Surface Water Quality in Coastal Watersheds Following the Woolsey Wildfire in California
Wildfires can have significant impacts on watersheds, affecting hydrology, geomorphology, water quality, and urban infrastructure. Wildfires are increasing in number and severity in California, which has the potential to impact waterways not only within the burned area, but also further downstream and into coastal waters. Surface water quality after wildfires in areas with anthropogenic influences have documented increases in various metal pollutants such as lead, mercury, copper, and zinc, as well as nutrients, total suspended solids (TSS), and sediment. However, there is very little research on bacteria, specifically Escherichia coli (E. coli), following wildfires. This research focuses on the 2018 Woolsey Fire in Malibu, California, to understand the seasonal trends of pollutant levels for E. coli, nitrogen, phosphorus, TSS, and copper, as well as analyze the impacts of other factors such as land use, discharge, precipitation, and septic tank locations on contaminant loading. Water quality data from monitoring stations three years (2016-2018) prior to the Woolsey Fire and one year after the fire (2018-2019) for three watersheds (the Santa Monica Bay Watershed, the Malibu Creek Watershed, and the Upper Santa Clara River Watershed) were compared. The Santa Monica Bay (5 monitoring stations) and Malibu Creek (14 monitoring stations) watersheds were directly impacted by the Woolsey Fire and approximately half of the stations were located within a burned drainage area. The Upper Santa Clara River watershed (12 monitoring stations) was unaffected by the fire and is used as a control watershed. Three wet weather events and two dry weather events were sampled each year for each watershed. Baseline data for 2016-2018 for the watersheds are generally stable and are within five times the local E. coli water quality standard of 235 MPN/100mL, as designated by the Los Angeles Regional Water Quality Control Board. Preliminary results show that the Woolsey Fire contributed to substantial increases of bacteria levels during multiple storms following the fire compared to pre-fire conditions. E. coli levels reached up to 54,000 MPN/100mL in the Santa Monica Bay watershed after post-fire storm events, over 200 times larger than the E. coli standard. Ongoing work includes an additional year of monitoring data for 2019-2020 and a change point method to detect if shifts in pollutant concentrations due to the fire are statistically significant. This work has implications for managing coastal watersheds after fire and mitigating the impacts of contaminated waterways.