Scientiﬁc or Technical Note

Radiation Safety Management Vol. 15 (9–14)

Observation of radiocesium in seabed soil at the Notsuke Strait of the southern
Okhotsk Sea derived from the Fukushima Daiichi Nuclear Power Plant
Yasushi NABAE1), 2), Sunao MIYASHITA1), and Satoru NAKASHIMA1), 3)
Department of Chemistry, Graduate School of Science, Hiroshima University,
1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan
2)
Hydrographic and Oceanographic Department, Japan Coast Guard,
3-1-1 Kasumigaseki, Chiyoda-ku, Tokyo 100-8932, Japan
3)
Natural Science Center for Basic Research and Development, Hiroshima University,
1-4-2 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan
1)

Received Jan. 8, 2016; accepted Apr. 25, 2016

Seabed soils of the southern Okhotsk Sea at the Notsuke Strait, Hokkaido were collected, and the radioactivity was measured with a Ge semiconductor detector. Trace amount of radiocesium derived from the Fukushima
Daiichi Nuclear Power Plant was detected. It was suggested by combining other results that this radiocesium wasoriginally deposited in western Fukushima from the Fukushima Daiichi Nuclear Power Plant, and migrated to theJapan Sea with the Agano River, and then carried to the Notsuke Strait by Tsushima Warm Current and Soya
Warm Current.
Key words: fukushima daiichi nuclear power plant, okhotsk sea, radiocesium, agano river, notsuke strait[doi:10.12950/rsm.160108]

1.

Introduction

Tokyo Electric Power Company (TEPCO) Fukushima

Daiichi Nuclear Power Plant (FDNPP) accident occurred in

March 2011. Large quantity of radioactive material was spreadin environment, and the area around Fukushima was contami-

nated with the radioactive material . The direction of the wind1)

the early time. 38 Bq/kg (wet) of 134Cs was detected at theAgano River mouth mud in the investigation of August 2011. In

addition, 72 Bq/kg of 134Cs was observed in the seabed soil of 20

m in depth at the sea area of the Agano River mouth offing4–5).

In June 2011, 1.9 mBq/L of 134Cs was detected at surface

seawater of Ishikari Bay in the northern Japan Sea, and 3.14

at the time of the release, a passage course of plume and its pre-mBq/L of 137Cs which was 3.3 times of the preceding year was

the earth surface soil by rain or snow migrates with the wind or

and the Okhotsk Sea, and 134Cs was detected at the northeastern

cipitation were related to the quantity of sediment of radioactive

material in each place2). The radioactive material deposited onflow of water. Ministry of Education, Culture, Sports, Scienceand Technology detected radioactive material deposited in theupper reaches of the Agano River by plane monitoring3). The
Agano River flows from Fukushima and Gunma to Niigata.

Niigata Prefecture continued the environmental radioactiv-

ity investigation to know the influence by FDNPP accident fromNatural Science Center for Basic Research and Development,
Hiroshima University,
1-4-2 Kagamiyama, Higashi-Hiroshima 739-8526, Japandetected in the NO-10 point in the southern Okhotsk Sea6–7). In

addition, some investigations were carried out in the Japan Sea

part of the Japan Sea, Hokkaido coast and Tohoku coast 8–9).

In the present study, seabed soils at the Notsuke Strait in thesouthern Okhotsk Sea were collected, and an extremely small

amount of the radioceasium migrated from FDNPP was con-firmed.

TEL: 082-424-6291 FAX: 082-424-0700
E-mail: snaka@hiroshima-u.ac.jp

9

Scientific or Technical Note

2-2. Method for measurement of radiocesium

A germanium semiconductor (GEM40-76-XLB-C made by

Okhotsk Sea

SEIKO EG & G Co., Ltd) was used for the measurement of the

NO-10radioactivity of the sample.

Japan Sea

The g radioactivity of samples (St.1–St.9) on February 23,

2015 was measured for 86,400 seconds. Sample of St.2 showed

the highest count of 134Cs in all samples, therefore it was re-

Pacific

measured as St.2* for 250,000 seconds. The concentrations of

Cs and 137Cs were calculated by official method 10), and the134

results of the measurements were corrected to the quantity ofradioactivity of the sampling day. Because the emission ratio ishigh and the separation with other nuclides is easy, 605 keV of apeak was chosen for 134Cs. And 662 keV of a peak was chosen

Notsuke Strait

for 137Cs. The samples were dried after the measurement and theweights were measured to obtain water ratio of the sample, andthen the radioactivity concentration in dry sample was calcu-

St.31

lated. This measurement was carried out in Hiroshima approxi-

St.30

mately 800 km away from Fukushima. In addition, this detector

was not affected by the Fukushima accident.

