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-47- <br />The elastomeric membrane system on the ground surface beneath the <br />vats should be combined with a 4 -inch layer of free draining gravel <br />and 6 inches of recompacted on -site clays which meet the criteria in <br />the Site Grading section below. The gravel drain blanket should be <br />sloped to a sump where fluids can be collected. <br />In our opinion, the potential for ground water contamination <br />resulting from an accidental spill during plant operation is virtually <br />nonexistent for the following reasons: <br />(1) Accidental spills, should they occur during the vat leaching <br />process, will be cleaned up within a short time. Cleanup of <br />spills will be in the interest of the operators of the plant so <br />that processing can continue and the values from the spilled <br />materials reclaimed. <br />(2) The natural soils at the site and the virtually impermeable <br />elastomeric membrane system which will be installed will absorb <br />almost no fluid during a shoat duration spill. <br />(3) Experience indicates that seepage from low head impoundments, <br />into the clays typical of those underlying the plant, even over <br />long periods of time, is very low. This low seepage is <br />attributed to the low differential fluid head which is not <br />capable of providing the energy required for fluid flow through <br />the fine - grained materials. <br />GRADING <br />'ite grading should be carefully planned to provide positive <br />drainage away from all buildings, surface facilities and waste <br />