What is Acid Mine Drainage?
Acid Mine Drainage (AMD), also known as acid rock drainage, is just that—acidic drainage generally produced by abandoned metal mines or coal mines. AMD is a toxic byproduct very much like battery acid and has been deemed one of the most serious threats to water quality by the U.S. Environmental Protection Agency.AMD may flow from any part of the mine where sulfides are exposed to air and water, including waste rock piles, tailings, open pits, underground tunnels, and leach pads. If uncontrolled, the acid mine drainage may runoff into streams or rivers or leach into groundwater.
How is it formed?
The mining process exposes iron sulfide (pyrite) and unremoved coal contained in the unused portions of the mined earth to air and water. This exposure to the air and water creates oxidation, which then leads to an increase of acidity. The water’s natural pH decreases and the concentrations of dissolved metals increases. Conditions such as this create very poor water quality and the inability to support aquatic life.Where does AMD occur?
Although abandoned underground mines produce most of the AMD, there are recently mined and reclaimed surface mines that have produced AMD. Local ground-water and surface-water resources are harmed by this process. A major culprit of AMD is mined ore from copper.Other areas where the earth has been disturbed such as construction sites and transportation corridors may also contribute to AMD.
AMD also occurs naturally in some environments as a part of the rock weathering process.
What are some of the common impacts of AMD?
One of the most noticeable signs of AMD is an occurrence called Yellow boy. It is a solid orange-yellow film visually present in surface water. Yellow boy is produced through the contact with fresh water or neutralizing minerals, resulting in low pH levels. Yellow boy smothers plant and animal life on the streambed, disrupting stream ecosystems.AMD directly impacts the macroinvertebrate community (bottom-dwelling bugs in streams). Most macroinvertebrates are very sensitive to pollution and serve as clear indicators as to the health of a stream. They are collected and studied in many areas around the U.S. for this purpose. http://www.benthos.org/database/allnabstracts.cfm/db/Anchorage2006abstracts/id /581
How is AMD controlled?
For active mines, the acidic water produced must be neutralized to achieve pH 6-9 before discharge from a mine site to a stream is permitted.It takes many years to clean up the effects of AMD. There are a few forms of treatment: Chemical, biological, or physical treatments may be used to remove the negative impact to the watershed.
Biofiltering can be done through a man-made wetland (or artificial marsh). It is designed to emulate a natural wetland, which consists of biofilters. This process removes sediments and pollutants such as heavy metals from the water.
Other forms of treatment include the chemical treatment of acid neutralization, ion exchange, or aeration.
http://www.wvu.edu/~agexten/landrec/chemtrt.htm acid neutralization
http://www.ene.gov.on.ca/programs/3354e40.pdf biofiltration
Where can I learn more about Acid Mine Drainage?
USEPA Region 3
http://www.epa.gov/region3/acidification/what_is_amd.htm
Virginia Tech
http://ewr.cee.vt.edu/environmental/teach/gwprimer/acidmine/acidmine.html
Pennsylvania Department of Environmental Protection
http://www.dep.state.pa.us/dep/deputate/minres/bamr/amd/science_of_amd.htm
Montana Tech
http://multimedia.mtech.edu/elc/LinksLibrary/AcidRock/default.htm
Other sites referenced:
http://www.cotf.edu/ete/modules/waterq/wqacidmine.html
http://www.earthworksaction.org/pubs/FS_AMD.pdf
http://www.northern.org/artman/publish/acidminedrainage.pdf