Byrne, P, Zhang, H, Ullah, S, Binley, A, Heathwaite, AL, Heppell, CM, Lansdown, K and Trimmer, M (2015) Diffusive equilibrium in thin-films (DET) provides evidence of suppression of hyporheic exchange and large-scale nitrate transformation in a groundwater-fed river. Hydrological Processes, 29 (6). pp. 1385-1396. ISSN 0885-6087

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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 (DET) 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 DET 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.

Item Type: Article
Additional Information: This is the final published version of the article (version of record). It first appeared online via Wiley at - please refer to any applicable terms of use of the publisher.
Uncontrolled Keywords: nitrate cycling, riverbed sediment, water quality, pollution, hyporheic zone, groundwater, pore water
Subjects: G Geography. Anthropology. Recreation > GB Physical geography
Divisions: Faculty of Natural Sciences > School of Physical and Geographical Sciences
Depositing User: Symplectic
Date Deposited: 03 Mar 2020 10:18
Last Modified: 03 Mar 2020 10:23

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