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High concentrations of trace metals are often found in
aquatic ecosystems exposed to direct inputs from mines, smelters and
industries. Sulfuritic materials found in coal and associated rock and
shale strata are the main producers of acid found in acid mine drainage.
These sulfuritic materials are principally pyrite and marcasite, both sulfides
of iron, which, in the presence of air and sufficient water, are capable of
being oxidized to iron sulfates, and eventually, free sulfuric acid. The pyrite
is found in association with coal, clay, shale and other rocks adjacent to and
including the coal being mined.
Other constituents found in mine drainage are produced by secondary reactions of
sulfuric acid with minerals and organic compounds in the mine and along the
stream valleys. Such secondary reactions produce high concentrations of aluminum,
manganese, calcium, sodium, and other constituents in drainage water. These mine
drainage constituents, along with iron and sulfate, are indicators of mine
drainage pollution that may persist long after the acid in the drainage has been
neutralized.
Acid mine drainage is largely a problem associated with older,
abandoned mines. Water enters mine shafts or gob piles (unused coal) and
dissolves iron sulfides, exiting as an acid effluent of continually varying
character.
When these waters from the coalmines and adjacent land are allowed to drain
directly into the stream system, a polluted stream can result. The extent of
pollution depends upon the quantity of pollutant as well as the stream’s
discharge and chemical properties. Generally, the result is
1) a lowering of hydrogen ion concentration (pH) and dissolved oxygen content,
2) increased acidity and iron content, including iron precipitates, and 3) other
chemical changes to the extent that streams may no longer be able to sustain
their natural flora and fauna. In many cases the stream is no longer
satisfactory as a water supply source or for recreational use. Acid mine
drainage not only causes discoloration of the water, makes a sludge deposit in
the receiving streams, and destroys fish and aquatic life essential for fish
survival, but continues to lower the
productive capacity of the stream for many miles downstream. Where the extent
of coal mining is great, the problem is of particular concern.
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