The hydrogeochemistry of the Nickel Rim mine tailings impoundment, Sudbury, Ontario

From 1953 to 1958, mine tailings were deposited in an elevated impoundment at the Nickel Rim mine, an abandoned Ni–Cu mine near Sudbury, Ontario. The oxidation of sulfide minerals, principally pyrrhotite (8 wt.% of tailings), has generated low-pH waters and released high concentrations of Fe (up to 9.8 g/l), SO4 (up to 24 g/l), dissolved metals (1130 mg/l Al; 698 mg/l Ni) and other dissolved constituents. Groundwater flow through the tailings is mainly horizontal with velocities of 4–8 m/yr. Iron, SO4 and Mn are moving at groundwater velocities and a low-pH front (pH≤4.5) is moving more slowly than the groundwater velocity. A series of pH zones with pH values of 3.0, 4.1, 5.6 and 6.7 is present within the impoundment. The occurrence of these zones is attributed to the successive dissolution of iron (oxy)hydroxides, aluminum hydroxide and aluminosilicates, siderite and calcite. Concentrations of dissolved metals, including Al, Co, Cr, Ni and Zn are controlled by the pore-water pH, except for Cu, which is controlled by the precipitation of covellite. The dominant secondary minerals are goethite, jarosite and gypsum. Accumulations of gypsum and jarosite in the oxidation zone have been great enough to cement the tailings. The cemented layers limit O2 diffusion, resulting in longer oxidation times for the tailings. High concentrations of Fe and SO4 presently in the tailings pore-water will discharge from the impoundment for at least 50 years.