IMPACT OF MINE DRAINAGE ON A LAKE FROM AN ABANDONED TAILINGS IMPOUNDMENT

An integrated groundwater and surface water study was conducted to evaluate the geochemical evolution of a lake in northern Manitoba, Canada, which has been severely impacted by acid mine drainage. A sulfide-rich tailings impoundment adjacent to the impacted lake has undergone over 70 years of sulfide oxidation. Concentrations of sulfide oxidation products are extremely high in the tailings pore water (pH<1, 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). The highest concentrations and lowest pH conditions are observed in the zone of active oxidation. Extensive accumulations of secondary minerals, jarosite, goethite, melanterite and gypsum, have formed a massive continuous hardpan approximately 1 m below the tailings surface. The highest concentrations of dissolved metals are observed directly above and within the massive hardpan layer. During precipitation events, surface seeps along the flanks of the impoundment develop and discharge pore water with a geochemical composition that is similar to the composition of water directly above the hardpan. These results suggest that shallow lateral flow of water due to a transient perched water table is resulting in higher contaminant loadings than predicted assuming discharge is only derived from the deeper primary water table. Groundwater and surface water from the tailings flows directly into a lake adjacent to the impoundment. Concentrations of metals and SO₄ in surface waters of the lake are elevated, with maximum concentrations observed in an isolated embayment (8,500, mg/L Fe, 20,000 mg/L SO₄, 30 mg/L Zn, and 100 mg/L Al). An abrupt increase in metal and sulfate concentrations at a 2 meter depth in the embayment suggests that higher density metal-laden water is accumulating at depth, limiting normal mixing processes.