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."
