SOURCE AND NATURE OF SULFATE TOXICITY OF TWO PIT LAKES AT THE FORMER STEEP ROCK IRON MINE, NORTHWESTERN ONTARIO

Flooding of two open pits of the Steep Rock iron mines near Atikokan, Ontario, has lead to the formation of Hogarth and Caland pit lakes. Hogarth pit lake is characterized by non-stratified to weakly stratified and oxygenated water column, near neutral pH (6.4-8.0), extraordinarily high SO₄^2- concentrations (1200-2000 mg/L), lower alkalinity (50-125 mg/L), increased hardness (1200-1800 mgCaCO₃/L). Caland pit lake is well stratified with an upper oxygenated fresh water lens that overlies an anoxic and moderately saline (200-500 mg/L SO₄^2-)water column. The pH of Caland water is similar to that of Hogarth, but is more alkaline (100-200 mg/L) and has a lower hardness (400-1000 mgCaCO₃/L).

Toxicity testing has shown that Caland water is non-toxic, whereas Horgarth water has varied from being acutely toxic in 1998 to chronically toxic in 2004. Recent toxicity identification and evaluation tests, using Ceriodaphia dubia and Lemna minor, indicate that the chronic toxicity of Hogarth pit lake is restricted to the winter and is the result of high sulfate levels.

Sulfur isotopes and water-rock reaction experiments were used to determine the source of sulfate toxicity. The sulfur isotope composition of dissolved sulfate of the two pit lakes (-3.7 to -3.2‰) is depleted relative to pyrite from the ore zone and waste rocks (-1.5 to 10‰). However, the isotopic composition of pit lake sulfate is consistent with leaching of ore zone pyrite with subsequent fractionation due to precipitation of saturated sulfate minerals from the water column. Batch and column leaching experiments were performed using material from pit wall rock units and surface waste dumps, and reacted with local ground water and rain water, in order to further assess if the high sulfate levels are the result of ground water or surface water inflow. Results from the leaching experiments indicate that the high sulfate concentration of the pit lakes could only be produced from ground water oxidation of pyrite lenses within the buried ore zone, whereas the near neutral pH reflects subsequent interaction with carbonate wall rock. The differences in water chemistry between Hogarth and Caland pit lakes are attributed to differences in the pyrite content of the buried ore zone, with the ore zone beneath Hogarth pit lake containing a higher abundance of pyrite.