Acid drainage is one of the more common problems associated with mining activity, especially in the Yukon. It's also among the more difficult problems to solve.

It happens when sulphide compounds, like the iron sulphide material in a mine, are broken up and exposed to the atmosphere, says Benoit Godin, head of Environment Canada's Environmental Contaminants section in Whitehorse.

"On contact with water and oxygen, the sulphide compound starts to oxidize. That means that the sulphide dissociates itself from the iron, integrates some water and oxygen, and releases some hydrogen ions," he explains. "In short, it's oxidation of sulphide material, producing sulphuric acid and dissolved metal."

The acid drains away from the rock that produced it, and that's when the serious environmental damage begins to happen. As the acid drainage moves through the surrounding rocks, it dissolves metals from them and carries them along into the natural drainage system.

Almost any metal in the area might be dissolved and swept into the water system, says Godin. In the Yukon, the metals most likely to be found in acid mine drainage are zinc, lead, arsenic, and copper.

The acid mine drainage makes the water in the streams themselves more acidic, which means that the metals stay in solution and flow downstream, travelling many kilometres from their sources.

The metals will stay in their dissolved form until the water passes through alkaline rock that can reduce the acidity of the water enough to precipitate them out of the solution.

Dissolved metal can do a lot of damage to the inhabitants of the stream.

"It basically kills fish," says Godin. Metal ions in the water attach themselves to the fish's gills, interfering with respiration. They cause damage to liver and kidneys, and weaken the fish so that they are less fit to grow and reproduce.

"There's a gamut of lethal and sub-lethal effects," Godin says. Besides fish, invertebrates, algae, and every aspect of life in the stream can be affected. In high enough concentrations, the acid-dissolved metals can even endanger larger animals and humans who drink the water.

Acid rock drainage is not a new phenomenon, Godin says. In fact, it's a natural occurrence.

"These sulphide deposits are there naturally. It's a geological formation, and the water and oxygen always have had some effect on it. It's just that the more you break the rock, the more surface area you expose, and the more sites that can produce the acid. The actual volume of acid that you can generate is much bigger if you have a pile of broken rock."

The problem of acid drainage from mines is not new either. It has been recognized in Europe at least since the seventeenth century, but only recently has it become a major concern in planning new mines.

"I think what happened was our mines got bigger and the problems got bigger," says Godin. "Big open pit mines can generate acid for a very long time."

The Yukon already has a number of acid-generating mines, both active and abandoned. Several mines that are now in the planning stages are designing measures to prevent the generation of acid mine drainage.

The problem can't be ignored, says Godin. "I don't think it's philosophically right to have all the benefits in this generation and all the problems in the next."

In next week's column, we'll look at some of the techniques mining companies and environmental scientists are hoping will limit the impact of acid mine drainage, now and in the future.

For more information about acid mine drainage, contact the Environmental Protection Branch of Environment Canada in Whitehorse.