Acid mine drainage (AMD) is a major global issue, with the UN declaring it the second largest environmental problem behind global warming 2017.
AMD (also acid rock drainage - ARD) is a particularly toxic and corrosive type of wastewater characterised by the following contaminants:
Metals (As, Cu, Fe, Pb, etc)
Hardness (Ca, Mg)
It is caused when water (rainwater or groundwater) and oxygen contacts exposed earth and rocks containing sulphide minerals. These minerals dissolve into the water creating acidity and producing some amazingly bright water colours.
AMD is typically associated with industries such as mining and construction, where large-scale earth disturbances are part of operations.
If left untreated, AMD has a devastating effect on the environment; corroding waterways, staining riverbeds and killing local wildlife.
Acid Mine Drainage from surface mining, Ohio. Source: Wikimedia Commons
AMD is a particularly difficult issue at legacy mine sites which have been abandoned without proper rehabilitation. Many of these sites are decades-old historical mines which ceased operations when government regulations and community oversight were scarce.
These legacy sites generate an extensive amount of AMD that leaches into the surrounding environment, with no private sector owners liable to fix the issue.
The USA reports approximately 35,000 mine sites generating AMD on forest service lands, polluting over 20,000 km of streams and rivers.
A well-known example is the Berkeley Pit located at a legacy copper mine in Montana, USA. The mine was shut down in 1982 and has since accumulated 200 billion litres of acidic (pH 2.5) water laden with toxic metals.
Accumulation of AMD in the Berkeley Pit, Montana USA. Source: Wikimedia Commons
The pit is now a major tourist attraction for the area, however local authorities are required to constantly monitor and manage water levels to ensure it doesn’t overflow.
Australia also has many legacy mines generating AMD and creating environmental issues; with some of the most problematic sites for each state listed below:
Australian legacy mine sites producing AMD
With most mining companies gone from these sites, it is left to local governments and councils to fix the AMD issues. Unfortunately, this is often neglected.
Moreover, most legacy mines are polluting in remote locations - so the general public is either unaware or unconcerned with the damage.
That is not to say the problem of AMD is easily solved. One of the biggest challenges is finding economical ways to treat such large volumes of acidic liquid.
A simple approach is to add an alkaline reagent (usually limestone, slaked or hydrated lime), which neutralises the acid (raises pH) and precipitates the toxic metals into a solid sludge. This process is called lime precipitation and is shown by the following diagram:
Typical lime precipitation process for AMD treatment
Lime precipitation is effective in removing the acid and metals from AMD, but requires significant infrastructure: pumps, reactors, clarifiers, reagent silos and a sludge handling system.
The precipitation reactors produce a large amount of solid materials (mostly gypsum and metal hydroxides), which are often dewatered and sent to landfill.
Once treated, the water is far less toxic; and can either be discharged to the environment or further polished (using reverse osmosis or ion exchange) to upgrade the water for potable use.
A world-class example of lime precipitation is the Eastern Basin AMD Treatment Facility located in South Africa.
Construction of the Eastern Basin Facility cost just under ZAR 1 billion (AU$85 million). It has the capacity to treat over 100 million litres per day of AMD from local mine shafts, returning high-quality treated water to the environment.
In Australia, local governments are fully aware of the AMD issues in their regions; but despite examples of successful treatment around the world, our legacy mines continue to discharge AMD into the environment.
Part 4 – Acid mine drainage