St.29

Shibetsu

St.28 St.9
St.8
St.27 St.1
St.33
St.3
St.2
St.32

The samples (St.27–St.33) of the additional investigation on

St.6
St.7
St.4
St.5

February 6, 2016 of dry seabed soil (2,000 cm3) were measured

for 250,000 seconds. And the sample of St.33 was measured

again as St.33* for 600,000 seconds, because the 134Cs concentration was the highest among them. This measurement was car-Fig. 1. Sampling points.
Seabed soils were collected at the Notsuke Strait in Okhotsk
Sea. Seabed soils of St.1–9 were collected on February 23,
2015. Those of St.27–33 were collected on February 6, 2016.
The Japan Coast Guard investigates NO-10 every year.

2.

Sampling and experimental method

2-1. Sampling

On February 23, 2015, we collected seabed soil at the

Notsuke Strait of the Sibetsu-cho, Hokkaido. The Notsuke Strait

is a sea area where a part of the Tsushima Warm Current of theJapan Sea flows out into the Pacific. Figure 1 shows detailed

sampling points of seabed soil. The seabed soil samples werecollected using Ekman-Berge bottom sampler, and the stones

and the visible creature with naked eye were removed. Samples

were stirred and were put in U8 container in a wet state. As an

additional investigation, we collected seabed soil of surface

layer (2 cm) by a diver on February 6, 2016.

10

ried out in Tokyo. The background measurement was carriedout, and the influence from environment was removed.

3.

Results and discussion

Table 1 shows the latitude, longitude, and depth of theseabed soil sampling points. The depth in this sea area is partic-

ularly shallow in the Notsuke Strait, and the sea area shallower

than 20 m is widely spread. The sampling points were chosen in

the southwest side of the Notsuke Strait, because these sampling

points were supposed that the sediment is thick. The depth of theshallowest sampling point was 4.0 m, and that of the deepest

sampling point was 15.2 m. Some seabeds of the sampling

points included sand, some included few clay and silt, and someincluded small stones. Because the seabed soils of St.6, 30, 31

were small in quantity and did not include the clay, the seabedsoils were not collected. A lot of clay and silt were included in

St.32 and St.33.

Table 2 showed the sampling day, the volume of measure-ment, the time of measurement, 134Cs concentration and 137Csconcentration for the sample of the seabed soil. 0.033 ± 0.0084

Bq/kg of 134Cs was detected for a sample of St.33.

Nabae et al.

2.0

145c
12ö
15÷E

8.0

St. 2

43c
38ö
21÷N

145c
13ö
23÷E

8.6

St. 3

38ö
46÷N
43c

145c
14ö
31÷E

5.4

St. 4

43c
38ö
43÷N

145c
17ö
14÷E

5.0

St. 5

43c
38ö
14÷N

145c
19ö
26÷E

8.4

St. 6

43c
40ö
36÷N

145c
17ö7÷E

10.4

St. 7

43c
39ö
30÷N

145c
17ö
31÷E

12.3

St. 8

43c
40ö
44÷N

145c
14ö
21÷E

15.2

St. 9

43c
41ö
12÷N

145c
11ö
26÷E

13.2

St. 27

43c
40ö
20÷N

145c7ö
56÷E

4.0

St. 28

43c
41ö
30÷N

145c7ö7÷E

5.0

St. 29

43c
43ö
42÷N

145c5ö
37÷E

9.0

St. 30

43c
45ö
32÷N

145c4ö2÷E

5.0

St. 31

43c
47ö
21÷N

145c3ö
55÷E

5.0

St. 32

43c
38ö
21÷N

145c
13ö
23÷E

10.0

St. 33

43c
39ö
56÷N

145c8ö8÷E

5.5

Figure 2 shows relation between the concentrations of 134Cs

and 137Cs for each sample for the additional investigation.

0.0materials were spread in environment by FDNPP accident wasapproximately 1. And the slight difference in the ratio among

FDNPP 1–3 nuclear reactors was reported. The ratio of the firstnuclear reactor was around 0.89–0.93 (average: 0.91). The ratioof the second nuclear reactor was around 0.96–1.05 (average:

1.0). The ratio of the third nuclear reactor was around 0.97–1.04(average: 1.01)11). The 134Cs/137Cs ratio means that we can esti-

mate concentration of 137Cs from the concentration of 134Cs in
the Fukushima accident. At St.33 on February 6, 2016, the con-centration of 134Cs was 0.034 ± 0.0086 Bq/kg, and the concen-

tration of

Cs was 1.43 ± 0.015 Bq/kg. When these137

concentrations were corrected to the values of March 11, 2011,

0.0

0.5

1.0
Cs [Bq/kg]

1.5

2.0

Fig 2. Relation of concentrations between 134Cs and 137Cs atseabed soil of Notsuke Strait.
The data were corrected on the values of March 11, 2011.
Although the concentrations of 134Cs except for St.33* were
judged to be ND, the obtained values were included in Fig. 2 toknow the relation.

