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.