Direct observation of <sup>134</sup>Cs and <sup>137</sup>Cs in surface seawater in the western and central North Pacific after the Fukushima Dai-ichi nuclear power plant accident

Abstract

Abstract. The horizontal distribution of radioactive cesium (Cs) derived from the Fukushima Dai-ichi nuclear power plant (FNPP) in the North Pacific is still unclear due to the limitation of direct measurement of the seawater in the open ocean. We present the result of direct observation of radioactive Cs in surface seawater collected from a broad area in the western and central North Pacific in July 2011, October 2011 and July 2012. We also conducted a simple particle tracking experiment to estimate the qualitative spatial distribution of radioactive Cs in the North Pacific. 134Cs was detected at 94 stations out of 123 stations, and 137Cs was detected at all stations. High 134Cs and 137Cs concentrations more than 10 m Bq kg−1 were observed in the area of the northern part of Kuroshio Extension at 144° E and 155° E in July 2011, in the area 147–175° E around 40° N in October 2011, and the northern part of Kuroshio Extension at 155° E and 175°30´ E in July 2012. Combining the result of direct observations and particle tracking experiment, the radioactive Cs derived from the FNPP had been dispersed eastward to the central North Pacific during 2011. It was considered from the horizontal distribution that radioactive Cs was dispersed not only eastward but also north- and southward in the central North Pacific. Pronounced dilution process of radioactive Cs from the FNPP during study period is suggested from temporal change in the activity ratio of 134Cs / 137Cs, which was decay-corrected on 6 April 2011, and relationships between radioactive Cs and temperature.

Horizontal distribution of Fukushima-derived radiocesium in zooplankton in the northwestern Pacific Ocean

“…These were possibly due to advection of the contaminated waters from the coastal area of the FNPP1. Aoyama et al (2013) and Kaeriyama et al (2013) also reported eastward movement of direct discharged radiocesium from FNPP1 in the transition region.…”

Lagrangian simulation and tracking of the mesoscale eddies contaminated by Fukushima-derived radionuclides

“…The large anticyclonic Tohoku eddy (TE,) with the center at around 39 • N, 144 • E in March 2011, was sampled after the accident in the two R/V cruises in June (Buesseler et al, 2012) and July 2011 (Kaeriyama et al, 2013), showing large concentrations of 137 Cs and 134 Cs. The anticyclonic Hokkaido eddy (HE), genetically connected with the TE, originated in the middle of May 2011 with the center at around 40 • N, 145 • E. After that it captured some contaminated water from the TE.…”

“…2a by the dashed line), after the day of the accident on 11 March 2011. The TE was sampled on 10 and 11 June 2011 by Buesseler et al (2012) along the transect 35.5-38 • N, 144 • E shown in (c) and at the end of July 2011 by Kaeriyama et al (2013) …”

“…The locations of stations with surface seawater samples (collected by Buesseler et al, 2012 andKaeriyama et al, 2013) with measured radiocesium concentrations at the background level are indicated by the green diamonds. Stations where the concentrations were measured to be much higher are marked by the magenta diamonds.…”

“…137 Cs and 134 Cs isotopes with 30.17 and 2.06 years half-life, respectively, were detected over a broad area in the western North Pacific in 2011 and 2012 (Honda et al, 2012;Buesseler et al, 2012;Inoue et al, 2012a, b;Tsumune et al, 2012Tsumune et al, , 2013Kaeriyama et al, 2013;Oikawa et al, 2013;Aoyama et al, 2013;Kameník et al, 2013;Kumamoto et al, 2014;Kaeriyama et al, 2014;Budyansky et al, 2015). 137 Cs concentration levels off Japan before the accident were estimated at the background level to be 1-3 mBq kg −1 , while 134 Cs was not detectable.…”