PREDICTIVE MODELING OF WATER QUALITY ISSUES OF TWO PIT LAKES AT THE FORMER STEEP ROCK IRON MINE, NORTHWESTERN ONTARIO

Continual flooding of the two primary open pits of the Steep Rock iron mines near Atikokan, Ontario, Canada has lead to the formation of Hogarth and Caland pit lakes, which are located in the former middle and eastern arms of Steep Rock Lake, respectively. At present both lakes are ~200 m deep and are filling at a rate of ~3 m/year. It is estimated that sometime between 2030 and 2040 the pit lake will attain the original lake level and begin to outflow into the Seine River. Outflow into the Seine River system could adversely affect the aquatic life because of the toxicity arising from unusually high concentrations of dissolved sulfate (1200-2000 mg/L) in Hogarth pit water.

In order to assess the impact of an outflow event, empirical and computational methods are used to determine the water quality of the mixed waters under the influence of the water-rock interactions with the surrounding geologic formations. A series of water mixing and water-rock experiments have been designed to determine the composition of the mixed waters resulting from joining of the two pit lakes and subsequent outflow. Toxicity identification evaluation (TIE) methods, using Ceriodaphia dubia and Lemna minor are used to assess the impact of the experimental waters on aquatic faunal and floral biota, respectively. Computational modeling of water chemistry using the software PHREEQC is also being used to predict changes in water chemistry arising from mixing and water-rock interaction.

One water-rock interaction experiment revealed that Hogarth's sulfate source may be becoming exhausted. Preliminary results of a TIE test using a series of mixed waters indicate that the high sulfate levels in Hogarth (1292 mg/L) will be diluted by inflow from Caland (236 mg/L) to levels that have much less impact on the growth of Lemna minor (769 mg/L at 1:1 ratio). However, these levels still cause chronic adverse effects. The geological influence may also alter sulfate levels. Results from empirical and computational modeling will allow accurate predictions of the composition of the mixed waters under geological influences, and will be used to prescribe appropriate mitigation measures.