PHYSICAL AND GEOCHEMICAL PROCESSES IN A NORTHERN CANADIAN LAKE IMPACTED BY ACID MINE DRAINAGE

Camp Lake, located in Northern Manitoba, Canada, has been severely impacted by acid mine drainage. Geochemical and physical processes in Camp Lake are similar to many pit lakes, however, Camp Lake is unique in that it has a low relative depth (1%). A sulfide-rich tailings impoundment adjacent to the impacted lake has undergone 70 years of sulfide oxidation. Concentrations of sulfide oxidation products are extremely high in the tailings pore water (pH<1, up to 129,000 mg/L Fe, 280,000 mg/L SO4, 55,000 mg/L Zn, 5,700 mg/L Al, 1,620 mg/L Cu, 97 mg/L Cd, 55 mg/L As, and other metals). Groundwater discharging from the tailings flows directly into the anoxic zone of a semi-isolated bay in Camp Lake. Geochemical profiles of the water column in the semi-isolated bay were sampled in detail on three occasions, in June (2001), August (2002) and April (2003, beneath 1 m of ice). Sampling results suggest that the water column in the bay is permanently stratified throughout the year and exhibits characteristics of a meromictic lake, showing poor mixing processes. Three distinct layers were observed during the study; a mixolimnion, chemocline, and monimolimnion. Low concentrations of dissolved ions and a pH of 3.2 were observed in the mixolimnion (0-2 m). At a depth between 2 and 3 m, a sharp increase in sulfate and dissolved metals occurs forming a chemocline. During the spring and summer, a thermocline coincided with the chemocline layer. The thermolcline, however, was not present during winter sampling. Below the chemocline, concentrations of metals and SO4 in the monolimnion are elevated, with maximum concentrations up to 8,500 mg/L Fe, 20,000 mg/L SO4, 30 mg/L Zn, and 100 mg/L Al, including elevated concentrations of Cu, Cd, Pb and Ni. The water density in the monolimnion ranges between 1.02 and 1.03 g cm-3, suggesting that this layer will not undergo seasonal mixing processes. Mixing is likely confined to the upper 2 m in the mixolimnion. The pH increases with depth from 3.1 at the surface of the water column to 5.9 at the bottom of the bay (6 m). Within the lower 2 m of the water column, a decrease in some dissolved metals occurs. Speciation model calculations suggest that the formation of siderite, ferric oxy(hydroxides), jarosite, gibbsite, aluminum hydroxysulfate, and gypsum may be limiting the concentrations of Fe, Al, and SO4.