Acid Mine Drainage
Acid mine drainage is one of mining's most serious threats to water. A mine draining acid can devastate rivers, streams, and aquatic life for hundreds, and under the "right" conditions, thousands of years.
How does it form?
At metal mines, the target ore (like gold, silver, copper, etc) is often rich in sulfide minerals.
When the mining process exposes the sulfides to water and air, together they form sulfuric acid.
This acid can and often does dissolve other harmful metals and metalloids (like arsenic) in the surrounding rock.
Acid mine drainage can be released anywhere on the mine where sulfides are exposed to air and water -- including waste rock piles, tailings, open pits, underground tunnels, and leach pads.
Acid drainage is often marked by "yellow boy", an orange-yellow substance (visible in the photo on this page), that occurs when the pH of water drops low enough so that previously dissolved iron precipitates out.
Harm to fish & other aquatic life
Acid mine drainage can have severe impacts on fish, animals and plants. Many impacted streams have a pH of 4 or lower -- similar to battery acid.
For example, acid and metals runoff from the Questa molybdenum mine in New Mexico has harmed biological life in eight miles of the Red River.
Acid mine drainage is especially harmful because it can occur indefinitely -- long after mining has ended. Hardrock mines across the western United States may require water treatment in perpetuity.
For example, government officials have determined that acid drainage at the Golden Sunlight mine will continue for thousands of years.
Water treatment can be a significant economic burden if a company files for bankruptcy or refuses to cover water treatment costs.
For example, acid runoff from the Summitville Mine in Colorado killed all biological life in a 17-mile stretch of the Alamosa River. The site was designated a federal Superfund site, and the EPA is spending $30,000 a day to capture and treat acid runoff.
For more information:
- EARTHWORKS: Acid Mine Drainage. Fact sheet
- U.S. Fish and Wildlife Service: Acid Mine Drainage and Effects on Fish Health and Ecology: A Review Commissioned by the Anchorage office of USFWS to better determine the potential impacts of several proposed large mine projects in Alaska.
- U.S. Geological Survey: Watershed contamination from hardrock mining