Solute mixing in the hyporheic zone: impact of dispersion, upwelling and heterogeneity.

The hyporheic zone is a key interface where surface water and groundwater interact, triggering a wide range of biogeochemical reactions. This interaction is governed by mixing processes whose dynamics remain poorly understood in hyporheic environments. Here we investigate hyporheic mixing using numerical simulations of Darcy-scale flow and solute transport beneath river bedforms in heterogeneous porous media. We consider a range of conditions that vary solute advection and dispersion, groundwater upwelling intensity, and the ratio between the heterogeneity correlation length and bedform size. Mixing is quantified through the scalar dissipation rate and by assessing the emergence of ergodic behavior in hyporheic transport. Finally, we interpret the resulting mixing dynamics within a lamellar framework, in which the stretching and elongation of material elements traveling through the hyporheic zone control the efficiency of mixing.