CU Boulder study finds minor toxic soil levels following Marshall Fire

Mar. 4—Concentrations of toxic metals at properties burned in the Marshall Fire were measured at slightly elevated levels compared to unburned properties, according to a University of Colorado Boulder study. However, the elevated concentrations were minor and well below thresholds of concern established by the Environmental Protection Agency.

"Given the extent and severity of the Marshall Fire, it is reassuring that soil contamination is not one of the problems residents or city officials need to worry about," Sierra Jech, CU Boulder doctoral student and co-lead author of the study, said in a release.

CU Boulder Professor Noah Fierer said he hopes the study gives people some "peace of mind" and data that is useful.

"We did the study because people were worried," Fierer said. "People who had their properties burned or people who had homes near the burned area were worried whether their soils were contaminated. They were worried about after the fire, could their kids play around in the backyard in the soil? Could they grow food in their backyard and have a garden? Could their pets be on their properties, or was there some sort of risk?"

The researchers sampled soil from 58 properties, half in the burned area and half outside the fire perimeter. They collected multiple soil samples from each property, working with volunteer property owners in Boulder County who allowed them to do soil sampling.

The researchers, part of CU Boulder's Cooperative Institute for Research in Environmental Sciences, worked with a team at Colorado State University that completed a lot of the soil analyses for the project.

"Studies like this one are important to share broadly because it would be very easy for residents to be fearful or concerned about soil contamination when rebuilding on their properties, in the absence of data," CIRES Fellow Eve-Lyn Hinckley said in the release.

Soils in properties within the burned area had higher concentrations of copper, zinc, lead and chromium, which are typically considered toxic. Fierer said this happened because the fire didn't just burn grass, trees and shrubs. It burned houses, cars, batteries, installation, wiring and more which released those metals into the environment.

The team also collected soil from open space that was burned in the fire containing mostly grassland and no homes. Fierer said they wanted to know if they saw the same pattern of elevated metals, and they did not.

"These fires that occur in residential areas are really different because they're partly or largely fueled by structures burning, so there's a lot more potential for soil contamination than in a wildfire," he said.

Fierer said the team can't say there's no concern, but based on EPA and other estimates of what concentrations of metals in soil are cause for concern, it was well below the threshold.

"Science can never say there's absolutely no risk, but hopefully we provided some useful data to people in the area," he said.

For more information, visit marshallsoils.colorado.edu/.

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