Radioactive cesium dynamics derived from hydrographic observations in the Abukuma River Estuary, Japan Shigeho Kakehi a, * , Hideki Kaeriyama b , Daisuke Ambe b , Tsuneo Ono b , Shin-ichi Ito c , Yugo Shimizu b , Tomowo Watanabe a a Tohoku National Fisheries Research Institute, Fisheries Research Agency, 3-27-5 Shinhama-cho, Shiogama, Miyagi 985-0001, Japan b National Research Institute of Fisheries Sciences, Fisheries Research Agency, 2-12-4, Fukuura, Kanazawa, Yokohama, Kanagawa 236-8648, Japan c Atmosphere and Ocean Research Institute, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8564, Japan article info Article history: Received 7 August 2015 Received in revised form 30 November 2015 Accepted 30 November 2015 Available online xxx Keywords: Radioactive cesium Fukushima Dai-ichi Nuclear Power Plant Accident Mixing diagram Desorption Deposition Abukuma river estuary abstract Large quantities of radioactive materials were released into the air and the ocean as a result of the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident, caused by the 2011 Tohoku earthquake and the subsequent major tsunami off the Pacific coast. There is much concern about radioactive contami- nation in both the watershed of the Abukuma River, which flows through Fukushima Prefecture, and its estuary, where it discharges into the sea in Miyagi Prefecture. We investigated radioactive cesium dy- namics using mixing diagrams obtained from hydrographic observations of the Abukuma River Estuary. Particulate radioactive cesium dominates the cesium load in the river, whereas the dissolved form dominates in the sea. As the salinity increased from <0.1 to 0.1e2.3, the mixing diagram showed that dissolved radioactive cesium concentrations increased, because of desorption. Desorption from sus- pended particles explained 36% of dissolved radioactive cesium in estuarine water. However, the dis- solved and particulate radioactive cesium concentrations in the sea decreased sharply because of dilution. It is thought that more than 80% of the discharged particulate radioactive cesium was deposited off the river mouth, where the radioactive cesium concentrations in sediment were relatively high (217 e2440 Bq kg 1 ). Radioactive cesium that was discharged to the sea was transported southward by currents driven by the density distribution. © 2015 Elsevier Ltd. All rights reserved. 1. Introduction The Tohoku earthquake and subsequent major tsunami off the Pacific coast of Japan on 11 March 2011 triggered the Fukushima Dai- ichi Nuclear Power Plant (FDNPP) accident, which resulted in the release of large quantities of radioactive materials to the air and ocean (Buesseler et al., 2011; Chino et al., 2011; Yasunari et al., 2011; Tsumune et al., 2012; Kumamoto et al., 2015). Although a variety of radioactive materials were emitted into the environment (Povinec et al., 2013), 134 Cs and 137 Cs have received much attention because of their potential impacts on public health and ecosystems, and their relatively long half-lives (2.07 y and 30.07 y, respectively). The total amount of radioactive cesium directly discharged into the sea was estimated to be between 3 and 6 PBq (Kawamura et al., 2011; Estournel et al., 2012; Tsumune et al., 2013). Moreover, more than 10 PBq of radioactive cesium was deposited into the ocean from the atmosphere (Morino et al., 2013; Miyazawa et al., 2013). The Abukuma River, a major river in the Tohoku Region of Japan, is 234 km long. It has a watershed area of 5400 km 2 , and has an average discharge of 200 m 3 s 1 in its lower reaches. While it flows through both Fukushima and Miyagi Prefectures (Fig. 1), most of its catchment is in Fukushima Prefecture. Relatively higher 137 Cs concentrations than before the FDNPP accident have been observed in river water and sediments (Iguchi et al., 2013; Sakaguchi et al., 2014), and 137 Cs has been detected in freshwater fish caught in the river (Iguchi et al., 2013; Mizuno and Kubo, 2013). The river flows into Sendai Bay, where relatively higher 137 Cs concentrations than the oceanic region off the bay have been observed in seawater and zooplankton (Kaeriyama et al., 2015). The 137 Cs concentration of 300 Bq kg 1 dry was recorded for coast and off-shore marine sediments in the Abukuma River estuary by a towed gamma-ray spectrometer (Hirao et al., 2014). In contrast, however, it is re- ported that radioactive cesium concentrations in marine sediments were lower in the area north of the FDNPP than south (Ambe et al., * Corresponding author. E-mail address: kakehi@affrc.go.jp (S. Kakehi). Contents lists available at ScienceDirect Journal of Environmental Radioactivity journal homepage: www.elsevier.com/locate/jenvrad http://dx.doi.org/10.1016/j.jenvrad.2015.11.015 0265-931X/© 2015 Elsevier Ltd. All rights reserved. Journal of Environmental Radioactivity 153 (2016) 1e9