Abandoned Mine Drainage Education

The Definitions

Provided by Hedin Environmental


AMD most commonly refers to Acid Mine Drainage.  This means water that is net acidic, usually containing metals such as iron, manganese, and aluminum, as well has high levels of sulfate.  However, not all polluted water that comes from mines is acidic.  

Therefore, AMD can also means Alkaline Mine Drainage.  While this water may still be contaminated with iron, manganese, and sulfate, it is net alkaline and does not contain aluminum.

In order to encompass both of these pollution types, some people define AMD as Abandoned Mine Drainage.  This terminology, however, fails to acknowledge pollution that is generated on active mines (thus the possibility Active Mine Drainage).

 

So What Is AMD?

AMD is polluted water that has resulted from some type of mining or earth disturbance.  In Pennsylvania, AMD is most often associated with surface or deep coal mining.  AMD is caused when iron and sulfur-containing minerals are exposed to air and water.  Pyrite, commonly called "Fool's Gold,"  is one such mineral.  Pyrite is often found in rocks near coal seams, so when the coal is mined, the pyrite is exposed, beginning a series of chemical reactions.  Although the pH of AMD may be nearly neutral, it is not uncommon for the pH of AMD to be around 2. 

 

The Chemistry

For the chemically inclined, the following reactions detail steps in the formation of AMD.  Not all reactions occur on all sites, but these are the primary reactions that occur.

pyrite oxidation

FeS2 + 7/2 O2 + H2O à Fe2+ + 2SO42- + 2H+

acid dissolution of clays

Clay + H+ à SiO2 + Al3+ + Mg2+ + Ca2+ + Mn2+

acid neutralization by calcite

CaCO3 + 2H+ à Ca2+ + H2O + CO2

ferrous iron oxidation

Fe2+ + ¼ O2 + H+ à Fe3+ + ½ H2O

ferric iron hydrolysis (produces "Yellow Boy")

Fe3+ + 3OH- à Fe(OH)3

aluminum hydrolysis

Al3+ + 3OH- à Al(OH)3

summed iron oxidation and hydrolysis reactions

Fe2+ + ¼ O2 + 5/2 H2O à Fe(OH)3 + 2H+

acid neutralization by bicarbonate ion

H+ + HCO3- à H2O + CO2

acid neutralization by hydroxyl ion

H+ + OH- à H2O

 

Why Is AMD Harmful?

AMD can be harmful to humans and ecosystems for the following reasons:

  • Aluminum is toxic to humans (though this is not well understood) and is highly toxic to fish and other aquatic organisms.

  • Most aquatic organisms require net alkaline, neutral pH conditions for survival, and AMD is often net acidic with very low pH.

  • Iron forms a solid and coats the bottoms of streams, smothering aquatic bugs and organisms that form the base of the food chain.  This solid can also coat the gills of fish, choking them.

  • High sulfate levels cause bad tasting drinking water.

  • Iron and manganese cause problems with domestic water systems (scale, clogged pipes, orange clothing).

  • AMD-polluted water cannot be used for industrial purposes.

 

AMD Treatment

The type of AMD treatment that is most appropriate depends upon the type of AMD (chemical parameters), the amount (flow rate of AMD), and the space available for treatment. Before the 1980's, expensive chemical plants were the only way to treat AMD. However, techniques developed since that time, collectively called Passive Treatment, use nature's own products, such as wetlands to treat AMD. Several engineering and consulting companies specialize in the design and implementation of effective, reliable passive wetland treatment systems.




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