The flames have been beaten back from Slave Lake in northern Alberta.
But as residents slowly return to their fire-scarred town, scientists are wondering about the environmental legacy of a blaze that consumed hundreds of homes and businesses — and everything in them.
“If there’s a really heavy fire and you burn up plastic ... you’ll end up with some of that residual, partially burnt plastic in the soil,” said Anne Naeth, a biologist and expert in land reclamation at the University of Alberta.
Studies have been done on what happens when materials such as vinyl siding are burned, but there’s little research on what’s left behind from large-scale conflagrations. And large-scale certainly describes what happened in Slave Lake.
More than 400 lots were either levelled or badly damaged. Entire neighbourhoods were burned to their foundations. Dozens of vehicles were reduced to metal skeletons. Gas stations exploded.
“I would expect all sorts of nasty toxins, both inorganic and organic,” said David Schindler, an ecologist at the University of Alberta.
“Certainly lots of zinc, lead, mercury, copper, cadmium from burned wiring, piping, auto parts, broken lights and thermometers; dioxins and furans from burning tires; PCBs and PAHs, probably phthalates and polybrominated compounds from other burned plastics, upholstery, etc.”
Many of those compounds, including some of the most toxic, are likely to dissipate quickly.
But one of the few studies done on the environmental impact of wildfires in residential areas suggests that heavy metals will probably stick around. The U.S. Geological Survey looked into what happened after fires in southern California roared through residential developments in 2007.
“For arsenic, lead and antimony, concentrations in one or both of the residential composite ash samples approach or exceed U.S. Environmental Protection Agency residential soil preliminary remediation guidelines,” the report says.
The study also found elevated levels of chromium. As well, it found the remaining ash was so alkaline it was caustic — significantly more so than ash from woodland fires.
Exposure to heavy metals has been linked with developmental retardation, various cancers and kidney damage.
Intense wildfires also damage soil. The heat bakes the clay component of dirt almost as if firing it in a kiln, leaving it unable to absorb water.
“You have to break up that hardpan that’s been formed and add organic material,” said Naeth. “You wouldn’t just be able to pop your house back on there and expect you’re going to be able to grow the plants you normally have around the home.”
Hydrocarbons from burning or residual fuels aren’t likely to be a long-term problem, said University of Alberta microbiologist Al Jobson.
“There’s going to be surprisingly little in the way of residual hydrocarbon left if it’s had a good chance to burn,” he said.
Many such chemicals, such as antifreeze from vehicle radiators, will simply biodegrade.
“There’ll be household residues that burned to various degrees, so a lot of that residue will be sitting in the basements.”
Much of that will have to be trucked off to safe landfills, he said.
Alberta Environment may conduct soil testing in Slave Lake.
“They don’t think cleanup and remediation will be a huge issue,” said provincial spokeswoman Michelle Davio. “They will go in and assess it.”
But the environmental cleanup is unlikely to be simple, said Naeth.
“Whatever the compounds are that are not released will be in the residue,” she said. “All of those materials that are left will have to be removed and either hauled off to a dump or remediated.”