Sources and fate of nitrate in a groundwater‐fed estuary elucidated using stable isotope ratios of nitrogen and oxygen

Abstract

We used δ15N‐nitrate (NO−3) and δ18O‐NO−3 to unravel the provenance and fate of NO−3 in a groundwater‐fed estuary. A total of 13 monthly and two time series surveys were undertaken in the Werribee River estuary near Melbourne, Australia. The different survey timescales provided a comprehensive evaluation of the hydrological effects on the biogeochemistry of NO−3, which accounted for tidal variability and episodic runoff events. The distribution of δ15N and δ18O of the estuarine NO−3 along a mixing line between shallow and deep groundwater provided strong evidence for the predominance of groundwater‐derived NO−3 in the estuary. During dry periods when water residence time in the estuary was extended, shallow groundwater contributed 60% to 76% of the NO−3 (calculated from δ15N‐NO−3 and δ18O‐NO−3) to the estuary, and assimilation removed ∼ 70% of this groundwater‐derived NO−3. During wet periods, deep groundwater provided more (62%) NO−3 than shallow groundwater, and there was no indication of NO−3 consumption. Occasional sources of NO−3, which were also reflected by their δ15N‐NO−3 and δ18O‐NO−3 values, included NO−3 from nitrification and sewage‐derived NO−3 particularly at the bay entrance to the estuary. Greater emphasis should be placed upon the role of groundwater as a substantial NO−3 end member when assessing NO−3 biogeochemistry using methods relying on the dual isotopic composition of NO−3.