Modeling the Regional Dispersion of Continuous Multi-Source Tritiated Water Discharges in Surface Waters Around Taiwan

The discharge of ALPS-treated water from the Fukushima Daiichi Nuclear Power Plant (FDNPP) in August 2023 renewed concerns regarding radionuclide dispersion in the North Pacific, particularly in the waters surrounding Taiwan. This event highlighted the need to assess not only releases from Fukushima but also the cumulative influence of multiple nuclear power plants operating within the region. To investigate potential dispersion patterns under simultaneous multi-source discharges, this study employed a particle tracking model coupled with the Semi-implicit Cross-scale Hydroscience Integrated System Model (SCHISM), together with a two-dimensional Gaussian diffusion model, to simulate tritium dispersion in surface seawater from six facilities located in the western North Pacific region during 2023–2024: FDNPP, Wolsong, Qinshan, Fuqing, Daya Bay, and the Maanshan Nuclear Power Plant (NPP3 in Taiwan). Also, the modeled tritium concentrations in the Pacific area were compared with background seawater levels reported in the IAEA Marine Radioactivity Information System (MARIS) database. This comparison provided a baseline consistency check to examine whether the simulated tritium distributions were influenced by large-scale ocean circulation and cumulative multi-source discharges.

To further evaluate potential local impacts around Taiwan, seven representative monitoring sites were selected to capture spatial variability across different coastal sectors and offshore regions, including Kinmen, Matsu, the Tamsui River Estuary, Cijin, the Zhuoshui River Estuary, Guishan Island, and FRI-ST-15 (a Fisheries Research Institute monitoring station). These sites were used to examine seasonal concentration responses associated with eastern, western, northern, and southern waters, as well as offshore island environments. The results indicate that tritium released from multiple sources was transported northward by the Kuroshio Current, reaching southern Japan and extending eastward to approximately 180°E. In the northwestern waters of Taiwan, including Kinmen and Matsu, contributions from Fuqing and Qinshan were dominant. At Kinmen, Fuqing’s contribution reached maximum values immediately after discharge and remained significant into early spring, whereas the contribution from Qinshan was comparatively smaller. At Matsu, Qinshan’s contribution increased approximately one month after discharge, decreased by late winter, and reached a secondary maximum in the subsequent winter, while Fuqing’s contribution increased during late winter and maintained a moderate influence thereafter.

Finally, some sensitivity analyses assuming a 50-fold increase in discharge concentrations were conducted to assess potential variability and relative influence among sources. The results indicated negligible influence from Wolsong and FDNPP, whereas discharges from Qinshan, Fuqing, Daya Bay, and NPP3 produced more pronounced, seasonally modulated signals that diminished with increasing distance from Taiwan.