What are the effects of acid mine drainage?

Acid mine drainage refers to the surface runoff formed when rainwater comes in contact with either mine spoils (the reject from ore processing activities) or overburden (the material removed to get at the ore-bearing layer). It can also form when groundwater floods abandoned mine shafts, adits, and galleries underground. Spoil or overburden piles that have high levels of sulfide minerals and/or metals are especially problematic. In the presence of oxygen and with the help of microorganisms, high levels of metals and very acidic runoff (pH < 2 in some cases) is created, which results in acid mine drainage. This is a problem in parts of the country where high sulfur coal is mined (such as the East, e.g. WV and KY) as well as in areas of metals mining. Perhaps the most famous example of AMD is the Tar Creek Superfund Site, one of the oldest and largest in the country. It is located in Oklahoma and part of Missouri within the Tri-State Mining District - mostly lead and zinc minerals were mined there (former Picher field). It was formed in part due to the impacts of AMD following closure of the mine and cessation of water treatment. Sub-surface workings flooded with water, came into contact with sulfide minerals and waste piles, and greatly contaminated the local watershed, even threatening local groundwater supplies at one point. The area has yet to - and may never - fully recover.