Wildfires can turn harmless minerals in soils into contaminants

EUGENE, Ore. — Dec. 15, 2025 — In the wake of wildfire, a vital micronutrient can become a toxic heavy metal — and could eventually make its way into groundwater.

New research from the University of Oregon breaks down how chromium converts from a benign form in rocks and soil to a carcinogenic one in the presence of extreme heat during wildfires.

Simulated wildfire experiments done on soils naturally rich in chromium revealed that fires reaching 750 to 1,100 degrees Fahrenheit created the highest amount of the harmful contaminant. The location of soil — a summit or valley, for instance — influenced what temperature caused the most conversion.

The research, published Nov. 25 in the journal Environmental Science & Technology, underscores the need for a better understanding of how fires influence pollutants in the environment. It also suggests potential value in broader testing for soil contaminants that could leach into groundwater after a wildfire.

“In the Pacific Northwest, the number of fires and severity of them has been increasing,” said study lead author Chelsea Obeidy, a soil scientist now at California State Polytechnic University, Humboldt. “We were motivated to figure out if there was any contaminant link there, if fires could be mobilizing contaminants of interest.”

She led the study as a doctoral student in the lab of professor Matthew Polizzotto, an earth scientist and environmental chemist at the UO.    

The predominant form of environmental chromium is chromium 3, an element that supports metabolic function in humans. But chromium 6, often a byproduct of industrial processes, is a Class A carcinogen linked to lung, sinus and nasal cancer. Oxidation converts chromium 3 to chromium 6, a process that can happen over time as rocks weather away or are exposed to extreme heat.

Obeidy knew that there were serpentinite rocks rich in chromium 3 in southwestern Oregon, an area that is increasingly at risk for wildfires. But what wasn’t clear was to what degree wildfires would transform that chromium into something toxic — or if it would end up in the groundwater.

So Obeidy and her colleagues collected soil samples across Eight Dollar Mountain, a hill in the Rogue River-Siskiyou National Forest riddled with chromium 3 deposits. They sampled a variety of elevations to capture a range of soil weathering, with more weathering found near the summit. Then, they brought the samples back to the lab and burned them for two hours at temperatures between 400 and 1,500 F.

To recreate the effect of leaching from rainwater, the team also packed plastic columns with the burned soil and pumped rainwater through them for a week. That represented around half of a year’s worth of rain trickling through. They collected the water that drained out and analyzed it for chromium 6 to see which locations on the hill could affect groundwater.

Polizzotto noted that soils tend to be very variable. “They change over really small spatial scales,” he said. “If we want to assess risks, we have to know the extent to which things might vary from place to place.”

Soils from at and near the summit contained the most chromium 6 when burned around 750 F. Because summits experience the most weathering, the rocks break down further and release more chromium 3 into the soil, leaving more available that can be converted to chromium 6 under the right conditions.

Closer to the bottom of the slope, chromium 6 emerged at higher temperatures, around 1,100 F. Wildfires can vary in temperature, but they can fall in that range. Lower-intensity temperatures, such as those found in prescribed or cultural burns, didn’t seem to create much chromium 6. But that needs to be investigated further, Obeidy said.

Depending on the slope position the soil came from, chromium 6 could taint groundwater above EPA standards for six months to almost 2.5 years.

“This could have a lasting impact on a burned landscape,” Obeidy said. “Maybe we need to be sampling after burned environments in these certain rock types.”

After wildfires, the U.S. Forest Service assesses environments for things such as erosion risk and human safety issues. But chromium 6 isn’t a contaminant they currently look for.

There’s already a push to look at other heavy metals in post-fire environments; elements like manganese, lead and nickel can end up in soil after fires too, eventually seeping into water sources. That means having an array of metal testing after wildfires could provide valuable information, Obeidy said. The types and levels of contaminants in a burned area will vary, particularly in landscapes that are human-influenced.

“We're really at the infancy of establishing all the things we need to know,” Polizzotto said. 

— By Jude Coleman

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