In 2020, when a backcountry campfire in Alberta’s Rocky Mountains was not properly put out during a high-risk wildfire season, aircraft rushed to the scene to prevent a bigger blaze. Soon, a creek ran the colour of rust, dyed by fire retardant that was accidentally dropped directly into it. It created an otherworldly scene, and a rare opportunity for a group of scientists.
Long-term retardants, like those dropped on North Racehorse Creek near the B.C. border in southern Alberta, are key tools for fighting wildfires: they dampen the intensity of the flames and slow it down, giving fire fighters a chance to get on top of the blaze. Retardants are often dropped by helicopter or airplane on forests or on infrastructure when a fire is raging nearby, though there are restrictions on where it can be dumped, including a prohibition on spraying it over waterways.
But accidents happen.
North Racehorse Creek in southern Alberta was dyed a rusty red after being accidentally hit by a fire retardant drop in 2020. Crews are supposed to avoid hitting water bodies, but accidents happen. Photo: Supplied by Alberta Forestry and Parks
The North Racehorse Creek incident was reported by an Alberta Forestry and Parks employee to researchers with the Southern Rockies Watershed Project at the University of Alberta. They were able to get to the site two weeks after the incident to measure the impacts on the aquatic ecosystem — a unique opportunity as most studies on the effects of fire retardants on water occur in labs.
The scientists were particularly interested in phosphorus, a main ingredient in the retardant that is also a key fertilizer in agriculture. Too much phosphorus has the potential for negative impacts, including algae blooms and dead fish. To get a sense of the scope of the contamination, researchers sampled the creek above the release and at the site. Then they returned a year later, in 2021, to measure again, this time taking samples downstream.
Their peer-reviewed results, published in June in Environmental Science & Technology, showed a lasting impact from the release, as the phosphorus clung to sediment and travelled far downstream, contradicting previous studies that suggested retardants quickly wash away and dissipate. In fact, the scientists found that the phosphorus became more concentrated downstream over time.
Another study, published months earlier from U.S. researchers, showed another impact from retardants: the potential for heavy metal contamination. Cadmium, one of the heavy metals found in retardants, is a known carcinogen that can impact aquatic and human health.
Retardants are a common response to intense wildfires — they were dropped as wildfires raged in Jasper National Park last year, for example. They help establish a barrier by making the forest less flammable, and are distinct from short-term retardants and suppressants that are used to put out fires. But the decision to drop the retardant in the first place is part of a complex balance of inevitable trade-offs, as decisions have to be made quickly with limited information and time.
“There’s a fair bit of chaos, sometimes, at the start of some quite major incidents,” Trevor Howard, the president of the International Association of Wildland Fire, said in an interview with The Narwhal. “And it really does take some time to get that order in the chaos and start to get ahead of the game.”
Long-term fire retardants are dropped as a protective measure on forests and infrastructure when wildfires are burning nearby. They dampen the intensity of flames and slow a wildfire’s spread. They also leave a rusty residue behind — as is seen in this 2017 photo from the aftermath of a B.C. wildfire. Photo: Jonathan Hayward / The Canadian Press
A creek runs red, and scientists ‘still finding impacts one year later’
The retardant that stained Racehorse Creek is a popular product, called Phos-Chek, captured in so many pictures of wildfire-fighting efforts that it has become widely recognizable: an iconic red plume falling from an airplane.
Once dropped, it coats the forest in a protective slurry that resists fire, the red dye acting as a marker. Phos-Chek, like all retardants, has to pass government tests in both the United States and Canada before its use is approved — but that doesn’t mean it’s benign.
And it also doesn’t mean the impacts are always well understood.
“There’s lots of work that’s been done on the toxicology of retardants, but most of that work has been done in buckets or aquarium tanks,” Uldis Silins, a professor of forest hydrology at the University of Alberta and co-author of the paper on phosphorus impacts, said. “What there is not much of — in fact, what is extremely rare — is real-world studies of retardant impacts in the field.”
