The transport, distribution, and budget of anthropogenic 129I in the Bohai and North Yellow Seas, China

Abstract

The potential release of radionuclides threatens marine ecosystems with the rapid development of coastal nuclear power plants in China. However, transport, dispersion, and final budget of anthropogenic radionuclides remain unclear, especially in the Bohai and North Yellow Seas, which are semi-enclosed marginal seas with poor water exchange. This study analyzed anthropogenic ¹²⁹I concentration (a typical product of nuclear power plant operations) in seawater samples from this area. ¹²⁹I/¹²⁷I ratios (1.61–6.71 × 10^-10) fall within the typical background range from similar latitudes, indicating the safe operation of nuclear power plants currently. Direct atmospheric deposition from European nuclear fuel reprocessing plants accounted for 66% of the total ¹²⁹I, which far exceeded riverine inputs (34%). Riverine ¹²⁹I acts as a clear tracer, depicting that high ¹²⁹I concentrations were primarily confined within 50 km of the shore, and strong seawater dilution occurred at 50–100 km. Marine sediment serves as the largest sink (59%) of ¹²⁹I, while 27% is retained in the water column. Additionally, surface outflow and deep-layer inflow occur in autumn in the southern strait of the Bohai Sea. These findings enhance understanding of the diffusion of water-soluble radioactive contaminants in semi-enclosed marginal seas and provide a quantitative perspective for tracing marine environmental processes.