Appearances of Fukushima Daiichi Nuclear Power Plant-Derived ¹³⁷Cs in Coastal Waters around Japan: Results from Marine Monitoring off Nuclear Power Plants and Facilities, 1983–2016

Monitoring of ¹³⁷Cs in seawater in coastal areas around Japan between 1983 and 2016 yielded new insights into the sources and transport of Fukushima Daiichi Nuclear Power Plant (FDNPP)-derived ¹³⁷Cs, particularly along the west coast of Japan. Before the FDNPP accident (1983–2010), the activity concentrations of ¹³⁷Cs, mainly from fallout, were decreasing exponentially. Effective ¹³⁷Cs half-lives in surface seawater ranged from 15.6 to 18.4 yr. After the FDNPP accident (March 2011) ¹³⁷Cs activity concentrations in seawater off Fukushima and neighboring prefectures immediately increased. Since May/June 2011, ¹³⁷Cs activity concentrations there have been declining, and they are now approaching preaccident levels. Along the west coast of Japan remote from FDNPP (i.e., the Japan Sea), however, radiocesium activity concentrations started increasing by 2013, with earlier (May/June 2011) increases at some sites due to airborne transport and fallout. The inventory of ¹³⁷Cs in the Japan Sea (in the main body of the Tsushima Warm Current) in 2016 was calculated to be 0.97 × 10¹⁴ Bq, meaning that 0.44 × 10¹⁴ Bq of FDNPP-derived ¹³⁷Cs was added to the estimated global fallout ¹³⁷Cs inventory in 2016 (0.53 × 10¹⁴ Bq). The net increase of ¹³⁷Cs inventory in the Japan Sea through the addition of FDNPP-derived ¹³⁷Cs accounts for approximately 0.2% of the total ¹³⁷Cs flux from the plant to the ocean from the accident