Transport Mechanisms and Persistence of Wildfire-related Polycyclic
Aromatic Hydrocarbons in a Southern California Coastal Watershed
Abstract
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. We collected soil
and water samples periodically and during rain events over 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. However, 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 an interplay of multiple
geographic factors contribute to the observed variations. Understanding
these complex mechanisms is critical to predicting long-term wildfire
impacts to water quality in diverse settings.