The impact of wildfires on drinking water can continue for months or even years after the flames have been extinguished, according to a global review led by researchers at the University of British Columbia (UBC). The study warns that ash, sediment, nutrients, metals and chemicals can enter rivers long after a fire, creating prolonged challenges for water supplies and treatment facilities.
Researchers examined 23 scientific studies covering 28 watersheds worldwide comparing water quality before and after wildfires. They analysed changes in sediment, nutrients, heavy metals, organic carbon, dissolved ions and chemicals used in firefighting.
The findings revealed that contamination does not always peak immediately after a wildfire. Instead, it can intensify over time, particularly when heavy rainfall, storms or snowmelt wash accumulated ash and debris into rivers.
The warning is especially significant for Canada, where wildfire activity has increased sharply. In 2023, more than 15 million hectares of land burned across the country, exceeding twice the previous national record.
Alberta Wildfires Reveal Long Term Water Contamination
The review examined studies that monitored water quality for at least six months after wildfires to understand whether contamination gradually declined or persisted.
Following the 2016 Fort McMurray wildfire in Alberta, rivers recorded elevated levels of sediment, nitrogen, phosphorus and lead. These changes were observed even in areas where less than one quarter of the watershed had burned.
The contamination also increased water treatment expenses. The Regional Municipality of Wood Buffalo reportedly spent approximately $500,000 more annually on treatment chemicals to manage changes in raw water quality.
Long lasting effects were also documented after the 2003 Lost Creek wildfire in Alberta’s southern Rocky Mountains. Phosphorus and nitrogen levels remained elevated for years. Flooding in 2013 washed previously deposited ash and soil back into rivers, pushing phosphorus concentrations to seven to nine times their earlier levels. Some elevated levels continued downstream for more than 14 years.
Researchers compared the process to ash settling at the bottom of a bathtub and resurfacing when the water is disturbed. In the same way, stored wildfire debris can be carried into waterways long after the original fire.
Smoke and Firefighting Chemicals Add to Water Risks
The review found that wildfires can increase concentrations of sediment, nutrients, heavy metals and polycyclic aromatic hydrocarbons (PAHs), which are produced when vegetation and other materials burn.
Smoke can also transport contaminants into watersheds that have not been directly affected by flames, expanding the potential area of water quality impacts.
Firefighting chemicals present another concern. Canada uses long term fire retardants, including Phos Chek in British Columbia and Alberta. These products contain nutrients and trace metals that may contribute to algal blooms and increase the complexity and cost of drinking-water treatment.
Following the Fort McMurray wildfire, water treatment facilities had to increase chemical dosing to manage the affected water.
Researchers emphasise that the severity and duration of contamination depend on several factors, including fire intensity, the amount of land burned, vegetation, weather conditions and the design of local water treatment systems.
While some utilities can adjust their treatment processes, smaller communities with limited financial and technical resources may struggle to manage prolonged changes in water quality.
The research team is developing a model that connects wildfire behaviour, smoke movement and river systems. The goal is to help Canadian water utilities anticipate risks that may continue for several years after major fires.
Scientists are calling for coordinated, long term water monitoring and stronger preparedness plans particularly in wildfire prone provinces such as British Columbia and Alberta.
The study highlights the need to treat wildfire related water contamination as a long term environmental challenge rather than a temporary consequence of burning forests.
Researchers say monitoring must continue well beyond the immediate aftermath of a fire, as rainfall and snowmelt can release stored contaminants into rivers long after emergency response operations have ended.
As Canada faces growing wildfire risks, protecting drinking water sources will require closer coordination between researchers, water utilities and local authorities.
The findings reinforce a critical message: extinguishing a wildfire does not necessarily end its environmental impact, particularly for the rivers and communities that depend on them for clean drinking water.