Processes affecting long-term changes in 137Cs concentration in surface sediments off Fukushima

Temporal changes in cesium-137 (Cs-137) concentrations in the surface (0-10 cm) layer of seabed sediment were quantified from continuous observation data at 71 stations within a 150-km radius of the Fukushima Daiichi Nuclear Power Plant, and the primary processes affecting temporal changes were identified. From March 2011 to the end of 2015, about 80% of the initially deposited Cs-137 in the surface sediment in the coastal region (bottom depth <= 100 m) region has dissipated (radioactive decay is not included). Such a remarkable change in the Cs-137 concentration was not observed in the offshore (> 100 m) region. This paper focuses on the following three processes that affected the decrease in the Cs-137 concentrations, and assesses their relative importance; (1) resuspension and transport of Cs-137-bound sediment, (2) desorption of Cs-137 from the sediment, and (3) dilution of Cs-137 by vertical mixing of sediment. Consequently, it was estimated that the first two processes together have potentially contributed to reduce the Cs-137 inventory in the top 10 cm of the coastal region by at most 35%. Furthermore, by applying a pulse input sediment mixing model to the observed vertical distribution of sedimentary Cs-137, it was also estimated that more than 43% of the Cs-137 in the surface sediment was transported to deeper sediment layers by vertical mixing of the sediment. This indicates that the decrease of Cs-137 concentrations in coastal sediments was mainly affected by mixing of Cs-137-bound surface sediment with less contaminated sediment in the deeper layers.