The federal team tasked with caring for the Yukon's defunct Faro mine say they are moving closer to a plan to close and clean up the site, but worry about pollution.

Located near the town of Faro, the mine produced lead, silver and zinc from about 1970 until 1998. The federal government has since been maintaining the mine site, in part by treating water flowing from the site.

The mess at the Faro site is massive, including a toxic waste tailings pond that stretches four kilometres along a former creek bed.

Officials must deal with about 55 million tonnes of toxic acid-generating waste in the pond, either by securing it in place or by pumping it into the pit it originally came from.

Otherwise, experts say it could pollute everything flowing into the Pelly River.

They also have to contend with 320 million tonnes of waste rock piled around the mine's three open pits. The heaps have to be levelled and covered with earth.

Bill Slater, the technical manager with the Faro Project Management Team, told CBC News that the current water treatment plant, which has contained the waste from the water to date, won't be able to do that job forever.

"The water quality has not deteriorated to the extent that it will. But we predict that in the longer term, it will deteriorate more and at that point we simply won't be able to manage the concentrations and the amount of water," Slater said.

"We need to … take some other measures to make sure that we can prevent that in the long term."

The federal government is working on a final remediation plan with an estimated cost of $900 million.

While a firm plan is not expected to be ready for about four years, Slater said the government will likely have to care for the Faro mine site long after the cleanup is done.

"We can't really walk away from it because the types of rocks that we have here are subject to acid generation at levels that are very damaging to fish," he said.

That assessment has made the Faro mine site one of Canada's biggest environmental liabilities, alongside sites like the former Giant Mine in Yellowknife.

"Sometimes I hear we're the No. 1 environmental disaster in Canadian history. We go between one, two and three," Faro Mayor Michelle Vainio said.

"Certainly, there should never be anything like Faro again."