Article pubs.acs.org/est
Southwest Intrusion of 134Cs and 137Cs Derived from the Fukushima Dai-ichi Nuclear Power Plant Accident in the Western North Pacific Hideki Kaeriyama,*,† Yugo Shimizu,† Daisuke Ambe,† Masachika Masujima,† Yuya Shigenobu,† Ken Fujimoto,† Tsuneo Ono,† Kou Nishiuchi,‡ Takeshi Taneda,§ Hiroaki Kurogi,∥ Takashi Setou,† Hiroya Sugisaki,‡ Tadafumi Ichikawa,† Kiyotaka Hidaka,⊥ Yutaka Hiroe,† Akira Kusaka,† Taketoshi Kodama,† Mikiko Kuriyama,† Hiroshi Morita,† Kaoru Nakata,‡ Kenji Morinaga,† Takami Morita,‡ and Tomowo Watanabe† †
Research Center for Fisheries Oceanography and Marine Ecosystem, National Research Institute of Fisheries Sciences, Fisheries Research Agency, 2-12-4, Fukuura, Kanazawa, Yokohama, Kanagawa 236-8648, Japan ‡ Headquarters Research Management Department, Fisheries Research Agency, 2-3-3, Minato Mirai, Nishi-ku, Yokohama, Kanagawa 220-6115, Japan § Stock Enhancement and Aquaculture Division, Seikai National Fisheries Research Institute, Fisheries Research Agency, 1551-8, Taira-machi, Nagasaki, Nagasaki 851-2213, Japan ∥ Coastal Fisheries and Aquaculture Division, National Research Institute of Aquaculture, Fisheries Research Agency, 6-31-1, Nagai, Yokosuka, Kanagawa 238-0316, Japan ⊥ Fisheries Agency, Ministry of Agriculture, Forestry and Fisheries, 1-2-1, Kasumigaseki, Chiyoda-ku, Tokyo 100-8950, Japan S Supporting Information *
ABSTRACT: Enormous quantities of radionuclides were released into the ocean via both atmospheric deposition and direct release as a result of the Fukushima Dai-ichi Nuclear Power Plant (FNPP) accident. This study discusses the southward dispersion of FNPP-derived radioactive cesium (Cs) in subsurface waters. The southernmost point where we found the FNPP-derived 134 Cs (1.5−6.8 Bq m−3) was 18°N, 135°E, in September 2012. The potential density at the subsurface peaks of 134Cs (100−500 m) and the increased water column inventories of 137Cs between 0 and 500 m after the winter of 2011− 2012 suggested that the main water mass containing FNPP-derived radioactive Cs was the North Pacific Subtropical Mode Water (NPSTMW), formed as a result of winter convection. We estimated the amount of 134Cs in core waters of the western part of the NPSTMW to be 0.99 PBq (decay-corrected on 11 March 2011). This accounts for 9.0% of the 134Cs released from the FNPP, with our estimation revealing that a considerable amount of FNPPderived radioactive Cs has been transported to the subtropical region by the formation and circulation of the mode water.
