Water Resources & Mining

Executive summary

This report provides an overview of the mine void issue, the creation of pit lakes and associated hydrogeological processes, an assessment of the potential impacts on groundwater resources, and water management considerations at mine closure. The report outlines the technical information required for the compilation of recommended guidelines, which will assist the mining industry in gaining environmental approvals related to proposed developments below the water table. Eighteen case studies (detailed in Appendix 1) have also been completed to highlight differences between groundwater environments, regional setting and mine closure options.

Mining is leaving a legacy of hundreds of mine voids throughout the State. There are numerous safety issues that must be addressed as part of mine closure and, until recently, there had been no assessment of the potential long-term environmental impacts of mining below the water table. The mine void issue is vitally important to both the Government and mining industry, as neither wishes to be liable for rehabilitation or stabilisation of a mine void over a period of decades or possibly millennia. In Australia, mine void issues have previously focused on the coal mining industry, sand mining, and politically sensitive mines in the tropical areas of the Northern Territory.

Mine void-related impacts are a long-term concern for Western Australia, as there are currently about 1800 existing mine voids and more than 150 mines operating below the watertable. The size of mine voids varies from borrow pits (about 100 m in diameter) to the enormous pits in the Goldfields and Pilbara. The larger mines require substantial groundwater abstraction (dewatering) from sumps or in-pit/perimeter bores to facilitate dry-floor mining practices. On cessation of dewatering, the waterlevel recovers to create a ‘pit lake’ within the mine void, thus initiating geochemical and hydrological processes that evolve with time. The infilling of the void with water may take centuries, with chemical evolution via evaporation continuing much longer.

There is potential for many pit lakes in Western Australia to become point sources of hypersaline water and impact on the surrounding groundwater resources. The low annual rainfall and high evaporation experienced over much of the State produces a rainfall deficit, which contributes to the development of hyper-saline water bodies. There are also potential problems with the generation of acidic conditions in pit lakes, particularly for the coal mining industry in the higher rainfall, southwest region and a few isolated metalliferous mines.

The salinisation and acidification of pit lakes has the ability to affect local and regional groundwater resources, as well as the broader natural environment. The extent of impact on the surrounding groundwater environment is largely dependent on the local hydrogeology, as to whether the mine void will act as a (1) groundwater sink or (2) groundwater through-flow cell. In the groundwater sink regime, evaporation exceeds the rate of groundwater inflow into the void and is typical of most hard-rock mines throughout Western Australia. Mine voids where groundwater inflow exceeds evaporation act as a ‘groundwater through-flow’ type, forming potential environmental hazards with saline plumes moving out of the void and affecting other groundwater resources.

Read more at www.dmp.wa.gov.au/documents/HG9.pdf