Beach Water Bacterial Responses to Wildfires in California: a Twenty-year Investigation

This work integrates spatiotemporal datasets in California over a twenty-year period (2003-2023) to identify links between anomalies in fecal indicator bacteria (FIB) — Escherichia coli, total coliforms, fecal coliforms, and Enterococcus — at beaches and the occurrence of recent wildfires. Average FIB concentrations, land cover, precipitation, and burn histories are quantified with available data for every month across all watersheds flowing into the California Coast. Monthly non-burn background FIB concentrations are calibrated in coastal areas. FIB anomalies that are statistically distinct from these background levels are identified for both burn periods (>10% of the watershed burned within a two-year window) and non-burn periods and are reported as the anomalous FIB concentrations normalized by the background concentrations. Correlations between FIB anomalies and monthly precipitation and land cover fractions are identified by comparing data from all watersheds impacted by large fires. During burn periods, anomalous total coliform concentrations relative to baseline values (anomalies) exhibit a negative correlation with urban and positive correlations with coastal oak woodland, mixed chaparral, and redwood land cover fraction and monthly precipitation. Fecal coliform anomalies exhibited a positive correlation with urban land cover fraction after fires. Results indicate prominent post-fire export of total coliform bacteria (perhaps from decaying plant material and soils) in watersheds with less urbanization, and prominent export of fecal coliform bacteria from humans and animals in urbanized watersheds to beach waters. This is the first effort to generalize post-fire FIB responses in beach waters. The results may inform future risk assessments and proactive measures.