Acid mine drainage

DEFINITION: The flow of acidified waters from mining operations and mine wastes

Acid mine drainage can pollute groundwater, surface water, and soils, producing adverse effects on plants and animals.

During mining, rock is broken and crushed, exposing fresh rock surfaces and minerals. Pyrite, or iron sulfide, is a common mineral encountered in metallic ore deposits. Rainwater, groundwater, or surface water that runs over the pyrite leaches out sulfur, which reacts with the water and oxygen to form sulfuric acid. In addition, if pyrite is present in the mining waste materials that are discarded at a mine site, some species of bacteria can directly oxidize the sulfur in the waste rock and tailings, forming sulfuric acid. In either case, the resulting sulfuric acid may run into groundwater and streams downhill from the mine or mine tailings.

Acid mine drainage (AMD) pollutes groundwater and adjacent streams and may eventually seep into other streams, lakes, and reservoirs to pollute the surface water. Groundwater problems are particularly troublesome because the reclamation of polluted groundwater is very difficult and expensive. Furthermore, AMD dissolves other minerals and heavy metals from surrounding rocks, producing lead, arsenic, mercury, and cyanide, which further degrade the water quality. Through this process, AMD has contributed to the pollution of many lakes.

AMD runoff can be devastating to the surrounding ecosystem. Physical changes and damage to the land, soil, and water from AMD directly and indirectly affect the biological environment. Mine water immediately adjacent to mines that are rich in sulfide minerals may be as much as 100,000 to 1,000,000 times more acidic than normal stream water. AMD water poisons and leaches nutrients from the soil so that few, if any, plants can survive. Animals that eat those plants, as well as microorganisms in the leached soils, may also die. AMD is also lethal for many water-dwelling animals, including plankton, fish, and snails. Furthermore, people can be poisoned by drinking water that has been contaminated with heavy metals produced by AMD, and some people have developed skin cancer as a result of drinking groundwater contaminated with arsenic generated by AMD leaching.

Alterations in groundwater and surface-water availability and quality caused by AMD have also had indirect impacts on the environment by causing changes in nutrient cycling, total biomass, species diversity, and ecosystem stability. Additionally, the deposition of iron as a slimy orange precipitate produces an unsightly coating on rocks and shorelines.

AMD was so severe in the Tar Creek area of Oklahoma that the US Environmental Protection Agency designated the area as the nation’s foremost hazardous waste site in 1982. The largest complex of toxic waste sites in the United States (US) was produced by the mines and smelters in Butte and Anaconda, Montana, with much of the pollution attributed to direct and indirect effects of AMD. AMD is also a widespread problem in many coal fields in the eastern US. In 2013, Earthworks, a nonprofit environmental organization, issued a report, based on its review of extensive government documents, revealing the size and scope of the problems created because of AMD. The forty hard-rock mines studied were projected to contaminate between 17 and 27 billion gallons of water. The Environmental Protection Agency (EPA) has spent billions of dollars cleaning up the damage caused by acid runoff from the Summitville Mine, Colorado, alone.

Into the mid-2020s, new technology was developed to both treat AMD and recover useful minerals from it. Studies highlighted advances such as adsorption treatments using industrial byproducts, passive systems like constructed wetlands and permeable reactive barriers, and membrane distillation-crystallization methods that could simultaneously purify water and precipitate minerals. Long-term ecological monitoring showed that diverted or treated AMD sources could lead to measurable improvements in stream recovery. The 2015 Gold King Mine spill in Colorado underscored the risks of abandoned mines, when it released millions of gallons of toxic wastewater into rivers and affected downstream communities. In the US, the EPA's Superfund program continued to address AMD-related contamination, with more than $1.1 billion in private-party commitments to site cleanups in 2024. Globally, research shifted toward sustainable remediation strategies that integrated waste reuse, resource recovery, and reduced treatment costs.

Bibliography

"Abandoned Mine Drainage." United States Environmental Protection Agency, 1 Aug. 2025, www.epa.gov/nps/abandoned-mine-drainage. Accessed 20 Sept. 2025.

Bell, F. G. Basic Environmental and Engineering Geology. CRC, 2007.

Gestring, Bonnie, and Lisa Sumi. "Polluting the Future: How Mining Companies Are Polluting Our Nation’s Waters in Perpetuity." Earthworks, May 2013, earthworks.org/wp-content/uploads/2021/09/PollutingTheFuture-FINAL.pdf. Accessed 20 Sept. 2025.

Jacobs, James A., Jay H. Lehr, and Stephen M. Testa. Acid Mine Drainage, Rock Drainage, and Acid Sulfate Soils: Causes, Assessment, Prediction, Prevention, and Remediation. Wiley, 2014.

“Superfund Remedial Program Accomplishments and Metrics.” United States Environmental Protection Agency, 1 Aug. 2025, www.epa.gov/superfund/superfund-remedial-annual-accomplishments-metrics. Accessed 20 Sept. 2025.

“Watershed Contamination from Hard Rock Mining.” US Geological Survey, 16 May 2017, www.usgs.gov/centers/colorado-water-science-center/science/watershed-contamination-hard-rock-mining. Accessed 20 Sept. 2025.

Younger, Paul L., Steven A. Banwart, and Robert S. Hedin. Mine Water: Hydrology, Pollution, Remediation. Academic, 2002.