Spatial distributions and potential ecological risks of legacy and emerging antifouling additives in sediments of Onsan and Okpo Harbors, South Korea

Antifouling paint additives in surface sediments were investigated in Okpo Harbor and Onsan Harbor, South Korea, two harbors characterized by contrasting dominant anthropogenic activities. Okpo Harbor is a shipbuilding-oriented harbor with a large shipyard, whereas Onsan Harbor represents a multi-use industrial harbor. Organotin compounds were detected at substantial levels in both harbors; however, their spatial distributions and compositional characteristics differed markedly. Okpo Harbor exhibited relatively low mean concentrations, with distinct inner-harbor hotspots dominated by tributyltin, suggesting relatively fresh TBT inputs or remobilization of TBT-containing legacy residues associated with shipyard activities. In contrast, Onsan Harbor showed higher mean concentrations and broader spatial variability, with monobutyltin predominating, reflecting aged residues derived from advanced degradation of historical TBT inputs. Booster biocides were enriched in inner-harbor areas of Okpo Harbor, likely associated with hull coating and repair activities, whereas concentrations in Onsan Harbor were generally lower and spatially restricted. Potentially toxic elements were elevated in zones influenced by ship repair in Okpo Harbor and by adjacent industrial complexes in Onsan Harbor, consistent with surrounding land-use characteristics. Ecological risks were generally low, with TBT-driven risks localized in inner Okpo Harbor and medium-high mERM-Q values, driven by Zn and Pb, at two industrial sites in Onsan Harbor. These findings highlight the importance of harbor-specific management strategies that consider dominant anthropogenic activities and local environmental conditions.