Nitrogen transformations within a tropical subterranean estuary

A detailed geochemical groundwater survey was conducted within a carbonate sand subterranean estuary (STE) system on the tropical island of Rarotonga, Cook Islands, to identify N sources and transformation along the groundwater flow paths. There were two distinct sources of N to the STE: (1) local organic material which produced NH₄⁺ in the anoxic deeper groundwater (3.5–4 m below the aquifer surface), and (2) an NO₃⁻ rich upper saline plume located in the shallow groundwater near the high tide mark of the STE. Ammonium concentrations decreased away from the organic source and the δ¹⁵N-NH₄⁺ signature became increasingly enriched. The calculated kinetic fractionation factor for NH₄⁺ loss was much less than that reported for nitrification in other systems, implying that mixing was important in reducing concentration. A simple fractionation/mixing model showed that NH₄⁺ loss via nitrification increased towards the surface where it contributed up to 80% of the observed decrease in NH₄⁺ concentration. A fast moving (~ 2 m d^{− 1}) upper saline plume of shallow NO₃⁻ rich groundwater was a major feature of the studied STE. Denitrification was estimated to account for up to 88% of the decrease in NO₃⁻ concentration in this plume. The potential denitrification rate based on an isotope tracer addition experiment was 270 μmol L^{− 1} d^{− 1}. Assuming that similar NO₃⁻ rich plumes occur all along the foreshore of the studied lagoon, we estimate that groundwater can contribute 4–15% of the daily lagoon N standing stock. We conclude that submarine groundwater discharge of shallow N rich porewater into the studied lagoon system is strongly regulated by denitrification within the STE.