Source apportionment of dissolved Ni contamination in Gyeonggi Bay, Korea, using the Ni/Cu ratio and anthropogenic Gd

Abstract

This study investigated the spatial distributions of dissolved heavy metals (Mn, Co, Ni, Cu, Zn, and Cd) and rare earth elements (REEs) in Gyeonggi Bay, which is exposed to various pollutants, identified the sources of contaminating metals in seawater, and quantitated the contributions of each source using end-member mixing relationships. Surface seawater samples were collected in Gyeonggi Bay, including the Han River estuary and Shihwa Lake (SH), during spring (41 stations) and summer (27 stations). Dissolved metal concentrations were generally higher in the inner bay areas influenced by riverine and coastal inputs and decreased seaward across Gyeonggi Bay, whereas salinity generally increased toward the offshore region. In both seasons, the levels of most metals, including nickel, were highest in SH. The Ni/Cu ratio was also elevated in SH and the ISMA, indicating distinct Ni-enriched signatures in these areas. Anthropogenic Gd (Gd_anth) showed clear enrichment in the Ganghwa region across both seasons. The Ni/Cu ratio and the anthropogenic gadolinium level (Gd_anth) showed approximately conservative behavior and exhibited markedly different values among the sources, supporting their use as effective tracers of contamination sources. Three end-members (possible contamination sources) were defined: inner SH (E_SH), the Ganghwa waterway (the modified Han River signal; E_GH), and the background seawater in Gyeonggi Bay (E_Gout). End-member mixing analysis showed that the seasonal mean end-member contributions to dissolved Ni in the seawater of the Incheon Special Management Area (ISMA) followed the order E_Gout (62.4%) > E_SH (30.1%) > E_GH (7.5%) in spring and E_Gout (83.4%) > E_SH (14.9%) > E_GH (1.7%) in summer. Across both seasons, the high E_Gout contribution reflects strong tidal mixing, short residence time, and dilution by offshore waters in the ISMA. Nevertheless, the E_SH contribution indicates that anthropogenic inputs from Shihwa Lake remained the major source of dissolved Ni enrichment in the ISMA.