1993-11-03_REVISION - M1988112 (2)

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'l 2 could 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. BMRI points to testing that indicates that the soil liner and the tailings are less permeable 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 rationale is not acceptable, is that maintenance of minimal head on the composite liner is the only way to guard against loss of containment in the event that a large flaw exists, or develops in the liner. BMRI should achieve the worst case permeability for Type 2 material referenced in the design documents throughout Phase II of the facility. Preliminary testing indicates that none of the drainage layer material placed to date will meet this permeability criterion. Demonstration that minimum hydraulic conductivities of 5 x 10_4 cm/sec have been achieved within the drainage layer should take the form of one laboratory scale permeability test per acre of installed liner. Failing tests may be indicative of a need to increase the testing frequency. The Division staff that completed the original design review for the tailing impoundment recommended approval of a facility that incorporated a number of redundant design features to assure minimal environmental impact or loss of containment. Among the notable features stated as achievable in the original design documents are: 1. A high quality composite liner. 2. A drainage layer that would minimize head on the liner. This would reduce seepage to an inconsequential level through the small flaws that can be anticipated in any synthetic liner installation, and would also minimize seepage in the case of an unanticipated flaw or rupture of significant dimension. 3. Relatively benign material discharged to the impoundment. A commitment to detoxify the tailings to a low level of cyanide prior to routing them to the impoundment. 4. Large depth to water table. BMRI made the point that if there were unexpectedly high rates of seepage from the impoundment, it was unlikely that the contaminant plume would ever reacYi the water table. It was the redundant nature of the design concept that assured minimal or zero environmental impact. The safeguards provided by item 3 in the foregoing list were not in place for some period o1'' time, in that the AVR system failed, and a high cyanide slurry was discharged to the impoundment. It now appears that item 2, minimization of head on the liner, will also be less effective than if a high permeability material were to be utilized as called for in the design documents. EMBANKMENT RAISE FILL BMRI has proposed a change from the approved specification for gradation limits for the Phase II, Raise 1 embankment materials. The