Acid mine drainage and environment

Shabir Ahmad Dar
Acid mine drainage (ADM) is the most significant environmental pollution problem associated with mining industry. The main cause of acid mine drainage is the occurrence of pyrite and other sulphide minerals like pyrrhotite, chalcopyrite, arsenopyrite, galena etc., with the rock of coal seams. During mining these minerals get exposed to air and mine water, then oxidation and hydrolysis results in the generation of acid mine drainage. The low pH value of the discharged mine water results in the further dissolution of minerals and release toxic metals. When this discharged water is allowed to get into other water bodies, it results in acidity and release of high toxic metals. The high concentrations of these metals are harmful to the vegetation, aquatic life, human life and wild life.
Acid mine drainage (AMD) is an inevitable bye product of the mining and mineral industry which is generally characterized by high concentration of dissolved heavy metals, sulphate and low pH as low as two and continues to be an important water pollution problem in mining industry around the world. Although the generation of AMD occurs naturally, mining and processing of metal ores and coals can promote AMD generation through exposing sulphide minerals to both oxygen and water.AMD pollutes the receiving streams and aquifers, when it allowed to discharge without any treatment. Contaminants from mine discharge can persist for a long time after mining has stopped.
AMD also results from outflow of acidic water from thousands of abandoned sulphide mineral sand microbial activity. This results in high acidity and high dissolved metal concentration in the mine drainage which has adverse effects on aquatic ecosystems of the receiving rivers, corrosion of mining machinery and other environmental degradation.
The amount of AMD produced depends on the size of the exposed surface area of sulphide minerals. The concentration of dissolved metal ions in AMD also depends on the type and quantity of sulphide mineral present as well as the host rock composition. The AMD problem is severe in metal mining areas, such as Iberian Pyrite Belt in South West of Spain, the largest sulphide deposits in the world. ).
In India the coal mines that face a serious pollution problem due to AMD are Assam (NECL), Meghalaya, Chhattisgarh (Churcha, West Chirimir i), Maharashtra (Ambora), Baragolai ( ECL), Rikhikol (WCL), Margarita group of mines and Gorbi (MP).
AMD is a unique pollutant because acid generation and discharge continue to occur even after the mining is ceased. As a result of this acidity it is unsuitable for the use of animals, plants, mankind and aquatic life.The problem of AMD is not restricted to the local area at the source, but may extend to distances if the acid water is allowed to get discharged into the main water stream. AMD impacts more frequently on the quality of ground water than that of surface water. If acid producing mines located in permeable formation, water with low pH percolates into the aquifers and gets spread over a wide area through ground water movement which is ultimately consumed in different ways by human being through wells and bore wells. The acidic and ferrous water are not only responsible for the corrosion of mine plant and equipment and formation of scales in the delivery pipe range, but also pollution of the mine surface environment, thus affecting the surface ecology.AMD causes serious threat to human health and ecological systems because it contains heavy metal contaminant which is not biodegradable and thus tends to accumulate in living organisms causing various diseases and disorders. Low pH of mine drainage results in dissolvable of heavy metals in water and its high concentration causes toxicological effects on aquatic ecosystems. AMD also has direct effects on fish by causing various physiological disturbances. High acidity may adversely affect fish growth rates and reproduction. The primary cause of fish death in acid water is loss of sodium ions from the blood. Less availability of oxygen to the cells and tissues leads to anoxia and death as acid water increases the permeability of fish gills to water, adversely affecting the gills function. Besides chemical effects of mine drainage, physical effects, such as increased turbidity from soil erosion, accumulation of coal fines and smothering of the stream substrate from precipitated metal compounds may also occur. Precipitation of ferric hydroxide may outcome in a complete layering of the stream bottom, filling in crevices in rocks and making the substrate unstable and unfit for habitation by benthic organisms. Trace metals, such as zinc, cadmium and copper which may also be present in mine drainage are toxic at extremely two concentrations.
In summing up, Acid mine drainage is presently one of the most severe environmental pollution problem associated with mining activities. AMD is generated from active underground and open cast coal mines containing pyrite mineral. It is also generated from abandoned mines and heaps containing pyrite. The discharge of acid mine water causes direct severe impact on water environment and indirectly on human health.
(The author is Student of Environmental Science & Technology at Central University of Punjab, Bathinda.)
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