The waste generated from coal mining and beneficiation is contaminated with carcinogens, heavy metals, and radioactive elements.
It's extremely dangerous to the environment and human health, and an important consideration for mining companies.
While each miner has its own plan to tackle the problem Virotec Solutions CEO Lee Ferguson says the industry's treatment "has at times been found wanting".
In a report on coal mining Ferguson said the risks of improper waste treatment were "comprehensive and well-documented".
She listed arsenic and heavy metal contamination, alarmingly high cancer threats, and the devastation of local flora and fauna as some of the worst impacts.
Nevertheless Ferguson's report acknowledged coal as one of the world's most plentiful energy resources with demand expected to quadruple in the coming decade.
As a necessary resource, the challenge for miners is to understand the risks and pick the right solutions.
Acid mine drainage
The Department of Sustainability, Environment, Water, Population and Communities has ranked acid mine drainage as "one of the main strategic environmental issues facing the mining industry".
The problem occurs after the rock in coal stockpiles, handling facilities, and overburden reacts with air and water.
In many cases the reactions can be highly acidic, with the acid then mobilising trace metals contained within the coal.
Ferguson said the cocktail of chemicals produced in runoff was a serious threat if released to the environment untreated.
"The combination of acid and metals seeps into waterways where it can effectively sterilise many kilometres of streams," she said.
In the worst cases groundwater can be contaminated with metals such as arsenic, cadmium, and thallium, damaging not only the environment but eventually making its way into drinking supplies.
Ferguson explained that such contaminants were "known carcinogens" and posed a "significant risk to human and environmental health".
She said in most operations run-off was either collected and impounded at the mine site or impounded as run-off during storms.
"In some cases, neither of these methods works effectively, due to the large volumes of liquid to be captured or the complexity of the systems involved."
While acid mine drainage is a difficulty facing all of the world's coal mines, Ferguson said Virotec's operations in Australia pointed to how the problem could be effectively contained.
At the closed Aberdare East Colliery near Cessnock, New South Wales, the company was contracted to rehabilitate the site's contaminated coal tailings.
At Aberdare tailings from the mine were impounded in a small catchment area on site.
The run-off was highly acidic and had significant contamination from a number of heavy metals.
After treatment Ferguson said Virotec's chemical reagent, used as an additive to the tailings, had "resulted in significant reductions of all relevant metals and an increase in pH to within regulatory limits".
At Aberdare East the remediation of aluminium, iron, and manganese was particularly effective.
Aluminium contamination went from 36.9 mg/L in the initial seepage to 0.03mg/L after treatment.
Similarly iron contamination went from 168 mg/L to 0.001mg/L and manganese went from 9.5 mg/L to 2.3mg/L.
The water's pH was also raised from its initial high acidity of 2.6 to 8.2, close to the normal range of river water and within regulatory guidelines.
Queensland's underground barriers
For another miner in Queensland Virotec used the chemical additive outlined above and a permeable reactive barrier (PRB) to remediate the groundwater.
A PRB is a wall built below the ground used to clean water as it passes through.
Permeable means the wall has tiny holes that allow groundwater to flow through it, and the reactive material in the wall helps trap and remediate contaminated particles.
"The option of using PRBs to treat acid mine drainage at coalmine sites has the potential advantage that well-designed barriers can be constructed and left unattended," Ferguson said adding that there are challenges with designing barriers that work effectively over the long term.
Ferguson said selecting the right materials for the barrier was one of the hardest challenges, with most substances presenting their own setbacks.
While limestone is used widely Ferguson said it had "many problems".
She said the slightly soluble nature of lime meant it could be leeched out of high rainfall areas, and some contaminants were able to coat lime particles and prevent them from continuing to neutralise water.
Ferguson said Virotec had developed its own material to form PRBs that remediated contaminants without impeding water flow.
But she said like all materials Virotec's had its limits.
"As with all PRBs, there is a limit to their capacity to effectively treat water at which point they need to be replaced," she said.
Ultimately Ferguson's report highlighted the importance of environmental management for the coal mining industry.
While coal is a valuable and necessary resource, it has the potential to seriously damage the environment.
But with miners aware of the dangers, and companies working to improve treatment technology, our natural resources can be used without undue pressure on the environment.