CRITICAL DRIVERS OF POST-WILDFIRE WATER QUALITY: APPLICATION OF A CONCEPTUAL FRAMEWORK TO CHARACTERIZE GEOCHEMICAL SIGNATURES AND BIOLOGICAL RESPONSE IN FOUR WESTERN U.S. WATERSHEDS

Due to ongoing risks to water supplies from wildfires in the western U.S., a conceptual framework was developed to understand the critical drivers that influence water quality. “Drivers” are defined categorically and broadly include characteristics associated with climate, fire, ecology, land cover, and soils. We demonstrate the utility of this framework by considering the relationship among “drivers”, “factors” (e.g., precipitation, burn severity, land use, soil properties), “stressors” (e.g., discharge, water quality, sediment metal concentrations) and “effects” (e.g., metal bioaccumulation as a measure of biological condition). Studies identifying links between wildfire critical drivers and ecosystem health are limited, but the conceptual framework provides a roadmap that can identify the interconnectedness among water quality variables and biological effects. Here we use metal concentrations in bed sediment and aquatic macroinvertebrate tissue to differentiate post-fire chemical stressors on biological health in four western watersheds, located in Montana, Colorado, New Mexico, and California. The transport of burned material from hillslope runoff is characterized by sediment metal concentrations and represent a unique geochemical signature in each burned watershed. In this study, arsenic concentrations in bed sediment were highest in the Montana watershed (14.7 ± 4.4 µg/g) and lowest in the California watershed (3.6 ± 1.1 µg/g). Copper and zinc concentrations were similar among all sites (31.1 – 41.4 µg/g Cu, 92.0 - 138 µg/g Zn), suggesting geochemical similarities among these western watersheds. Metal concentrations in aquatic macroinvertebrate tissues can indicate biological effects and provide insight into sources and potential contaminant risk in receiving waters. In this study, all four watersheds were dominated by detritivores, a tolerant taxonomic group common in disturbed systems. Arsenic concentrations in this group ranged from 1.5 – 2.8 µg/g. Herbivores are less tolerant and had higher tissue arsenic concentrations (1.9 – 4.9 µg/g). These data, combined with the conceptual framework, provide insight into how post-wildfire environmental stressors influence biological indicators and demonstrates the utility of the approach for wildfire mitigation strategies.