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<br />June 3, 1992 <br />Page 3 <br />STEFF~CiERTSON AND KIRSTEN <br />Amendment) conservatively assumes a drainage layer permeability of 5 x IOy cm/sec and a design <br />objective of a maximum head between pipes of 1.75 feet. Based upon a tailings permeability of 5 <br />x 10' crrt/sec a minimum spacing of 35 feet is indicated. For a tailings permeability of 10' cnt/sec, <br />a minimum spacing of 78 feet is indicated. The calculations also consider a drainage layer <br />permeability of 10"' crrt/sec which for a tailings permeability of 5 x 10-0 and 10" cTrt/sec indicates <br />drain spacing of 49.5 and 110 feet, respectively. A design spacing of 40-foot maximum was adopted. <br />Actual drain pipe locations are shown on the drawing No. 14711-105 issued with the as-built report. <br />Based on the selected spacing of 40 feet, the hydraulic head on the synthetic liner would be limited <br />to less than two feet following approximately one year of tailings deposition. The average hydraulic <br />head on the liner can be estimated to be approximately one foot following one year of tailings <br />deposition and consolidation of the initial tailings. <br />3.0 LEAKAGE ESTIMATES AND GROUND WATER IMPACT'S <br />Potential leakage from the tailings impoundment is controlled by a number of factors. These factors <br />are discussed in response to Questions 10 and it of the CMLRD Adequacy Letter Responses <br />(SRI{, January 1990). The calculations of theoretical leakage quantities and seepage rates into the <br />underlying foundation soils are attached to this memorandum. <br />The most important factors in limiting leakage are the underdrain layer, drainage pipe network and <br />the synthetic liner. The existence of the drainage layer and underdrain pipe network will limit the <br />hydraulic head on the underlying foundation soils. For the purpose of analysis, the average <br />hydraulic head of one foot, discussed above, was adopted. Using the EPA approach outlined in the <br />"Draft Leak Detection Rule", one 10 mm2, flow or hole per acre of geomembrane liner is assumed. <br />This represents the probable frequency of flaws in the geomembrane following the high level of <br />quality control exercised during construction. <br />To analy2e flow through each potential flaw in the liner, it is necessary to consider the permeability <br />of the material both above and below the liner. Flow will occur in a hemispherical pattern to the <br />flaw and, once saturated, in a similar hemispherical pattern in the soil component of the composite <br />liner system. The calculations performed during design (attached) conservatively assumed that the <br />permeability of the soil liner beneath the hole was at 10' cm/sec. Based upon this assumption, each <br />flaw would result in a theoretical seepage rate of 0.018 gpm. For the 167.6 acre impoundment area, <br />this is equivalent to 4740 gallons per year. Actual permeability measurements obtained during <br />construction quality control (Appendix A, Construction Report) indicate a consistent value of less <br />