1900
1700
1500
1300

Cs137 662 keV

1100
900

to know the relation. The values were corrected on the values of

The 134Cs/137Cs radioactivity ratio when the radioactive

St.27
St.32

134

judged to be ND, the obtained values were included in Figure 2

out.

St.28
St.29

1.0

0.5

Although the concentrations of 134Cs except for St. 33 were

March 11, 2011 to estimate the concentration of the global fall-

St.33*

1.5

x

43c
40ö
12÷N

1.
0

St. 1

=

Depth [m]Cs134 605 keVLongitudeCs [Bq/kg]

Latitude

137

No.

y

Position and Depth at each sampling points

Count

Table 1

700
500
550

600

650
700
Energy [keV]

750

800

Fig 3. Gamma-ray spectrum of the seabed soil from St.33* of
the Notsuke Strait.
Energy and Counts of 134Cs and 137Cs are shown in the ﬁgure.
Sampling position: St.33* of the Notsuke Strait (43 ° 39¢ 56¢¢ N,
145 ° 8¢ 8¢¢ E)
Sampling date: February 6, 2016. Amount for the measurement:
2187.56 g.
Measurement date: February 26, 2016. Measurement time:
600,000 s.
Germanium semiconductor: GEM40-76-XLB-C made by SEIKO
EG & G Co., Ltd

the concentration of 134Cs is expected to be 0.17 ± 0.045 Bq/kg,

and the concentration of 137Cs is 1.60 ± 0.017 Bq/kg. Whenradiocesium was released by the Fukushima accident, theCs/137Cs ratio is expected to be approximately 1.0 at the time

134

11

Scientific or Technical Note

Table 2

Results of the measurement of the sample

No.

Sampling day

Volume [cm3]Time [s]134Cs [Bq/kg]

137Cs [Bq/kg]

St. 1

Feb 23, 2015100

86,400ND (0.19)

ND (0.33)

St. 2

Feb 23, 2015100

86,400ND (0.33)

ND (0.20)

St. 2㈙Feb 23, 2015100

250,000

ND (0.14)

0.41d0.14

St. 3

Feb 23, 2015100

86,400ND (â)ND (0.25)

St. 4

Feb 23, 2015100

86,400ND (0.22)

ND (0.39)

St. 5

Feb 23, 2015100

86,400ND (â)ND (0.16)

St. 6

Feb 23, 2015

â

â

â

â

St. 7

Feb 23, 2015100

86,400ND (0.19)

ND (0.19)

St. 8

Feb 23, 2015100

86,400ND (â)ND (0.02)

St. 9

Feb 23, 2015100

86,400ND (0.09)

ND (â)

St. 27

Feb 6, 2016

2,000

250,000

ND (0.01)

0.35d0.012

St. 28

Feb 6, 2016

2,000

250,000

ND (0.02)

0.95d0.018

St. 29Feb 6, 2016

2,000

250,000

ND (0.01)

0.86d0.015

St. 30Feb 6, 2016

â

â

â

â

St. 31

Feb 6, 2016

â

â

â

â

St. 32

Feb 6, 2016

2,000

250,000

ND (0.02)

0.28d0.013

St. 33

Feb 6, 2016

2,000

250,000

LTD (0.03)

1.45d0.023

St. 33㈙Feb 6, 2016

2,000

600,000

0.034d0.0086

1.43d0.015

Gamma ray of the seabed soil of the Notsuke Strait was measured. Asterisk(㈙) shows the results measured for longer
time again. Although the concentrations of 134Cs except for St. 33㈙were judged to be ND, the obtained values were included in the parenthesis to know the relation. We were not able to measure the samples of St. 6, St. 30, and St. 31 because the amount is very few.

of accident. From this fact, the concentration of 137Cs from theglobal fallout at St.33 is supposed to be 1.60–0.17 = 1.43 Bq/kg

on March 11, 2011. In addition, on Figure 2, the vertical lengths

to St.27, 28, 29, 32 points from line of y = 1.0 x are supposed tobe the concentration of 137Cs from global fallout.

