ES-03. Potential for soil contamination from fire at the wildland-urban interface: a case study of the 2021 Marshall Fire in Colorado

Abstract
The risk of wildfires at the wildland-urban interface (WUI) is expected to increase in the future due to expansion of residential development and elevated wildfire activity. However, most previous studies on the environmental impacts of fire events have focused exclusively on fires occurring in wildland areas. In contrast, the environmental consequences of wildfires at the WUI, including the potential for post-fire soil contamination, have not been well-characterized. The objective of this study was to determine the potential for soil contamination following the 2021 Marshall Fire—a particularly disastrous example of the types of wildfire events that can occur at the WUI—to better assess how wildfires at the WUI may uniquely impact soil quality. We sampled soils from burned and unburned residential properties and from non-residential sites (grasslands) that were within or adjacent to the perimeter of the 2021 Marshall Fire. We hypothesized that, given the presence of anthropogenic materials in residential areas, burned residential soils would have significantly higher concentrations of metals and polycyclic aromatic hydrocarbons (PAH) than unburned residential and non-residential soils. We found that burned residential properties had significantly higher soil concentrations of zinc, chromium, and copper as compared to unburned properties (p < 0.001). For the non-residential soils, there were no differences in heavy metals in the burned sites as compared to the unburned non-residential sites (p > 0.05). Although concentrations of toxic metals did increase in fire-impacted residential soils, the levels of contamination appear to be below thresholds of likely concern for human health. However, we cannot exclude the possibility that fire increased mobilization of metals and made them more bioavailable. Contrary to our initial hypothesis, PAH concentrations were not elevated in burned versus unburned residential sites (p > 0.05). Fire events at the WUI can uniquely impact soil quality and such impacts should be studied more extensively to guide policy decisions and safeguard the re-population of fire-impacted residential areas.