Spatiotemporal distributions and mass budgets of artificial radionuclides (137Cs and 239,240Pu) in the seas surrounding Korea

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In order to quantitatively evaluate and predict the behavior of the anthropogenic radionuclides, this study examined the spatiotemporal distributions and mass budgets of 137 Cs and 239,240 Pu in the seas surrounding Korean Peninsula (i.e., the East/Japan Sea, the Yellow Sea, and the southern sea of Korea) from 2018 to 2024. Surface activities of 137 Cs ranged from 0.56 to 2.43 mBq kg −1 , while those of 239,240 Pu ranged from 1.28 to 5.70 μBq kg −1 . 137 Cs showed a gradual decline over time, with surface distributions exhibiting slight spatial and temporal variations. The vertical distribution of 137 Cs showed surface or subsurface (< 100 m) peaks followed by a decrease with depth. In contrast, 239,240 Pu activities were depleted near the surface and increased with depth, displaying a distinct maximum at mid-depth (750–1000 m). Mass balance calculations suggest the negative inventory change rates ( d I / d t ) across all regions, indicating net losses of 137 Cs primarily due to radioactive decay and particle scavenging. The corresponding residence times of 137 Cs were 70.7 ± 1.0 years in the East/Japan Sea, 40.1 ± 0.7 years in the Yellow Sea, and 53.8 ± 1.2 years in the southern sea of Korea—all exceeding the radiological half-life of 137 Cs (30.17 years). These findings offer quantitative insights that can enhance future predictions of the variability in artificial radionuclide activities, such as 137 Cs and 239,240 Pu, in Korean seas, where external inputs from surrounding regions play significant roles. • Decline of 137 Cs by ~21 % in Korean seas from 2018 to 2024 • 239,240 Pu shows mid-depth maxima at 750–1000 m in East/Japan Sea. • Residence times of 137 Cs exceed its half-life in all Korean seas. • Box-model reveals net 137 Cs loss due to decay and particle sinking. • Pu/Cs ratio could be an effective tracer for water mass origin.

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