Diffusive equilibrium in thin films provides evidence of suppression of hyporheic exchange and large-scale nitrate transformation in a groundwater-fed river.

The hyporheic zone of riverbed sediments has the potential to attenuate nitrate from upwelling, polluted groundwater. However, the coarse-scale (5-10 cm) measurement of nitrogen biogeochemistry in the hyporheic zone can often mask fine-scale (<1 cm) biogeochemical patterns, especially in near-surface sediments, leading to incomplete or inaccurate representation of the capacity of the hyporheic zone to transform upwelling NO₃⁻. In this study, we utilised diffusive equilibrium in thin-films samplers to capture high resolution (cm-scale) vertical concentration profiles of NO₃⁻, SO₄²⁻, Fe and Mn in the upper 15 cm of armoured and permeable riverbed sediments. The goal was to test whether nitrate attenuation was occurring in a sub-reach characterised by strong vertical (upwelling) water fluxes. The vertical concentration profiles obtained from diffusive equilibrium in thin-films samplers indicate considerable cm-scale variability in NO₃⁻ (4.4 ± 2.9 mg N/L), SO₄²⁻ (9.9 ± 3.1 mg/l) and dissolved Fe (1.6 ± 2.1 mg/l) and Mn (0.2 ± 0.2 mg/l). However, the overall trend suggests the absence of substantial net chemical transformations and surface-subsurface water mixing in the shallow sediments of our sub-reach under baseflow conditions. The significance of this is that upwelling NO₃⁻-rich groundwater does not appear to be attenuated in the riverbed sediments at <15 cm depth as might occur where hyporheic exchange flows deliver organic matter to the sediments for metabolic processes. It would appear that the chemical patterns observed in the shallow sediments of our sub-reach are not controlled exclusively by redox processes and/or hyporheic exchange flows. Deeper-seated groundwater fluxes and hydro-stratigraphy may be additional important drivers of chemical patterns in the shallow sediments of our study sub-reach. © 2015 The Authors. Hydrological Processes Published by John Wiley & Sons Ltd. [ABSTRACT FROM AUTHOR]