Temporal variation of dissolved137Cs in seawater collected in the western north Pacific in 2011-2013

• Main Authors: 福田 美保, 青野 辰雄, 山崎 慎之介, C. Honda Makio, Kawakami Hajime, Saino Toshiro
• Format: Conference Object
• Language: English
• Published: 2019
• Subjects: Fukushima; Oyashio; Pacific; Subarctic
• Online Access: https://repo.qst.go.jp/?action=repository_uri&item_id=72819; http://id.nii.ac.jp/1657/00072807/

The 2011 off the Pacific coast of Tohoku Earthquake and subsequent tsunami on 11 March 2011 caused damages that led to the accident at TEPCO’s Fukushima Dai-ichi Nuclear Power Station. Large amounts of radionuclides were dispersed by the resulting hydrogen explosions, and highly contaminated water also leaked from unit 2 of FDNPS into the terrestrial and marine environments (e.g. United Nation Scientific Committee on the Effects of Atomic Radiation (UNSCEAR), 2014). The most recent estimation for total atmospheric fallout of 137Cs amount from the accident has been estimated as 15–20 PBq, and about 80% of the total amount of emitted 137Cs was transported to the sea (Aoyama et al., 2016). Many researchers have revealed that dissolved 137Cs in seawater have transported by several factors such as ocean current, semi-permanent eddy and subduction (e.g. Buesseler et al. 2012, Aoyama et al., 2016?; Kumamoto et al.,2014; Kaeriyama et al., 2014). However, there is some controversy about the factors affecting radiocaesium in the open ocean, particularly in subarctic area. In this study, we observed dissolved radiocaesium in seawater collected from the western north Pacific (subtropical to subarctic zone) during the four sampling periods in 2011-2013. In upper-layer seawater (water depth: 0-10 m), the 137Cs activity concentrations at stations near the coast of Japan, particularly June-August in 2011, were relatively high (>500 mBq/L). In July 2013, the activity concentrations were the same level compared to those before this accident and had not significant difference in this study area. It seems that the spatial and temporal distributions of the activity concentrations, particularly in 2011, were caused by seawater transportation with not only the Kuroshio and Oyashio current but also the semipermanet eddy. The effective environmental half-life in upper-layer seawater was 139-330 days and these values were slower than those in coastal area in 2011-2013. Annual Meeting Asia Oceania Geoscience Society 2018