It’s why Silins and his team raced out to the creek to collect samples after the retardant was dropped.
Madelyn Lux, then an undergraduate student studying environmental science, took a close look at those samples, which showed 33 per cent more phosphorus in the water where the retardant was dropped and 133 per cent more at a location one kilometre downstream, compared to upstream samples.
One year later, there was 167 per cent more phosphorus in the water six kilometres downstream, and the sediments were enriched with it.
While fires themselves can lead to a rush of phosphorus into water bodies through erosion, this drop happened in an area with limited-to-no fire impacts, making it a prime case study.
“It shows that phosphorus is, first of all, being stored in stream-bed sediments, because we’re still finding impacts one year later, and it’s starting to move downstream, because we’re finding impacts six kilometres down from where the drop was,” Lux said in an interview.
Melissa Kim, the vice-president of research and development at Perimeter Solutions, the company that manufactures Phos-Chek, doesn’t dispute the findings in the paper and said the company is constantly striving to improve its products and make them more environmentally friendly.
“Perimeter Solutions continuously improves its fire retardant solutions and phases out older generations of products,” she wrote in response to emailed questions from The Narwhal.
Adding small amounts of nutrients to water will have a minimal impact, according to Silins, but too much can have a significant impact on water quality and the aquatic ecosystem. He said this particular drop is “probably the worst-case scenario” when it comes to direct impacts from retardant.
It’s no small matter, either. Silins hasn’t seen any figures on how often a creek or water body is blasted with retardant in Canada and the Canadian Interagency Forest Fire Centre, which is owned and operated by the federal, provincial and territorial agencies responsible for wildland fire management in Canada, told The Narwhal it does not have any national data on long-term retardant use. U.S. data shows there were 259 reported accidental drops into U.S. waterways between 2012 and 2021. Less than half a per cent of all drops mistakenly end up into waterways there — in total, there were more than 80,000 drops in the U.S., or more than 500 million litres of retardant during that time frame.
In B.C., more than 162 million litres of retardant was used between 2010 and 2024, according to statistics provided to The Narwhal by the provincial wildfire service. The amount varies wildly from year to year, from as low as 1.7 million litres in 2011, to as much as nearly 21 million litres in 2017.
“The B.C. Wildfire Service prioritizes the protection of areas with high environmental values, such as community watersheds or habitats for species at risk,” spokesperson Emelie Peacock said by email. “It is our policy not to release or drop fire retardant where it can enter a body of water.” A spokesperson in Alberta did not respond to a request for similar information.
Both Silins and Howard with the International Association of Wildland Fire said retardant use is increasing in tandem with fire intensity and frequency, particularly near cities and towns.
“We would imagine this probably happens more often than we’re aware of,” Silins said of accidental drops.
Fire retardants contain heavy metals — and ‘trade secrets’
Another study, co-authored by Daniel McCurry, a professor of civil and environmental engineering at the University of Southern California, Los Angeles, found impacts can also come from heavy metals in retardants.
McCurry specializes in water treatment, but said he was chatting with a journalist a few years ago about another water-quality topic when the journalist asked him about the red plumes he’d seen falling from planes on Los Angeles-area fires.
“He dug up the publicly available material safety data sheets for the product, and he sent it to me,” McCurry said in an interview. “It’s primarily ammonium phosphate, which is basically just fertilizer, but I think something like 10 per cent is listed as a trade secret, and I was like, ‘Well, I don’t like that, Jake, they’re dumping millions of gallons of this stuff every year all over the environment.’ ”
McCurry suspected those ingredients included anti-corrosive agents, a key requirement for retardants that are carried in the metal tanks inside wildfire-fighting airplanes.
He reached out to companies for samples of their products, but was rejected and had to buy the products himself, some of which are available for homeowners seeking to protect their properties from fire.
