A modeling study on the oceanic dispersion and sedimentation of radionuclides off the coast of Fukushima

We developed a three-dimensional prognostic oceanic dispersion model that accounted for the phase transfer of radionuclides between seawater, suspended particles, and seabed sediments with multiscale grain sizes. A detailed hindcast of 137Cs in the seabed sediment off the Fukushima coast was conducted to investigate the transfer mechanism of dissolved 137Cs derived from the Fukushima Daiichi Nuclear Power Plant (FNPP1) accident toward the seabed sediment. Extensive model-data comparison demonstrated that the model could satisfactorily reproduce the oceanic structure and 137Cs concentrations in the seawater and seabed sediment. The model successfully reproduced the major features of the observed spatial variation of the 137Cs activities in the sediment, which represented more than 90% of the sedimentary radiocesium existing in the coastal area off Fukushima several months after the accident. Shear stress associated with the resuspension of the seabed sediment was induced by waves near the shore and by current velocity offshore of the study area. The adsorption of 137Cs on the seabed sediment differed depending on the particle size, with adsorption on clay being the most substantial. The distribution of 137Cs in the sediment off the Fukushima coast was formed mainly owing to adsorption from the dissolved phase by June 2011, when the impact of the direct oceanic 137Cs release from FNPP1 was remarkable. After June 2011, seabed sediment became a source of 137Cs released to the seawater owing to resuspension with and desorption from the sediment. •Oceanic nonconservative radionuclides dispersion model was developed.•A hindcast of 137Cs in the seabed sediment off the Fukushima coast was conducted.•The model successfully reproduced observed 137Cs concentrations in sediments.•137Cs in sediment was mainly adsorbed via dissolved phase by June 2011.