Effect of low-permeability layers on spatial patterns of hyporheic exchange and groundwater upwelling

Bed form-induced hyporheic interactions are characterized by a nested system of flow paths that continuously exchange water, solutes, momentum, and energy. At the local scale, sediment heterogeneity plays a key role in the hydrodynamics and potential for biogeochemical transformations within the hyporheic zone. This manuscript explores the role of low-permeability sedimentary layers on the interplay between bed form-induced hyporheic exchange and groundwater upwelling. A hydrodynamic conceptualization that sequentially couples fully-turbulent flow in the water column and Darcian flow in the sediment is used. Low-permeability layers are characterized by long residence times and solute accumulation. Furthermore, these layers induce hydrodynamic sequestration due to the relocation and, in some cases, emergence of new stagnation zones. Spatial patterns of residence time distributions and flushing intensities indicate that the interface of the low-permeability layers has the potential to be a hot spot for biogeochemical transformations and flow acceleration near such interface can increase the mobilization capacity for the products of redox chemical and microbial processes. A discussion about the possible implications that hydrodynamic changes have on the biogeochemistry of hyporheic zones is presented; however, further biogeochemical experimentation and modeling are needed to validate these arguments.