Montana State professor studies how wildfires can threaten drinking water quality

BOZEMAN - When a wildfire raged near Bozeman in the Bridger Mountains in 2020, destroying 28 homes and scorching more than 8,000 acres of forested foothills, one residual side effect was largely avoided, thanks to the city having groundwater and surface water sources in different mountain ranges. This limited the negative impact that post-fire runoff containing contaminants such as ash, soil, and organic matter could have had on Bozeman's drinking water.

"If a community is relying on a surface water source, or they only have one drinking water supply, they may not have as much flexibility with their drinking water treatment plant operations to deal with emergency response following a fire," said Amanda Hohner, assistant professor of civil engineering in Montana State University's Norm Asbjornson College of Engineering. "We're fortunate in Bozeman; we have multiple water sources, and the watershed is not highly developed. So, we're pretty lucky here, since there's not a lot of human or agricultural impacts on the water quality."

Hohner is head of MSU's Water Quality and Drinking Water Treatment Lab, where she focuses much of her work on how wildfires can adversely affect drinking water quality. Two doctoral students, one master's student and one undergraduate student join Hohner in the lab. Much of the work is funded by grants from the U.S. Forest Service and the National Science Foundation.

"As a former water utility director, I understand firsthand the risks that wildfires pose to community water systems," said Craig Woolard, head of the Department of Civil Engineering in MSU's Norm Asbjornson College of Engineering. "Dr. Hohner's research provides critical insights that help water utilities manage this ever-increasing threat to their water supplies."

When wildfires occur, especially in mountainous regions like western Montana, much of the groundcover that would otherwise absorb and partially filter water runoff can be destroyed, allowing contaminants to be carried into surface water sources such as streams, rivers and lakes, Hohner said. These contaminants can overwhelm drinking water treatment plants.

"Those contaminants can be naturally occurring organic matter and particles that drinking water treatment plants have to remove from the water," Hohner said. "So, if a water system has a reservoir and there's a big increase in sediment and nutrients, that can lead to algal blooms, which can also enter into treatment plants."

The impact can be catastrophic. In August 2022, Las Vegas, New Mexico, faced a major water crisis after the largest wildfire in state history scorched more than 531 square miles - an area nearly half the size of Rhode Island. Heavy summer rains then washed ash, debris and other contaminants into the Gallinas River, the city's primary water source. With just 30 days of clean water remaining, the city declared a state of emergency while officials scrambled to install filtration systems and secure alternative supplies. The crisis, which lasted months, highlighted the long-term dangers wildfires can pose to water infrastructure, especially in mountain communities dependent on vulnerable watersheds downstream from burn scars.

"Some areas can experience short-term problems related to wildfires impacting drinking water treatment facilities," Hohner said. "But sometimes, the impact can be felt much longer. There's a study out of Colorado that showed water quality effects of a fire from the early 2000s, 15 years later."

Even if the direct effects aren't necessarily large in scale, smaller communities might not have the resources to keep the sediment, ash and contaminants out of the treatment plants, she said.

"Plants will need to adjust their operations by increasing their measures to better remove the particles and organic carbon," Hohner said. "All of that can clog filters more frequently, so they'll have to clean their filters more regularly."

Hohner first became interested in how wildfires can affect the quality of drinking water when she was a doctoral student at the University of Colorado Boulder.

"I was in graduate school when a wildfire moved through the area of the source water supply for the city of Fort Collins [Colorado]," she said. "I've always lived out West, where we have these beautiful watersheds that we rely upon for drinking water supply. I like to help communities have safe drinking water."