Protecting Water from Mine Waste

Mining ore produces large quantities of waste rock, tailings and other refinery by-products that are usually stored on site. With exposure to the atmosphere, the sulfide-rich waste oxidizes, releasing acid and metals into the environment and putting nearby water bodies at risk.

Scientists from Environment Canada's National Water Research Institute (NWRI) are leading studies on the mechanisms that control the release and transport of these contaminants—information needed to develop effective management and restoration strategies.

The NWRI researchers and their partners are carrying out their investigations at several Canadian sites, including the Sherridon Mine near Flin Flon, Manitoba. Closed since 1951, the copper, zinc, gold, and silver mine generated some 7.4 million tonnes of high-sulfide tailings during its 24-year life span, covering an area of more than 50 hectares.

During the summers of 2000 and 2001, the research team conducted hydrogeological and geochemical studies on the Sherridon tailings to find out how much sulfide oxidation has occurred and to evaluate the waste's neutralizing capacity. Various minerals have the capacity to neutralize acid released through oxidization, and thereby stabilize trace metals. If insufficient neutralizing minerals are available, however, acidic waters with elevated concentrations of metals and sulfate will migrate from the waste piles to surface waters—where they could kill fish and invertebrates—and to underlying geologic formations, where their effects may appear many years later.

To find out what is happening below the surface, the researchers collected continuous cores of the tailings, coring from the surface to the bottom of the tailings piles. When they hit a cemented layer of consolidated tailings, known as hardpan, they used a backhoe to dig through it to the unconsolidated layer below. Two cores were collected at each location—one to analyze the water in the pores of the tailings, and one to analyze the solids. They also collected samples of groundwater at each location, and took water samples at one-metre intervals from the nearby lake.

The research team's results provide irrefutable evidence that abandoned mines can continue to release elevated concentrations of metals and other elements to surface and groundwater long after they are closed. After seven decades of oxidation, the pore waters in the tailings contain very high concentrations of dissolved metals, sulfate, and acid. Nearly all of the neutralizing minerals have been depleted, but less than half of the sulfide-mineral content has been consumed—suggesting that metals and acids will be released for decades or possibly centuries to come.

The highest concentrations of dissolved metals were found directly above and within the hardpan layer located about a metre below the surface of the tailings. During rainfall, pore water with a geochemical composition similar to that found above and within this layer seeps from the edges of the waste piles. Because of this, researchers suspect that the hardpan causes water to flow laterally, rather than downward, carrying with it much higher contaminant loads than they would expect to find if the discharge were coming only from the deeper water table.

The discharge from the tailings flows directly into the nearby lake, and the water samples from the lake show an abrupt increase in metal concentrations at a two-metre depth, indicating that higher-density, metal-laden water is accumulating at this depth and degrading water quality.

Results of these studies will be used to improve models for predicting the duration of the oxidation process, the rates at which metals are transported, and their long-term release into receiving waters. More accurate predictions of the degree of environmental damage likely to occur and how it can be avoided through improved disposal methods will lead to more effective tailings management programs and a more sustainable mining industry.

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