Geophysical surveys of leaking tailings impoundments

Abstract

Alcoa of Australia's bauxite treatment plant at Kwinana discharges alkaline liquor to impoundments constructed on Quaternary formations of the Perth Basin. Contaminated groundwater has been detected outside some of these impoundments. The impoundments are constructed with an impermeable basal clay layer overlain by a permeable, thin sand blanket. Wet tailings slurry is poured in, the fine residue fraction is dried and stacked higher than the dyke walls allowing access of tracked vehicles and geophysical equipment to the impoundments. Leakage is initially detected from monitoring bores placed outside the impoundments. Approximate leak locations can then be detected from water level lows in piezometers placed within the semi-confined aquifer of the sand blanket. The mise-a-la-masse technique uses current electrodes placed within the impoundment and within conductive, leaking water at some distance away. A current pathway is provided between the impoundment and the conductive groundwater via the leakage zone and this can be mapped as voltage highs using potential electrodes within piezometer holes near the base of the impoundment. In some cases the detection of these leakage points is complicated, but not impossible, by multiple leaks, PVC liners placed above the basal clay layer and the strongly varying conductivity within the impoundment. The mise-a-la-masse anomalies only give leak location with differences in intensity between anomalies not being diagnostic of leak properties.