PERSISTENCE AND TRANSPORT MECHANISMS OF WILDFIRE-RELATED POLYCYCLIC AROMATIC HYDROCARBONS IN A SOUTHERN CALIFORNIA COASTAL WATERSHED

As climate change causes wildfires to become more numerous and destructive, it is necessary to understand the impacts they have on burned watersheds and the fate of wildfire-produced contaminants. Here, we focus on wildfire-associated water quality impairment by assessing the generation of polycyclic aromatic hydrocarbons (PAHs), a class of organic contaminants with carcinogenic, mutagenic, and ecotoxic properties. We investigated PAH impacts associated with the 2018 Woolsey Fire, which burned over half of the Malibu Creek Watershed near Los Angeles, California. We collected soil and water samples periodically and during rain events over three years following the fire. Sampling sites were distributed throughout the watershed to incorporate all major tributaries of Malibu Creek, and to capture varying topography, geology, land use, and fire intensity. The PAH concentrations found exceeded 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 linked with suspended particulate matter, which was in turn directly related to stream discharge and precipitation intensity. Significant geographic variability was observed during the second wet season, suggesting topography, burn intensity, and other factors play a role in post-fire recovery and the persistence of fire-derived PAHs in the watershed. We investigated the contribution of these factors through a combination of long-term soil and water sampling, assessment of suspended particulate material, analysis of the variation and evolution of PAH compound distributions, and evaluation of geographic controls. Results suggest that a combination of multiple geographic factors contribute to the observed data. Understanding these complex mechanisms is imperative to predicting long-term wildfire impacts on water quality in a variety of settings.