Study unearths new assessment of arsenic in Yellowknife soil

Current guidelines for the remediation of contaminated NWT sites are based in part on a five-fold overestimation of naturally occurring arsenic levels in Yellowknife soil, according to a new study.

The study, published in the journal Science of the Total Environment, found elevated levels of arsenic trioxide in soils as far as 30 km from the site where Giant Mine roasters once released billows of the toxic dust.

“We call that a smoking gun, or a fingerprint of Giant Mine or Con Mine emissions, because that’s the form released by old roaster stacks in the region,” said Mike Palmer, manager of Aurora College’s North Slave Research Centre and the study’s lead author.


Giant and Con, both no longer operational, were Yellowknife’s biggest gold mines. Giant is considered one of Canada’s most contaminated sites and a billion-dollar, federally led cleanup operation is beginning.

The new study establishes a lower figure for how much arsenic would naturally occur in Yellowknife soil if it weren’t for human activity like those mines, known as the background concentration.

Soil quality guidelines are based in part on background concentrations of arsenic. NWT soil guidelines outlined in a 2003 report reference an estimate that placed the background concentration of arsenic in Yellowknife-area soils at 150 parts per million.

Palmer’s study instead sets the upper range at just 30 parts per million, a fifth of the earlier estimate – which is still in use.

He said previous attempts at estimating naturally occurring arsenic in Yellowknife soil used different methods and likely sampled too close to the roasters to get a meaningful number.


“It answers a longstanding question in Yellowknife about whether our soils within the region are just naturally elevated,” said Palmer.

“We’re essentially saying no, that’s not necessarily the case. The general concentrations and soils in our region aren’t that much different from the rest of Canada.”

No change planned at Giant cleanup

The plan to clean up Giant Mine is based on existing soil quality guidelines. That plan aims to restore the Giant Mine site to the current industrial standard for arsenic contamination, which is 340 parts per million, and the townsite to the current residential standard of 160 parts per million.

“That’s what our approved plan is, so we don’t envision any of those changing,” said Natalie Plato, deputy director of the Giant Mine remediation project.


“We did a human health ecological risk assessment, which looks at the levels we’re cleaning to – post-remediation – to determine if there are any residual risks,” said Plato. “The overall summary of that risk assessment determined that risks are low or, in some cases, even very low.”

A sign warns of a contaminated area next to the highway passing Giant Mine
A sign warns of a contaminated area next to the highway passing Giant Mine. Sarah Pruys/Cabin Radio
A building on the contaminated Giant Mine site
A building on the contaminated Giant Mine site. Ollie Williams/Cabin Radio

Palmer said his findings do not necessarily indicate elevated risk to the public. He said his team is working closely with the NWT government’s Department of Environment and Natural Resources to ensure future risk assessments reflect the newest data.

“What it really does is allows those risk assessments to be based in the most contemporary and the most up-to-date information,” he said.

By email, the department said it is working to revise its soil remediation guidelines and will be sharing an updated human health risk assessment alongside Crown-Indigenous Relations and Northern Affairs in a series of public webinars this summer.

The department said the NWT’s soil remediation criteria differ in part from Palmer’s evaluation because the existing criteria focused on a tighter area surrounding Yellowknife.

But Palmer says samples must be taken further from the sites of past emissions to get an accurate measurement of what the arsenic concentration would be without human pollution.

“All the evidence presented in the paper shows that soils within 20 km of town are just not a reliable source of background,” said Palmer.

How the study worked

Palmer’s team collected almost 500 soil samples in the Yellowknife area over three summers, starting in 2015. Team members collected samples within a 30-km radius of Yellowknife by pushing aluminum tubes into the ground, or driving them in with a sledgehammer where the terrain was dense or rocky. Samples came from undisturbed locations outside city or industrial sites.

The tubes were sealed, frozen, and the samples shipped to Queen’s University. There, researchers sawed the tubes open and chopped them into smaller pieces for analysis.

The study found arsenic trioxide in 80 percent of samples as far as 30 km away from Yellowknife. The concentration of arsenic in the soil was higher closer to Giant Mine’s roasters.

Palmer said his team was surprised to find arsenic trioxide persisting in the soil 60 years after most of the dust was emitted. He said Giant Mine released nearly 90 percent of its lifetime emissions in its first 10 years of operation after opening in 1948.

Giant Mine and the neighbouring Con Mine released an estimated 22,000 pounds of arsenic trioxide per day in the early 1950s. Giant Mine accounted for the bulk of emissions and took longer than the smaller Con Mine to install scrubbers to limit its environmental impact.

The primary job of the Giant Mine remediation project is to freeze 237,000 tonnes of arsenic trioxide in underground chambers beneath the mine.

The Yellowknives Dene First Nation is calling for an apology and compensation from the federal government for the lasting impacts of the mine. A petition and TikTok awareness campaign prompted a recent statement from the federal government, which the First Nation says falls short of correcting historical wrongs.