S11-6	

Oceanic dispersion of radioactive cesium around Japan !
and western North Pacific after the Fukushima Dai-ichi
Nuclear Power Plant Accident!

1*11
2
1
Hideki Kaeriyama , Daisuke Ambe , Masachika Masujima , Kou Nishiuchi , Ken Fujimoto ,Tsuneo Ono1and Tomowo Watanabe1	
1 National Research Institute of Fisheries Science, FRA 2 Seikai National Fisheries Research Institute, FRA!

*E-mail: kaeriyama@affrc.go.jp	

Introduction!
The Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident after the Great East Japan Earthquake and tsunami on 11 March 2011 resulted in elevation of thelevel of anthropogenic radioactivity in the marine environment in the western North Pacific. Radioactive cesium (Cs) with relatively long half-life time (ca. 2.0 yr forCs-134 and 30 yr for Cs-137) is a serious concern for people involved in fisheries and seafood safety. In this study, we report the concentrations of radioactive Cs(especially Cs-137) in seawater collected from western North Pacific, Japan Sea and East China Sea collected during June 2011 and August 2012. The
concentrations of Cs-137 in the surface water around Japan before the accident (Background level) were almost 1-2 mBq/kg. !

Materials and Methods!
Seawater samples of 20L were mostly collected from sea surface around Japan and western North Pacific. In October-November 2011, samples were verticallycollected from surface to 500 m depth off the coast of Tohoku area. The seawater samples were not filtrated and acidified to pH = ca 1.6 with HNO3. Radioactive Cs
in 18 kg of seawater with stable Cs added as carrier were concentrated by adsorption onto ammonium phosphomolybdate (AMP). The concentrations of Cs-134
and Cs-137 were measured by the HPGe detector. The detection limits of Cs-134 and Cs-137 were varied from 1.2 mBq/kg to 5.0 mBq/kg, due to the length ofmeasurement time between 7,200 sec and 85,000 sec. All activities were decay corrected to the sampling date. 	

1. Around Japan	

2. Western North Pacific!

Off the coast of eastern Japan, Cs-134and Cs-137 were detected from almost
samples, and those were ranged from
2.4 to 208 mBq/kg and from 4.0 to 220
mBq/kg, respectively. On the otherhands, only Cs-137 was detected atbackground level (up to 2.0 mBq/kg) inthe Japan Sea, East China Sea and Seto
Inland Sea. !
The obvious elevation of theradioactive Cs in seawater due to theFDNPP accident seemed to be mostly
limited in the area off the eastern Japan.	

Cs-134 and Cs-137 were detected from broad range of latitude at 144˚E,
155˚E and 175˚E. In 2011, high concentrations of Cs-137 were observed at
northern part of the Kuroshio extension in 144˚E and 155˚E, and Cs-137 atthe 175˚E line were almost one order of magnitude lower than those of144˚E and 155˚E (Fig. 2(a) and (b)). The concentrations of Cs-137 at theKuroshio extension and its southern part were lower than those of thenorthern stations at144˚E. The Kuroshio extension seemed to prevent thesouthward dispersion of radioactive Cs. After one year, the highconcentration area seemed to be moved to 155˚E and 175˚E, and the levelof Cs-137 at 144˚E line was declined one order of magnitude. Our results
showed similar dispersion patterns with the simulation model (Fig. 2 (c),Kobayashi et al., 2012), and these results strongly suggest that the highCs-137 water masses (ca. 10 mBq/kg) are located at the center of North
Pacific after one year from the FNPP accident. 	

FDNPP	Fig. 1. Results of the radioactive Cs in the sea surface water around Japan. Samples were collectedduring June 2011 and July 2012. Red closed circles indicate Cs-134 and Cs-137 were detected, blue
closed circles do only Cs-137 was detected at background level (~2 mBq/kg), whereas both Cs-134
and Cs-137 were under the detection limit (< 5 mBq/kg) in the green open circles as results of shorttime measurements.	

(a)	

3. Off Tohoku area (Vertical distribution)!

2011/6/29-7/29	

Cs-134 and Cs-137!
(mBq/kg)	

Cs-134 and Cs-137!
(mBq/kg)	

Depth (m)	FDNPP

Depth (m)	ME05	 MO12	

St. ME05	

Depth (m)	(c) 37˚25ʼN	

St. MO12	

(d) 147˚E	

Fig. 3. (a) Location ofsampling stations.
Satellite image of sea
surface height wassuperimposed. (b)Vertical profiles of
Cs-134 (red circle)
and Cs-137 (blackcircle) at two stations(ME05 and MO12)
near the cycloniceddy. White symbols
indicate detection
limit, since Cs-134
and Cs-137 were notdetected. (c) Contourof Cs-137 at 37˚25ʼNtransect. Seawater
samples werecollected at black
circles. (d) same as(c) at 147˚E transect. 	

