Passive pore water sampling provides evidence of suppression of hyporheic exchange and ntrate transformation in a groundwater-fed river

In recent years, the potential of riverbed hyporheic sediments to attenuate nitrate from polluted groundwater has received much attention. However, transformation of reactive nitrate along upwelling flow paths is dependent on redox conditions and the availability of electron donors; these biogeochemical conditions are in turn strongly dependent on hydro-morphologic drivers including riverbed structure and water flux. We worked on a 50m stretch of the River Leith, Cumbria, UK, which is dominated by upwelling nitrate-rich groundwater. Previous investigations of hyporheic connectivity and pore water chemistry in our sub-reach suggest strong groundwater upwelling might suppress the hyporheic zone, possibly restraining its ability to attenuate nitrate in the upwelling groundwater. However, this hypothesis could not be verified previously due to the difficulty of measuring pore water chemistry at depths in the riverbed