Water is considered mining’s biggest casualty of all time given the huge scale at which mining of different types are advancing in today’s world. Acid mine drainage (AMD) is one of the largest repercussions of mining on water resources. It is a risk associated with all kinds of hardrock mines for extraction of useful metals and a significant number of coal mines suffer from acid mine drainage.
What is acid mine drainage?
As the name suggests, it is the outflow of acidic water from mining areas. It is identified by appearance of brownish-yellow precipitate with a foul smell and increase in acidity of water. Target ores for metals such as gold, copper, silver etc. or coal sometimes are sulphur bound which are known as ‘metal sulphides’.
Acid mine drainage is most commonly caused by iron sulphide called “pyrite” or “fool’s gold” which is abundantly found with almost all the metal ores. These sulphides are exposed during mining, and undergo oxidation reaction in presence of air and water to form sulphuric acid that is corrosive. The acid run off further dissolves harmful metals such as copper, aluminium, manganese, mercury, lead etc. and poison the ground or surface water system.
Acid mine drainage is an irreversible process that turns undisturbed and relatively unreactive ores into volumes of hazardous waste whose containment is extremely difficult once it reaches a water body. Acid drainage can also occur in non-mining areas such as construction sites, artificially dug areas or naturally weathered rocks termed as ‘acid rock drainage’. The process of pyrite oxidation and acid formation can be further catalysed by bacteria called as ‘extremophiles’ which can thrive in extreme conditions such as acidic environment.
Environmental impacts of acid mine drainage
Acid mine drainage can occur indefinitely for hundreds of years even after the mining activities have ended and thus affects a large horizon – ecologically and economically. The mines continue to produce acidic drainage as long as the rocks are exposed or become devoid of acid producing minerals. It is one of the single biggest non-point sources of pollution.
Some Roman mining sites in Great Britain are still producing problematic acid mine drainage, 2000 years after the stoppage of mining (CNHE, 2017). Moreover, AMD can develop at any point of the mining process – underground working, open pit mine faces, waste rock dumps, tailings deposits and ore stockpiles. In India, acid mine drainage usually comes from coal mines.
Most of the living organisms thrive well in the pH of near neutral which is 7. Acid mine drainage in some of the worst cases could decrease the pH of water to as low as less than 3. Thus it degrades streams, rivers and aquatic life forms in varying degrees depending upon the severity of pH change. It limits the growth of the river ecology in addition to unsuitable use of water for human, agriculture, industrial and recreational activities.
The increase in acidity can cause loss of sodium ions from the blood of fishes, which adversely affects the gill’s function. Prolonged exposure to low pH can be lethal for plants and animals with effects such as stunted growth, lesions, low reproductive rates and deformities (Vyawahre and Rai, 2016). Acid mine drainage is also responsible for loading a huge amount of heavy metals like lead, aluminium, manganese and more into our waterways. Aluminium is toxic to many aquatic animals and limits root development in plants leading to nutrient deficiency.
In addition to increase of acidity and release of toxic metals, neutralisation of the acid water by alkaline rocks could create an additional problem. In case of decrease in acidity, the iron which was in ferrous form turns into ferric and comes out of the water as a yellowish precipitate (solid) called ‘yellowboy’. It effectively chokes the streambed, crevices and prohibits the growth of benthic (bottom dwelling) life forms.
AMD have renouncing affects on the quality of groundwater system if the mines are located in permeable soil formations. The acidic and heavy metal containing water may percolate inside the water aquifers (underground water pockets) and gets spread throughout the underground water system which ultimately reaches human consumption via bore wells.
Prevention, mitigation and treatment of acid mine drainage
Acid mine drainage does bear a significant economic cost and irreversible environmental effects. Prevention of the acid-generating processes seems to be the only way to completely wipe out the problem of acid mine drainage but, stopping of mining is not the viable solution in most cases.
However, sulphuric acid could be stopped from forming by preventing the waste rock and mine tailings to come in contact with air by strategies such as submerging the tailings under water, sealing them behind barriers or burying them underground (groundtruthseeking.org, 2010). Nonetheless, isolating and containing huge amount of the generated waste is practical either above or below ground thus storing it for a future possibility or treating them with the help of technological advancement.
In most cases it is impossible to completely treat the contaminated water from acid mine drainage due to its high cost, inefficient technology and easily diffusible quality of water. But, its consequences could be limited by treating the water before releasing it into the environment through passive and active technologies (Johnson and Hallberg, 2005).
Passive Treatment- It is the use of naturally occurring biological and chemical reactions that could be employed to reduce the acidity of AMD. For example, ‘bioreactors’ which works on use of sulphate-reducing microbes that precipitate out dissolved harmful metals. Another method is the creation of artificial wetlands where microbes can remove the heavy metals from the mine water. Both these methods are limited to small scale mines since it is expensive and requires a huge area of land. However, operation and maintenance of passive treatment system is comparatively lesser as compared to active treatment.
Active Treatment- It is the typical water treatment system involving the use of alkaline chemicals such as caustic soda, hydrated lime, sodium carbonate etc. to decrease the acidity of mine drainage. Limestone is one very common chemical used due to its low cost and relative abundant availability. On the other hand, continued active treatment due to precipitation increases the rate of total dissolved solid (TDS) which is an important water quality parameter. Moreover, active treatment technologies are expensive due to chemical usage, equipments and huge manpower.