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

We used δ 15 N‐nitrate (NO − 3 ) and δ 18 O‐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 δ 15 N and δ 18 O 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 δ 15 N‐NO − 3 and δ 18 O‐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 δ 15 N‐NO − 3 and δ 18 O‐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 .