The separation of sulphide minerals and the disposal of low sulphur tailings is a promising strategy for the prevention of acid mine drainage. A detailed hydrogeochemical study is underway at the INCO Ltd. Copper Cliff Tailings Area comparing the geochemical evolution of pore waters derived from low sulphur tailings with that derived from higher sulphide tailings. In June 1993, three 10 m x 15 m field lysimeters were constructed containing low sulphur tailings (0.35 wt.% S), main tailings (0.98 wt.% S), and total tailings (2.3 wt.% S), respectively. The lysimeters were monitored semi-annually to determine the pore-water composition, the pore-gas composition, and the bulk physical properties of the tailings. Geochemical modelling was conducted to interpret mineral phases that limit concentrations of dissolved constituents. Pore-gas oxygen levels are depleted within the upper 20 cm of the main tailings and total tailings lysimeters, indicating the zone of active oxidation. Complete gas-phase oxygen depletion is not observed in the low sulphur tailings lysimeter. Oxidation in the main and total tailings lysimeters has resulted in the development of acidic conditions, whereas pore-water pH in the low sulphur tailings has remained near neutral. Substantial increases in the concentrations of dissolved constituents in the pore water began within the first year in the total and main tailings lysimeters, but has yet to be observed in the low sulphur lysimeter. Maximum pore-water concentrations, measured in October 1995, exceed 13 g/L SO4, 1,500 mg/L Fe, and 1,500 mg/L Ni in the total tailings lysimeter, whereas the low sulphur lysimeter exhibited lower dissolved concentrations (< 2 g/L SO4, < 5 mg/L Fe, and < 40 mg/L Ni). The initial results of this study suggest that pore-water acidification and metal loading to the environment could be substantially decreased through the production of low sulphur tailings.

In conjunction with the field lysimeter experiments, laboratory experiments are underway to evaluate acid neutralization reactions in unoxidized tailings. A series of saturated column experiments are being conducted, using an acidic input solution and tailings samples from the three lysimeters, to study the migration of low-pH conditions through unreacted tailings.