Processes affecting long-term change in Cs abundance in seabed sediment off Fukushima

Otosaka, Shigeyoshi  ; Kobayashi, Takuya 

The Cs abundance in surface sediment in the coast off Fukushima decreased at the rate of about 30%/year. In this study, we assessed the three processes, (1) vertical mixing of sediment, (2) offshore transport of Cs bound particles, and (3) desorption of Cs from sediment, that may affect the decrease in the Cs abundance. The process (1) was assessed by applying time-series data of vertical distribution of sedimentary Cs into a pulse input sediment mixing model. The model estimated up to 70% of Cs in the coastal sediment was efficiently transported to the lower sedimentary layers in the first 5 years after accident. In conclusion, we consider that the primary process decreasing the Cs abundance in surface sediment is vertical mixing of sediment due to bioturbation or storm events.