Primary weathering rates, water transit times, and concentration‐discharge relations: A theoretical analysis for the critical zone

Abstract The permeability architecture of the critical zone exerts a major influence on the hydrogeochemistry of the critical zone. Water flow path dynamics drive the spatiotemporal pattern of geochemical evolution and resulting streamflow concentration‐discharge (C‐Q) relation, but these flow paths are complex and difficult to map quantitatively. Here we couple a new integrated flow and particle tracking transport model with a general reversible Transition State Theory style dissolution rate law to explore theoretically how C‐Q relations and concentration in the critical zone respond to decline in saturated hydraulic conductivity ( K s ) with soil depth. We do this for a range of flow rates and mineral reaction kinetics. Our results show that for minerals with a high ratio of equilibrium concentration ( ) to intrinsic weathering rate ( ), vertical heterogeneity in K s enhances the gradient of weathering‐derived solute concentration in the critical zone and strengthens the inverse stream C‐Q relation. As decreases, the spatial distribution of concentration in the critical zone becomes more uniform for a wide range of flow rates, and stream C‐Q relation approaches chemostatic behavior, regardless of the degree of vertical heterogeneity in K s . These findings suggest that the transport‐controlled mechanisms in the hillslope can lead to chemostatic C‐Q relations in the stream while the hillslope surface reaction‐controlled mechanisms are associated with an inverse stream C‐Q relation. In addition, as decreases, the concentration in the critical zone and stream become less dependent on groundwater age (or transit time).