The Northern Territory government is investigating ways to clean up an old copper mine, described as "one of Australia's most polluting sites" — a job which could cost taxpayers hundreds of millions of dollars.
The remote Redbank mine, 1,200 kilometres south-east of Darwin, only operated for two years, in the mid-1990s.
But its legacy is still visible in a nearby creek, which has turned green from acid leaching out of the mine.
The mine's owner, Redbank Copper, was allowed to start mining in 1994 without paying an environmental bond.
But after a crash in copper prices, the mine was placed in care and maintenance, and the company later went bankrupt, ultimately leaving the Northern Territory government responsible for the site in 2016.
The Northern Territory's Department of Industry, Tourism and Trade (DITT) last week released a tender to develop "a remediation strategy for the mine to improve water quality downstream of the mine, and to rehabilitate the mine to a condition that is safe and acceptable to stakeholders".
According to details in the tender, acid generated by waste rock and tailings at the mine is leaching into surface water around the mine.
"Elevated copper concentrations have been measured within Settlement Creek approximately 40 kilometres downstream at the Queensland and Northern Territory border," the tender states.
Associate professor Gavin Mudd, an environmental engineering expert from RMIT University, calls Redbank mine "one of the most intensely polluted sites" he has ever visited.
"For such a small site ... it's caused an extraordinary legacy," he said.
"Redbank is certainly amongst one of Australia's most polluting sites.
"[Surrounding waterways have] really high concentrations of copper — you're talking about 300 milligrams per litre, and similar concentrations of things like aluminium — whereas normally, for an aquatic ecosystem we aim for values of 0.001 milligrams per litre."
Options for backfilling pit
The DITT tender includes four options for rehabilitation of the mine site, outlining how the 48-metre-deep pit could be remodelled and acid leaching stopped.
The options comprise backfilling the pit with mine waste and covering the tailings, two methods of partially backfilling the pit or relocating all the mine waste to a new above-ground waste storage facility.
Copper-processing infrastructure at the mine will also need to be demolished and disposed of offsite or onsite.
DITT would not comment on how expensive the clean-up of Redbank might be.
"Work to minimise or rectify environmental harm caused by legacy mining activities at Redbank is being progressed by the legacy mines unit through the mining remediation fund," a DITT spokesperson said.
But Mr Mudd said the total cost of the rehabilitation would likely be in the hundreds of millions of dollars.
"Whether it is closer to $100 million or $300 million ... it's probably somewhere in that order of magnitude," he said.
"The exact time frame depends on the options taken and therefore the amount of work involved.
"It's not a huge site, so it is something that could be done in a couple of years, but these things do need to be very carefully staged.
"As always, the better approach is to take the time and make sure it is done right ...we need to make sure it gets done to a very high standard."
The tender indicates a "kick-off meeting" with the department could occur in early January 2023.
