Nickel mine Wikipedia
From 1953 to 1958 Nickel Rim mines operating around Sudbury, Ontario deposited their mine tailings in an impoundment near the mine. The water in the impoundment was later tested and found to have low pH and high concentrations of iron, sulphur and dissolved metals such as aluminum and nickel. This is due to the oxidation of the tailings (usually sulfide minerals like pyrrhotite).