Heavy rains are prompting a controlled water release at Vista Gold’s Mount Todd gold mine located about 50 kilometres north of Katherine in the Northern Territory.
The first release this wet season, Vista said it has been working with the NT Mines Department and began pumping treated water from Retention Pond 3 (Batman Pit) on Saturday.
By Monday morning about 114 megalitres had been released in accordance with the company’s waste discharge licence.
A Vista spokesperson said torrential rain and heightened flow through the nearby Edith River made it an opportune time to release the treated water.
The ABC reports General Manager Brent Murdoch said the Batman Pit contained 10.6 gigalitres of water which the company began treating in October 2012.
"The quality of that water is substantially better, we dropped out 99 per cent of the elements in there,” he said.
The contaminated water contains heavy metals including zinc, copper and cadmium.
"It's still got elements in there that need to be considered when we are releasing water, but it allows us to release water at a rate that ensures the environment is protected,” Murdoch said.
He explained controlled water releases help mitigate the risk of uncontrolled spills during periods of heavy rain which have happened in the past.
"Our other catchment ponds are filling up, the one of particular note is RP1, the one right down the bottom of the mine," he said.
"That has currently got about half a metre of freeboard.
"To put that in context, this is adequate if we only get average rainfall in the future.
"We're pumping continuously out of that into another pond that has a lot more capacity, RP7, which is the old tailings dam.
"But if we get higher than average rainfall in the near future, there is a real risk that it will over-top, which is something we are doing everything we can to avoid."
Pumping is expected to continue for several days.
Vista is currently preparing to reopen the gold mine at Mt Todd, and the site is estimated to hold around 7 million ounces of gold, with a further 2 million ounces inferred.
“The Mt Todd site is an environmental legacy mine; what Vista proposes to do is to turn a problem into an opportunity,” NT mining minister Willem Westra van Holthe has previously said.
Excess water also forced Vista to discharge two million litres of water into the Edith River in April last year.