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Memo to Larry Oehler 3 October 24, 1993 that, in fact, the assumed worst case hydraulic conductivities have not been achieved. Preliminary laboratory permeability testing results for the drainage blanket material are more in line with the assumed worst case, but still show the assumptions to be optimistic rather than conservative. The hydraulic conductivity of the drainage blanket has been an item of concern to the Division for the last 1.5 years (e.g. memo from Posey to Humphries dated 4/24/92, memo from Sorenson to Humphries dated 4/29/92, memo from Sorenson to Humphries dated 6/24/92). In the 6/24/92 memo, I stated: "unless you are dealing with clean sands or gravels (which we are not), or unless you have a hydrometer analysis of particles within the soil smalle¢~ than .074 mm diameter (which we do not), that any attempt to relate grain- size distribution to permeability is tenuous". The memo goes on to say that "samples of the sub-drain layer material should be subjected to additional laboratory scale, constant head permeability analysis, to provide reliable data on the permeability of this vital drain layer material". Given that the drainage layer materials may be much less permeable than the approved design anticipates, laboratory scale falling head permeability analysis may also be indicated. The operator asserts that lower permeability layers within the tailing will control the rate at which water can pass through to the drainage layer because the ability for water migrating through the tailings to bypass low permeability zones in the tailings is negligible due to the greater seepage path distance and tortuosity required. It is the Division's view, that considering the complexity of the inter-bedded sands and slimes, the likely presence of facies changes within individual layers, and the likely presence of lenses of variable material within the tailing pile, it would not be prudent to utilize the lowest permeabilities found within the tails as the controlling factor for flow to the drain layer. The operator presents 1 x 10-6 cm/sec as a "conservatively high controlling tailings permeability". Given that 12 of the 19 tailing permeability data points presented to the Division thus far indicate hydraulic conductivities in excess of 1 x 10-6 cm/sec, the Division would not view this as a conservatively high figure for modelling. (Perm. data points are from app. B, Phase II, Raise 1 Design Report and app. F, Construction Status Report). Finally, the confirmation by the design engineer, and the concurrence by BMRI, that it is integral to the performance of the impoundment that the drainage layer be installed to specification, indicates the need for oversight by the design engineer, the Division, and now the third party engineer, and for consideration by Division management in making the decision as to whether or not BMRI's past or future actions constitute a reason to believe that a violation has occurred.