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Volume 22, issue 7 | Copyright
Hydrol. Earth Syst. Sci., 22, 4083-4096, 2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 30 Jul 2018

Research article | 30 Jul 2018

Nitrogen attenuation, dilution and recycling in the intertidal hyporheic zone of a subtropical estuary

Sébastien Lamontagne1, Frédéric Cosme2, Andrew Minard2, and Andrew Holloway3 Sébastien Lamontagne et al.
  • 1CSIRO Land and Water, PB 2, Glen Osmond 5064, Australia
  • 2Golder Associates, Richmond, VIC 3121, Australia
  • 3Golder Associates, St Leonards, NSW 2065, Australia

Abstract. Tidal estuarine channels have complex and dynamic interfaces controlled by upland groundwater discharge, waves, tides and channel velocities that also control biogeochemical processes within adjacent sediments. In an Australian subtropical estuary, discharging groundwater with elevated (>300mgNL−1) NH4+ and NO3 concentrations had 80% of the N attenuated at this interface, one of the highest N removal rates (>100mmolm−2day−1) measured for intertidal sediments. The remaining N was also diluted by a factor of 2 or more by mixing with surface water before being discharged to the estuary. Most of the mixing occurred in a hyporheic zone in the upper 50cm of the channel bed. However, groundwater entering this zone was already partially mixed (12%–60%) with surface water via tide-induced circulation. Below the hyporheic zone (50–125cm below the channel bed), NO3 concentrations declined slightly faster than NH4+ concentrations and δ15NNO3 and δ18ONO3 gradually increased, suggesting a co-occurrence of anammox and denitrification. In the hyporheic zone, δ15NNO3 continued to become enriched (consistent with either denitrification or anammox) but δ18ONO3 became more depleted (indicating some nitrification). A high δ15NNO3 (23‰–35‰) and a low δ18ONO3 (1.2‰–8.2‰) in all porewater samples indicated that the original synthetic nitrate pool (industrial NH4NO3; δ15N ∼ 0‰; δ18O ∼ 18‰–20‰) had turned over completely during transport in the aquifer before reaching the channel bed. Whilst porewater NO3 was more δ18O depleted than its synthetic source, porewater δ18OH2O (−3.2‰ to −1.8‰) was enriched by 1‰–4‰ relative to rainfall-derived groundwater mixed with seawater. Isotopic fractionation from H2O uptake during the N cycle and H2O production during synthetic NO3 reduction are the probable causes for this δ18OH2O enrichment. Whilst occurring at a smaller spatial scale than tide-induced circulation, hyporheic exchange can provide a similar magnitude of mixing and biogeochemical transformations for groundwater solutes discharging through intertidal zones.

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Short summary
The dual nitrate isotope technique is one of the most commonly used approaches to study the origin and fate of N introduced in aquifers. In this study, we first demonstrate a large attenuation of groundwater N at a former industrial site, especially at the interface between surface and groundwater. We also provide evidence for a switch in the oxygen isotopic signature of groundwater due to this extensive N attenuation. This could be used to better quantify N attenuation processes in aquifers.
The dual nitrate isotope technique is one of the most commonly used approaches to study the...