Acid mine drainage (AMD), also called acid rock water or acid rock drainage (ARD), refers to the outflow of acidic water from (usually) abandoned metal mines or coal mines. However, other areas where the earth has been disturbed (e.g. construction sites, subdivisions, transportation corridors, etc.) may also contribute acid rock drainage to the environment. In many localities the liquor that drains from coal stocks, coal handling facilities, coal washeries, and even coal waste tips can be highly acidic, and in such cases it is treated as acid rock drainage. Acid rock drainage occurs naturally within most environments as part of the rock weathering process but is exacerbated by large-scale earth disturbances characteristic of mining and other large construction activities, usually within rocks containing an abundance of sulfide minerals.

Occurrence

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Subsurface mining often progresses below the water table, in which case water must be constantly pumped out of the mine in order to prevent flooding. When a mine is abandoned, the pumping will cease and the water table will return to its former position, flooding the mine. The introduction of water is the initial step in most acid rock drainage situations. Tailings piles or ponds may also be a source of acid rock drainage.

Metal mines may generate highly acidic discharges where the ore is a sulfide or is associated with pyrites. In these cases the predominant metal ion may not be iron but may be zinc, copper, or nickel. The most commonly mined ore of copper, chalcopyrite, is itself a copper-iron-sulfide and occurs with a range of other sulfides. Thus, copper mines are often major culprits of ARD. Metal sulfides (often pyrite which is iron-sulfide) newly exposed to air and water are broken down into metal ions and sulfuric acid by colonies of bacteria and archaea. These microbes, called extremophiles for their ability to survive in harsh conditions, occur naturally in the rock, but limited water and air (oxygen) supplies usually keep their numbers low. Special extremophiles known as acidophiles especially favor the low pH levels of abandoned mines. Acidithiobacillus ferrooxidans in particular has been identified as a key contributor to the oxidation of pyrites ^*.

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