Tracing the contributions of different nitrate sources associated with submarine groundwater discharge in coastal seawaters off Jeju Island, Korea

We measured the concentrations of dissolved inorganic nutrients and the dual isotopic composition of nitrate (δ15N-NO3− and δ18O-NO3−) in coastal waters off Jeju, a volcanic island in Korea, to trace its main sources. Sampling of seawater and fresh groundwater was conducted in four different coastal areas of Jeju Island: Haengwon (HW), Pyoseon (PS), Ilgwa (IG), and Sagye (SG) in May 2020 and 2021. The significant negative correlations between NO3− and salinity in the four areas indicate that the main source of NO3− is fresh submarine groundwater discharge (FSGD), with the extrapolated fresh groundwater endmember values ranging from 170 to 300 μM (δ15N-NO3−: 4.1–10.8 and δ18O-NO3−: 1.7–6.4). The actual sources of SGD-driven NO3− in these coastal waters were determined using a bi-plot diagram (δ15N-NO3− vs. δ18O-NO3−) and a Bayesian stable isotope mixing model (MixSIAR). The results showed that, besides the background contribution from open-ocean waters, the main sources of NO3− in HW were fertilizer (69 ± 5%) and manure and sewage (24 ± 7%) and those in PS, IG, and SG were manure and sewage (PS: 53 ± 11%, IG: 57 ± 12%, SG: 63 ± 13%) and fertilizer (PS: 27 ± 8%, IG: 24 ± 5%, SG: 22 ± 5%). Our extrapolation approach for NO3− dual isotopes provides a better way to evaluate the main sources of NO3− in coastal waters off volcanic islands where SGD-driven NO3− is significant, but its actual source groundwater cannot be located. [Display omitted] •Fresh groundwater is the dominant source of NO3− in coastal waters off Jeju Island.•Dual NO3− isotopes revealed different NO3− sources in groundwater and coastal water.•A Bayesian stable isotope mixing model distinguished the source of NO3− in SGD.•In the northern Jeju, fertilizer was the main source of NO3− (69%).•In the southern Jeju, manure and sewage were the main sources of NO3− (53–63%).