Acid Mine Drainage is acidic water (pH <5.0), laden with iron, sulfate and other metals, that forms under natural conditions when geologic strata containing pyrite are exposed to the atmosphere or oxidizing environments.

Acid mine drainage is often the most visible source of environmental impact from an abandoned mine, as it discharges from the mine adit (tunnel) as discolored water. However, hydrothermal activity often creates zones of mineral alteration that extend well beyond the immediate location of concentrated ore mineralization. In fact, large areas of hydrothermally altered terrane extend from mined areas to areas without any associated mineral production. Mineral alteration can be a source of hydrologic contamination on a scale that is not immediately evident. Thus, the extent of environmental impact derived from hydrothermally altered areas unaffected by economic mineral production or other  significant anthropogenic influence is a subject that historically has not received much attention.