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INTRODUCTION Fukushima Dai-ichi Nuclear Power Plant Accident. After the Tohoku earthquake of magnitude 9.0 and subsequent tsunami on 11 March 2011, a loss of electric power at the Fukushima Dai-ichi Nuclear Power Plant (hereafter FNPP) resulted in overheated reactors and hydrogen explosions. Radioactive materials were then released into the ocean through both atmospheric fallout and the direct release and leaking of the heavily contaminated coolant water.1,2 Due to its relatively long half-life (2.07 years for 134Cs and 30.07 years for 137 Cs), evaluation of this radioactive Cs in the marine environment is important in addressing risks to both marine ecosystems and public health through consumption of fisheries products. Although dispersion patterns in surface seawater are already reported, understanding the ocean distribution patterns of radioactive Cs throughout the water column is key in assessing its effects on marine ecosystems.3−5 FNPP-derived radioactive Cs dispersed eastward broadly in the North Pacific, © 2014 American Chemical Society
but the southward dispersion was not reported during March 2011 and August 2012, probably because the strong current of the Kuroshio Extension (KE) transported the Cs far eastward before dispersing southward, or simply due to the lack of observations in the southern region.2,4,5 The FNPP-derived radioactive Cs plume was transported eastward with an estimated speed of 8 cm s−1.4 Some model experiments have already discussed the dispersion of radioactive Cs derived from the FNPP,6−11 with estimated amounts of 137Cs discharged directly into the ocean ranging from 2.3 to 14.8 PBq.12 The large variation of these amounts is mainly due to the lack of oceanic observation. Furthermore, little is known of the contribution of atmospheric fallout into the Pacific Ocean Received: Revised: Accepted: Published: 3120
August 19, 2013 February 10, 2014 February 28, 2014 February 28, 2014 dx.doi.org/10.1021/es403686v | Environ. Sci. Technol. 2014, 48, 3120−3127
Environmental Science & Technology
Article
Figure 1. Sampling locations for radioactive cesium in the western North Pacific. Red circle indicates the Fukushima Dai-ichi Nuclear Power Plant, and green circles indicate the sampling stations. Brue arrows indicate surface velocity fields (>1.0 m/s) estimated from a satellite altimeter. The date of the fields is a closer one to the cesium observation. The red bold line and red broken line indicate the Kuroshio Current (KC) and Kuroshio Extension (KE), respectively. The positions of KC and KE were estimated by satellite altimeter data and sea surface drifter data.5 3121
dx.doi.org/10.1021/es403686v | Environ. Sci. Technol. 2014, 48, 3120−3127
Environmental Science & Technology
Article
35° 30′N and 36° 30′N, respectively, located north of, south of, and directly within the KE by the R/V Shoyo-maru. The station located within the KE was sampled in situ with the acoustic Doppler current profiler. A total of 233 samples were collected at stations mainly located south of Japan in the western North Pacific (Figure 1 and SI Table S1). The unfiltered seawater sample was transferred into a 20 L plastic bag and acidified to pH 1.6 by adding 40 mL of concentrated nitric acid. In the open ocean, the differences in the concentrations of radioactive Cs after the FNPP accident between filtered seawater and unfiltered seawater were negligible.13,25 Thus, the results of this study could be comparable to previous studies. Since cesium in seawater mostly exists in a dissolved form, water properties such as temperature (T), salinity (S), and dissolved oxygen (DO) data were also obtained by CTD (T: SBE3, S: SBE4, DO: SBE43 and depth: SBE9plus, Seabird co., USA) at each water sampling station, except for station A00 in SH1209. DO data in SY1210 were not used for the analysis because the DO sensor was broken during the cruise. Analysis of 134Cs and 137Cs in Seawater. The 134Cs and 137 Cs in seawater samples were concentrated by adsorption onto ammonium phosphomolybdate (AMP) using a modified method described elsewhere.5,26,27 CsCl of 0.52 g was added to the sample as a carrier and then 8.0 g of AMP were added to the sample and stirred for at least one hour. After settling the AMP, the supernatant was decanted and the AMP/Cs compound was collected onto a glass fiber filter (GA-100, ADVANTEC co ltd.) by filtration and washed with nitric acid. The AMP/Cs compound was dried at 60−70 °C for more than 48 h and then weighed. The weight yield of the AMP/Cs compound exceeded 95%. The chemical yield was not determined in this study, though 100% chemical yield was assumed (see Supporting Information). A high purity coaxial germanium (HPGe) semiconductor detector with multichannel analyzer (Seiko EG & G, ORTEC) measured the 134Cs and 137Cs radioactivity in the AMP/Cs compounds. The HPGe semiconductor detector had a resolution of 1.44 keV at a peak of 662 keV (137Cs). The energy dependent efficiency calibration for the HPGe semiconductor detector was conducted with five gamma ray reference sources in a 100 mL plastic container identical to that used for sample measurement (Japan Radioisotope Association). These