English

Abstract

The interaction between geochemical and hydrological processes in waste rock was evaluated in an instrumented waste rock pile as part of ongoing studies into the scale-up of water chemistry predictions. Sulfate and metal concentrations were determined in both pore water and outflow waters collected in sixteen basal lysimeters over a four-year period. The waste rock contained approximately 0.5% sulfur as pyrite/pyrrhotite and was acid generating. The correlation between outflow rate and concentrations was negative during times when macropore flow was active and positive over longer periods. Sulfate concentrations greater than 40,000 mg/L and elevated metal concentrations including cobalt, manganese, nickel, strontium, uranium and zinc were observed in the acidic (pH ~ 3.6) pore water and outflow water. Over the four–year experiment approximately 5% of the initial sulfur was released at the base of the pile. Predicted sulfide oxidation rates determined from laboratory experiments were up to four times higher than the rates inferred from the field experiment (0.1 mg SO4 -2 /kg rock/week to 19 mg SO4 -2 /kg rock/week). A lower-permeability cover placed on the surface of the waste rock pile three years after the experiment started induced a decrease of the dissolved load in the effluent from a pre-cover average of 4.8 mg SO4 -2 /kg rock/week to 1.2 mg SO4 -2 /kg rock/week. Additional