Giant Mine arsenic trioxide at ‘low risk’ from approaching wildfire

After this story was initially published, a Cirnac spokesperson reached out to better clarify how arsenic trioxide is currently being stored and why it’s not at risk. We’ve added these details to this story.

The toxic arsenic trioxide stored underground at Yellowknife’s Giant Mine remediation site is considered low-risk as a wildfire approaches the city, officials say.

Giant Mine is a former gold mine that operated for decades at the city’s northern perimeter before being shut down in the early 2000s.

The gold roasting process used at the mine, and some of its other operations, have left a lasting toxic legacy. It is routinely referred to as one of the most toxic sites in Canada, and remediation work is scheduled to last for more than a decade and cost more than $4 billion.

“The arsenic trioxide at Giant Mine, of which the majority is stored underground, is considered low-risk,” said Megan MacLean, a spokesperson for Crown-Indigenous Relations and Northern Affairs Canada (Cirnac).

Another Cirnac spokesperson, Myranda Bolstad, clarified the arsenic is stored in underground chambers surrounded by solid rock and plugged with cement bulkheads. The arsenic is stored between 80 and 250 feet below the surface.

According to Cirnac’s website, “237,000 tonnes of arsenic trioxide dust is currently safely contained and managed” as it is stored underground. However, to minimize the risk of the arsenic escaping, thermosyphons are being used to freeze arsenic trioxide in place.

The thermosyphons act like heat pumps to cool the rock around the chambers, freezing water around the rock and ensuring arsenic trioxide cannot leave.

The only thermosyphons currently on-site are test thermosyphons, and are not currently being used as the primary means to secure the arsenic trioxide.

Ben Nind, the executive director of the Giant Mine Oversight Board, said there are “very limited thermosyphons on the site” and they are in a cleared area. He said any risk would be handled by staff on site and “at this time there are no concerns.”

More: How Giant Mine’s thermosyphons work

MacLean said a crew has been working over the past week to prepare Giant Mine for a total shutdown, which has included firesmarting the property.

“Evacuation preparations included collecting brush and clearing materials from recent construction and moving these materials to the tailings ponds and away from structures,” she wrote.

“Additionally, materials from the previous deconstruction of the roaster are stored inside locked shipping containers in the central tailings pond away from vegetation.”

She said power has been disconnected with the exception of three underground pumps that are able to safely operate. The pumps will ensure any water that comes into contact with the underground storage chambers is pimped to the surface and treated.

The manager of the mine was comfortable with the state of the remediation site before evacuating, MacLean said.