Soil sampled from the yards of homes burned in the Marshall fire does not contain high levels of toxins that would threaten human health for those rebuilding there, according to the first major study of soil hazards after the devastating fire in late 2021.
The study led by the Cooperative Institute for Research In Environmental Sciences at the University of Colorado did find slightly elevated levels of some metals such as zinc, chromium and copper, but “well below the estimated thresholds of concern.” The study appeared in the February edition of the academic journal “Environmental Science and Technology.”
The researchers, from CU and Colorado State University, felt personally compelled to carry out helpful science where they live, answering urgent questions from neighbors, said CU soil ecology professor Noah Fierer.
The impetus for the detailed study “was actually pretty simple,” Fierer said. “People were worried. Obviously, the Marshall fire was devastating. Could their kids and pets run around in their backyard? Could they grow gardens in their backyard? There was clearly a need.”
Frankly, Fierer said, the researchers were prepared to find worse, knowing how many potential toxins are contained in batteries, car engines, household paint and cleaning fluids, insulation and more. The results should be reassuring, said Fierer, who lives just a few miles from the Marshall burn area that killed two people, destroyed more than 1,000 homes and damaged $2 billion in property.
“I’m confident in saying that people living in burnt areas shouldn’t be worried about metal contamination in their backyards,” he said.
Soil results from the Marshall fire can’t be universally applied to other wildland urban interface areas, such as Maui or other high-profile catastrophes, because of differences in materials burned and the speed and duration of flames, the researchers said. But the CIRES study does point the way on how soil samples can be taken. It also provides new information on wildfire impact on suburban open space, since Marshall fire homesite samples were compared with unburned residential areas and surrounding public lands, Fierer said.
How the research was conducted
The researchers found Marshall-area homeowners eager to cooperate and gathered multiple samples from each of 58 sites. The sites included fully burned homes, unburned residential areas and natural grasslands in the surrounding open space, to help determine background levels of any metals.
About four soil samples were taken from any one site, Fierer said.
In addition to looking for toxic metals, the study said “the authors expected to find elevated levels of polycyclic aromatic hydrocarbons (PAHs), a group of often toxic chemicals as observed following wildland fires.” No such elevated levels were found.
“I wouldn’t feel comfortable using the results from this study to make any claims about potential soil contamination at another site,” Fierer said. “But now we at least have a protocol in place for investigating that and answering this question and next time.”
And with climate change and the growth of suburbs into more wildland urban interface areas across the United States, more fires are sure to come, researchers said.
Results from the open space soil samples should also prove interesting to researchers studying the impact of wildfires, Fierer added. No surprising levels of toxins were found in burned open space areas, but samples did include elevated levels of potassium, for example.
“So we do see changes in the soils with burning versus not burning in the open space, but what we don’t see is the elevated metals,” Fierer said. Heavy metals can bind to organ cells and block their proper functions.
In assessing the potential health impacts, the researchers assumed humans would be exposed primarily on their skin, through children playing in yards or wind-blown dust. The researchers call for more study of how the sampled levels of toxins would impact humans if ingested, for example, by eating garden vegetables grown in affected soil.
Also, the study concludes, “we sampled soils four months after the fire event, making it possible that contaminants were transported away from surface soils by wind, surface water runoff, postfire debris removal, or movement of contaminants deeper into the soil profile with melting snow or rainfall.”
“Every fire,” Fierer said, “is quite different.”