Acid mine drainage
Acid mine drainage (AMD) refers to the outflow of acidic water from coal or metal mines, often abandoned ones where ore- or coal mining activities have exposed rocks containing the sulphur-bearing mineral pyrite. Pyrite reacts with air and water to form sulphuric acid and dissolved iron, and as water washes through mines, this compound forms a dilute acid, which can wash into nearby rivers and streams.
Environmental and Health Effects
Bright orange-colored water and stained rocks are usually signs of AMD. The orange color is caused by ferric hydroxide precipitating out of the water, which forms as the AMD becomes neutralized. At low pH values, the metal ions remain soluble, and when the pH rises, the iron oxidizes and precipitates out. Depending on the conditions, the orange-colored precipitates may form inside the mine or several miles downstream, and can be harmful to aquatic life: the clumps reduce the amount of light that can penetrate the water, affecting photosynthesis and visibility for animal life. Also, as the precipitate settles it blankets the stream bed, smothering the bottom-dwellers and their food resources.
Water discharging from deep-mine openings and surface mine seeps results in elevated concentrations of acidity, iron, manganese, aluminum, and sulfate in receiving streams and rivers, depleting the buffering ability of water by neutralizing the carbonate and bicarbonate ions that form carbonic acid. Streams and rivers with low buffer capacity are not able to neutralize the acid load and consequently become acidic. An estimated 2,390 miles of streams in the Allegheny and Monongahela River Basins have been degraded by AMD to the point of not being able to support fish communities.
Production of AMD can occur long after mines have been abandoned if piles of waste rock are in contact with air and water, and this effluent can be a large water quality problem for river and stream ecosystems for states with a history of coal mining, such as West Virginia and Pennsylvania. AMD dissolves heavy metals such as copper, lead, and mercury into ground and surface water.
Treatment and Coal Waste
Active treatment involves installing a water treatment plant, where the AMD is dosed with lime to try and neutralize the acid, and then passed through settling tanks to remove the sediment and particulate metals, creating coal waste.
- "Coal mining and the environment: Acid Mine Drainage" World Coal Institute, accessed June 2010.
- "Acid Mine Drainage: Chemistry" Center for Educational Technologies, accessed June 2010.
- James I. Sams III and Kevin M. Beer, "Effects of Coal-Mine Drainage on Stream Water Quality in the Allegheny and Monongahela River Basins—Sulfate Transport and Trends" U.S. Department of the Interior Water-Resources Investigations Report 99-4208, 2000.
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