Sedimentation and remobilization of radiocesium in the coastal area of Ibaraki, 70 km south of the Fukushima Dai-ichi Nuclear Power Plant
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Sedimentation and remobilization processes of radiocesium were investigated from time-series observations at nine stations in the coastal area of Ibaraki, 70–110 km south of the Fukushima Dai-ichi Nuclear Power Plant (1FNPP). Sediment samples were collected four times between June 2011 and January 2012, and concentrations of radiocesium as well as sediment properties such as grain size and elemental compositions were analyzed. Cumulative inventory of 137Cs in sediment (0–10 cm) ranged between 4 × 103 and 3 × 104 Bq/m2 as of January 2012. This amount was generally higher at stations nearer 1FNPP and has remained at the same level since August 2011. From these results, it can be inferred that dissolved radiocesium advected southward from the region adjacent to the 1FNPP and was deposited to the sediment of the study area in the early stage after the accident. The incorporation of radiocesium into sediments was almost irreversible, and higher concentrations of 137Cs were obtained from the finer-grained fraction of sediments. In the northern offshore stations, resuspension of the fine-grained sediments formed a high-turbidity layer 10–20 m above the seabed. These results indicate that radiocesium-enriched fine particles were transported from the coast to offshore regions through the bottom high-turbidity layer.
KeywordsRadiocesium Fukushima Dai-ichi nuclear power plant Seabed sediment Coastal area Redistribution
Field and analytical supports were provided by staff members of Radiation Protection Department, Nuclear Fuel Cycle Engineering Lab., JAEA. This investigation benefited enormously and was vastly improved through discussions with Drs. M. Chino, A. Endo, O. Togawa, H. Nagai, T. Matsunaga, H. Kawamura, T. Suzuki (JAEA), S. Igarashi (Fukushima Pref.), Y. Kato, and H. Narita (Tokai University). We are also grateful to two anonymous reviewers for their constructive comments on the paper.
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