What does wildfire do to arsenic? These researchers are working to find out

A small team of researchers are examining an area near Boundary Creek outside of Yellowknife to figure out what last year's wildfires did to arsenic in the landscape. 

A modelling study published this spring in the peer-reviewed scientific journal Environmental Research Letters suggested the N.W.T.'s North Slave Complex of fires released between 69 and 183 tonnes of arsenic into the air and water. 

Mike Palmer, a scientist at Aurora Research Institute, said it shows that there's the "potential" for arsenic to be redistributed if fire were to burn closer to the city than it did in 2023. But, he pointed out, the recent paper did not take actual measurements from the land — and that's exactly what he and Ontario grad student Chloe Earnshaw-Osler have been doing. 

"We're measuring, essentially, those things that those authors were trying to estimate through their modelling exercise," he said.  

In the early 1950s, the territory's Giant Mine and Con Mine are estimated to have together spewed 22,000 pounds of arsenic emissions per day. That arsenic, which came from the ore roasting process, settled into the land and water around Yellowknife and little is known yet about what happens to it when those landscapes catch fire. 

In a late-August interview, Palmer said learning what fire does to arsenic and other mining contaminants is a "big issue" across North America right now, as wildfires become more frequent and severe due to the changing climate. 

Carrying out a local study was something Palmer was already considering as he returned to Yellowknife from last year's wildfire evacuation and first spotted the area where he and Earnshaw-Osler, a student at Queen's University in Kingston, Ont., are now carrying out their work. 

"It's a fantastic site," he said. "We have this area that was intensely burned behind us and then across the road we've actually got an unburned area. So it's a really nice spot where we can contrast processes in burned landscapes versus unburned landscapes." 

Changes to soil, vegetation, water

Fire changes the type of vegetation that grows in an area, and thereby changes the way water moves through that landscape too. 

Palmer and Earnshaw-Osler's goal is to find out how much arsenic was released into the atmosphere, and if water is carrying arsenic into new places — or deeper into the ground. 

"Around Yellowknife and other places that are influenced by atmospheric emissions from roaster stacks and stuff, this is where we see most of our stuff," said Palmer, pointing to the top five centimetres of a soil sample laid out on the ground. 

But when the land burns, that top layer of soil is removed.  

"The metals and the other contaminants associated with that layer here, they've either gone to the atmosphere or they've resettled as ash. They're then available to be, you know, they can migrate down the soil column when there's water mixed with them, or they can run off the soil." 

The small team of researchers are taking measurements at the site to quantify these changes. They've taken samples of soil from burned and unburned land on either side of the highway and are analyzing them to see, among other things, if the species of arsenic in the soil has changed. 

Earnshaw-Osler said she and another student have also dug a 50-centimetre-deep soil pit, taken horizontal soil samples, and installed sensors before backfilling the hole to measure moisture and temperature. 

They also built a weir between a tree and the edge of a rocky outcrop, where water is expected to flow into the creek in the spring. The weir and some of the equipment attached to it will help them measure how much water is flowing into the creek, and the concentration of arsenic in that water. 

"You combine the flow with the concentration data, and you get a load which is your total amount, so we can estimate the amount of kilograms or grams of arsenic … that are moving into the landscape here, per unit area, versus the same thing in an unburned area," Palmer said. 

He said knowing what wildfire does to arsenic and other contaminants is "top of mind" for people living near legacy mine sites, and is a natural question to have after fire reaches one's doorstep. 

Palmer said the territorial government is helping fund the research through its cumulative impact monitoring program. He expects to be reporting results from the study in two years.