomembrane liner is assumed. This assumption is for the pudpose of seepage <br />' impact analysis and does not imply that these holes or flaws will actually be present. <br /> If the average head on the liner is 7 ft. and the soil beneath the liner exhibits a permeability of <br />' l x 10~ cm/sec, each lOmmr hole will pass 1.99 gallons of seepage per year or, over the 167 <br /> acre lined area, a total seepage of 332 gallons per year. This is less than the potential seepage <br />' previously calculated for holes in the liner which was presented in the permit adequacy responses <br /> (response No. 11). If the permeability of the liner sub-base clay liner layer is assumed to be <br />' 6 x 10-' cmisec, the avenge value from 37 field permeability tests conducted during the Phase <br />allons per <br />h the liner flaws or holes reduces to 199 <br />n <br />the total see <br />e thro <br />[I con <br />wcti <br />a <br /> g <br />, <br />p <br />g <br />ug <br />s <br />o <br />' October, 1993 70 SRK Project No. 14723 <br />