Decline in radiocesium in seafloor sediments off Fukushima and nearby prefectures

Radioactivity has been monitored in seafloor sediments off Fukushima and nearby prefectures regularly. During the initial monitoring period (May-September 2011), ¹³⁷Cs concentrations in the surface sediments (0-3 cm) generally increased to 8-580 Bq/kg. Subsequently, concentrations decreased at variable rates. In the latest data, from February 2016, concentrations were still higher at 0.8-141 Bq/kg than the pre-accident level. The geometric mean concentration declined steadily from 47 Bq/kg in September 2011 to 13 Bq/kg in February 2016. The ¹³⁷Cs abundance (Bq/m²) in the surface sediment at each station decreased similarly. The rate of decrease of surface abundance varied spatially by almost one order of magnitude, ranging from 1.1 × 10^-4 to 1.7 × 10^-3/day, equivalent to halving times of 16-1.1 years, respectively. The rate of decrease was related to the median sediment grain size at each station. In addition, bottom-water dynamics, through the redistribution of bottom sediments, may have caused spatial variability in the rate of decrease, whereas vertical profiles of ¹³⁷Cs concentrations in the sediment suggest that vertical migration of ¹³⁷Cs was not a major mechanism reducing the surface ¹³⁷Cs concentration. From September 2011 to February 2016, the overall halving time of ¹³⁷Cs in the surface sediment in the monitoring area, excluding the area inside a 30-km radius from the Fukushima Dai-ichi Nuclear Power Plant, was 2.3 years. Thus, 76% of the originally deposited ¹³⁷Cs (46 × 10¹² Bq) in the surface sediment was transported out of the area during that period. (author)