Weathering products and geochemistry of waste residues at the Brukunga pyrite mine, Adelaide Hills, S.A., in relation to environmental impacts

Abstract

The Brukunga Mine is located approximately 50 km east-south-east of Adelaide and was established in 1952 as a source of sulfur for superphosphate. Due to low sulfur prices, the mine was closed in 1972 and since that time has been a source of Acid Mine Drainage (AMD), which has in turn had a dramatic effect of the local and downstream environments.

Investigations into the waste dump and mine bench rocks indicate that large quantities of sulfides (up to 22%) are present. Oxidation of sulfides, catalysed by bacteria, quickly form a strongly acidic environment. The lack of significant quantities of rapid neutralising minerals, such as calcite, accentuates and enhances these low pH conditions. Resultant large quantities of acid and heavy metals produced through this oxidation are subsequently released into the surroundings. The contamination is enhanced by the release of lithophile elements formed during slow aluminosilicate neutralisation reactions that are taking place. The pollutants are released into Dawesley Creek, a tributary of the Bremer River, which flows through the mine site.

The tailings also add to the acidity problems through similar processes, but here sulfide quantities are much lower. A large accumulation of soluble salts has developed at depth and their location away from the zone of active oxidation indicates that oxidation has not been consistent through time. The development of a cemented layer region at 1-1.4m depth within the tailings is thought to be the preliminary stages of a hardpan. Calculations indicate that the formation of the cemented region is reducing the movement of the oxidation front and therefore the oxidation rate. Combining the two main observations suggests that the oxidation of the tailings is decreasing with time, through a self sealing action.

The current environmental hazard the tailings represents is significant. The groundwater movement through the tailings represents a major flushing effect, allowing water with high quantities of ferrous iron to be transported to the dam wall where it is readily oxidised. This, combined, with oxygen from the atmosphere, causes the rapid oxidation of the rocks which make up the dam wall, and represents a major problem. A mechanism to reduce the ground water through-flow needs to be considered.