137

Summary of Radioactive Cs Dynamics Studies in Coastal Areas andAssessment of River Impacts
Toshiharu MISONOU*, Tadahiko TSURUTA, Takahiro NAKANISHI and Yukihisa SANADA
Remote Monitoring Research Group, Fukushima Environmental Monitoring Division, Japan Atomic Energy Agency
45-169 Sukakeba, Kaibama, Haramachi-ku, Minamisouma-shi, Fukushima, 975-0036, Japan*E-mail: misono.toshiharu@jaea.go.jp

Abstract
A large amount of radioactive Cs was released into the environment from the Fukushima Daiichi nuclear
power plant (FDNPP) following the 2011 Great East Japan Earthquake. Nine years have passed since this
accident, and the radioactive Cs concentration in seabed sediments has decreased. The behavior of radioactive
Cs in coastal areas, however, is complicated due to the influence of rivers, waves and so on. The Japan Atomic
Energy Agency has conducted several studies at the mouth of the Ukedo River. Here, we review previous
studies and evaluate the impact of the radioactive Cs supplied by rivers using a sediment trap. In this study, amooring system consisting of a sediment trap was installed in the Ukedo River estuary near the FDNPP in
2017. The results showed 137Cs flux in winter accounting for 60% of the annual flux. This suggests thatmobilization of radioactive Cs in the coastal area is primarily due to resuspension and not river discharge.
Key words :	estuary, Fukushima Daiichi nuclear power plant accident, resuspension, sinking particle,
Ukedo River

1. Introduction
The Tokyo Electric Power Company Holdings(TEPCO) Fukushima Daiichi Nuclear Power Station(FDNPP) released a large amount of radioactive Cs into
the environment following the 2011 Great East Japan
Earthquake and Tsunami. The released radioactive Cs was
deposited over wide land and sea areas (Hirose, 2016;
Mishra et al., 2019). The amount of 137Cs released fromthe FDNPP has been reported to be 18 PBq (Katata et al.,2015). In particular, the 137Cs inventory of seabedsediments shallower than 100 m is 0.16 PBq (Otosaka &Kato, 2014) and tends to be higher in coastal areas(Kusakabe et al., 2013). The 137Cs inventory of surface
sediments shallower than 100 m decreased by 27%
annually from 2011 to 2015 (Otosaka, 2017). While theradioactive Cs concentration in coastal seabed sediments
has been decreasing yearly, seabed sediments withrelatively high radioactive Cs content have been observed
locally (NRC, 2018). Radioactive Cs in seabed sediments
is strongly adsorbed on fine sediment particles and has alow elution rate to seawater (Otosaka & Kobayashi,2013). Therefore, it is suggested that seabed sedimentfunctions as a sink for radioactive Cs in marine
environments. Previous sampling of seabed sediments
has been as shallow as the layer to approximately 20 cm
from the surface (Arita et al., 2014; Ambe et al., 2014). It
137
is difficult, however, to evaluate the Cs inventory of
Global Environmental Research
24/2020: 137-144
printed in Japan

seabed sediments, because information concerning
radioactive Cs in the depth direction has been insufficient.
Therefore, it is important to collect deep cores andanalyze the amount of radioactive Cs with depth.
As mentioned above, the coastal area is expected to
be a dynamic area with respect to radioactive Cs evenseveral years after the accident because of the impact ofriverine 137Cs discharge. For evaluating the future
distribution of radioactive Cs throughout the entire sea
area, it is important to predict radioactive Cs distribution
in the estuary. Therefore, in this paper, we first review
several studies conducted by the Japan Atomic Energy
Agency (JAEA) in the Ukedo River estuary. In addition,
to determine the relationship between seafloor sediments
and radioactive Cs supplied from rivers, we focused on
evaluating the amount of radioactive Cs from rivers andsinking particles collected using sediment traps (Fig.1).

2. Distribution of Radioactive Cs in
the Ukedo River Estuary
JAEA has conducted several studies focusing on the
behavior of radioactive Cs in the Ukedo River area since2012. From calculation of the amount deposited using
data from the 4th Airborne Monitoring Survey (MEXT,
2011), the amount of radioactive Cs deposited in the
Abukuma River basin (578 TBq) and the Ukedo Riverbasin (536 TBq) is almost the same. However, the©2020 AIRES

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T. MISONOU et al.

River inflow(Particulate 137Cs)

Sinking

Sediment trap isinstalled to understandthe behavior of sinking
particles.
(This study)

Variation of 137Cs concentrationin seabed sediments(Sanada et al., 2018)

Resuspension
Distribution of seabed sediments and 137Cs inventory(Tsuruta et al.,2017)

Vertical disturbanceFig. 1 Positioning of this study.

