Fate and Transport of Metals from an Abandoned Tailings Impoundment after 70 Years of

Hydrogeological and geochemical studies of an abandoned high-sulfide tailings impoundment at Sherridon, Manitoba, were conducted during the summers of 2000 and 2001. The Sherridon tailings have undergone oxidation for more than 70 years. Mineralogical analysis indicates that the unoxidized tailings contain nearly equal proportions of pyrite and pyrrhotite that together make up to 60 wt. % of the tailings. Sulfides are extensively depleted in the upper few centimeters of the tailings. Sulfide oxidation has led to very high concentrations of dissolved iron and sulfate, elevated concentrations of trace metals, and the generation of low-pH conditions. Extensive accumulations of secondary melanterite, rozenite, jarosite, goethite, and gypsum have formed a massive continuous hardpan approximately 1 m below the tailings surface. The highest concentrations of dissolved metals are observed directly above and within the massive hardpan layer. During rainfall or other precipitation events, surface seeps along the flanks of the tailings develop and discharge pore water that has a geochemical composition resembling that of the water directly above the hardpan. These results suggest that shallow lateral flow of water due to a transient perched water table is resulting in higher contaminant loadings than are predicted by assuming that discharge is derived only from the much deeper primary water table. Groundwater and surface water from the tailings flow directly into a small lake adjacent to the impoundment. Samples from the water column of the lake show an abrupt increase in the metal concentrations at a 2-m depth, suggesting that higher density metal-laden water is accumulating at depth. Although the tailings have oxidized for>70 years, the presence of a significant amount of residual sulfide minerals suggests that metals and acid will be released for decades to centuries