Ten-year temporal changes in the supply source of Cs released by the Fukushima Daiichi Nuclear Power Plant to the coastal area

Sato, Shun ; Wakiyama, Yoshifumi*; Takahashi, Fumiaki  ; Takata, Hyoe*

We compared the ongoing direct release of Cs from the Fukushima Daiichi Nuclear Power Plant (FDNPP) and riverine inputs of Cs that have contributed to the persistent elevated Cs in coastal waters around the plant since 2012. The direct release of Cs clearly decreased from 17.5 TBq during 20122014 to 0.6 TBq during 20192021 because of measures implemented by the FDNPP. The riverine input decreased slightly from 2.2 TBq during 20122014 to 2.0 TBq during 20192021. The ratio of direct release to the total Cs flux into the coastal water around the FDNPP decreased from 89% during 20122014 to 23% during 20192021. This change resulted in part from a lowering of the limit of detection of Cs activities to avoid overestimation of Cs fluxes from the facility, but also from installation of an impermeable seaside wall. The trend of decreasing riverine Cs input might have been due entirely to environmental remediation. Our results indicate that the contribution of riverine input to the coastal waters will continue for a long time. Precise prediction of the long-term distribution of Cs in the marine environment will require consideration of a variety of environmental factors and anthropogenic effects.