1993-11-03_REVISION - M1988112 (2)

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The Division is concerned not only with the gradation of the drainage blanket materials, but also with the hydraulic conductivity. BMF2I has stated that there is no specification in the permit for hydraulic conductivity; the Division does not agree with that statement. The calculations included in the 1989 amendment for performance of the drainage blanket rely on a certain minimum hydraulic conductivity, and in the June 3, 1993 memorandum from Rob Dorey (SRK) to .Andre Douchane (BMG) that was provided to the Division in response to concerns regarding seepage from the facility, it was stated that the calculations "conservatively assumes a drainage layer permeability of 5 x 10_y cm/sec". The Division has no alternative but t:o look at such wording as an assessment of the worst case scenario. Recent in situ permeability testing of drainage blanket material shows 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. BMRI 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 tailing to bypass low permeability zones 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, it would not be prudent to utilize the lowest permeabilities found within the tails as the controlling factor for flow to the drain layer. BMF2I 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 Divisic>n would not view this as a conservatively high figure for modelling. (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