Dissolved and particulate metals, and Pb isotopes in Oehwang River estuary and Onsan port seawaters, Korea

This study investigated dissolved and particulate metals (Cu, Zn, Cd, and Pb), together with Pb isotopic compositions, in coastal seawater from the Oehwang River estuary and Onsan Port, which have been reported as severely contaminated areas due to industrial complexes and harbor activities. A total of 53 surface seawater samples were collected during each of four sampling periods (April, July, August, and October 2025). Water quality parameters, including salinity, pH, and turbidity, were measured in situ, and dissolved and particulate metal concentrations as well as Pb isotopic ratios were analyzed.

Overall, metal concentrations within Onsan Port were consistently higher than those in the estuarine zone. Concentrations of Cu, Zn, and Pb in Onsan Port frequently exceeded the chronic toxicity water quality guidelines. In addition, three sites sampled in August, located in front of wharfs B and C, showed Zn concentrations exceeding the acute toxicity guideline.

Pb isotopic compositions plotted against the inverse of Pb concentrations for both dissolved and particulate phases revealed three distinct two–end-member mixing relationships involving four isotopic end members. The ²⁰⁷Pb/²⁰⁶Pb ratios of the dissolved phase (particulate phase in parentheses) were 0.915 (0.942), 0.892 (0.894), 0.888 (0.883), and 0.870 (0.865). These mixing relationships correspond to three zones: the area in front of wharfs B and C within Onsan Port, the inner and outer areas of Onsan Port, and the Oehwang River estuary.

Pb isotopic signatures in the hotspot area indicate that the sources of elevated Pb and Zn concentrations were zinc concentrates imported from Australia and unloaded at wharf B during the sampling period. In contrast, other isotopic end members with distinct Pb isotope ratios represent metal concentrates that were unloaded during previous periods.

The strong linear correlations observed between dissolved and particulate metal concentrations suggest that dissolved Cu, Zn, Cd, and Pb in this area primarily originate from the dissolution of imported metal concentrates during unloading activities at the wharfs, while particulate metals represent residual materials remaining after partial dissolution in seawater.

Temporal variations in both the spatial distribution patterns and concentration ranges of metals were pronounced and were mainly controlled by harbor-related inputs rather than freshwater discharge. Because local metal sources could be clearly identified using spatial distributions of dissolved and particulate metals together with Pb isotopic compositions, key characteristics of the mixing processes—such as source-receptor area, mixing time scale, and spatial extent—could be effectively constrained in this complex coastal environment.