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Memo to Larry Oehler 5 October 24, 1993 soil wet of the optimum moisture content to achieve minimum hydraulic conductivity. The soil liner at San Luis is being compacted dry of optimum. The operator points to the low permeabilities measured (one in situ permeability test per acre of soil liner) for approved soil liner as the critical parameter that assures adequate soil liner performance. The Division agrees, but given the foregoing discussion (items 1, 2, and 3 above), as well as the lack of an ASTM guidance document for the use of the BATS in situ permeameter, the Division will require that the permeability of the soil liner be further verified by laboratory scale, falling head permeability tests for soil samples collected from approved soil liner areas (one sample per acre), and prepared to the same specifications (moisture content, density, etc.) as in situ soil liner. Conclusions The Division believes that. there are two components of any liner system that are integral to containment, they are: 1. A composite liner system consisting of a low permeability soil liner directly overlain by, and in intimate contact with, a geomembrane. 2. A method by which the hydrostatic head on the composite liner can be minimized. One of the reasons that the Division did not require a more verifiable head minimization system at the San Luis facility, such as geogrid sandwiched between two geomembranes for leak collection and removal, was the operator's assurance that the tailing liquidus, the fluid intended to be collected by the drainage blanket, would be low in cyanide. The early failure of the AVR system to eliminate cyanide from the tailing entering the impoundment, has created a situation where a considerable volume of high cyanide fluids may be migrating through the tailing pile. It is now evident that the hydraulic conductivity of the drainage blanket is lower than was anticipated by the design engineer. This will result in higher head on the liner and increased seepage from the impoundment than would occur if the drainage blanket material exhibited the hydraulic conductivity that was presented to the Division as "worst case" in the design documents. The operator points to testing that indicates that the soil liner and the tailings are less permeab:Le than was anticipated in the design assumptions, and that this will compensate for the less effective drainage blanket in terms of total seepage from the facility. The reason that this rational is not acceptable, is that maintenance of minimal head on the composite liner is the only way to guard against catastrophic loss of containment in the event that a large