TERRAPUB Geochemical Journal
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Geochemical Journal, Vol. 46 (No. 4), pp. 311-320, 2012

Low levels of 134Cs and 137Cs in surface seawaters around the Japanese Archipelago after the Fukushima Dai-ichi Nuclear Power Plant accident in 2011

Mutsuo Inoue,1 Hisaki Kofuji,2 Seiya Nagao,1 Masayoshi Yamamoto,1 Yasunori Hamajima,1 Ken Fujimoto,3 Keisuke Yoshida,1 Atsuo Suzuki,4 Hideaki Takashiro,5 Kazuichi Hayakawa,6 Kazuhito Hamataka,1 Shota Yoshida,6 Masayuki Kunugi7 and Masayuki Minakawa3

1Low Level Radioactivity Laboratory, Institute of Nature and Environmental Technology, Kanazawa University, Nomi, Ishikawa 923-1224, Japan
2Japan Marine Science Foundation, Minato, Mutsu, Aomori 035-0064, Japan
3Fisheries Research Agency, National Research Institute of Fisheries Science, Fukuura, Kanazawa, Yokohama 236-8648, Japan
4Shizuoka Prefectural Environmental Radiation Monitoring Center, Omaezaki, Shizuoka 437-1612, Japan
5College of Bioresource Science, Nihon University, Shimoda, Shizuoka 415-0014, Japan
6Institute of Medical, Pharmaceutical and Health Sciences, Kanazawa University, Kakuma, Kanazawa 920-1192, Japan
7Environmental Safety Center, Tokyo University of Science, Noda, Chiba 278-8510, Japan

(Received March 2, 2012; Accepted June 18, 2012)

Abstract: A total of 150 surface seawater samples were collected from around the Japanese Archipelago between July 2009 and February 2012, with sample collection peaking in July and October 2011. Low-background measurements revealed that, except for coastal and offshore areas around western Japan, 134Cs and 137Cs concentrations in surface waters were significantly higher than those recorded before the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident, although their concentration levels were generally very low (<~0.1 to 1 mBq/L for 134Cs). In June 2011, concentration peaks of 1 mBq/L for 134Cs and 2.5 mBq/L for 137Cs were recorded in the Tsugaru Strait and off western Hokkaido, toward the Japan Sea approximately 150-300 km north of the high-deposition area of Akita in northern Honshu. In October 2011, 134Cs was detected only in the surface waters off western Hokkaido at a concentration of 0.2 mBq/L, which is markedly lower than the values observed in June 2011. These findings indicate that the observed radionuclide distributions could primarily be attributed to the atmospheric deposition of radionuclides emitted from the FDNPP and transported by ocean currents.
Key words: radiocesium, surface seawater, water circulation, deposition, Fukushima Dai-ichi Nuclear Power Plant accident


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