Researchers at the University of Colorado Boulder found that wildfires in Colorado and throughout the western United States can dramatically alter the water quality of rivers and streams up to eight years after a fire.

Results of a new CU Boulder study show high levels of contaminants such as organic carbon, phosphorus and nitrogen can remain in water for years after a fire.

“We don’t really know too much as a field about the lingering effects of wildfires on water quality,” said Carli Brucker, lead author and former CU Boulder doctoral student. “One surprising thing was definitely seeing that some of these contaminants had significantly elevated concentrations even eight years after the wildfires.”

The researchers from CU Boulder’s Cooperative Institute for Research in Environmental Sciences collected more than 100,000 water samples from 500 watershed sites in Colorado and across the western United States. About half of the watersheds where samples were collected were places where a wildfire had already burned, and the other half were untouched by a wildfire.

CU Boulder professor, CIRES fellow and study co-author Ben Livneh said the team sampled up and down the Front Range, including in the Poudre River.

“We were really wanting to know what the impact of wildfires on our water quality and typically how long does that impact last,” Livneh said.

Livneh said prior studies have shown that water quality changes in the immediate aftermath of a fire, but not a lot is known about the impacts on water quality multiple years after a fire. Additionally, those studies looked at specific locations rather than across state lines. Livneh said a big part of their goal was to compile a large data set.

“We knew there might be more to this story,” Livneh said. “There’s a mystery of how long does it last.”

Impacts to water quality, the study found, were worse in more forested areas and in areas where the fire burned closer to the main stem of the river.

Overall, the results showed that wildfires on average impact deeper layers of soil, burn larger vegetative structures and disrupt nutrient cycles to a greater extent than previously suspected, according to the study. This can have negative implications for people’s drinking water and freshwater systems.

Livneh said this is important for water suppliers and utility companies to know so they can better prepare and respond to wildfires. He added that organizations may reconsider their type of water treatment plan to use and whether water treatment needs to continue further into the future than a year or two.

“The biggest implications of these wildfires are for our water utilities,” Brucker said. “For our water utilities, the ones that I’ve talked to, this is already very much a priority … especially around Boulder and other wildfire-prone areas. I think my advice to community members in Boulder is to really just listen to your local water utilities. They have a lot of plans in place to deal with these impacts, and they’re going to ensure we’re always safe from these contaminants.”