Wildfire-induced elemental changes in drinking water reservoir sediments

Wildfires in forested source water areas increasingly threaten drinking water supplies amid climate and land use change. Fire-driven erosion and pollutant runoff can substantially degrade downstream aquatic systems, including drinking water reservoirs, by introducing toxic contaminants, increasing nutrient loading, and elevating turbidity. The Sooke Lake Reservoir, located within the Greater Victoria Water Supply Area (GVWSA) in British Columbia, Canada, supplies high-quality drinking water to approximately 440,000 residents. Using lake sediment cores, we reconstruct the geochemical impacts of four past wildfires that burned under varying environmental conditions in the GVWSA at sub-decadal temporal resolution and derive implications for water supply management. All fires produced significant shifts in sediment element concentrations that closely tracked independent fire indicators, including charcoal. Some elements (e.g., Al, Ba, Mg) exhibited consistent increases following inferred severe fires, whereas others varied substantially. Post-fire geochemical changes fell into two broad groups: (1) elements that increased following severe fires and were positively correlated with magnetic susceptibility, reflecting enhanced inorganic inputs, and (2) elements weakly correlated with magnetic susceptibility but positively correlated with total carbon and nitrogen, indicating association with organic inputs. These perturbations persisted from <20 to >100 years and aligned closely with documented changes in vegetation. The results indicate that severe catchment fires can potentially drive long-lasting increases in erosion and fluxes of toxic elements, underscoring the need for proactive wildfire risk mitigation and water supply preparedness as regional fire activity intensifies. The approach presented here is broadly applicable to surface water sources with intact sedimentary records and provides a robust method for evaluating wildfire risks to drinking water systems.