Wildfires Are Contaminating Water Supplies

If you stood on the banks of the Cache la Poudre River in Colorado after the 2020 Cameron Peak Fire, the rumbling water may have appeared black. This slurry of ash and charred soil cascaded toward the reservoirs that supply drinking water for the downstream city of Fort Collins, home to around 170,000 people. Although the water looked clear again several weeks later, Charles Rhoades, a research biogeochemist at the US Forest Service Rocky Mountain Research Station, says he is still seeing contaminants from the fire in the watershed.

Recent studies have found that while some watersheds begin to recover within five years of a fire, others may be fundamentally altered, never fully returning to their pre-fire conditions. And with wildfires becoming more common, much larger, and burning for longer as the world warms, hydrologists, ecologists, and water-management officials are scrambling to understand and mitigate the consequences fire-contaminated water can have on humans and ecosystems.

In a healthy forest, there’s a lot of “litter” on the ground—pine needles, dead leaves, debris. “It acts like a sponge,” says Rhoades. “As rainfall comes in, it moves through that layer slowly and can trickle into the soil.” When fires scorch the land, they burn that vegetation and organic matter, leaving behind a bare landscape that’s highly susceptible to erosion. Instead of filtering into the ground, rain will slide right off the surface, moving quickly, picking up soil, and carrying it into streams and rivers. Not only does this cause sediment build-up, but it can disrupt the water chemistry. Rhoades found elevated levels of nutrients, like nitrogen, in rivers almost 15 years after a high-severity fire. These nutrients can lead to harmful algal blooms, although they don’t directly impact drinking water quality. But other sites show increased levels of heavy metals like manganese, iron, and even lead after a major fire, which can complicate water-treatment processes.

Other regions across the western US, like Taos, New Mexico, and Santa Cruz, California, have faced similar issues, as wildfires increase in frequency and duration due to climate change and decades of fire-suppression practices. For much of the 20th century, the US Forest Service and other land management agencies aimed to keep all fires from burning, believing it was the best way to protect forests. But naturally occuring, low-severity fires improve forest health, preventing the accumulation of dense underbrush and dead trees that act as fuel.

“We have this huge buildup of fuel on the landscape from 140 years of fire suppression, and we know that the consequences of that—combined with increases in severe weather—make the likelihood of really intense fire behavior much higher than it used to be,” says Alissa Cordner, an environmental sociologist and professor at Whitman College in Washington state and volunteer wildland firefighter. “We also have more and more people living next to forests and migrating to places in the wildland-urban interface.” Any municipality is at risk of water contamination if a wildfire burns through its watershed.

“Consumers rarely know about all this stuff that’s going on under the hood,” says Rhoades. After a wildfire, water providers work tirelessly to ensure residents don’t experience the effects in their taps, which requires collaboration between land agencies, like the Forest Service, USGS, and local governing bodies. They perform regular water testing, install sediment-control structures, and sometimes, alter water treatment protocols to deal with the increased load of contaminants.

Sediment is one of the biggest concerns for utilities managers, as it clogs up filtration systems that prepare water for treatment. “We have to slow down the treatment process for sediment to drop out,” says Alison Witheridge, watershed planning manager at Denver Water. “It takes more energy and more chemicals to treat the water.” When there’s too much sediment, water-treatment plants can’t take that water, says Rhoades.

After the 2012 Hayman Fire, the Poudre river had so much sediment that the water provider needed to temporarily switch to an alternative water supply. Carbon, another wildfire contaminant, can cause serious problems for water managers, too. “When you chlorinate that water in a treatment plant, you can create some carcinogenic compounds,” says Rhoades. “So if you get big pulses of carbon going into treatment plants, especially ones that aren’t used to that, they have problems.”

Municipalities with simple water-treatment plants that pull water from one source may be at the greatest risk. “If you rely on a really clean water supply, your treatment plant is not really super extensive because you don’t need it, right?” says Sheila Murphy, research hydrologist for the USGS. “So some watersheds, if there ever is a huge impact, they’re not set up for that. Now that we’re getting more fires in areas that aren’t used to fires, that’s a big challenge for water providers.”

But keeping the watershed clean is only one piece of the puzzle. If a fire damages infrastructure, cities may have to deal with potential chemical contamination from burnt pipes. When the Marshall Fire swept through Louisville, Colorado, in 2021, it left hundreds of homes and buildings burned. The city knew about the risks of burned infrastructure from the experiences in fire-impacted communities like Paradise, California, so they initiated a large water-sampling campaign to test for volatile organic compounds (VOCs) and semi-volatile organic compounds (SVOCs), two common fire-related contaminants and carcinogens.

“This systematic testing and flushing allowed the city to clear neighborhood by neighborhood, block by block, and house by house,” says Cory Peterson, deputy director of utilities for the City of Louisville. “The water-distribution system was able to come back relatively quickly, but we also wanted to make sure the individual service lines to homes were safe.” They continued to test throughout the home-rebuild process and created an interactive map for the community to see their property’s water-quality status.

While cities now have many tools and extensive knowledge to adequately respond in an emergency, “the long-term effects that these ecosystem shifts have on our water supply is sort of unknown and underexplored,” says Matt Ross, an ecosystem scientist and assistant professor at Colorado State University.

Proper response and rehabilitation is tricky because it depends on the region’s terrain, existing water supply, and infrastructure. The USGS is currently performing research in California, Oregon, New Mexico, Montana, and Colorado to see how each region’s watershed responds. “Some places may be more resilient to the effects of fire, whereas in some areas where you have extreme storms, such as New Mexico, we often see a very large sediment response that can cause a lot of damage and fill reservoirs,” says Murphy.

In addition to geology and precipitation, there’s also land use to consider. “Are there legacy mines in the region or is there agricultural use? Are there urban-wildland interfaces? All those factors will affect what you see in the watershed,” she says. Once their research is complete, they hope to give water providers a better idea of what the potential risks are for a particular region so cities can better prepare.

Data collection and monitoring are also essential, says Ross, who has been working to install remote sensors in streams to measure water quality in real time. These sensors are increasingly common and capture basic parameters like dissolved oxygen, acidity, and suspended sediment. “It’s really helpful to know what’s coming into the system and can provide an early warning,” says Witheridge.

There’s also the long-term work that extends years after the fire itself. Land managers may plant trees to stabilize soils or establish vegetation closer to streams and restore watershed function, says Rhoades. And there’s work on the front end, too, like tree thinning and forest clean-up to prevent fires—or at least keep them from reaching high severity.

Overall, there is a shift toward a more holistic approach to fire management. “We’re thinking about the impacts of fire not just in how many acres burned or how many structures destroyed, but what happens to watersheds or what happens to the health of people in smoky communities a thousand miles away,” says Cordner. “All of those ripple effects do raise a sense of urgency for us to do as much mitigation work as we can.”