Distribution and ecological risk of antibiotics and herbicides in the Seto Inland Sea, Japan

The global use of antibiotics and herbicides has significantly increased, yet their environmental impacts in coastal waters remain insufficiently explored. This study examined the dynamics of these compounds in the Seto Inland Sea of Japan, with a focus on ecological risks relative to predicted no-effect concentrations (PNECs). Surface water samples were collected between 2019 and 2023 from multiple sites. In total, 249 seawater samples were analyzed using solid-phase extraction followed by liquid chromatography-tandem mass spectrometry to target three herbicides and eleven antibiotics. The results showed that sulfamethoxazole had the highest detection frequency (65.9%), while bromacil, diuron, and several antibiotics, including clarithromycin and azithromycin, were also frequently detected. Higher concentrations were observed in the inner zones of Hiroshima Bay and at river mouths, indicating significant land-based contamination sources. Comparative analysis revealed that clarithromycin, diuron, sulfamethoxazole, and azithromycin were present at levels exceeding their respective PNECs, highlighting potential ecological risks, particularly near sewage treatment plants and constricted estuary regions. Notably, over half of the diuron samples surpassed the PNEC, raising serious concerns for ecological health in coastal regions.