Abstract
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.
Highlights
The 2011 Tohoku Earthquake off the Pacific coast of Japan occurred on 11 March 2011, and the associated tsunami damaged the Fukushima Daiichi Nuclear Power Plant (FDNPP) of the Tokyo Electric Power Company (TEPCO), resulting in the release of large amounts of radioactive material to the marine environment, including the seawater and bottom sediments
The concentration data for 137Cs and 134Cs in the surface sediments were published on the Nuclear Regulation Authority of Japan (NRA) website as soon as they became available
We examined temporal changes of the 137Cs concentrations in the surface sediment to a depth of 3 cm in this study as well as the changes during the initial monitoring period (May 2011–September 2011) reported previously (Kusakabe et al 2013) (Fig. 4)
Summary
The 2011 Tohoku Earthquake off the Pacific coast of Japan occurred on 11 March 2011, and the associated tsunami damaged the Fukushima Daiichi Nuclear Power Plant (FDNPP) of the Tokyo Electric Power Company (TEPCO), resulting in the release of large amounts of radioactive material to the marine environment, including the seawater and bottom sediments. The 137Cs concentration in seawater outside a 30-km radius from the FDNPP rose from a pre-accident level of 0.0011–0.0019 Bq/l to as much as 186 Bq/l in April 2011 (Oikawa et al 2013). Radioactivity in seafloor sediments has been systematically monitored by the Marine Ecology Research Institute, under contract with the Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT), from May 2011 to March 2013, and with the Secretariat of the Nuclear Regulation Authority of Japan (NRA) from April 2013 to the present. There were eight monitoring stations, but the number has increased gradually; since May 2012, 32 monitoring stations have been in operation off Fukushima and nearby prefectures. Kusakabe et al (2013) have already reported monitoring results obtained during the period from
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