Understand the risks
During mining operations, pit water management is closely monitored for safety, regulatory and economic reasons. Dr Linklater said similar focus is needed to develop and monitor pit lakes, and identify beneficial outcomes to help communities after mine closure.
In many cases, the water sits at the base of a deep pit and never rises to a level where it would discharge to groundwater or spill over the pit crest. In arid areas of Australia, the dry climate and high evaporation rates reduces these risks.
But in wetter areas, the lake water may rise above the local water table, causing release to groundwater, or may even spill from the pit, and may impact nearby receptors such as groundwater supply bores, surface waterways, flora or fauna.
Other factors that could cause pit lakes to overflow include intense rainfall events or unstable pit walls.
Such releases are manageable if pit lake water quality is acceptable, or if there are long distances between the lake and local receptors (which could allow significant dilution of any contaminants as the outflow mixes with other water types downstream of the pit).
However, in geologies with high sulphide content, acidic and metalliferous reaction products can form on exposed pit walls.
These products can wash down into the pit lake and adversely affect water quality, especially if combined with evapo-concentration. If this acidic metalliferous water is released, and receptors are nearby, the damage to local flora, fauna and communities can be considerable.
Problems range from animals and birds drinking contaminated water to loss of vegetation. Accumulation of elevated heavy metal concentrations in waterways near pit lakes can also cause health problems in local communities.
“Sustainable pit lakes are integral to successful closure planning for open pit mines,” Dr Linklater said. “If pit design and water quality are poor – and the risk of water overflow is high – mining companies face significant financial and reputational risks.”
Hypersalinity is another threat. In areas with low rainfall and high evaporation, saline groundwater entering a pit lake can evapo concentrate, forming a brine. Over time, salinity in the pit lake increases. High evaporation from the pit lake, and low lake water levels may draw groundwater water from the surrounding aquifers. This causes a fall in groundwater levels and may damage the surrounding habitat.
Dr Linklater said climate change is compounding the challenge of designing pit lakes and modelling water quality.
“Higher frequency of intense rainfall events increases the risk of water outflow from some pit lakes. At the same time, climate change will increase hypersalinity risks at some pit lakes as temperatures rise,” she said.
An important response is a greater focus on data collection to test water quality prediction models and refine them if needed.
“Mining companies are doing more work on modelling water quality,” Dr Linklater said. “The challenge now is to invest in data collection and have an agile approach to modelling as the climate changes.”
