2021 年 55 巻 4 号 p. 132-143
Radiocesium (137Cs),derived from Global Fallout, in the surface seawater showed a uniform value nationwide (range: 3.3–5.6 Bq/m3; mean: 4.1 Bq/m3) in 1984–1985, decreased with a half-life of 16–18 years, and then, converged into～1.5 Bq/m3 as of June 2010. After the Fukushima Dai-ichi Nuclear Power Plant accident in March 2011, there was a remarkable change in 137Cs activities in surface waters during the first year period; the activity reached a maximum in the middle of April and thereafter decreased exponentially with time. Since then, 137Cs of surface seawater showed different dynamics depending on the coastal waters of Japan. The activity of 137Cs in the coastal waters around the Fukushima Daiichi Nuclear Power Plant has been maintained by (i) continuous direct releases from the facility, (ii) desorption/dissolution from marine sediments, and (iii) riverine transport of the facility fallout deposited on land including salinity-induced desorption of 137Cs from fluvial particles. On the other hand, along the west coast of Japan remote from the facility (i.e., the Japan Sea),showed different behaviors. Immediately after the accident, a part of the 137Cs subducted into the subsurface of the North Pacific, and then the entrainment of subsurface 137Cs into the surface waters through vertical mixing. The 137Cs activities started increasing by 2013 and its effect has been continuing.