Scale issues and the effects of heterogeneity on the dune-induced hyporheic mixing

Hyporheic mixing plays a vital role in physical, biogeochemical, and thermal processes in river networks. While many studies have emphasized the role of streambed heterogeneity in hyporheic mixing processes, this study articulates the scale issues inherent in concentration, mixing, heterogeneity, and modeling approaches. It subsequently conducted simulations of hyporheic mixing in synthetic, heterogeneous, 2-D cross-sectional river beds based on prescribed hydraulic conductivity defined at the local-scale. It then investigated the flux variation distribution and the mixing zone under different degrees of heterogeneity and flow scenarios. Since the characterization of the heterogeneities in detail at the local scale is practically impossible, Monte Carlo simulation based on stochastic theory was used to demonstrate the hyporheic mixing under the large-scale control volume (macromixing). Afterward, a first-order stochastic analysis was undertaken to explore the relationship between local-scale flux variance distribution and the mixing zone under different heterogeneity and flow scenarios. The results of this study show that the flux variance in the streambed is an appropriate metric for assessing the magnitude of hyporheic mixing at all scales. Further, surface water velocity and upwelling groundwater are found to be the dominant controlling factors of the flux variance and in turn, the mixing process, followed by the streambed heterogeneity. In addition, it demonstrates that the hyporheic mixing process is significantly affected by the complex surface and groundwater circulation regimes and the stagnation zone under steady-state flow conditions.