Canada’s drinking water can remain vulnerable to contamination for months or even years after wildfires have been extinguished, according to a global review led by researchers at the University of British Columbia (UBC). The study found that water quality problems can intensify long after a fire, particularly when heavy rainfall or snowmelt washes accumulated ash and debris into rivers.
Researchers examined 23 studies covering 28 watersheds worldwide, comparing water quality before and after wildfires. They assessed changes in sediment, nutrients, metals, organic carbon, ions and chemicals used to fight fires. The findings showed that contamination does not always peak immediately after a wildfire and can worsen over time.
The research is particularly relevant to 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.
The review focused on studies that monitored water quality for at least six months after wildfires to determine whether contamination declined or persisted. Researchers repeatedly observed delayed increases in pollutants entering waterways.
After the 2016 Fort McMurray wildfire in Alberta, rivers recorded elevated levels of sediment, nitrogen, phosphorus and lead, even in areas where less than a quarter of the watershed had burned. The Regional Municipality of Wood Buffalo subsequently increased its annual spending on water-treatment chemicals by approximately $500,000 to address changes in raw water quality.
Long term effects were also documented following the 2003 Lost Creek wildfire in Alberta’s southern Rocky Mountains. Elevated nitrogen and phosphorus levels persisted for years. Flooding in 2013 washed stored ash and soil into rivers, causing phosphorus concentrations to rise to seven to nine times their previous levels. Some increases continued more than 14 years downstream.
Qingshi Tu, an assistant professor in UBC’s Faculty of Forestry and Environmental Stewardship compared the process to ash settling in a bathtub and resurfacing when the water is disturbed. Similarly, ash and other materials can remain stored in watersheds after a wildfire before being carried into rivers by later weather events.
Smoke and Firefighting Chemicals Add to the Risk
The studies found that wildfires can increase sediment, nutrients, heavy metals and polycyclic aromatic hydrocarbons, which form when vegetation and other materials burn. Smoke can also transport contaminants into watersheds that were not directly affected by flames.
Firefighting chemicals present an additional challenge. Canada relies on long term fire retardants, including Phos Chek in British Columbia and Alberta. These products contain nutrients and trace metals that can contribute to algal blooms and increase the cost of treating drinking water.
Following the Fort McMurray wildfire water treatment facilities needed higher doses of chemicals to manage wildfire-related changes in the affected water. Researchers warn that such costs can continue long after emergency response operations have ended.
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.
Not all communities have the same capacity to adapt. Smaller municipalities with limited budgets and treatment infrastructure may be particularly vulnerable when contamination persists for extended periods.
To help utilities prepare the research team is developing a model that links wildfire behaviour, smoke and river systems. The aim is to help Canadian water providers anticipate risks that may continue for several years after a fire.
Researchers are also calling for coordinated, long term water quality monitoring and stronger preparedness planning in wildfire prone provinces such as British Columbia and Alberta, particularly in areas where fires burn near drinking water sources.
Loretta Li, a UBC Civil Engineering professor and the study’s senior author, said Canada is entering a new era of wildfire risk and that protecting drinking water requires treating wildfire impacts as a long term challenge rather than a short term emergency.