Fig. 2 Predicted seabed sediment map (Tsuruta et al, 2017). Location of FDNPP and Ukedo River estuary.

catchment area of the Ukedo River is 1/10 that of the
Abukuma River, meaning that the concentration ofradioactive Cs supplied to the ocean by the Ukedo Riveris much higher than that supplied by the Abukuma River(Sakuma et al., 2019a). Therefore, the Ukedo River likely
has a large impact on the surrounding coastal area.
The distribution of seabed sediments (silt, fine tomedium sand, coarse sand to granules, bedrock, andartifacts) in the Ukedo River estuary has been
investigated using two methods, depth measurements andacoustic soundings (Tsuruta et al., 2017). The surface
area ratios of bedrock and seabed sediments are 65% to2
35% (the total area investigated was about 30 km ),

respectively (Fig. 2). Sediment accumulation is restricted
to the area south of the Ukedo fishing port in two
elongated zones. Fine to medium sand (with a surface
ratio of 24%) is the main constituent of the seabedsediment. Fine sediment containing a large amount ofclay minerals (such as silt) is thought to have highradioactive Cs concentrations (Bostick et al., 2002), but
the proportion of fine sediments (≦0.064 mm) was foundto be only 0.4%. From results of core sampling of seabedsediments using vibrocoring techniques (to a maximum
depth of 100 cm), an inventory of radioactive Cs in the
Ukedo River estuary sediment was made (Tsuruta et al.,1372017). Figure 3 shows the relationship between Cs

Summary of radioactive Cs dynamics studies in coastal areas and assessment of river impacts

139

Fig. 3 Relationship between 137Cs inventory and water depth (Tsuruta et al., 2017).

Fig. 4 Contour maps of 137Cs concentrations in sea sediment (surface) near the Ukedo River estuary (Sanada et al., 2018).

inventory and water depth. The results obtained fromJanuary to March 2014 at the mouth of the Ukedo River(Tsuruta et al., 2017) are compared with the results of
Otosaka and Kato (2014) (sampling period from 2011 to2012) and Black and Buesseler (2015) (sampling period
from 2012 to 2013). The 137Cs inventory in the shallow
area (water depth: < 30 m) is remarkably larger than that
offshore. This suggests that the 137Cs distribution near theestuary, where the basin has a high level of radioactive
contamination, is significantly affected by discharged
137
Cs from the Ukedo River.
A secular change in the 137Cs concentration in seabedsediments has been reported (Sanada et al., 2018). An in
situ measurement of the radioactive Cs concentration inseabed sediments has been attempted by focusing on
unmanned surface vehicle technology (Rathour et al.,2015) to enable continuous and wide-area monitoring.
The distribution of radioactive Cs concentrations in theseabed sediments was measured using an unmanned
observation ship, Windy3S, at the mouth of the Ukedo
River seven times from October 2015 to October 2017
(Sanada et al., 2018; Fig. 4). In the measurement area set
137
in the estuary, the Cs concentration in the surface layer
tended to differ for each observation period. Also, the137
Cs inventory tended to decrease over time. This was ingood agreement with the physical decay of 137Cs. There
are also reports that radioactive Cs has been detected
relatively deep in sediments (Tsuruta et al., 2017). These

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Table 1 137Cs discharge ratio from different catchments (Sakuma et al., 2019a).

River
AbukumaOdaka
Ukedo
Takase (Ukedo River tributary)
Maeda
Kuma
Tomioka
Total of 13 rivers in Fukushima coastal regionaa

Cs discharge (TBq) and discharge ratio (%)

137

11 March 2011 – 27 September 2011
18 TBq (3.1%)
0.12 TBq (0.34%)
3.2 TBq (0.89%)
1.6 TBq (0.88%)
1.0 TBq (1.1%)
0.91 TBq (1.0%)
0.25 TBq (0.64%)
11 TBq (0.79%)

11 March 2011 – 31 December 2017
25 TBq (4.3%)
0.27 TBq (0.75%)
6.3 TBq (1.8%)
3.7 TBq (2.0%)
1.9 TBq (2.0%)
2.0 TBq (2.3%)
0.55 TBq (1.4%)
23 TBq (1.7%)

 stimated using catchment137 Cs inventory at each other catchment in the Fukushima coastal region and the average137 Cs discharge ratio from six riverEbasins such as the Odaka, Ukedo, Takase, Maeda, Kuma, and Tomioka Rivers.

