Hyporheic residence time distribution: scanning heterogeneity in riverbed morphology, hydraulic conductivity, and groundwater flux.

Residence time distribution of solute in the hyporheic zone plays a key role in controlling the downstream transport and biogeochemical transformation of dissolved substances in river corridors. Riverbed morphology, hyporheic hydraulic conductivity, and groundwater upwelling/downwelling condition are crucial factors that control the resulting residence time distribution. In particular, these factors can exhibit spatial variations at different length scales leading to a complex multi-scale organization of hyporheic flow paths.  In this context, we analyze the dominance or otherwise of spatial heterogeneity in the morphology of the riverbed, in the arrangement of hyporheic hydraulic conductivity, and in the organization of upwelling/downwelling flows. We frame our work in a stochastic context, i.e., we treat each factor as a spatially correlated random field, characterized by its degree of heterogeneity and its spatial correlation scale. We explore different combinations, in relative terms, of the degree of heterogeneity and the size of the correlation lengths of the different factors. We use a Monte Carlo approach for each combination to solve flow and solute transport for different realizations numerically. Results indicate the transition between the dominance of a given single factor and the complex interactions of all factors depending on (i) the target feature of the residence time distribution (e.g., mean, variance) and (ii) the relative degree of heterogeneity and spatial correlation of different single factors.