134Cs and 137Cs in the North Pacific Ocean derived from the March 2011 TEPCO Fukushima Dai-ichi Nuclear Power Plant accident, Japan. Part one: surface pathway and vertical distributions
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Activities of radiocaesium released by the Fukushima Dai-ichi Nuclear Power Plant (FNPP1) accident were measured by surface sampling at 408 stations and in vertical profiles at 24 stations in the North Pacific Ocean, and time-series samples were collected at two coastal stations. After July 2012, 137Cs activity in the surface water near FNPP1 remained around 1000 Bq m−3, which corresponds to a discharge rate of about 10 GBq day−1. FNPP1-derived radiocaesium spread eastward in surface water across the mid-latitude North Pacific with a speed of 7 km day−1 (8 cm s−1) until March 2012, and of 3 km day−1 (3.5 cm s−1) from March 2012 through August 2014. In June 2012, 134Cs activity reached a maximum of 6.12 ± 0.50 Bq m−3 at a 151-m depth (potential density, σ θ = 25.3 kg m−3) at 29°N, 165°E. This subsurface maximum, which was also observed along 149°E, might reflect the southward transport of FNPP1-derived radiocaesium in association with the formation and subduction of subtropical mode water (STMW). In June 2012 at 34°N–39°N along 165°E, 134Cs activity showed a maximum at around σ θ = 26.3 kg m−3, which corresponds to central mode water (CMW). 134Cs activity was higher in CMW than in any of the surrounding waters, including STMW. These observations also indicate that the most effective pathway by which FNPP1-derived radiocaesium is introduced into the ocean interior on a 1-year time scale is CMW formation and subduction.
KeywordsFukushima Dai-ichi Nuclear Power Plant accident Radiocaesium Surface pathway Subduction Subtropical mode water Central mode water Inventory
This study was supported in part by the "Radioactive Survey and Research Fund" of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan (Houshanou-chousa-kenkyuhi, FY2011–2014) and in part by the J-RAPID fund of the Japan Science and Technology Agency for the project entitled “Investigation and Prediction of Impacts of the 2011 off the Pacific coast of Tohoku Earthquake on Marine Environment, FY2011–2012”. The authors thank Takashi Ishimaru for water sampling during the UM1103 cruise of the Umitaka-maru, Toshitaka Gamo for water sampling during the KT1106 cruise of the Tansei-maru, researchers at the Marine Division of the Japan Meteorological Agency for water sampling during cruises KS1202 and KS1205 of the Keifu-maru and the RF1205 cruise of the Ryofu-maru. We thank the commercial ship company, captains and crew of VOS ships for their voluntary work to collect surface seawater samples. We also thank Satoshi Nakamura for his kind collaboration to collect seawater samples at the research pier of Hazaki Oceanographical Research Station of the Port and Airport Research Institute. We also thank you Junko Inomata for his support to collect seawater samples at Tomioka. A part of coastal observations at Tomioka and Hasaki presented in this article was supported by EC 7th Framework project COMET-FRAME (COordination and iMplementation of a pan-Europe instrumenT for radioecology) (Grant Agreement Number 604974) and a Marine Project of Institute of Environmental Radioactivity, Fukushima University, Japan. We also thank Tomomi Onda, Aoi Mori, Yukiko Suda, and Tomoko Kudo for creating the database, drawing graphs, and making tables for this paper.
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