Assessing wildfire risk to municipal drinking water using paleoecology

Wildfire regimes are changing, with fires becoming more frequent and intense globally as climate change advances, putting natural capital such as surface water at risk. Wildfires can detrimentally impact surface water quality by burning vegetation and altering soils, leading to increased erosion and release of contaminants into waterways, including reservoirs. Here, a multi-proxy paleoecological investigation assesses the risk posed by wildfire to water supply in a municipal drinking water catchment serving 400,000 people in the Capital Regional District (CRD) on Vancouver Island, British Columbia, Canada. Sediment records from Sooke Lake Reservoir and adjacent Begbie Lake were analyzed for charcoal, magnetic susceptibility, geochemistry, pollen, and diatoms to quantitatively reconstruct the terrestrial and aquatic biogeochemical impacts of five past fire events at sub-decadal resolutions. Severe fires have burned in the Sooke watershed under differing climates, including modern conditions and past warmer, drier states. These past climates serve as analogues for anticipated future conditions. All five fires had statistically significant effects on terrestrial and aquatic systems in the study watershed across diverse physical, chemical, and biological fire-related proxy indicators. Magnetic susceptibility, sediment chemistry, and fossil diatom evidence reveal post-fire increases in soil erosion and water column turbidity. Sediment concentrations of potentially toxic elements, including barium, chromium, and lead, also increased after several fires. These fire-induced changes likely persisted for decades, in some cases perhaps more than a century. Overall, this study suggests that Sooke Lake Reservoir and Begbie Lake are vulnerable to post-fire effects which last for many years before pre-fire conditions return. Wildfires pose a significant threat to water quality and water supply security in the CRD. These insights into location-specific catchment fire characteristics and impacts inform evidence-based water supply management recommendations. The novel approach developed and demonstrated in this study can be applied to assess the wildfire risk to any surface water basin with a sufficiently long and intact sedimentary record.Science, Irving K. Barber Faculty of (Okanagan)Earth, Environmental and Geographic Sciences, Department of (Okanagan)Graduat