When a mine is dug, and materials are removed there can at times be environmental damage. If a mine yields acid that runs off, it can be a source of acid mine drainage.
Acid mine drainage happens when surface water, rainwater or other sources of water contact the waste piles or mine spoils. This contact results in the water reacting in substances in the material, usually iron sulfide or similar materials.
The resulting liquid is acid, and it drains off and can do damage to plants, ground water, and to even animals and people. In the presence of oxygen and micro organisms, the ground water that is a little acidic can be made to be very acid based, and it can do a large amount of damage.
Acid Mine drainage can also occur when abandoned mines are flooded for whatever reason, and the water acts with the sides of the mine, causing this same acidic condition. High sulfur content soil, and high sulfur content coal can cause this kind of acid drainage, as can other materials and substances that are exposed to the open air and water by mining.
There are parts of the country such as West Virginia and Kentucky that mine different metals, and where coal is mined, that Acid Mine Drainage is a real problem. One site example is the Tar Creek Superfund Site, where one of the oldest and largest mines in the country was located.
Tar Creek is in Oklahoma and Missouri, and it lead and zinc, in addition to some other metals were mined there. When the mine closed and water was allowed to come into the mine area, sub surface regions became flooded, and contacted sulfide minerals.
This contaminated the watershed and groundwater, and while the government and authorities are trying to solve this site of Acid damage, it is so extensive and serious that the area may never really fully recover.