reference sources contained quantified concentrations of gamma ray radionuclides, 54 Mn, 57Co, 60Co, 88 Y, 109Cd, 137Cs, and 139Ce, and had different radioactive concentrations and heights from one another. The efficiency curves in the energy interval from 514 to 1836 keV were estimated to quantify the activity concentration of the detected photopeaks of 134Cs and 137Cs (SI Figure S1a). The 134Cs and 137 C radioactivity was determined by analyzing the photopeak area corresponding to 605 and 796 keV for 134Cs and 662 keV for 137Cs, obtained by more than 35000 s counting (SI Figure S1b). The concentrations of 134Cs and 137C were corrected for decay from the sampling date. Coincidence summing effects of 134 Cs were corrected with 134Cs standard solutions purchased from the Japan Radioisotope Association. The concentration of three times the standard deviation from counting statistics was defined as the detection limit concentration. The detection limit of 137Cs at an 82000 s count was nearly 1.3 Bq m−3, which was almost the same concentration as was obtained in North Pacific surface water prior to the FNPP accident (1.0−2.5 Bq m−3).18−20 The water column inventory of 137Cs from 0 to 500
owing to limited information and lack of open ocean observational data.4,13 The vertical dispersion of FNPP-derived radioactive Cs in the open ocean with depths greater than 1000 m is also still largely unknown. In February 2012, Kumamoto et al.14 and Kitamura et al.15 observed the FNPP-derived radioactive Cs at subsurface depths, below the surface-mixing layer south of the KE. Oceanic Background and 137Cs in the North Pacific before the FNPP Accident. The Kuroshio Current (KC) and its extension, KE, are the strongest currents off the south and east coasts of Japan (Figure 1).16 The KC and KE play important roles in reproduction, dispersal, and migration of pelagic fish species.17 The largest contributor to 137Cs deposition in the Pacific Ocean before the FNPP accident was in the early 1960s as part of the global fallout from atmospheric nuclear weapons testing.18,19 In the North Pacific, the concentration of 137Cs in surface water ranged from 2.0 to 2.5 Bq m−3, decay-corrected in 2000, and was a horizontally homogeneous distribution.20 The southward transport of 137Cs from the subarctic region (north of KE) to the subtropical and tropical regions (south of KE) was observed at 20°N, 165°E in 2002.21 There were two 137Cs concentration maxima (2.87 Bq m−3 at 250 m and 2.91 Bq m−3 at 400 m), located at the density range of North Pacific Subtropical Mode Water (NPSTMW) and Lighter Central Mode Water.21 The winter mixed layer south of KE, which forms the NPSTMW core layer, develops and reaches its deepest depth from February to March and the newly formed NPSTMW south of the KE is subducted and advected southwestward by the Kuroshio recirculation.22 It then begins to appear in the southernmost Japanese islands within a few months.23 The 137Cs core waters observed at 20°N, 165°E in 2002 were formed by movements of mode waters during four decades between the 1960s and 2000s.21 In this study, we focus on the vertical distribution of 134Cs and 137Cs in the western North Pacific, especially south of Japan and the KC. Our observational data was obtained during August 2011 and March 2013. We discuss the possible dispersion of FNPP-derived radioactive Cs to the far southern region of the KC in the western North Pacific.
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MATERIALS AND METHODS Sample Collection. Seawater samples were collected with 12 L Niskin-X bottles (General Oceanic Inc.) at the stations shown in Figure 1 and Supporting Information (SI) Table S1. A repeat observation was conducted four or five times a year between 27°N and 34°N along 138°E by the R/V Soyo-maru, Fisheries Research Agency (FRA). The north−south transect at 138°E had been set as a project of FRA to monitor the Kuroshio ecosystem from 2001 onward.17 The transect was located south of Omaezaki City, Shizuoka prefecture, and was referred to as the “O-line”.17 Seawater samples were collected from five depths between 0 and 500 m at three or four stations in the nine O-line observations during August 2011 and March 2013. The sampling stations on the O-line were set north of, south of, and directly in the KC (Figure 1). The station located within the KC was sampled in situ as part of the indicator isotherm at 15 °C and 100 m depth.24 During O-line sampling, two deep-water samples (750 and 1000 m) were also collected at the stations south of the KC from August 2012. In September 2012, five depths were sampled by the R/V Shoyomaru between 0 and 500 m, located far south of Japan between 13°N and 26° 50′N, around 135°E. In October 2011 and November 2012, seawater samples were collected at 34° 30′N, 3122
dx.doi.org/10.1021/es403686v | Environ. Sci. Technol. 2014, 48, 3120−3127
Environmental Science & Technology
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Figure 2. Vertical profiles of 137Cs at three or four stations along the O-line during August 2011 and March 2013. Arrows indicate the detection of 134 Cs. Error bars indicate counting error (±1σ). When 137Cs was under the detection limit (