The Notsuke Strait is located at the offing of Shibetsu-cho,

Hokkaido in southern part of the Okhotsk Sea. Ministry of

Education, Culture, Sports, Science and Technology investi-

gated the radioactivity of the whole Japan by plane and carried itout in Hokkaido from April to May, 2012. The air dose rate of

the altitude of 1 m from the earth surface of Hokkaido is shownin the investigation report12), and this shows sedimentation ofradiocesium on the earth surface soil. According to the report ofthis investigation, 134Cs and 137Cs were less than 10 kBq/m2 inwhole Hokkaido. It was the lowest level in the whole Japan. The

12

remarkable deposition was not reported in it. According to thereportofthe

Hokkaido

Government

Administration Department, the radioactivity ofAgriculturalCs was all

134

ND in the earth surface soil radioactivity investigation including

farmland of the whole Hokkaido. And the radioactivity concen-

tration of 137Cs was less than the value before the FDNPP accident13). From the result above, it is thought that there were very

little 134Cs which flowed out from the river of the neighborhoodof Shibetsu-cho into the sea area.

In the sea area, in April 2010 of the preceding year of the

FDNPP accident, the Japan Coast Guard collected surface sea-water in offing of Monbetsu, Hokkaido (northeast offing approx-

imately 74 km far from Monbetsu, 44 ° 50¢ N, 144 ° 00¢ E) in thesouthern

Okhotsk

Concentration of

Sea,

and

investigatedradioactivity.

Cs was 0.96 ± 0.04 mBq/L, but in June 2011137

Nabae et al.
Soya Warm
Current

Notsuke Strait

Tsu
sh

C

urrent

ima

m
W ar

FDNPP

Agano river mouth

Fig 4. Tsushima Warm Current and Soya Warm Current 14).
The Tsushima Warm Current and the Soya Warm Current contribute to the migration of Cs from the Agano
River mouth to the Notsuke Strait.

of the next year, 3.14 ± 0.07 mBq/L that was 3.3 times higher

than the preceding year was detected in the investigation of thesame sea area

. The method of Japan Coast Guard was that the

6–7)

Cs of the surface seawater was adsorbed by ammonium

phoshomolybdate and was separated with the cation exchange

resin. From this, it can be judged that ionic cesium exists insurface seawater, and Cs of NO-10 point migrated to the

Notsuke Strait by the ocean currents and was adsorbed to theseabed soil. In addition, there is the ocean current in this nearby

sea area, which is shown in Figure 414). The Tsushima Warm
Current flows near the Agano River mouth and a part of the

Tsushima Warm Current flows through the Soya Strait. The

Soya Warm Current flows to the Pacific. The physical property(Results of Surveys in 2010) by the Japan Coast Guard, the aver-age concentration of 137Cs of the seabed soil was 2.5 Bq/kg on

the Japanese peripheral sea area. However, the concentration of

Cs of the seabed soil at the Notsuke Strait was extremely low

137

than the average concentration of 137Cs of the seabed soil on theJapanese peripheral sea area. It is known that Cs is easily

adsorbed by clay. A lot of coarse sand and shells were includedin the seabed soil collected in the Notsuke Strait, but there were

few clay and silt. Therefore, it is supposed that there was little

quantity of adsorption of the Cs by the global fallout. Similarly,

the Cs derived from FDNPP is supposed that there was a little

quantity of adsorption. Japan Coast Guard investigates theradioactivity of the surface water every year. The concentrationof these ocean currents gradually changes, but these ocean cur-of 137Cs at NO-10 point (Figure 1) increased in 201116). It is

fallout deposited in the seabed soil of the Japanese peripheral

the present study is reached from the FDNPP accident. We canrents are a consecutive ocean current for the material transporta-tion15). It is known that the radioactive materials by the global
sea area. According to a Report of Radioactivity Surveys

thought that some of these 137Cs were adsorbed in the seabed soilat the Notsuke Strait. It can be judged that the 134Cs obtained insuppose that radioactive material released by TEPCO FDNPP
13

Scientific or Technical Note

accident reached this sea area by the Tsushima Warm Current

and the Soya Warm Current from the Agano River mouth. We

continue the sampling and the radioactivity measurement of theseabed soils at Sakata offing and Soya offing.

4.

Conclusions

For the investigation of the migration pathway, we sampled

the seabed soil at the Notsuke Strait and measured the g-ray

spectrum with the Ge semiconductor detector. The g-ray spec-

trum had peak of 134Cs at 605 keV and that of 137Cs at 662 keV.
We measured concentration of 134Cs with 0.033 ± 0.0084 Bq/kg,

and the concentration of 137Cs with 1.4 ± 0.015 Bq/kg. The

radioactivity investigation with the plane and the farmland earth

surface soil radioactivity investigation did not show the remark-able deposit of artificial radioactive materials in Hokkaido.

