Exploring the Complex Effects of Wildfire on Stream Water Chemistry: Insights From Concentration-Discharge Relationships

Wildfires are a worldwide disturbance with unclear implications for stream water quality. We examined stream water chemistry responses immediately (<1 month) following a wildfire by measuring over 40 constituents in four gauged coastal watersheds that burned at low to moderate severity. Three of the four watersheds also had pre-fire concentration-discharge data for 14 constituents: suspended sediment (SS_fine), dissolved organic and inorganic carbon (DOC, DIC), specific UV absorbance (SUVA), major ions (Ca²⁺, K⁺, Mg²⁺, Na⁺, Cl⁻, (Formula presented.), (Formula presented.), F⁻), and select trace elements (total dissolved Mn, Fe). In all watersheds, post-fire stream water concentrations of SS_fine, DOC, Ca²⁺, Cl⁻, and (Formula presented.) changed when compared to pre-fire data. Post-fire changes in (Formula presented.), K⁺, Na⁺, Mg²⁺, DIC, SUVA, and total dissolved Fe were also found for at least two of the three streams. For constituents with detectable responses to wildfire, post-fire changes in the slopes of concentration-discharge relationships commonly resulted in stronger enrichment trends or weaker dilution trends, suggesting that new contributing sources were surficial or near the surface. However, a few geogenic solutes, Ca²⁺, Mg²⁺, and DIC, displayed stronger dilution trends at nearly all sites post-fire. Moreover, fire-induced constituent concentration changes were highly discharge and site-dependent. These similarities and differences in across-site stream water chemistry responses to wildfire emphasize the need for a deeper understanding of landscape-scale changes to solute sources and pathways. Our findings also highlight the importance of being explicit about reference points for both stream discharge and pre-fire stream water chemistry in post-fire assessment of concentration changes.