A new study examining the aftermath of California’s Eaton and Palisades wildfires has found significant variations in potentially harmful contaminants left behind in residential soils and ash, highlighting the risks posed by urban wildfires and the benefits of large-scale clean-up efforts.
Researchers analysed hundreds of soil and ash samples collected from homes destroyed in the January 2025 fires, which swept through the Los Angeles communities of Altadena and Pacific Palisades. The blazes killed 31 people and destroyed more than 16,000 structures, the vast majority of them homes.
The study, published in Environmental Science and Technology Letters, found widely varying levels of lead and arsenic contamination, with the highest concentrations of lead detected in structural ash from some older properties.
Professor Daniel Richter, of Duke University’s Nicholas School of the Environment, who oversaw the research, said: ‘Homes often contain metals in structural materials and household products, and fires release metals in smoke but also concentrate metals in structural ash that can potentially contaminate residential soils.’
The project began while the fires were still burning. After speaking with a colleague in Los Angeles, Professor Richter connected with local garden designer Robin Jones, founder of Honey Girl Grows, who helped organise a team of volunteer homeowners to collect samples from more than 30 burned properties.
By March, the volunteers had gathered around 300 soil and ash samples, which were sent for analysis. The researchers discovered that lead levels were particularly high in the ash of some homes built before the 1970s, presumably because of the presence of lead-based paint used at the time.
The study also assessed the effectiveness of the extensive clean-up operation carried out by the US Environmental Protection Agency and the US Army Corps of Engineers. During June and July, the team collected more than 100 additional samples from 17 properties after remediation work had been completed.
Richter said: ‘Together the agencies removed nearly 2.4 million tons of burned and hazardous materials – a massive clean-up that targeted house foundations and structural ash where six inches of ash and soil were scraped for removal.’
The follow-up analysis showed that contaminant levels had fallen substantially in areas that underwent remediation, particularly at sites where lead and arsenic concentrations had initially been highest.
Richter added: ‘These data are especially important because they are some of the only comparisons of pre- and post-clean-up, as the USACE did no testing following these fires.’
The research also raises questions about differing standards for lead contamination in residential soil. California has its own threshold for lead levels, which is less than half the federal standard used by the US Environmental Protection Agency.
‘The two lead concentrations needlessly confuse the public, and they critically need a technical review,’ Richter said.
Beyond the immediate impact of the wildfires, the findings point to a broader issue affecting urban environments across the United States. According to the researchers, many cities still contain traces of historical metal contamination buried in their soils.
As Richter noted, quoting lead author Dossou: ‘Soils have long memories.’
The full research can be read here.
Photo: Jessica Christian
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