“The Phos-Chek that we tested, we literally bought from Home Depot,” he said. The researchers then calculated the impact of those retardants if they were released into water bodies. The resulting paper found the retardants they tested had concentrations of heavy metals sometimes well in excess of safe drinking water levels. That could pose a direct threat to aquatic and human health in places like Los Angeles, where fires raged earlier this year, depending on how much retardant is used and where it’s dropped.
Impacts on water will vary greatly depending on the surface area of the water body and its flow.
“In the case of direct surface water contamination, we estimate that to remain below U.S. National Recommended Aquatic Life Criteria standards, for every 100 gallons [380 litres] of retardant dropped into surface water, the receiving water body would need to contain at least 800,000 gallons [over three million litres] of water to remain below aquatic toxicity thresholds,” according to the paper.
Phos-Chek, in particular, the same retardant dropped on the creek in the Canadian study, showed “pretty high concentrations of chromium, cadmium and vanadium,” according to McCurry, all heavy metals.
In B.C., more than 162 million litres of fire retardant was used between 2010 and 2024, according to statistics provided to The Narwhal by the provincial wildfire service. “It is our policy not to release or drop fire retardant where it can enter a body of water,” the service stated. Photo: Jonathan Hayward / The Canadian Press
Back in Alberta, Silins said his team might try to recreate similar tests of their own samples.
“It’s not quite a smoking gun, but it’s, at least to us, pretty plausible that the fire suppressants were a major contributor to the observed spikes of heavy metals in the environment after wildfires,” McCurry said.
Kim said the heavy metals identified in McCurry’s study are “naturally occurring, as they are in all ammonium phosphate fertilizers,” and Phos-Chek usage is significantly lower than agricultural uses.
“No heavy metals are intentionally added to the retardant formulations,” she wrote, adding retardants undergo extensive testing by federal agencies before being approved for use in Canada or the United States.
“Our scientists are constantly working to develop a fire retardant that is both highly effective in stopping wildfires and as environmentally responsible as possible,” she wrote. “Meeting this balance is also what makes qualification so rigorous.”
Neither Parks Canada nor Alberta Forestry and Parks responded to questions and requests for comment by publication time.
Better forest management could reduce the need for fire retardant: wildfire expert
There are rules around where retardants should be dropped, particularly when it comes to water bodies. Those rules are an acknowledgement of the potential impacts. But when faced with the destructive power of a fire, often the choice to use retardant is clear.
Howard, who outside of his role with the wildland fire association is the national manager of the prescribed burning strategy with the Australasian Fire and Emergency Service Authorities Council, said poor land management and climate change are both contributing to increased fire intensity and frequency, which is linked to the increased use of retardants.
“If those areas are well-managed with prescribed fire [and other tactics including selective logging] then there’s not the same necessity to put bulldozers or large air tankers with retardant into some of those areas, because the wildfires don’t have the same level of impact,” he said.
While he points to success with prescribed burning in northern Australia, much of it led by Indigenous communities, southern Australia is seeing more large fires, as is Canada.
Still, Howard said retardants are critical for managing fires, particularly in hard-to-access places like Canada’s mountains and forests. He said the localized impacts of a retardant drop have to be assessed against the broader impacts of a wildfire left to burn.
“As these big fires unfold, particularly with the intensity and the scale that we’re seeing nowadays, they can move through the landscape and impact threatened ecological communities and threatened species,” he said. Hills denuded of the vegetation that helps filter water as it trickles and streams into creeks, rivers and lakes carry their own pollutants and sediment, and can also lead to sharp increases in phosphorus.
“The environmental issues of not taking action to control the wildfire can be very, very significant,” he said.
In the aftermath of a blaze, once calm returns and lessons are learned, he said research like that conducted by Lux and Silins is vital.
“I think it’s a really important piece of work, because, as the authors actually point out, there’s really no body of literature on this particular topic already, so it’s quite pioneering stuff.”
