Acid mine drainage (AMD) refers to the process by which sulphides found in mining waste are oxidised, leading to the subsequent release of pollutants. Sulphide is typical present in the form of pyrite (FeS2), which is the largest contributor of AMD, though other metal sulphide ores are common sources.
Oxidation of pyrite occurs after it is exposed to oxygen and water, releasing aqueous iron, sulphate, and hydrogen ions into the water. These ions can react to form sulphuric acid, lowering the pH of the solution. As the water becomes more acidic it can dissolve other metals, further contaminating connected waterways. AMD is a significant cause for concern as it has the potential to become a constant source of mine site pollution over a long period of time.
The potential threat of AMD stems from its ability to occur years after a mine site has been closed. For example, Mt Goldsworthy, an iron ore mine in the Pilbara which closed operations in 1982, was rehabilitated by the owner, BHP Billiton, from the mines closure until 1992. Due to the highly pyritic waste rock, AMD potential was identified in 2000; studies in 2009 and 2013 confirmed that AMD was present at the site, and spreading. The treatment and rehabilitation cost for Mt Goldsworthy has been estimated at $100m.
Other examples include the Redbank copper mine in the Northern Territory, which has gradually leached copper sulphate into Hanrahan’s Creek long after its closure in 1996. High levels of toxic chemicals have been found up to 42km from the mine site, and ecologically important wetlands 80km from the site are at risk of contamination. Australia’s first uranium mine, Rum Jungle (also in the Northern Territory), ceased mining operations in 1971, and buried waste rock has been leaching acid and metals into nearby waterways ever since. The total clean-up cost has been estimated at $200m.
Strategies to reduce the impact of acid mine drainage fall under two broad categories: management and treatment. Management refers to actions taken to minimise the oxidation of sulphide materials, to prevent or minimise contaminants polluting the environment, and treatment to allow the re-use of materials. Treatment refers to methods and practices used to rehabilitate polluted areas by containing the spread of contaminants and removing them from the environment.
Waste rock dumps are simply the areas of stored waste rock from mining activities. The way waste rock is stored plays a major part in its potential for oxidation and AMD production. For example, sulphide rich waste stored in loose piles of rubble on the surface have a high potential for oxidation, as they are exposed to both oxygen and water. Covers can be used to restrict the exposure to air and/or water. Wet covers may be necessary for areas with high rainfall; they involve placing the waste underwater, removing its exposure to air. Dry covers are suitable in more arid regions, involving a thick layer of compacted soil covering the waste, and loose soil on top to encourage vegetation. These methods require long-term monitoring to ensure the integrity and effectiveness of the covers.
If a low percentage of waste rock contains a significant proportion of sulphide and the rest of the waste is relatively benign, one method of management may be to separate it out and disposing it by burying it in alkaline material, such as limestone. This method will reduce the potential for AMD generation.
Treatment of AMD is classed as passive or active. Active treatment may require power, mechanical equipment, and/or constant maintenance, while passive treatment does not, and is typically cheaper than active treatment. Common passive treatment methods involve constructing ponds designed to oxidise and precipitate metals, and reduce acidity. A common form of active treatment is chemical neutralisation via materials with a high pH, such as caustic soda, in waste management plants.
The threat of acid mine drainage stems from its capacity to occur decades after a mine has been shut down, and to continue generation for decades if left untreated. These factors give AMD the potential to become an environmental and financial disaster. Waterways and aquifers saturated with toxic chemicals can cause irreparable harm to their related ecosystems, and the price of rehabilitation can reach up to hundreds of millions of dollars, which is an incredibly high cost for corporations and governments to bear. Compared to this, the costs of prevention, management, and quick treatment of AMD is a small price to pay.