For hundreds, even thousands of years, human beings have mined for metals and stones, and with the advent of greater technology as well as greater needs, the demands for these resources continue to grow. While these resources benefit our lives in many ways, the effects of mining can be detrimental, and one such effect is the topic of this essay, acid mine drainage (A.M.D.). The causes of A.M.D. will be discussed, along with some of the physical and biological problems associated with it.
Some prevention and remediation treatments will also be considered.
Acid mine drainage refers to water (leachate, drainage or seepage) that has come into contact with oxidised rocks or overburden that contains sulphide material (coal, zinc, copper, lead). (Keller, 2000; U.S.G.S.; U.S.E.P.A., 2002). A common sulphide is pyrite, or iron disulfide (FeS2), and throughout this essay it will be pyrite that will be the primary sulphide considered. Acid mine drainage is not a new phenomenon, early mining techniques utilized gravity to avoid water pooling, resulting in the water becoming polluted by acid, iron, sulphur and aluminium (U.
S.E.P.A., 2002). It is most commonly associated with coal mining, especially with soft coal, coal that has high sulphur content. The pyrite that is present in coal seams will be accessible after surface mining when the overlying surfaces are removed or in deep mines that allow oxygen access to the previously inaccessible pyrite-containing coal (D.E.P. 1, 1997). After pyrite is exposed to air and water, sulphuric acid and iron hydroxide are formed, creating an acidic runoff (D.
E.P. 1, 1997; 2 2002).
When the water comes into contact with the pyrite, the chemical reactions that take place causes the water to increase in pH which will dissolve heavy metals which stay in solution. However, when the pH levels reach a certain stage, the iron can then precipitate out, coating sediments with the characteristic yellow, red or orange colourings (D.E.P. 2, 2002; U.S.G.S.; U.S.E.P.A., 2002). The rate that A.M.D. advances is also influenced by the presence of certain bacteria (Doyle; U.S.G.S). A.M.D that has dissolved heavy metals such as copper, lead and mercury can contaminate ground and surface water. Especially at risk are mines that are located above the water table (Keller, 2000; D.E.P. 2, 2002). The sources of water that get polluted can be surface water that permeates into the mine, shallow ground water flowing through the mine or any water that comes into contact with the waste tailings produced by mines.
Contamination of the water poses risks to health and integrity of structures, as well as economical loss. In high quantities, heavy metals can affect plant life. Not only is the A.M.D. detrimental to the health of the plant, plants that uptake heavy metals will pass them onto animals within the food chain. Growth and reproduction can be adversely affected in both aquatic animals and in terrestrial animals where drinking water is contaminated. Aquatic species are most at risk, as many are not tolerant of pH fluctuations, with most species having a defined pH tolerance range. Physical structures can be compromised as acid corrodes infrastructures such as bridges (D.E.P. 2, 2002; Keller, 2000; U.S.G.S.; U.S.E.P.A., 2002). Economically, areas affected by A.M.D. can suffer through reduced tourism due to pollution; a decline in recreational sports such as fishing, swimming and other outdoor activities (U.S.E.P.A., 2002).
Acid mine drainage is particularly hazardous in mines that are now abandoned. A poignant example is that of the Tar Creek area of Oklahoma in the United States of America. During the late nineteenth century the area was mined for lead and zinc and mining continued until around 1960. The mines reached down to 100m below the water table and while mining occurred the subsurface groundwater was actively pumped out. However, after 1960 and the closure of the mines the ground water naturally rose and caused some of the mines to flood. The resulting water had a high concentration of sulphuric acid and the overflow polluted streams. The results were so great that in 1982 it was designated by the United States Environmental Protection Association as the worst hazardous waste site in the United States (Keller, 2000). This highlights the problem of abandoned mines and A.M.D.
As with most things prevention is the best cure, and techniques and protocols can be implemented to reduce the risk of A.M.D. Two approaches commonly in use for the last 20 years are controlled placement of the over burden, and water management (O.S.M, 2002). Controlled placement involves either excluding oxygen or water from the pyritic material. The exclusion of oxygen can be achieved through submergence. By completely submerging the pyritic material in deep (several tens of feet), stagnant water, oxygen is unable to diffuse easily through the water and the pyrites therefore can not oxidate. Submergence has many limiting factors and does not work with all mines. The positioning of the mine and the water table must be considered, as must the likelihood of the mine failing under pressure. Pyrites can also be kept inaccessible to water by isolation above the water table. This is achieved by compacting the materials and then capping or sealing in clay. Although this reduces the permeability, it has in practice proven difficult to completely isolate the materials from water with breakages occurring.
Another approach is the management of water both during and after mining operations. Water flows can be altered so as to not come into contact with pyritic materials, over burdens can be placed land rough graded to prevent ponding and subsequent infiltrations (O.S.M, 2002). Also water that accumulates can be removed and polluted water can be isolated and treated so as to not further contaminate more water.
Other prevention techniques that are also remediation techniques are the construction of artificial wetlands (D.E.P. 2, 2002). Aerobic wetlands are only able to effectively treat water with net alkalinity. Through oxidation reactions metals are precipitated out to form oxides and hydroxides. In anaerobic or compost wetlands the rate of decomposition and mineralization are slowed. The wetland has a reducing nature with the organic environment promotes both microbial and chemical processes that generate alkalinity and increase pH levels. The oxygen is removed from the system by compost, allowing the sulphate to be reduced and also prevents the metals from oxidising.
In summary, acid mine drainage is the result of chemical reactions between oxygen, water and sulphide bearing materials that are the result of mining processes. The water is acidic and can contaminate ground and surface water, polluting streams. The results of this pollution can cause changes in the nutrient cycles affecting both plants and animals, and it can disrupt an ecosystem and alter the species diversity. The acidic water can also affect physical structures and have an effect on local tourism. Abandoned mines, such as the Tar Creek area, are of high risk in causing A.M.D., especially if the mines have altered the natural water table, or are located close to the water table. A.M.D. can be avoided through techniques such as submerging wastes to exclude oxygen contact, sealing wastes above the water table, as well as water management techniques. Also possible is the use of artificial aerobic or anaerobic wetlands to treat the affected water. Through mitigation techniques, it should be possible to reduce if not eliminate acid mine drainage as an environmental and social problem.