Contents lists available at ScienceDirect Marine Pollution Bulletin journal homepage: www.elsevier.com/locate/marpolbul 137 Cs and 90 Sr in surface waters of the Sea of Japan: Variations and the Fukushima Dai-ichi Nuclear Power Plant accident impact Katsumi Hirose a , Pavel P. Povinec b, ⁎ a Department of Materials and Life Sciences, Sophia University, Tokyo, Japan b Department of Nuclear Physics and Biophysics, Comenius University, Bratislava, Slovakia ARTICLE INFO Keywords: Sea of Japan Surface water 90 Sr 137 Cs Temporal variation Fukushima accident ABSTRACT 90 Sr and 137 Cs activity concentrations in surface waters of the Sea of Japan (SOJ) decreased during the period of 1993–2010 with effective half-lives of 18 and 15 y, respectively. The longer effective half-life of 90 Sr in the SOJ may suggest a surplus of 90 Sr to SOJ surface waters, however, no clear evidence of possible 90 Sr source has been found. After the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident, temporal variations of 137 Cs in the surface water of the SOJ have changed, while 90 Sr variations followed the pre-accident trends. The 90 Sr/ 137 Cs ratios reveal that increases of 137 Cs due to the FDNPP accident continued in surface waters of the SOJ until 2016. 1. Introduction The FDNPP accident in 2011 globally contaminated the terrestrial and marine environments by anthropogenic radionuclides (Hirose, 2016; Povinec et al., 2013b). The atmospheric deposition of FDNPP- derived radionuclides occurred mainly in the western North Pacific Ocean and its marginal seas, and direct liquid discharges mainly caused radionuclide contamination of the coastal environment near Fukushima Prefecture (Aoyama et al., 2013; Povinec et al., 2013a; Povinec and Hirose, 2015; Tsumune et al., 2012). The total releases of FDNPP-de- rived 137 Cs in the western North Pacific were estimated to be 12–15 and 3.5 PBq from atmospheric deposition and the direct discharge, respec- tively (Aoyama et al., 2016b). The FDNPP-derived radiocesium ( 134 Cs and 137 Cs), dominantly injected in March – April 2011, spread out in the North Pacific and its marginal seas, which has been documented by observational studies and model simulations (Aoyama et al., 2016a, 2016b, 2018; Inomata et al., 2018; Kaeriyama et al., 2016; Kumamoto et al., 2014; Nakano and Povinec, 2012; Povinec et al., 2013b, 2013c; Tsubono et al., 2016; Povinec et al., 2017; Takata et al., 2018). After the FDNPP accident, the SOJ was affected by atmospheric deposition of FDNPP-derived radionuclides, including radiocesium (Inoue et al., 2012a, 2012b, 2013, 2018; Ramzaev et al., 2014), al- though its levels in seawater were relatively low because major de- position of radionuclides occurred in the inland of Japan and in the western North Pacific(Aoyama et al., 2016b; Hirose, 2012, 2016; Povinec et al., 2013a). Oceanographic characteristics of the SOJ are quite different when compared with adjacent ocean and marginal seas (Hirose et al., 1999; Ikeuchi et al., 1999; Ito et al., 2003, 2005). To elucidate the complex circulation pattern and to determine potential radiological concern due to the FDNPP accident, it is important to have better understanding of oceanic behavior of FDNPP-derived radionuclides in the SOJ. The temporal variation of the FDNPP-derived radiocesium, which reflects transport and dispersion of radiocesium in the SOJ, were examined recently (Inomata et al., 2018; Inoue et al., 2018; Takata et al., 2018). However, there is still scant information on the FDNPP-derived 137 Cs in seawater in the SOJ. Cesium-134 is a unique tracer characterizing the FDNPP-derived radionuclides, which is, however, difficult to trace beyond more than a decade due to its short half-life of 2.06 y (Martin et al., 1997). On the other hand, 137 Cs is having suitable half-life (30.01 y; Juget et al., 2016), but sometimes it is difficult to evaluate its FDNPP-derived levels in seawater because all ocean has been contaminated by 137 Cs origi- nating from global fallout due to the atmospheric nuclear weapons tests (Povinec et al., 2014). Strontium-90 is also having suitable half-life (28.81 y; Woods and Lucas, 1996), and it has been frequently used as a tracer of seawater processes (Povinec et al., 2013a), however, in the case of the FDNPP accident its total atmospheric emissions were by about four orders of magnitude smaller than that of 137 Cs (Hirose, 2017; Igarashi et al., 2015). The oceanic input of 90 Sr due to direct liquid releases and atmospheric deposition in the western North Pacific were more than two orders of magnitude lower than that of 137 Cs (Povinec et al., 2012). These findings suggest that the 90 Sr/ 137 Cs activity ratio in the FDNPP releases should be different from that in global fallout https://doi.org/10.1016/j.marpolbul.2019.07.024 Received 16 April 2019; Received in revised form 10 July 2019; Accepted 10 July 2019 ⁎ Corresponding author. E-mail addresses: hirose45037@mail2.accsnet.ne.jp (K. Hirose), povinec@fmph.uniba.sk (P.P. Povinec). Marine Pollution Bulletin 146 (2019) 645–652 Available online 17 July 2019 0025-326X/ © 2019 Elsevier Ltd. All rights reserved. T