Reconstruction of radiocesium levels in sediment off Fukushima: Simulation analysis of bioavailability using parameters derived from observed Cs concentrations.

Radiocesium was released to the North Pacific coastal waters by the accident at the Fukushima Dai-ichi Nuclear Power Plant (1FNPP) of the Tokyo Electric Power Company (TEPCO) in March 2011. Since the radiocesium in the sediment off Fukushima was suggested as a possible source for the transfer of this radionuclide through the benthic food chain, we conducted numerical simulations of Cs in sediments off the Fukushima coast by using a model which incorporates dynamic transfer processes between seawater and the labile and refractory fractions in sediment particles. This model reproduced the measured temporal changes of Cs concentration in seabed surface sediment off Fukusima coasts, by normalizing the radiocsium transfer between seawater and sediment according to the particle diameter sizes. We found that the Cs level in sediment decreased by desorption during the first several months after the accident, followed by a reduction in the labile fraction until the end of 2012. The apparent decrease of the total radiocesium level in surface sediment was estimated to occur at rates of approximately 0.2 y within a 20 km distance from the 1FNPP. The comparison of Cs level decreases in the demersal fish and the simulated temporal labile fraction in fine sediment demonstrated that the consideration of radiocesium transfer via sediment is important for determining the Cs depuration mechanism in some demersal fish.