LONG-TERM WILDFIRE-RELATED CHEMICAL IMPACTS ON WATER QUALITY WITHIN THE MALIBU CREEK COASTAL WATERSHED

There has been a significant increase in the number and severity of wildfires in California within the last decade, and there is an expectation that climate change will further exacerbate their future occurrence, spatial distribution, and intensity. In addition to the loss of habitat and vegetation, wildfires can affect water quality in watersheds through physical and soil geochemical changes such as hydrophobicity, increased erosion, and generation or mobilization of harmful chemical compounds. Incomplete combustion of organic material during fires produces polycyclic aromatic hydrocarbons (PAHs), a class of organic compounds with known carcinogenic and mutagenic properties. The post-fire increase in erosion mobilizes particle-bound contaminants, including PAHs, into the watershed, potentially affecting wildlife and human health. In November 2018, the Woolsey Fire burned the majority of the Malibu Creek Watershed, a coastal watershed in the Santa Monica Mountains roughly 30 miles west of Los Angeles, California. We conducted multiple sampling events starting in December 2018 to assess PAH, nutrient, and suspended sediment concentrations at locations throughout the Malibu Creek watershed. We collected water and soil samples periodically and during storm events to evaluate spatial and temporal changes in contaminant concentrations and to characterize the source and environmental persistence of the pollutants. Results from the first wet season (2018-2019) show an increase in PAH concentrations within Malibu Creek and its tributaries during and immediately after storm events. The largest storm events were associated with high PAH concentrations, likely caused by precipitation-induced erosion of burned soil and the resulting increase in sediment loads within surface waters. PAH concentrations were low during the first dry season (2019) but rebounded during the first rain events of the second wet season (2019-2020). The results of soil analyses, combined with PAH molecular ratio approaches, provide evidence that burned soils may represent a continued source of PAHs in surface waters a year after the fire occurred.