Acid Mine Drainage: Sources and Treatment in the United States

Abstract

Acid mine drainage (AMD) occurs when metal sulfides are exposed to oxidizing conditions. Leaching of reaction products into surface waters pollute over 20,000 km of streams in the United States alone. Mining companies must predict the potential of creating AMD by using overburden analyses. Where a potential exists, special handling of overburden materials and quick coverage of acid-producing materials in the backfill should be practiced. The addition of acid-neutralizing materials can reduce or eliminate AMD problems. Placing acid-producing materials under dry barriers can isolate these materials from air and water. Other AMD control technologies being researched include injection of alkaline materials (ashes and limestone) into abandoned underground mines and into buried acid material in mine backfills, remining of abandoned areas, and installation of alkaline recharge trenches. Chemicals used for treating AMD are Ca(OH)2, CaO, NaOH, Na2CO3, and NH3, with each having advantages under certain conditions. Under low-flow situations, all chemicals except Ca(OH)2 are cost effective, whereas at high flow, Ca(OH)2 and CaO are clearly the most cost effective. Floc, the metal hydroxide material collected after treatment, is disposed of in abandoned deep mines, refuse piles, or left in collection ponds. Wetlands remove metals from AMD through formation of oxyhydroxides and sulfides, exchange and organic complexation reactions, and direct plant uptake. Aerobic wetlands are used when water contains enough alkalinity to promote metal precipitation, and anaerobic wetlands are used when alkalinity must be generated by microbial sulfate reduction and limestone dissolution. Anoxic limestone drains are buried trenches of limestone that intercept AMD underground to generate alkalinity. Under anoxia, limestone should not be coated with Fe+3 hydroxides in the drain, which decreases the likelihood of clogging. Vertical flow wetlands pretreat oxygenated AMD with organic matter to remove oxygen and Fe+3, and then the water is introduced into limestone underneath the organic matter. Open limestone channels use limestone in aerobic environments to treat AMD. Coating of limestone occurs, and the reduced limestone dissolution is designed into the treatment system. Alkaline leach beds, containing either limestone or slag, add alkalinity to acid water. At present, most passive systems offer short-term treatment and are more practical for installation on abandoned sites or watershed restoration projects where effluent limits do not apply and where some removal of acid and metals will benefit a stream. Keywords: acid-base accounting; acid-producing material; acid-neutralizing material; alkalinity-producing systems; anoxic limestone drains; chemical treatment; open limestone channels; passive treatment; wetlands