Acid mine drainage (AMD) is an environmental issue which costs Australia more than half a billion dollars a year to manage, and has been cited as a major pollutant of surface water in areas and rivers around the world.
AMD refers to the outflow of acidic water from mines, following an oxidative process that occurs when sulphide-containing minerals are exposed to air and water, as occurs during rainfall. Extensive mining activity over the past century has led to AMD occurring at unnaturally high rates, with acidic water run-off having long-term impacts on the surrounding environment.
In UniSA’s School of Natural and Built Environments, Dr Michael Short, together with colleagues Dr Russell Schumann and Prof Christopher Saint, is using two ARC Grants worth a combined $800,000 to research development of a new geochemical and microbial multi-barrier approach to tackle the issue of AMD.
It’s research that may lead to more sustainable and cost-effective practices within the mining industry. Dr Short explains.
AMD is a long-term legacy issue, with the potential environmental implications from acid drainage production often persisting for decades, centuries or even millennia after mining activities have ceased.
Perhaps the most infamous example here is the Rio Tinto (Red River) in Spain which, having been subjected to several thousand years of local mining activity, is today strongly acidic (pH around 2) and suffers from heavy metals pollution (arsenic, cadmium, lead and mercury) at around 100 times normal background levels. This pollution poses obvious environmental and human health risks.
The cost of AMD remediation at abandoned mines in North America alone has been estimated to be in the tens of billions US dollars. Across Australia, equivalent annual costs for AMD management are estimated to be around $150 million for operating mines and well over $500 million for abandoned mine sites.
A looming funding shortfall in so-called ‘rehabilitation securities’ held by Australian state governments to cover the cost of mine site rehabilitation means that both industry and government need to find effective, low-cost AMD management strategies to allow future mine site closures to be carried out sustainably. Existing management systems are inadequate for effective long-term AMD control and carry high potential costs for future remediation liabilities.
The primary focus is to control AMD ‘at–source’ rather than relying on expensive downstream treatment processes to remediate contaminated leachate prior to release from site.
The approach is still in its infancy but what we are researching is based on the more mature methodologies of geochemically-and microbially-assisted source control used in combination.
For instance the targeted addition of nutrient amendments to promote microbial colonisation could reduce oxygen availability at sulphide surfaces, lowering the rate of acid generation or perhaps preventing it altogether.
Types of nutrient additions being tested include simple and complex carbon organic sources in the form of simple sugars and more complex mixtures of organic carbon in industrial biosolids waste.
Examples of geochemical passivation include using limestone at a copper mine in Indonesia and an iron ore mine in Tasmania. The addition of organic and nutrient amendments to promote microbial colonisation has also been extensively tested, for example for treatment of copper tailings and also uranium tailings in Canada.
However, many of these case studies have been largely empirical, lacking the fundamental understanding required to further optimise the methodology to suit a range of applications.
This is where our research is looking to fill some of these knowledge gaps.
Fundamental to this approach is the use of materials readily available at site. Any treatment that can reduce or eliminate the need for costly importation of remediation materials to remote mine sites is more likely to be accepted by the industry. Thus the first step in the process is to assess what types of waste mineral resources are available at mine sites which could potentially be useful for AMD mitigation.
Until now, little or no attention has been given to potentially ‘useful wastes’ on-site which may be suitable for AMD control; such wastes are normally ignored in the overall mining operations. Similarly, other organic waste materials that can be applied to enhance the development of useful microbial communities (e.g. competitors to iron oxidisers) may also be available in close proximity to mine sites and these can be applied to complement the abovementioned geochemical passivation from mineral wastes.
These might include waste products from adjacent industries, such as sawdust and wood chips, animal manure or biosolids from municipal waste water treatment plants.
Current AMD management practices have so far been largely unsuccessful; as a result, the industry is looking to researchers, consultants and technology providers to develop new waste management strategies for cost-effective long-term site management strategies. Included in this is the industry’s recognition that complete geochemical assessment at a mine is now essential, including not only a description of the mineral resource, but surveying and characterising potentially acid-generating and acid-neutralising waste materials.
This represents a significant shift in thinking within the mining sector.
While any one approach is unlikely to offer the magic bullet for AMD control, the combined geochemical and microbial multi-barrier approach is a potentially useful addition to future AMD management practice.
Ultimately, having cost-effective long-term AMD mitigation strategies will help ensure that future mining operations are truly sustainable (i.e. economically, environmentally and socially responsible) and inter-generationally equitable.