THE PORE-WATER GEOCHEMISTRY AND THE MINERALOGY OF THE VADOSE ZONE OF SULFIDE TAILINGS, WAITE-AMULET, QUEBEC, CANADA

Low-pH waters, requiring treatment because of high concentrations of dissolved metals, are being discharged from a decommissioned tailings impoundment at the former Waite Amulet ZnCu mine in northeastern Quebec, Canada. A detailed study of the vadose zone of the tailings comparing mineralogical and geochemical analyses of the tailings solids with geochemical analysis of the tailings pore water and pore gas indicates the presence of three zones within the tailings. These zones include an upper sulfide-depleted zone in which the sulfides have been extensively removed and oxidation is largely complete, an intermediate zone, where sulfide oxidation and acid neutralization reactions are occurring, and an unoxidized zone below the depth of oxygen penetration, where sulfides are unoxidized. High aqueous concentrations of dissolved solids are being displaced downward from the sulfide-depleted zone, through the unoxidized zone toward the water table. Sulfide-oxidation and acid-neutralization reactions have generated concentrations of dissolved solids high enough to lead to the precipitation of a series of secondary solid phases. Precipitation-dissolution reactions involving these solid phases control the concentrations of the major ions in the tailings pore water. Concentrations of dissolved metals are controlled by precipitation-dissolution, crystal-structural replacement, and adsorption/coprecipitation reactions. Predictions based on the observed data and numerical simulations suggest that, in the absence of an effective remedial program, sulfide oxidation and H+ production will continue for several centuries. © 1990.