Fate and Transport of Metals from an Abandoned Tailings Impoundment after 70 Years of Sulfide Oxidation M.C. Moncur 1 , C.J. Ptacek 1, 2 , D.W. Blowes 1 , and J.L. Jambor 1, 3 1 Department of Earth Sciences, University of Waterloo, Waterloo, ON 2 National Water Research Institute, Environment Canada, Burlington, ON 3 Department of Earth and Ocean Sciences, University of British Columbia, Vancouver, BC Abstract 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. Introduction The former Sherritt Gordon Cu-Zn mine is located in Sherridon, Manitoba (Figure 1). Mining began in 1928 and ended in 1951 when ore reserves were depleted. Copper and minor amounts of Au and Ag were initially recovered until 1942, and then a Zn concentrate was also produced. Over the mine life, approximately 7.4 million tonnes of high-sulfide tailings were discharged, covering an area of 50.6 ha (Acres International Ltd, 1986). The bulk of the tailings was deposited in two separate impoundments; Camp Tailings and Woods Tailings (Figure 2). The Camp Tailings were deposited from 1931 to 1932 and have oxidized for more than 70 years. The Woods Tailings were deposited from 1937 to 1951 and have oxidized for the last 50 years. Groundwater and surface water derived from both tailings impoundments flow directly into Camp Lake, which has been seriously impacted by acid and metal loading. The objective of this study is to quantify the extent of sulfide oxidation and to determine acid neutralization reactions within the unsaturated zone of the tailings after 70 years of weathering. A second objective is to evaluate the impact of tailings pore-water discharging to Camp Lake. A detailed field investigation was conducted during the summers of 2000 and 2001 to characterize the tailings solids, pore waters, and the surrounding surface waters. Materials and methods Piezometer Network The Sherridon tailings impoundments were instrumented with 11 piezometer nests (Figure 3). Five nests were installed across the Camp Tailings and six nests across the Woods Tailings. Each piezometer nest consists of two to six drive-point piezometers and/or bundle piezometers. Twelve single point piezometers were installed around the perimeter of the Camp