Canadian drinking water at risk long after wildfires, UBC study warns
University of British Columbia
Canada’s drinking water can remain at risk long after wildfires burn out, according to a UBC-led global review that found water-quality impacts often emerge months or years later—not just immediately after a fire.
Researchers analyzed 23 studies across 28 watersheds worldwide, comparing pre- and post-fire levels of sediment, nutrients, metals, organic carbon, ions and wildfire-fighting chemicals. Across climates, contamination often intensified over time, particularly when storms or snowmelt washed stored ash and debris into rivers.
The findings carry particular weight for Canada, where wildfire activity has intensified. In 2023, over 15 million hectares burned, more than twice the previous national record.
Alberta studies show long-lasting impacts
The review focused on studies tracking water quality for at least six months to determine whether impacts fade or grow.
“The same delayed contamination pattern kept appearing,” said Raúl de León Rábago, study author and master’s student in civil engineering.
After the 2016 Fort McMurray wildfire, rivers showed elevated sediment, nitrogen, phosphorus and lead even where less than one-quarter of the watershed had burned. The Regional Municipality of Wood Buffalo increased annual treatment chemical spending by roughly $500,000 to manage wildfire-related changes in raw water.
In Alberta’s southern Rockies following the 2003 Lost Creek wildfire, phosphorus and nitrogen remained high for years. Floods in 2013 washed stored ash and soil back into rivers, causing phosphorus levels to jump to seven to nine times higher, with some increases persisting more than 14 years downstream. Similar long-term effects have been documented internationally.
“Imagine emptying a bucket of ash into a bathtub,” said Dr. Qingshi Tu, assistant professor in the faculty of forestry and environmental stewardship. “When the water is stirred, the ash resurfaces. That’s what can happen in watersheds after large fires.”
Smoke and firefighting chemicals add to risk
Across the reviewed studies, wildfire activity increased sediment, nutrients, heavy metals and polycyclic aromatic hydrocarbons—chemicals formed when vegetation and other materials burn. Smoke can also carry contaminants into unburned watersheds.
Canada relies heavily on long-term fire retardants such as Phos-Chek in B.C. and Alberta. These products contain nutrients and trace metals that can fuel algal blooms and raise treatment costs. After the Fort McMurray wildfire, higher chemical dosing was required to treat wildfire-affected water.
Protecting communities through long-term monitoring
Researchers note that water utilities’ ability to respond depends on fire intensity, duration, size, what burned, weather conditions and treatment system design. Not all systems have equal capacity to adapt, and smaller communities with limited budgets face greater risk from prolonged post-fire impacts.
The team is developing a model linking wildfire behaviour, smoke and river systems to help Canadian utilities anticipate multiyear risks. They say fire-prone provinces such as B.C. and Alberta need coordinated long-term water monitoring and preparedness planning, especially when fires burn near drinking water sources.
“Canada is entering a new era of wildfire risk,” said Dr. Loretta Li, senior author and UBC civil engineering professor. “If we want to protect drinking water, we have to treat wildfire impacts as long-term, not short-term.”
Journal
Science of The Total Environment
Method of Research
Literature review
Subject of Research
Not applicable
Article Title
Impacts of wildfire-related chemicals on surface drinking water sources: Status and research gaps
Large forest fire emissions are hidden underground
Lund University
image:
Forest fire in Sweden
view moreCredit: Johan A. Eckdahl
Researchers at Lund University have produced the most detailed map of carbon emissions from Swedish forest fires to date. The results show that the largest emissions occur below the ground surface, in peat and organic soils.
During the extremely hot summer of 2018, 324 forest fires were reported in Sweden. Using field measurements, models, and data from the Swedish Forest Agency, the Swedish Environmental Protection Agency, and the Swedish Meteorological and Hydrological Institute, researchers have now mapped where and why the carbon was released.
The study shows that emissions from intense above‑ground fires are overestimated in fire databases. Emissions from deep organic soils and peatlands, on the other hand, are underestimated - during the summer of 2018 by as much as 50 percent. These smoldering underground fires are rarely visible in satellite images, but can release very large amounts of carbon.
“What looks dramatic from above is not always what affects the climate the most. The significantly large emissions actually occur silently underground,” says Johan A. Eckdahl, forest fire researcher at Lund University and the University of California, Berkeley.
In boreal forests - the coniferous forest region that stretches around the Northern Hemisphere - more carbon is stored than exists in the atmosphere today. A large portion is found in deep peat soils that have built up over thousands of years. When these soils dry out and ignite, they can continue to burn below the surface for long periods. This means that traditional methods used in today’s fire databases, which rely on the size of the burning area, smoke density, and visible fire intensity, risk missing a crucial part of forest fires’ climate impact.
A telling example is the comparison between the 2018 year of fires and the 2014 forest fire in Sala, Sweden. Despite the total burned area being much larger in 2018, the results show that the Sala fire alone released roughly as much carbon as all 324 fires in 2018 combined.
“What matters is where it burns. A fire in deep peat soils can have a greater climate impact than hundreds of more intense fires on land with thin soil layers,” says Johan A. Eckdahl.
The study also provides new perspectives on forestry and land use. The high‑resolution maps show certain patterns suggesting that recently clear‑felled areas could serve as pathways for fire to spread into older, carbon‑rich forests and wetlands. The researchers also conclude that population density plays an important role in enabling early containment of high‑intensity fires. There were also some indications that early firefighting efforts and active forest management can reduce fire damage.
The researchers believe that the findings have relevance far beyond Sweden’s borders. If emissions have been underestimated during a Swedish year of fires of the scale seen in 2018, it raises the question of how large emissions may have been during recent extreme fires in North America and Siberia in 2021. In the latter case, adequate baseline data is lacking, making it difficult to calibrate satellites and models.
“Satellites that show burning above ground are important for understanding where underground burning will begin. However, we need to combine satellite overviews with field work. Only then can we understand how much carbon is actually released, and how to best protect the most vulnerable carbon stores in a warming climate,” concludes Johan A. Eckdahl.
Journal
Science Advances
Article Title
Reassessing boreal wildfire drivers enables high-resolution mapping of emissions for climate adaptation
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