Biogeochemical zonation due to intrameander hyporheic flow

The dynamics of chemicals within a catchment are strongly affected by the exchange between surface water and groundwater. The activity of hyporheic microorganisms plays a key role in these processes, as they are able to oxidize and reduce organic matter and nutrients. The interplay between the residence times in the hyporheic zone and the temporal scales of the microbial reactions must thus be understood in order to improve our understanding of the role of fluvial environments in nutrient cycling. In this paper we focus on the intrameander hyporheic region and investigate the links between river morphology, hyporheic flow field, and biogeochemical processes. We adopt a modeling framework that considers the planimetric evolution of a meandering river, the hyporheic flow field induced by the river sinuosity, and the main biogeochemical reactions of organic carbon degradation. The chemical zonation of the intrameander hyporheic zone clearly emerges as a result of the coupling between hyporheic flow and biochemical activity. The resulting patterns of nutrient concentrations are strongly influenced by the river morphology. We also show how to estimate the characteristic timescales of the redox reactions and that their interplay with the kinematic timescales of the hyporheic flow controls both the biochemical zonation and the overall rate of nutrient transformation.