Hydrogeochemical evolution of an inactive pyritic uranium tailings basin and retardation of contaminant migration in a surrounding aquifer

At the inactive Nordic tailings impoundment in the Elliot Lake uranium mining district, Ontario, hydrogeochemical investigations are being conducted to determine the effects of pyrite oxidation on the chemistry of pore water in the tailings and on the subsurface leaching of metals and radionuclides from the tailings. The rate of this downward migration is controlled by the rate of downward pore water flow and by geochemical processes, primarily those that consume acid, such as reactions involving residual lime-derived carbonate minerals and reactions involving iron and aluminium. In most of the tailings area, the zone of low-pH pore water has not yet reached the bottom of the tailings. In one of the areas where it has passed all the way through the tailings, a plume of tailings-derived water with high iron and sulphate concentrations has moved in the sand aquifer a horizontal distance of 400 m beyond the tailings dam. Although the iron-sulphate plume has moved this far, the front of the low pH water is still close to the toe of the tailings dam. Three years of monitoring has established that the low-pH front is advancing in the sand aquifer at a rate that is less than 1% of the groundwater velocity. The rate of acid-front advance is severely retarded because of pH neutralization caused by the dissolution of trace amounts of carbonate minerals in the sand. Although the plume segment beyond the low-pH front has high concentrations of iron and sulphate derived from the tailings, it does not contain hazardous levels of radionuclides or heavy metals, because these constituents are either insoluble or strongly adsorbed at neutral pH. A narrow zone, near the bottom of the tailings, was observed to have high solid-phase concentrations of various chemical and radioisotope constituents. This is attributed to the settling of fines in the bottom layer containing precipitates of gypsum and metal hydroxides, and co-precipitation and adsorption of trace metals and radionuclides from the residual process water