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ELSEVIER Journal of Environmental Radioactivity 217 (2020) 106206 Contents lists available at ScienceDirect Journal of Environmental Radioactivity journal homepage: http://www.elsevier.com/locate/jenvrad Mass balance and latest fluxes of radiocesium derived from the fukushima accident in the western North Pacific Ocean and coastal regions of Japan Michio Aoyama, Daisuke Tsumuneb, Yayoi Inomata, Yutaka Tateda *Center for Research in Isotopes and Environmental Dynamics, Univ. of Tsukuba, Tsukuba, Japan Environmental Science Research Laboratory, Central Research Institute of Electric Power Industry, Chiba, Japan Institute of Nature and Environmental Technology, Kanazawa University, Ishikawa, Japan. b ARTICLE INFO Keywords: Fukushima accident. Mass balance Radiocesiun 137Cs Inventory Flux ENVIRONMENTAL Check for updstoc ABSTRACT This article summarizes and discusses mass balance calculations of the activities of Fukushima-derived 137Cs released to the atmosphere and occan prior to 2018 as well as the 137Cs inventorics on land and in the occan, biota, and sediment. We propose that the consensus value of the total amount of 137Cs released to the atmosphere was 15-21 PBq; atmospheric deposition of 137Cs on land was 3-6 PBq; atmospheric deposition of 137Cs on the North Pacific was 12-15 PBq; and direct discharge of 137Cs to the ocean was 3-6 PBq. We also evaluated the movement of 137Cs from one domain to another for several years after the accident. We calculated that the amount of 137Cs transported by rivers might be 40 TBq. The annual deposition of 137Cs due to resuspension at Okuma during the period 2014-2018 was 4-10 TBq year 1. The 137Cs discharged to the ocean was 0.73-1.0 TBq year in 2016 2018. The integrated amount of FNPP1-derived 137Cs that entered the Sea of Japan from the Pacific Ocean from 2011 until 2017 was 270 ± 20 TBq, 6.4% of the estimated amount of FNPP1-derived 137Cs in Subtropical Mode Water in the North Pacific. The integrated amount of FNPP1-derived 137Cs that returned to the North Pacific Ocean through the Tsugaru Strait from the Sea of Japan was 110 ± 10 TBq. Decontamination efforts removed 134 TBq of 13/Cs from surface soil prior to February 2019, an amount that corresponded to 4% of the137Cs deposited on land in Japan. 1. Introduction The total amount of radionuclides released to the environment from the Fukushima Dai-ichi Nuclear Power Plant, hereafter FNPP1, as a result of the accident in March 2011 as well as the impact of those ra- dionuclides on biota, and especially humans, have been among the major concerns related to the FNPP1 accident. The radionuclide of principal concern with respect to human health has been radiocesium, and it is thus particularly important to know how much radiocesium was released to the environment. Many articles and several review articles have already been published concerning this issue (Buesseler et al., 2017; IAEA, 2015; Mathieu et al., 2018; Smith, 2014), but there has been no discussion based on mass balances between the atmosphere, land, and ocean. It is important to consider mass balances in discussions of the total amount of radionuclides released to the environment because the law of conservation of mass is a basic principle, and mass balance is one of the strongest constraints on estimates of the total amount of radionuclides released to the environment and to inventories in the air, on the land, and in the North Pacific Ocean. In this paper, we have summarized the results of studies of the amounts of Fukushima-derived 137 Cs that were released to the atmo- sphere and ocean as well as estimates of the 137Cs inventories on land, in the ocean, in biota, and in sediments. We propose consensus values of these inventories based on mass balance considerations. Finally, we discuss the fluxes of 137Cs between domains for several years after the FNPP1 accident. We consider in particular fluxes from the land to the ocean via rivers, releases from the accident site to the ocean, and delayed effects of the accident associated with resuspension from the land to the atmosphere, deposition from the atmosphere onto the land and ocean, and transport of FNPP1-derived 137Cs from the North Pacific Ocean to the Sea of Japan. Finally, we consider the total amount of 137 Cs in surface soil removed by human activity as a part of decontamination work. *Corresponding author. E-mail address: [email protected] (M. Aoyama). https://doi.org/10.1016/j.jenvrad 2020.106206 Received 10 October 2019; Received in revised form 6 February 2020; Accepted 12 February 2020. Available online 21 February 2020 0265-931X/2020 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Aoyama et a., 2020, JER, 10.1016/j.jenvrad.2020.106206

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