From these results and consideration of ocean current, it wassuggested that the radioactive material released from FDNPP

accident reached the Notsuke Strait from the Agano Rivermouth by the Tsushima Warm Current and the Soya Warm

Current.

5.

Acknowledgement

The present research was supported through the Hiroshima

University Phoenix Leader Education Program for the“Renaissance from Radiation Disaster”, funded by the Ministryof Education, Culture, Sports, Science and Technology.

6.

14

References

1) Nuclear Regulation Authority HP.
http://radioactivity.nsr.go.jp/ja/contents/5000/4901/24/1910_1216.pdf(Accessed Sep 17, 2015)
2) Kinoshita, N., Sueki, K., Sasa, K., Kitagawa, J., Ikarashi, S.,
Nishimura, T., Wong, Y. S., Satou, Y., Handa, K., Takahashi, T.,
Sato, M., and Yamagata, T.: Assessment of individual radionuclide distributions from the Fukushima nuclear accident covering
central-east Japan: Proc. Natl. Acad. Sic. USA, 108, 19526–19529(2011).
3) Habuki, H., Ishiyama, H., Yoko, T., Ono, T., Abe, A., Komata, O.,
Maruta, F.: Research of radioactive cesium distribution around
Agano River downstream due to Fukushima Daiichi Nuclear
Power Plant accident, Radioisotopes, 62, 649–658 (2013), (inJapanese)

4) Niigata Prefecture HP.
http://www.pref.niigata.lg.jp/HTML_Article/897/413/ver2011_all,0.pdf(Accessed Sep 17, 2015)
5) Inoue, M., Kofuji, H., Oikawa, S., Murakami, T., Yamamoto, M.,
Nagao, S. Hamajima, Y., Misonoo. J.: Spatial variations of lowlevels of 134Cs and 137Cs in seawaters within the Sea of Japan after
the Fukushima Dai-ichi Nuclear Power Plant accident: Appl.
Radiat. Isot. 81, 340–343 (2013).
6) Report of Radioactivity Surveys Results of Surveys in 2011, Japan
Cost Guard, 5–15 (2013).
7) Report of Radioactivity Surveys Results of Surveys in 2012, Japan
Cost Guard, 12–14 (2012).
8) Inoue, M., Kofuji, H., Hamajima, Y., Nagao, S., Yoshida, K.,
Yamamoto. M.: 134Cs and 137Cs activities in coastal seawater along
Northern Sanriku and Tsugaru Strait, northeastern Japan, after
Fukushima Dai-ichi Nuclear Power Plant accident: J. Environ.
Radioact. 111, 116–119 (2012).
9) Inoue, M., Kofuji, H., Nagao, S., Yamamoto, M., Hamajima, Y.,
Yoshida, K., Fujimoto, K., Takada, T., Isoda. Y.: Lateral variationof 134Cs and 137Cs concentrations in surface seawater in and around

the Japan Sea after the Fukushima Dai-ichi Nuclear Power Plant
accident: J. Environ. Radioact. 109, 45–51 (2012).
10) Japan Chemical Analysis Center HP.
http://www.kankyo-hoshano.go.jp/series/main_pdf_series_7.html(Accessed Jan 2, 2016)11) Komori, M., Shozugawa, K., Nogawa, N., Matsuo, M.: Evaluationof Radioactive Contamination Caused by Each Plant ofFukushima Daiichi Nuclear Power Station Using 134Cs/137Cs
Activity Ratio as an Index, Bunseki Kagaku, 62 (6), 475–483(2013), (in Japanese)
12) Nuclear Regulation Authority HP.
http://radioactivity.nsr.go.jp/ja/contents/6000/5847/24/203_0727.pdf(Accessed Aug 25, 2015)13) Hokkaido Prefecture HP.
http://www.pref.hokkaido.lg.jp/ns/gjf/dojomonitoring.htm(Accessed Sep 19, 2015)14) “Sailing Directions for South and East Coasts of Honshu”, Japan
Cost Guard, 7 (2012).
15) Nakata, A., Tanaka, I.: Seasonal and year-to-year variability ofbaroclinic volume transport of the Tsushima Warm Current, off
the west of Hokkaido: Scientific Reports of Hokkaido Fisheries
Experimental Station, 63, 1–8 (2002).
16) Report of Radioactivity Surveys Results of Surveys in 2010, Japan
Cost Guard, 8-16 (2011).