Canada’s drinking water can remain at risk long after wildfires burn out, according to a new review that found water quality impacts persist years—even a decade—after a wildfire. The findings carry particular weight for Canada, where wildfire activity has intensified. For example, in 2023, over 15 million hectares burned—more than twice the previous national record.
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. 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 7 to 9x higher, with some increases persisting more than 14 years downstream. Similar long-term effects have been documented internationally.
For this review, 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.
Across all 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.
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 behavior, smoke and river systems to help Canadian utilities anticipate multi-year risks. The researchers 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 Loretta Li, senior study author and University of British Columbia civil engineering professor. “If we want to protect drinking water, we have to treat wildfire impacts as long-term, not short-term.”
Data from University of British Columbia