GENERATION AND MOBILIZATION OF WILDFIRE-RELATED POLYCYCLIC AROMATIC HYDROCARBONS IN A SOUTHERN CALIFORNIA COASTAL WATERSHED

As wildfires become more prevalent and destructive, it is imperative to understand the impacts they have on the watersheds they burn. One particularly understudied aspect of wildfire-associated water quality impairment is the generation of polycyclic aromatic hydrocarbons (PAHs), a class of organic contaminants with carcinogenic, mutagenic, and ecotoxic properties. As a case study, we investigate PAH impacts associated with the 2018 Woolsey Fire, which burned over half of the Malibu Creek Watershed near Los Angeles, California. In addition to generating PAHs, the fire also increased erosion rates in the burned watershed. We collected soil and water samples periodically and during rain events over close to three years following the fire. Sampling sites were distributed through the watershed to incorporate samples from all major tributaries of Malibu Creek and to capture varying topography, geology, land use, and fire intensity. We found PAH concentrations exceeding EPA Ambient Water Quality Criteria, primarily during rain events, through the second wet season after the fire. Using molecular ratio approaches, the PAHs detected in water samples were directly linked to burned soil. Elevated PAH concentrations were associated with suspended particulate matter, which was in turn directly related to stream discharge and precipitation intensity. The first dry season following the fire showed low PAH concentrations, but higher concentrations returned at several sites during the second wet season. Questions remain as to why significant geographic variability was observed during the second wet season as wells as how long these PAH “fire signals” will persist. Understanding these complex mechanisms is critical to predicting long-term wildfire impacts to water quality in diverse settings.