Compatibility of slurry wall backfill soils with acid mine drainage

The use of slurry walls to contain oxidized tailings and provide cutoff below tailings dams is generally a cost-effective way of preventing environmental degradation due to seepage of acid water from tailings areas. Long-term environmental protection dictates that the slurry wall materials be compatible with the acid water. Six percent Avonseal bentonite by weight was added separately to two natural soils to represent slurry wall backfill materials, which were then permeated with several pore volumes of acid mine drainage (AMD) in the laboratory. Results using both flexible wall and fixed wall permeameters were similar. The carbonate-rich backfill gave an average hydraulic conductivity (K) of 1x10(-9) cm/s, buffered the AMD at circumneutral pH, and kept effluent metal concentrations to very low values (for example, less than 0.05 mg/1 zinc). Although the carbonate-free backfill also maintained low K (average 3x10(-8) cm/s) during AMD permeation, it could not neutralize the AMD as effluent pH decreased to approximately 3.5, and metal concentrations reached those of the influent or permeant after approximately 17 pore volumes. (C) 2000 Elsevier Science Ltd. All rights reserved.