2012/7/2-8/1	!
(b)	
44.5
44.0 !

Latitude (˚N)	

The concentrations of Cs-134 and Cs-137 were highest at surface layer anddecreased with depth at almost all stations. Near the cyclonic eddy front, Cs-134
and Cs-137 in surface water seemed to be dispersed to deeper layer due to theunstable state of water properties with mixing the differential water masses (Fig. 3(b)). !
Relatively complex pattern of Cs-137 were observed at the transects along with37˚25ʼN (Fig. 3(c)) and 147˚E (Fig. 3(d)). The highest Cs-137 (70 mBq/kg) wasobserved from surface water at 37˚30ʼN-147˚00ʼE. On the other hands, Cs-137 werevery low at 40˚N-145˚00ʼE, where the Oyashio water was dominant. The sub-surface
peaks of Cs-137 were observed at 150m and 300m at the stations located near theKuroshio axis (35˚30ʼN and 34˚30ʼN at 147˚E) and at 100m of 37˚25ʼN -147˚E, those
may be transported from shallower water of coastal area. !
The horizontal and vertical distribution pattern of radioactive Cs off Tohoku area
seemed to be affected by the water mass interaction among Oyashio water,Kuroshio water and heavily contaminated coastal water off the FDNPP.	
(b)	
(a)	

43.5 !

44.5 !
2011!
44.0 !
2012!
43.5 !

44.5 !
2011!
44.0 !
2012!
43.5 !

43.0 !

43.0 !

43.0 !

42.5 !

42.5 !

42.5 !

42.0 !

42.0 !

42.0 !

41.5 !

41.5 !

41.5 !

41.0 !

41.0 !

41.0 !

40.5 !

40.5 !

40.5 !

40.0 !

40.0 !

40.0 !

39.5 !

39.5 !

39.5 !

39.0 !

39.0 !

39.0 !

38.5 !

38.5 !

38.5 !

38.0 !

38.0 !

38.0 !

37.5 !

37.5 !

37.5 !

37.0 !

37.0 !

37.0 !

36.5 !

36.5 !

36.5 !

36.0 !

36.0 !

36.0 !

35.5 !

35.5 !

35.5 !

35.0 !

35.0 !

35.0 !

34.5 !

34.5 !

34.5 !

34.0 !

34.0 !
144˚E	
33.5 !

34.0 !
155˚E	
33.5 !

175˚E	

10!

10!

33.5 !
1!

10!

100! 1!

100! 1!

2011!
2012!

(c)	
2011/9/18	

2012/9/12	100!

Cs-137 (mBq/kg)	
Fig. 2. (a) Location of sampling stations in 2011 and 2012, sea surface current estimated from sea
surface height were superimposed. (b) Concentration of Cs-137 at each station. Error bars indicate
counting error (1σ). (c) Model results of Cs-137 dispersion in the North Pacific on September 2011 andSeptember 2012, model calculation based on JAMSTEC/DrC and WSPEEDI (Kobayashi et al., 2012) .
Sampling area of this study were shown as black lines. !
*Kobayashi et al., 2012: Public WS presentation file, http://nsed.jaea.go.jp/ers/environment/envs/FukushimaWS/jaea3.pdf	

Summary!
Ø The elevation of the radioactive Cs was observed off theeastern Japan coast, but no increase of radioactive Cs wasobserved in the Japan Sea, East China Sea and Seto InlandSea!
Ø After one year from the FDNPP accident, the high radioactive
Cs water seemed to be located at the center of North Pacific!
Ø Complex vertical distributions of radioactive Cs observed off
Tohoku area, are suggesting that Oyashio-Kuroshio transitionzone plays an important role to decide the vertical distribution
patterns of radioactive Cs in the North Pacific!
Ø Continuous monitoring of radioactive Cs in seawater shouldbe important to comprehensively understand the impact ofFDNPP accident to the oceanic environment	

Acknowledgements: We thank the captains, officers, crews and researchers of R/V Hokko-maru, R/V Wakataka-maru, R/V Mizuho-maru, R/V Soyo-maru, R/V Shirahuji-maru, R/V Yoko-maru (FRA), R/V Shoyo-maru (Fisheries Agency) and R/VKaiun-maru (Aomori prefecture) for their help and support collecting the seawater samples during the cruises. This work was supported in part by Fisheries Agency, Japan.