UCLA

Wildfires are increasing in frequency and severity across California, particularly in the wildland–urban interface, where they threaten not only structures but also environmental quality. Among the many toxic metals that can be generated during wildfires at wildland-urban interfaces, chromium is of concern because its hexavalent form [Cr(VI)] is soluble, mobile, and carcinogenic. This study aims to understand whether the 2025 Los Angeles wildfires have elevated Cr(VI) concentrations in residential soil and whether the debris and topsoil removal within the ash footprints lowered the chromium (VI) concentration. Soil samples were collected from burned homes, standing homes, and reference non-burned sites from two burned zones: Palisades Fire and Eaton Fire near Altadena. From burned properties, composite soil samples were collected from the area where debris and soil were scraped and from the surrounding perimeter area where soil was not remediated or removed. From standing homes, composite soil samples were collected from four locations: front yard, back yard, left side yard, right side yard. In some cases, two composite samples were collected, one from close to the standing home and one from far from the fence. Cr(VI) was extracted using phosphate buffer and analyzed using Inductively Coupled Plasma–Mass Spectrometry (ICP-MS). Data were analyzed in Python with nonparametric statistical tests to compare concentrations across site categories.Median Cr(VI) concentrations in both Altadena (0.22 mg/kg) and Palisades (0.12 mg/kg) were below the US EPA residential soil screening level of 0.95 mg/kg for carcinogens with a risk factor of 1 in one million. However, soils from some properties exceeded the more stringent California residential screening level of 0.3 mg/kg. These exceedances were observed in standing-home perimeters, burned-home perimeters, and even in some scraped soils. The results confirm that wildfires may have contributed to increased levels of Cr(VI). While scraping overall lowered Cr(VI) level, it did not result in lowering the level below the California health screening level in all properties. In general, Altadena soils contained higher Cr (VI) than Palisades, reflecting differences in recovery, washing off of Cr(VI) during ensuing rainfall events after the fires, and legacy contamination. Particle-size analysis revealed that fine fractions (<75 µm) were enriched in Cr (VI), underscoring their mobility and potential for inhalation exposure.Overall, this study demonstrates that Los Angeles wildfires elevated Cr (VI) levels in residential soils, which exceeded health-based thresholds under California guidelines in some properties. The debris removal or soil clean-up practices lowered the level, but they still left some properties with an elevated level of Cr (VI), which may pose a long-term health risk. These findings provide a scientific basis for targeted soil monitoring and site-specific remediation based on site history and terrain so that rebuilding efforts can protect public health in fire-impacted communities.