Acid mine drainage (AMD), also referred to as acid and metalliferous drainage, and acid rock drainage, can be defined as the outflow of acidic water from metal mines, or coal mines, that have, in most cases, been abandoned. The acidic run-off water can also be from mill tailing ponds or piles, coal spoils, and mine waste dumps that contain sulfide minerals.
AMD basically refers to water with a pH level that is less than 6.0, in which overall acidity surpasses the alkaline level.
In sub-surface mining, the activities happen below the water table. Hence, the water has to be frequently pumped out of the mine so as to prevent flooding. Once the mining activities are done, the mine is sometimes abandoned, causing the mine to fill up with water that soon turns acidic.
Likewise, during mining processes in metal mines, sulfides get exposed to water and air, causing them to form sulfuric acid. Sulfuric acid in run-off water can dissolve other dangerous metals and metalloids, e.g. arsenic, lead, and mercury, in the adjacent rocks.
AMD is a huge cause of water pollution in many areas around the world. It causes loss of aquatic life, and hampers the use of local streams for public drinking water, industrial water supplies, and recreation. It can also corrode parts of the infrastructure, such as bridges.
Some methods used for treating acid mine drainage include lime neutralization, and calcium silicate neutralization. Today, in mining locations, a geochemical assessment of mine materials during the initial stages of a project is performed, to assess the possibility for AMD, so that adequate precautions can be taken well in advance.