Acid mine drainage
About half of the coal-mine discharges in Pennsylvania are acidic, with pH <5. Generally, limestone or other calcareous strata that could neutralize acid are lacking or deficient at sites that produce acidic mine drainage (AMD). Although abandoned underground mines produce most of the AMD, some recently mined and reclaimed surface mines have produced AMD and have degraded local ground-water and surface-water resources. Acidic water produced at active mines must be neutralized to achieve pH 6-9 before discharge from a mine site to a stream is permitted.