STREAMBED STRUCTURAL HETEROGENEITY AS MAIN CONTROL OF AQUIFER-RIVER EXCHANGE FLUXES AND HOTSPOTS OF BIOGEOCHEMICAL CYCLING IN LOWLAND RIVER HYPORHEIC ZONES

Exchange fluxes and residence times of groundwater and surface water at aquifer-river interfaces are driven by hydrodynamic and hydrostatic forcings. While previous research, with a predominantly surface water perspective, has mainly focussed on the impact of bedform controlled advective pumping on hyporheic zone extent and residence times, little attention has been given to the impact of streambed structural controls on groundwater up-welling patterns and its implications for hyporheic exchange. Following a combined experimental and model-based approach, this paper highlights the impact of small-scale streambed structural variability on spatial patterns of hyporheic exchange flow, residence time distribution and the development of hotspots of biogeochemical cycling in the hyporheic zone of a lowland river. Combining Fibre-optic DTS and active Heat Pulse Sensing, this study identified distinct low conductivities peat and clay structures in the streambed to control patterns and quantity of groundwater up-welling. Streambed structure constraint patterns of groundwater up-welling superseded the impact of bedform driven fluxes on aquifer-river exchange flow patterns. In addition, enhanced residence times of up-welling groundwater in and around these organic rich structures lead to an increase in dissolved oxygen consumption and the development of anaerobic denitrification hotspots. The resulting increases in streambed nitrate attenuation as well as enhanced production of CO₂, CH₄ and N₂O as respiration end products highlight the importance of biogeochemical hotspots at aquifer-river interfaces and their dependence from the existence of streambed structural heterogeneity.