West Coast and Southland mine drainage workshop

01 October 2008

In July, the research consortium of scientists from CRL Energy, Canterbury University, Landcare Research and Otago University presented another successful workshop on their FRST-funded research programme into the environmental consequences of mine development on aquatic systems. The research programme is expected to take six years; currently the research group is at the start of year five. The workshop examined the ongoing development of a framework to assist data collection, interpretation and decision making by mining companies, regulatory agencies and other parties with regard to mine development proposals. The framework provides a set of standard guidelines so that regulators and industry can have greater certainty in environmental outcomes arising from mining activities.

The framework also provides guidelines on how to sustain and improve the surrounding environment when new and existing mineral deposits are being developed – setting the benchmark for good mining practices. The framework does not establish ‘acceptable’ and ‘unacceptable’ water quality criteria because these are likely to be different at different sites and because there are social, economic and cultural factors that may also influence decision-making. Instead the framework provides a robust scientific basis to determine the level of impact on aquatic systems.

The research programme involves detailed research into mine drainage and rock geochemistry, ecological impact on aquatic systems, as well as management and rehabilitation, and is overseen by a Governance Panel with representatives from mining companies, industry organisations, regional councils and Department of Conservation. The researchers report to the Governance Panel on a six-monthly basis.

Prediction of water quality impacts

Prediction of mine drainage chemistry and downstream water quality at proposed mine sites requires information on the target commodity, region, and geological formation. Acid-forming potential and trace element content of rocks to be disturbed by mining are often the key environmental considerations for both coal and gold mines. An assessment of this requires sample collection and acid base accounting analysis. More detailed information on mine drainage chemistry and trace element content can be had using XRF analysis and a variety of long-term-leaching simulation tests.

Once mine drainage chemistry is predicted, site specific hydro-geological information and background water quality information is used to predict downstream water quality. Hydrological data, background water quality and predicted mine drainage chemistry is integrated using a reactive transport approach rather than a simple dilution model.

Arsenic is the main environmental issue associated with hard-rock gold mines, and acidity is almost invariably controlled by surrounding rocks to pH 7-8. Typical gold mine ore contains about 2000 times as much arsenic (as arsenopyrite) as gold. The gold and arsenic are closely interlinked in the ore rocks, so that all gold-extraction techniques affect the arsenopyrite as well. The arsenopyrite will dissolve in water during all mining and processing activity and removal of arsenic from mine water before discharge is often necessary.

Predictions of ecological impacts

A healthy stream ecosystem usually includes a range of species of plants and animals which feed, prey and depend on each other in a number of ways. At the base of most stream foodwebs are plants (primarily algae) and terrestrial matter (e.g. leaves and wood) that are then decomposed by bacteria and fungi and eaten by a range of invertebrates (e.g. insects). These invertebrates are the main food for fish and crayfish. Mining discharges can directly and indirectly affect all components of a stream foodweb.

The most severe conditions are usually associated with acid-forming geology. These geological conditions have the potential to produce acidic waters, high in toxic metals (e.g. Fe, Al, Ni, Zn). They may also create metal hydroxide precipitates in streams and create highly turbid waters.

Severe impacts might mean only a few species of acid tolerant algae and invertebrates surviving in a stream and all fish dying. Gold mining is more likely to create sediment and turbidity issues and possibly release metals such as As, Sb & Hg. These metals can be toxic to many animals.

Management and remediation systems

Mine waste management can be a cost-effective means to minimise impact. Mine waste management goals are to; prevent or reduce the amount of water entering the mined area, reduce the contact of water and/or oxygen with acid-forming materials, and neutralise or reduce the level of contaminants present in any water runoff. To achieve these goals, factors influencing mine drainage at each site are evaluated, particularly background water quality, the volume, composition of mine waste material, and the position of the overburden and waste rock relative to surface and ground water. Appropriate site-specific management options are then applied to reduce the volume of mine drainage or minimise acidity and trace elements concentrations.

Remediation of mine drainage may be required even with good waste rock management. Remediation can be accomplished by either active or passive treatment systems, or a combination of both. Active systems typically require continuous dosing with chemicals, consume power and require regular operation and maintenance, but they are very reliable. Passive systems rely on natural physical, geochemical and biological processes but can fail if not carefully selected and designed. Site parameters to consider when selecting active and passive treatments include chemistry, flow rate, available land area, availability of power, and type of mine site.

Highly toxic arsenic is mobile throughout a range of pH conditions. Arsenic removal can be carried out by a number of processes, conventional techniques include oxidation, coagulation-precipitation, adsorption, ion exchange, membrane/reverse osmosis and biological processes. The choice of treatment depends on economics, availability of chemicals or adsorptive media and concentration of arsenic.

Publications on this research as well as a booklet on the July workshop can be found on this website.