Nitrogen attenuation, dilution and recycling at the groundwater – surface water interface of a subtropical estuary inferred from the stable isotope composition of nitrate and water

Estuarine environments have a dynamic groundwater – surface water interface driven by terrestrial groundwater discharge, tidal cycles, waves and other processes. This interface also corresponds to an active biogeochemical environment. An assessment of discharging groundwater with elevated (>300 mg N L−1) NH4+ and NO3− concentrations at such an interface located in a subtropical estuary indicated that 80 % of the N was attenuated, one of the highest N removal rates (>100 mmol m−2 day−1) measured for intertidal sediments. The remaining N was also diluted by a factor of two or more by mixing before being discharged to the estuary. Most of the mixing occurred in a hyporheic zone in the upper 50 cm of the riverbed. However, groundwater entering this zone was already partially mixed (12–60 %) with surface water via a tidal circulation cell. Below the hyporheic zone (50–125 cm below the riverbed), 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). The discrepancy between δ15NNO3 (23–35 ‰) and δ18ONO3 (1.2–8.2 ‰) in all porewater samples indicated that the original synthetic nitrate pool (δ15N ~ 0 ‰; δ18O ~ 18–20 ‰) had turned-over during transport in the aquifer before reaching the riverbed. 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.