Mining voids pose threat to groundwater systems

As the mining boom winds down, work will ramp up for water professionals grappling with the potentially toxic legacy of hundreds of coal mining pits.

“The party is now over and the question is: who is going to clean up after it?” said Energy and Resource Insights Principle Researcher Adam Walters.

Walters' recent report – The Hole Truth: the mess coal companies plan to leave in NSW – outlines State Government approved plans to leave more than 45 mining voids across the Hunter Valley, totalling an area bigger than Sydney Harbour.

These voids can extend more than 150 metres below the natural water table and there are plans for most to act as evaporative 'terminal sinks' once mining ceases.

But Walters' paper warned: “there remain many unknowns in the long-term fate and behaviour of such voids – such as hydrology and the effects on surface waters and groundwaters, water quality issues such as salt loads and heavy metals, wall stability in perpetuity.”

According to modelling frequently used in environmental impact statements, lakes will form in the voids but the water will not migrate off-site. Instead, it will evaporate gradually and draw down local groundwater.

However, Walters said much of the modelling was of “questionable quality”.

“Water professionals should be scrutinising the modelling: are they using good, accurate projections for rainfall in the future?” he said.

“With climate change, rainfall patterns are changing significantly and there is the potential for some of these voids to spill in the future.

“When they do, the water that spills will be highly saline.”

Another plan for some mine voids is to backfill them – an option Walters said would be cost-prohibitive in many cases, not to mention potentially impact groundwater.

“When you fill the void in you get water percolating down through the loose backfill in the pit, potentially becoming slightly more saline and more loaded with a range of contaminants, and then it migrating offsite into groundwater systems,” Walters said.

On the other side of the debate is New South Wales Mining, which argues the voids represent plenty of opportunities.

The group's soon to be released Upper Hunter Mining Dialogue study will reportedly recommend the voids become a series of lakes with the potential to attract thousands of tourists to the region.

Other solutions would see the holes used for wildlife conservation, irrigation, water storage, aquaculture and hydro-electric power generation.

Walters acknowledged some of the voids could be a positive asset.

“That potentially could be the case in some limited cases. I say limited because we have far more holes than we need,” he said.

“And most of the uses that would be beneficial require high-quality water.

“Yet all of the planning that's been done has basically been about creating the worst possible quality water by ensuring that not much is in the pit and it's evaporating so all the contaminants are concentrating.”

Those who see the voids as an opportunity point to success stories in Europe, where Germany has transformed its Soviet era coal mine 'lunar landscapes' into tourist-friendly environmental and recreational areas, which double as flood mitigation systems.

But Walters warned Australia did not have the huge amounts of rainfall required for such an outcome.

“That will require vast amounts of water to be coming into the pits to flush them and ensure the contaminants are at low concentrations. No studies have been done to show that's possible,” he said.

With coal mines set to close right around the country over coming decades, Walters said water professionals would need take notice.

“There is clearly going to be a lot of work to make up for the lack of studies and planning … and in terms of actively monitoring and managing these voids at mine closure,” he said.