Concentration‐discharge (C‐Q) relationships of solute export record the response of earth surface processes to changing hydrometeorological conditions. Contrasting C‐Q relationships have been pervasively observed, yet a mechanistic framework that can interpret diverse patterns remains elusive. This work tests the hypothesis that chemical contrasts in shallow and deep waters determine C‐Q patterns. We use data from Sleepers River, a catchment in Vermont with 25% - 50% snow fraction, and a recently developed watershed reactive transport model (BioRT‐Flux‐PIHM) to illuminate mechanisms.
Results/Conclusions
Stream data show that geogenic solutes, including Ca, Mg, K, and Na, exhibit dilution patterns (concentrations decreasing with discharge). Biogenic solutes, including dissolved organic carbon (DOC) and nitrate, exhibit flushing patterns (concentrations increasing with discharge). In contrast, solutes primarily derived from atmospheric deposition, including Cl, show chemostatic behavior with negligible changes in concentrations as discharge varies by orders of magnitude. Sulfate can come from atmospheric deposition and rocks at depth via the oxidation of sulfide-containing minerals. The C-Q pattern of sulfate exhibits a dilution pattern, indicating its major source from the deeper subsurface. Comparison of different water chemistry indicates that soil versus groundwater concentration contrasts determine the slopes of C-Q patterns. Reactive transport modeling illuminates that these patterns arise from different flow paths that bring soil and groundwater with distinct chemical signature into the stream under differing hydrological regimes. During the wet, snowmelt time, stream chemistry reflects the shallow soil water, whereas in dry summer and fall, stream chemistry reflect that of the groundwater. These results indicate broad regulation of subsurface biogeochemical heterogeneity in determining solute export patterns.