Abstract
AbstractThe 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.
Highlights
Of the major chemical elements necessary to sustain life, nitrogen (N) has the greatest total mass (4 × 1021 g) in Earth’s hydrosphere, atmosphere and biosphere (Galloway et al, 2003)
By analysing vertical concentration profiles captured by the diffusive equilibrium in thin films (DET) samplers, our study has revealed cm-scale changes in the concentration of redox-sensitive solutes at depths below surface water infiltration and demonstrated the important control of subsurface water flux on nitrogen biogeochemistry
Our deployment of the DET samplers in the upper 15 cm of river sediments filled a pre-existing ‘monitoring gap’ and provided strong evidence for the absence of surfacesubsurface water mixing in our study reach under baseflow conditions
Summary
Of the major chemical elements necessary to sustain life (nitrogen, carbon, phosphorus, oxygen and sulphur), nitrogen (N) has the greatest total mass (4 × 1021 g) in Earth’s hydrosphere, atmosphere and biosphere (Galloway et al, 2003). Recent research suggests that peak nitrate loading is being recorded in many catchments and that it will take decades for the leached nitrate to discharge into freshwaters due to storage and slow transport times in groundwater aquifers (Wang et al, 2012). This may be problematic in groundwater-fed rivers as predicted warmer and drier summers as a result of climate change (Wilby et al, 2006) will mean groundwater contributions to surface water will become more important, potentially leading to nitrate contamination of surface waters from groundwater.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
