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

In order to quantitatively evaluate and predict the behavior of the anthropogenic radionuclides, this study examined the spatiotemporal distributions and mass budgets of ¹³⁷Cs and ^239,240Pu 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 ¹³⁷Cs ranged from 0.56 to 2.43 mBq kg^-1, while those of ^239,240Pu ranged from 1.28 to 5.70 μBq kg^-1. ¹³⁷Cs showed a gradual decline over time, with surface distributions exhibiting slight spatial and temporal variations. The vertical distribution of ¹³⁷Cs showed surface or subsurface (< 100 m) peaks followed by a decrease with depth. In contrast, ^239,240Pu 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 ¹³⁷Cs primarily due to radioactive decay and particle scavenging. The corresponding residence times of ¹³⁷Cs 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 ¹³⁷Cs (30.17 years). These findings offer quantitative insights that can enhance future predictions of the variability in artificial radionuclide activities, such as ¹³⁷Cs and ^239,240Pu, in Korean seas, where external inputs from surrounding regions play significant roles.