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Pit lake planning critical to a sustainable mine site

Pit lake mine

Pit design and data collection are key to building a sustainable pit lake on mine sites.

For Claire Linklater, planning for sustainable mining pit lakes is not only about risk management. It’s also an opportunity to help communities after a mine closes.

Dr Linklater is a geochemist and principal consultant at SRK Consulting. Over more than three decades, she’s interpreted and modelled geochemical data, assessed acid rock drainage, predicted water quality at mines and advised on closure planning.

“Few issues in mine closure are as important as pit lake planning and monitoring,” she told Australian Resources & Investment. “More open-cut mines in Australia are expected to close this decade and next. The legacy will be an increase in pit lakes around this country.”

Operating mines in Western Australia range from one or two hectares in area and a few metres deep, to modern pits that are several hundred hectares in area and hundreds of metres deep. 

The Super Pit in Kalgoorlie, for example, is more than 600m deep, 1.5km wide and about 4km (about 600 hectares) long. 

Pit lakes form when open-cut operations stop and dewatering (groundwater removal) ceases. The remaining pit fills with ground, surface and rain water. 

Governments often prefer mining companies to backfill pits with waste rock, tailings or other suitable waste. But this is not always possible or too costly, and a pit lake is allowed to form. 

Dr Linklater said mining companies have increased their focus on pit lakes in mine-closure planning. 

“Companies know water quality in pit lakes can deteriorate over time and, if released, could potentially harm the surrounding community, wildlife and vegetation,” she said.

“Companies also know sustainable pit lakes can be achieved through an early focus on mine-closure planning and pit design. For existing pit lakes, the key is regular data collection to test water quality modelling – and good community engagement.”

Some pit lakes have been used for aquaculture. Others have been used for irrigation.

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.”

Pit lakes form when open-cut operations stop and dewatering ceases.

Consider opportunities

Dr Linklater said pit lake planning and monitoring have mostly focused on risk management. That work needs to continue, but mining companies should also consider commercial and community opportunities from successful pit lake developments. 

“Sustainable pit lakes can leave a positive legacy for remote communities,” Dr Linklater said. “We should think carefully about how these lakes, some of which could be quite large, can help remote communities and the environment.”

Some pit lakes have been used for aquaculture, others for irrigation (where the water has less salinity). Wildlife conservation has also featured through the construction of wetlands around some pit lakes. Recreation and tourism opportunities have emerged at larger pit lakes with good water quality.

“Pit lakes are potentially a valuable water source,” Dr Linklater said. “By engaging and collaborating with communities, government and other stakeholders, companies can maximise the value of pit lakes. 

“But that relies on sound planning and monitoring that ensures the long-term sustainability of pit lakes.”  

Best practice in pit lake planning

1. Plan early: Develop processes to identify water quality and pit lake risks. Ensure planning is integrated into the mine closure strategy at the start. 

2. Pit assessment: Understand which rocks are exposed on pit walls, pit characterisation and geologies that influence pit risks. Consider if final pit design can ensure sulphide rocks are not exposed on the pit wall surface, above the expected water level. 

3. Water balance: Model likely water inflows and outflows of a pit lake. Understand how they could change over time and the risk of contaminated pit lake outflows and impacts on local groundwater levels.

4. Linkages: Conduct source pathway receptor analysis to identify linkages between pit lakes, pit lake overflow and the surrounding habitat. Determine if those receptors are close to the pit lake. Identify all transmission pathways (e.g. surface water and groundwater).

5. Receptor analysis: This includes analysis of flora, fauna, or even groundwater or bore water. Which animals or birds might use the pit lake or be affected by its outflow? Are there Indigenous communities near the pit lake that could be impacted? Is there a nearby waterway that the local community uses for fishing or recreation? Identify primary and secondary receptors.

6. Monitoring: Develop a plan to monitor pit lake quality and implement monitoring points for surface water and groundwater. Determine the frequency of monitoring and aim to collect more water quality data to test and refine modelling.

7. Plan for climate change: Consider how greater variability in weather, an increase in extreme weather events, and hotter temperatures could increase the risk of water overflow at acid pit lakes – or hypersalinity that damages groundwater. Ensure modelling is agile and able to incorporate climate volatility. 

8. Think long-term: It can take decades before the water in a pit lake rises to a steady-state level, and the final amount of aerially exposed sulphide rock wall is known. Be prepared to invest in years or decades of monitoring pit lake quality and the stability of pit lake walls, as water levels rise, or as salinity increases. 

SRK Consulting is a leading, independent international consultancy that advises clients mainly in the earth and water resource industries. Its mining services range from exploration to mine closure. SRK experts are leaders in fields such as due diligence, technical studies, mine waste and water management, permitting, and mine rehabilitation. To learn more about SRK Consulting, visit www.srk.com

This feature appeared in the August issue of Australian Resources & Investment.

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