acid mine drainage

Acid mine drainage (AMD) refers to the outflow of acidic water from (usually) abandoned mines. In many localities the liquor that drains from coal stocks, coal handling facilities. Coal washeries, and even coal waste tips can be highly acidic, and in such cases it is treated as acid mine drainage. Metal mines can also generate highly acidic mine discharges where the ore is a sulfide or is associated with pyrites. In these cases the predominant metal ion may not be iron but may be zinc, copper or nickel. Such discharges may not seem visibly polluting to the naked eye, but the environmental damage may be considerably greater. Metal sulfides, often pyrite, newly exposed to air and water are broken down into metal ions and sulfuric acid by colonies of bacteria and archaea. These microbes, called extremophiles for their ability to survive in harsh conditions, occur naturally in the rock, but limited water and air supplies usually keep their numbers low. The chemistry of oxidation of pyrites, the production of ferrous ions and subsequently ferric ions, is very complex, and this complexity has considerably inhibited the design of effective treatment options. Many acid mine discharges contain elevated levels of toxic metals especially nickel and copper with lower levels of a range of other metals. In the coal belt around the south Wales valleys in the UK highly acidic nickel-rich discharges from coal stocking sites have proved to be particularly troublesome. In some AMD systems temperatures reach 120 degrees Fahrenheit (50 C), and the pH can be as low as -3.6http://ca.water.usgs.gov/water_quality/acid/. AMD-causing organisms can thrive in waters with pH very close to zero. Negative pH occurs when water evaporates from already acidic pools thereby increasing the concentration of hydrogen ions. About half of the coal mine discharges in Pennsylvania are acidic, with pH <5. Generally, limestone or other calcareous strata that could neutralize acid are lacking or deficient at sites that produce acidic mine drainage. Where limestone has been used, such as at Cwm Rheidol in mid Wales, the impact has been much less than anticipated because of the creation of an insoluble calcium sulfate layer on the limestone chips, blinding the material and preventing further neutralisation. Although abandoned underground mines produce most of the AMD, some recently mined and reclaimed surface mines have produced AMD and have degraded local ground-water and surface-water resources. Acidic water produced at active mines must be neutralized to achieve pH 6-9 before discharge from a mine site to a stream is permitted. In the UK many discharges from abandoned mines are exempt from regulatory control. In such cases the Environment Agency working with partners has provided some innovative solutions, including constructed wetland solutions such as on the River Pelena in the valley of the River Afan near Port Talbot

List of AMD sites worldwide

This list will never be complete, but it hints at the enormity of the problem. It includes both mines producing AMD and river systems significantly affected by such drainage.