In the Northwest Territories, new documents show the cleanup costs for Yellowknife’s arsenic-contaminated Giant Mine site will be close to a billion dollars.

That's double what officials with Aboriginal Affairs and Northern Development have said it would cost to remediate the former gold mine. The cleanup plan is currently in the final stages of an environmental review.

Kevin O'Reilly is with Alternatives North and was one of the people who pushed for an environmental assessment of the cleanup plan. He obtained the new information through an access to information request.

The documents show the Department of Aboriginal Affairs and Northern Development revised its cost estimate in March of 2012 — six months before public hearings on the cleanup.

But during the public hearings, federal officials never said they expected the cleanup to cost that much. Officials repeatedly said it would cost half that amount.

The document attributes the jump in price to a combination of inflation, additional maintenance, as well as fast-tracking some of the work to stabilize the site.

O'Reilly questions why they're not being more transparent.

"They knew back in March of that year the Treasury Board had approved a figure of $903 million. So I just can't figure out why the figure of $903 million wasn't mentioned to the board or told to the board reviewing this," he said.

No one from the department was available to talk about the new information.

O'Reilly wants an independent review of the cleanup plans.

"It's crying out for some kind of independent oversight with a lot more meaningful involvement from the community," he said.

The Mackenzie Valley Review Board is now deliberating on whether to approve the federal government's proposed cleanup.

Last week the cleanup team submitted new suggestions to the review board for how to treat water flowing over the mine.

"Costs seem to be mushrooming. We don't have the proper technology to identify or do some of the work, or new technologies suddenly drop from the sky," said O’Reilly.

O'Reilly said these last-minute changes are making him doubt the government's ability to manage the site.

Giant Mine operated as a gold mine from the late 1940s to 2004. The arsenic contamination is a result of how gold was mined at the site — over the years, more than 200,000 tonnes of arsenic trioxide, water-soluble dust was produced during the gold roasting process. The majority of the dust is underground.

The cleanup plan is to freeze the dust in place, and keep it frozen indefinitely.