VANCOUVER: The impact of wildfires on Canada’s drinking water may continue for years after the flames are extinguished, with delayed contamination posing growing challenges for water utilities and communities, according to a global review led by researchers at the University of British Columbia (UBC).
The study, which analysed 23 research papers covering 28 watersheds worldwide, found that wildfire related changes in water quality can intensify months or even years after a fire. Rainstorms, floods and snowmelt can wash accumulated ash, sediment and debris into rivers, increasing contamination risks long after the immediate danger has passed.
The findings are particularly significant for Canada, where wildfire activity has intensified in recent years. In 2023, wildfires burned more than 15 million hectares across the country, exceeding twice the previous national record.
Water Contamination Can Last for Years
The researchers examined long term changes in sediment, nutrients, metals, organic carbon, ions and chemicals associated with wildfire suppression. Their review focused on studies that monitored water quality for at least six months after fires.
“The same delayed contamination pattern kept appearing,” said Raúl de León Rábago, a study author and master’s student in civil engineering at UBC.
Evidence from Alberta illustrates the scale and duration of the problem. Following the 2016 Fort McMurray wildfire, rivers recorded elevated levels of sediment, nitrogen, phosphorus and lead, even in areas where less than one quarter of the watershed had burned.
The resulting changes in raw water quality increased treatment demands. The Regional Municipality of Wood Buffalo raised its annual spending on water treatment chemicals by approximately $500,000 to manage the wildfire-related impacts.
Long term effects were also observed after the 2003 Lost Creek wildfire in Alberta’s southern Rockies. Phosphorus and nitrogen levels remained elevated for years, while floods in 2013 washed stored ash and soil into rivers. Phosphorus concentrations surged to seven to nine times their earlier levels, with some increases persisting for more than 14 years downstream.
Explaining the phenomenon, Dr. Qingshi Tu, an assistant professor in UBC’s Faculty of Forestry and Environmental Stewardship, compared a burned watershed to a bathtub containing settled ash.
“When the water is stirred, the ash resurfaces,” he said, describing how storms and other disturbances can mobilise contaminants stored in the landscape.
Smoke and Firefighting Chemicals Add to the Risk
The review found that wildfires can increase sediment, nutrients, heavy metals and polycyclic aromatic hydrocarbons, a group of chemicals produced when vegetation and other materials burn.
Contamination is not necessarily restricted to areas directly affected by flames. Smoke can transport pollutants into watersheds that remain unburned, potentially extending the geographical reach of wildfire related water quality problems.
Firefighting chemicals can also contribute to the challenge. Long-term fire retardants, including PhosChek used in British Columbia and Alberta contain nutrients and trace metals that may encourage algal blooms and increase water treatment requirements.
These additional contaminants can place further pressure on water utilities already dealing with changes in the quality of their raw water supplies.
Smaller Communities Face Greater Challenges
The ability of water utilities to respond to wildfire related contamination depends on several factors, including fire intensity, duration, the size of the burned area, the materials consumed by the fire, weather conditions and the design of treatment systems.
Researchers warned that not all utilities have the same capacity to manage prolonged contamination. Smaller communities with limited financial resources may be particularly vulnerable when treatment systems require additional chemicals, monitoring or upgrades.
To help address these challenges, the research team is developing a model that connects wildfire behaviour, smoke movement and river systems. The aim is to help Canadian water utilities anticipate risks that may continue over multiple years and improve preparedness.
Researchers Call for Long Term Monitoring
The study highlights the need for coordinated, long term water quality monitoring and preparedness planning particularly in fire prone provinces such as British Columbia and Alberta, where wildfires can threaten drinking water sources.
Rather than focusing solely on immediate post fire assessments researchers say authorities should account for delayed contamination and the possibility that later storms or snowmelt could release pollutants stored in burned landscapes.
“Canada is entering a new era of wildfire risk,” said Dr. Loretta Li, the study’s senior author and a professor of civil engineering at UBC. “If we want to protect drinking water, we have to treat wildfire impacts as long-term, not short term.”
The findings underline a critical challenge for communities across Canada: protecting drinking water supplies requires sustained monitoring, effective treatment strategies and planning that extends well beyond the end of a wildfire.