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Description
The number, severity, and duration of wildfires are increasing across the Western United States, which can have significant impacts on urban infrastructure, watersheds, and water quality, especially in areas with anthropogenic influences. There is limited research on fecal indicator bacteria, specifically Escherichia coli (E. coli), following wildfires. This research focused on the North Santa Monica Bay Coastal Watersheds and the Malibu Creek Watershed burned by the 2018 Woolsey Fire in California. Nutrients (total nitrogen and total phosphorus), E. coli, and total suspended solids (TSS) years before fire (2015-2018) and after fire (2018-2020) were compared using a change point analysis and a single factor (one-way) analysis of variance. Water quality responses were evaluated with respect to precipitation, land cover, soil burn severity, and regulatory requirements. Precipitation was not a driver of surface water contamination before or after fire (R2 < 0.2). Additionally, land cover (shrub, forest, developed) was not a strong indicator of contamination. These unanticipated results are attributed to sample size, sampling procedures, and other confounding factors that may mute post-fire water quality signals. For all pollutants analyzed, storms within the first wet season after the fire generally had statistically higher concentrations compared to storms evaluated over the entire pre-fire study period (2015-2018) for both watersheds. Nutrients and TSS remained elevated during the second year after fire. The largest streamflows in the North Santa Monica Bay Coastal Watersheds correlated with peak TSS levels after fire. Moderate and high soil burn severity generally had up to twenty-four times larger pollutant levels compared to low and unburned areas. Pre-fire concentrations generally exceeded pollutant regulatory requirements, which was also exceeded post-fire with over a threefold increase of E. coli and nutrients after fire. This research has water quality implications for managing watersheds and mitigating the impacts of contaminated waterways, as well as serving as a resource for policy development in coastal areas after wildfires.
