Wildfires Threaten Water Quality for Up to Eight Years After They Burn

Water quality can remain compromised for nearly a decade following wildfires, according to a new study published in Nature Communications Earth & Environment.

Researchers from the Cooperative Institute for Research in Environmental Sciences (CIRES) at the University of Colorado Boulder analyzed over 100,000 water samples from 500 sites, making this the most comprehensive post-wildfire water quality study to date.

“We were attempting to look at notable trends in post-wildfire water quality across the entire U.S. West, to help inform water management strategies in preparing for wildfire effects,” said Carli Brucker, lead author and former University of Colorado Boulder and Western Water Assessment PhD student. 

“There’s been a lot of work, for example, in the National Climate Assessment and the International Panel on Climate Change talking about changes in global water supply,” added CIRES Fellow and Western Water Assessment Director, Ben Livneh. “But those assessments point to this gap in water quality assessments in a continental scale context, whereas people like me in physical hydrology have been thinking about the continental scale challenges for a while.” 

Prolonged contamination in fire-affected watersheds

The team investigated watersheds in burned and unburned regions, assessing concentrations of organic carbon, nitrogen, phosphorus, sediment and turbidity.

Their models revealed elevated levels of these contaminants in burned basins compared with unburned controls. While most impacts were strongest in the first five years post-fire, some contaminants, such as nitrogen and sediment, remained elevated for up to eight years.

This extended timeline challenges previous studies, which typically reported shorter recovery periods. The findings indicate that recovery of watershed health after wildfire events is a slow and variable process, influenced by a range of factors including fire proximity to rivers, regional soil composition and vegetation types.

Forest density intensifies water quality impacts

Fires in more heavily forested areas were associated with greater increases in water contaminants. Organic carbon, phosphorus and turbidity showed the most notable spikes within five years of fire exposure. In contrast, nitrogen and sediment levels often remained heightened beyond the five-year mark.

The study also highlighted the role of extreme weather events in contaminant transport. In some basins, pollutants remained dormant until a significant storm event triggered their movement into nearby water systems.

Variability complicates recovery predictions

Each watershed in the study responded differently to wildfire impacts. The team believes that this variation is likely due to where the fire struck — a fire closer to the river would bring worse impacts than an upstream fire. Different soils, vegetation and weather also change the impact in each watershed, making it difficult to plan for the future.

“There's a huge amount of variability in sedimentation rates,” said Brucker, who now works as a consultant. “Some streams are completely clear of sediment after wildfires, and some have 2000 times the amount of sediment.” 

“I'm hoping that providing concrete numbers is very impactful to water managers,” Brucker said. “You can’t fund resilience improvements on general concerns alone. Water managers need real numbers for planning, and that’s what we’re providing."

Reference: Brucker CP, Livneh B, Rosario-Ortiz FL, et al. Wildfires drive multi-year water quality degradation over the western United States. Commun Earth Environ. 2025;6(1):1-9. doi: 10.1038/s43247-025-02427-6

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