Distribution pattern and source tracing of Radionuclides in China's Coastal Seas: A multiscale perspective from Bohai...

Located within the world's most active land-sea interaction zones, sediments in China's offshore waters contain unique radionuclides that effectively trace terrestrial material migration, anthropogenic interference and environmental changes. Based on in-situ observations across China's coastal waters with a focus on Bohai Bay, this study characterizes the spatial distribution and regional variability of radionuclides, and clarifies the provenance constraints and land-sea interaction mechanisms that regulate their distribution. The results show that radionuclides in surface sediments of the Bohai Bay exhibit a terrestrial feature dominated by natural radionuclides with a low background of artificial radionuclides, with the activity order: ⁴⁰K ≫ ²³²Th ≥ ²³⁸U > ²²⁶Ra ≫ ¹³⁷Cs. The activity of natural radionuclides shows an inshore-high and offshore-low decreasing trend along the land-sea profile, reflecting the seaward declining influence of Yellow River terrestrial input. Meanwhile, the Th/U and Ra/U ratios remain relatively stable across the study area, suggesting possible similarity in sediment provenance. This pattern is highly consistent with the overall distribution of high values near large river estuaries and low values in open sea areas across China's coastal seas. Moreover, ¹³⁷Cs activity presents a macro latitudinal differentiation of being higher in the north and lower in the south. Local high values in the Bohai Bay do not alter the overall pattern, and comparison between nuclear power and non-nuclear power sea areas indicates that its distribution is irrelevant to nuclear power emissions. Based on the exogenous exposure screening of sediments from China's coastal seas, all radiological risk indices are well below internationally accepted threshold values, and no significant radiological risks were identified. This study clarifies the spatial distribution patterns and primary controlling factors of radionuclides in China's coastal seas, establishes an important radionuclide-tracer baseline for sediment dynamics and environmental assessments, and provides valuable insights into global material transport and land-sea interaction processes.