Water Quality of Coastal Watersheds Following the Woolsey Wildfire: Surface Water Quality of Pollutants of Concern Before and After Fire, and Mitigating the Impact of Wildfires Through Stormwater Management Techniques
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 (2015-2018) prior to the Woolsey Fire and two years after the fire (2018-2020) for three watersheds (the North Santa Monica Bay Coastal Watersheds, the Malibu Creek Watershed, and the Upper Santa Clara River Watershed) were compared. The North Santa Monica Bay Coastal Watersheds (5 monitoring stations) and Malibu Creek Watershed (18 monitoring stations) were directly impacted by the Woolsey Fire with approximately half of the stations 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 for comparison. 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 there were substantial increases of bacteria levels during multiple storms following the Woolsey Fire compared to pre-fire conditions and the control watershed. E. coli levels reached up to 54,000 MPN/100mL in the North Santa Monica Bay Coastal Watersheds after post-fire storm events, which is over 200 times larger than the local E. coli standard. A change point method to detect if shifts in pollutant concentrations due to the fire are statistically significant will be conducted. This research has implications for managing watersheds after fire and mitigating the impacts of contaminated waterways, as well as serve as a resource for policy development in coastal areas after wildfires. Various stormwater management techniques will be explored to identify potential mitigation measures after wildfire.