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In a nutshell, acid mine drainage is a form of water pollution that happens when rain, runoff, or streams come in contact with rock that is rich in sulfur. As a result, the water becomes very acidic and damages downstream aquatic ecosystems. In some regions, it is the most common form of stream and river pollution. Sulfur-bearing rock, especially one type of mineral called pyrite, is routinely fractured or crushed during coal or metal mining operations, and accumulated in piles of mine tailings. Pyrite contains iron sulfide which, when in contact with water, dissociates into sulfuric acid and iron. The sulfuric acid dramatically lowers the pH, and the iron can precipitate and form an orange or red deposit of iron oxide that smothers the bottom of the stream. Other harmful elements like lead, copper, arsenic, or mercury may also be stripped from the rocks by the acidic water, further contaminating the stream.
Where Does Acid Mine Drainage Happen?
It mostly occurs where mining is done to extract coal or metals from sulfur-bearing rocks. Silver, gold, copper, zinc, and lead are commonly found in association with metal sulfates, so their extraction can cause acid mine drainage. Rainwater or streams become acidified after they run through the mine's tailings. In hilly terrain, older coal mines were sometimes built so that gravity would drain out water from inside the mine. Long after those mines are closed, acid mine drainage continues to come out and contaminate waters downstream.
In the coal mining regions of the eastern United States, over 4,000 miles of stream have been impacted by acid mine drainage. These streams are mostly located in Pennsylvania, West Virginia, and Ohio. In the western U.S., on Forest Service land alone there are over 5,000 miles of affected streams.
In some circumstances, sulfur-bearing rock can be exposed to water in non-mining operations. For example, when construction equipment cuts a path through bedrock to build a road, pyrite can be broken up and exposed to air and water. Many geologists thus prefer the term acid rock drainage, since mining is not always involved.
- Drinking water becomes contaminated. Groundwater can be affected, impacting local water wells.
- Waters with a very low pH can support only severely reduced animal and plant diversity. Fish species are some of the first to disappear. In the most acidic streams, only some specialized bacteria survive.
- Because of how corrosive it is, acidic stream water damages infrastructure such as culverts, bridges, and stormwater pipes.
- Any recreational potential (e.g., fishing, swimming) and scenic value for streams or rivers affected by acid mine drainage are greatly reduced.
- Passive treatment of acidic streams can be conducted by routing the water into a purpose-built wetland designed to buffer the low pH. Yet, these systems require complex engineering, regular maintenance, and are applicable only when certain conditions are present.
- Active treatment options include isolating or treating the waste rock to avoid contact of water with sulfates. Once water has been contaminated, options include pushing it through a permeable reactive barrier that neutralizes the acid or routing it through a specialized wastewater treatment plant.
- Reclamation Research Group. 2008. Acid Mine Drainage and Effects on Fish Health and Ecology: A Review.
- U.S. Environmental Protection Agency. 1994. Acid Mine Drainage Prediction.