SUrface process Control on HYporheic function – SUCHY

Thin sedimentary layer at the interface between the river and its aquifer, the hyporheic zone strongly modifies biogeochemical signals at the watershed scale. This peculiar reactive behavior, which is associated to a high spatio-temporal heterogeneity, remains poorly understood. The SUCHY project aims at describing for the first time the role of surface processes in the structuring and the functionality of the hyporheic zone. SUCHY hypothesizes that hyporheic exchanges and reactivity are maintained by the intermittent alteration of the interface by floods and sediment transport, strongly structuring fluxes and biogeochemical gradients in space and time. To remedy the absence of quantitative spatio-temporal data of hyporheic processes, SUCHY proposes an original experimental methodology, based upon the optical properties of some granular materials, allowing for the first time the measure of transfers inside the sedimentary bed in the laboratory. Two experimental mesocosms will be fabricated, reproducing the natural conditions prevailing at the turbulent-porous sediment bed interface. These experiments will enable for the first time a spatio-temporal description of the mass and momentum exchanges generating hyporheic fluxes. Laser induced fluorescence imaging will allow quantifying surface and porous flow velocities, solute residence time and concentration gradients continuously through the interface, and follow the response to flood and sediment transport events for various groundwater level scenarios. The particular conditions at which a coupling between hyporheic fluxes, turbulence and sediment transport occurs will be evidenced. Spatio-temporal reactivity of transported solutes will be tested owing to fluorescent reactions, equivalent to the typical biogeochemical reaction kinetics observed in the hyporheic zone (denitrification, respiration, redox reactions,...). Eventually, living micro-organisms will be released in the mesocosm to highlight the principal feedbacks between biomass growth and surface process variability in the sedimentary bed. The SUCHY project ambitions, with novel experimental data, to offer an original vision of the hyporheic function of river beds, based on a dynamic equilibrium between surface processes and reactive transport in the hyporheic zone. At long term, this vision will allow for a better understanding of biochemical element cycling in the critical zone.