Acid mine drainage is one of the more common, and more serious, problems associated with hard rock mining activity in the Yukon.

When sulphide compounds, which are commonly found with mineral deposits, are broken up and exposed to water and oxygen, they start to oxidize, creating dissolved metal and sulphuric acid. The contaminated water drains into streams and rivers, where the dissolved metals can have disastrous effects on aquatic life.

The best way to deal with acid mine drainage is to prevent it, says Vic Enns, Head of Pollution Abatement for Environment Canada in Whitehorse. However, that is not as simple as it sounds.

One approach is to immerse the acid-generating rock under a permanent cover of water. "That is the ultimate fix that's widely accepted now," says Enns.

It works well in underground mines that have been developed along vertical shafts. When they are flooded, the water stops the oxidation. However, if the mine has been developed by means of horizontal tunnels, or adits, the water simply drains through and out at the lowest level, Enns says.

Open-pit mines are even more difficult to deal with.

"You have to take out a lot more material to get at the ore than you do with an underground mine," says Enns. That means much more sulphide material is exposed to the atmosphere, both in the pit and in the waste rock taken out of the pit.

Some open-pit mines can use water cover as an abandonment measure. The tailings and waste rock from the mining process may be dumped back into the pit, any exposed sulphide material on the upper sides of the pit is mined out and dumped into the pit as well, and the whole pit is flooded.

However, not every pit is suitable for flooding, says Enns. If the pit is dug into a hillside or a sloping valley, for example, water will pour out at the lower end before the sulphides at the upper end are flooded.

At the Faro lead-zinc mine, the waste rock cannot be flooded so the acid drainage water has to be collected in ponds and treated with lime to reduce the acidity and precipitate out the dissolved metals. The sludge of precipitated metals must then be disposed of safely.

"They'll be treating water at Faro basically forever, as far as we know," says Enns.

More experimental treatment techniques offer some hope for the future, at least for some mines.

Bacteria that reduce the sulphates in the drainage water to sulphides and create a more stable sludge may be used on a limited basis, Enns says. A pilot plant at the Anaconda Britannia mine near Squamish, B.C., has tested the process.

At other sites, acid-generating tailings have been covered by clay that is saturated with water.

"Again, the objective is to cut the supply of oxygen to the tailings," says Enns. However, the long-term effectiveness of the saturated cover solution is not yet proven.

Another approach being considered is wetlands treatment. Constructing a relatively natural wetland over the acid-generating material cuts it off from oxygen and allows wetlands vegetation and bacteria to immobilize at least some of the material. The technique has been used in the eastern United States at coal mines.

Wetlands treatment is still an emerging technology and there is no guarantee it will work well in a cold climate, says Enns. However, the owners of the United Keno Hill mine are interested in the potential of wetlands, particularly since some natural wetlands in the area are already helping to remove metals from some small drainages.

For more information about acid mine drainage and research into ways of mitigating it, contact the Environmental Protection Branch of Environment Canada in Whitehorse.