New research from the University of Oregon reveals that intense wildfires can transform a harmless form of chromium in some soils into a toxic contaminant that may linger in groundwater for years. The findings point to the need for expanded post-fire testing in fire-prone regions.
When wildfires roar through forests, the damage is obvious in the charred trees and smoky skies. Less visible is a chemical transformation that can quietly unfold in the soil beneath, turning a helpful nutrient into a dangerous contaminant that can seep into drinking water.
New research from the University of Oregon shows that intense wildfire heat can convert chromium-3, a common and generally harmless form of the element found in certain rocks and soils, into chromium-6, a carcinogenic form that can persist in groundwater for months or even years.
The study, published in the journal Environmental Science & Technology, focuses on serpentine-rich landscapes in southwestern Oregon, where naturally chromium-rich rocks sit in an area increasingly at risk for severe wildfires.
“In the Pacific Northwest, the number of fires and severity of them has been increasing,” lead author Chelsea Obeidy, a soil scientist who conducted the work as a doctoral student at the University of Oregon and is now at California State Polytechnic University, Humboldt, said in a news release. “We were motivated to figure out if there was any contaminant link there, if fires could be mobilizing contaminants of interest.”
Chromium-3 plays a role in human metabolism and is the predominant form of chromium in the environment. Chromium-6, by contrast, is a well-known industrial pollutant and a Class A carcinogen linked to lung, sinus and nasal cancers. The two forms are connected by oxidation, a chemical process that can occur slowly as rocks weather or rapidly under extreme heat.
Obeidy and her colleagues wanted to know how much wildfire heat it would take to drive that conversion in real-world soils, and whether the resulting chromium-6 could move into groundwater.
To find out, the team collected soil samples across Eight Dollar Mountain in the Rogue River-Siskiyou National Forest, a serpentine-rich hill laced with chromium-3 deposits. They sampled at different elevations, from the summit down the slope, to capture a range of soil conditions. Soils near the summit are more heavily weathered, meaning the rocks have broken down more and released more chromium-3 into the soil.
Back in the lab, the researchers subjected the soils to simulated wildfires. They heated the samples for two hours at temperatures ranging from 400 to 1,500 degrees Fahrenheit, then measured how much chromium-6 formed.
They found that the most intense conversion did not happen at the very highest temperatures, but in a specific range that real wildfires can reach. For soils from the summit and near the top of the slope, the largest amounts of chromium-6 appeared when samples were burned at around 750 degrees Fahrenheit. In less weathered soils lower on the slope, chromium-6 emerged mainly at higher temperatures, around 1,100 degrees.
The difference, the researchers say, comes down to how much chromium-3 is available in the soil and how the soil’s chemistry changes with weathering and heat.
To see whether the newly formed chromium-6 could actually move into water, the team ran a second experiment. They packed plastic columns with the burned soils and pumped rainwater through them for a week, simulating about half a year’s worth of rainfall trickling through the ground. The water that drained out was then tested for chromium-6.
The results suggested that, depending on where on the slope the soil came from and how hot it burned, chromium-6 levels in the leachate could exceed U.S. Environmental Protection Agency drinking water standards for six months to nearly two and a half years.
“This could have a lasting impact on a burned landscape,” Obeidy added. “Maybe we need to be sampling after burned environments in these certain rock types.”
The work also highlights how patchy and complex soil conditions can be, even within a single hillside.
“They change over really small spatial scales,” added senior author Matthew Polizzotto, an earth scientist and environmental chemist at the University of Oregon. “If we want to assess risks, we have to know the extent to which things might vary from place to place.”
That variability matters for land managers and communities trying to understand post-fire risks to water supplies. After major wildfires, the U.S. Forest Service already surveys burned areas for erosion hazards, flooding potential and other safety concerns. But chromium-6 is not currently on the list of contaminants they routinely check.
At the same time, scientists and agencies are beginning to pay more attention to heavy metals in post-fire environments. Elements such as manganese, lead and nickel can also concentrate in soils after fires and eventually reach streams, rivers and aquifers.
The new findings suggest that in landscapes underlain by chromium-rich rocks, like the serpentine formations in southwestern Oregon and similar geologies elsewhere, chromium-6 should be added to that list of concerns.
The study also offers a nuanced view of fire. While high-intensity wildfires appear capable of driving chromium-6 formation, lower-intensity burns — such as prescribed fires and cultural burns used by Indigenous communities to manage forests — did not seem to produce much chromium-6 in this experiment. Obeidy noted that this pattern needs further investigation, but it hints that carefully managed, cooler burns may reduce some chemical risks while still helping control fuel buildup.
For now, the researchers say, the work is an early step toward understanding how geology and fire behavior interact to shape water quality risks in a warming, drying climate.
“We’re really at the infancy of establishing all the things we need to know,” Polizzotto added.
Source: University of Oregon