Fig. 5 ( a) Bathymetric map and vibrocoring sampling points (Tsuruta et al., 2017). The black circle shows where the sediment trap was installed(St.B). (b) Water depth along Line 2. The solid line shows the surface of the seafloor, and the green range represents the distribution ofseabed sediments (Tsuruta et al., 2017). (c) The sediment trap was installed 1.5 m above the sea floor using the mooring system.

results suggest that radioactive Cs remains in themeasurement area. Therefore, it is suggested that there isa high possibility of local sediment movement near theestuary.
The discharge of 137Cs from the Ukedo River was
estimated using a tank model and L-Q equation (Sakuma
et al., 2019a). The release of 137Cs in the six months
following the FDNPP accident to the Ukedo Riveraccounted for approximately half of the total in thesubsequent period (approximately six years). After theaccident, 10 TBq (from the Ukedo and Takase rivers) of137
Cs was supplied to the Ukedo River estuary (Table 1).
The 137Cs inventory of seabed sediments in the Fukushimacoastal area (≦100 m) in 2011 was 160 TBq (Otosaka &Kato, 2014), so the influence of the Ukedo River cannot
be ignored.

3. Evaluation of Radioactive Cs Flux in the
Ukedo River Estuary
3.1 Materials and Methods
We installed a sediment trap to investigate themigratory behavior of sinking particles, which affect the
distribution of radioactive Cs in seabed sediments. Study
points were set up at the mouth of the Ukedo River,which is located approximately six kilometers from the
FDNPP (Fig. 5(a)). Our targets were points where studiesof the topographical shape and core sampling of theseabed sediments had been performed for a previous
report (Tsuruta et al., 2017). Because southward flow is
predominant along the Fukushima Prefecture coast (Yagiet al., 2013), we selected the area around Line 2 (Fig.
5(a)) on the south side of the Ukedo River. Line 2
includes flat and depressed terrain. Depressions tend tocontain high levels of radioactive Cs (Thornton et al.,Summary of radioactive Cs dynamics studies in coastal areas and assessment of river impacts2013), but they also suggest that sediment migration
might be restricted. Since depressions are a characteristic
place, we chose the area around No. 13 on the flat partand moored the earth and sand trap (Fig. 5 (b)).
We installed the mooring system shown in Fig. 5(c)at the study points (N 37.46866°, E141.0614°, water
depth: 16 m). The mooring system was equipped with asediment trap (SMC-7S500, NiGK Corporation, Japan).
During high water periods such as typhoons, sinking
particles were collected each day (measurement intervalof one day). Table 2 summarizes the sampling period forthe sediment trap. The 500-ml collection bottles in thesediment trap were pre-filled with 3.5% NaCl solution to
Table 2 Sampling period and measurement intervals of thesediment trap system

500

800

400

600

300

400

200

200

0

02/04
02/05
02/06
02/07
02/08
02/09
02/10

100

1600

1000

Mass flux (g m-2 day-1)

1400

800

1200

700

1000

600

800

500
400

600

300

400

200

200

08/31
09/01
09/02
09/03
09/04
09/05
09/06

100

0

1000

600
500

800

400

600

300

400

200

200

100
0

1600

1000
900

1400

800

1200

700

1000

600

800

500
400

600

300

400

200

200

100
0

0

0

1400

800

1200

700

1000

600
500

800

400

600

300

400

200

200

100
0

1600

1000
900

1400

800

1200

700

1000

600

800

500
400

600

300

400

200

200

100
0

0

0

1600

1000
900

1400

800

1200

700

1000

600

800

500
400

600

300

400

200

200

100
0

0

1000

9000

900

8000

800

7000

700

6000

600

5000

500

4000

400

3000

300

2000

200

1000

100
0

137Cs concentration (Bq kg-1)

900

0

0

1200

700

1600

900

08/22
08/23
08/24
08/25
08/26
08/27
08/28

1000

600

1400

800

1000

10/27
10/28
10/29
10/30
10/31
11/01
11/02

Mass flux (g m-2 day-1)

1200

700

900

137Cs concentration (Bq kg-1)

1400

800

1600

1000

06/06
06/07
06/08
06/09
06/10
06/11
06/12

900

3.2 Results
The eight sediment trap observations from February
2017 to November 2017 collected 55 sinking particle
samples. The mass fluxes and 137Cs concentrations of thesinking particles are shown in Fig. 6. Mass flux showed a

1day

03/08
03/09
03/10
03/11
03/12
03/13
03/14

1600

1000

measurement pitch

prevent backflow of the sinking particles.
The sinking particles were separated from theseawater using a centrifuge (CR22N, Hitachi, Japan).
Centrifugation was performed for 20 min at 3,000 rpm,
and the supernatant was discarded. To remove the salt
from the sample, deionized water was added to theremaining sample, which was then stirred andcentrifuged. This process was repeated three times. The
centrifuged sinking particles were dried at 105°C for
24 h, homogenized and then placed in a U-8 container.
137
The radioactive concentration of Cs in the sinking
particles was determined via gamma-ray spectrometry
using a high purity germanium detector (GMX40P4-76,
ORTEC, USA). The measurement times were from3,600 s to 302,400 s. The activity concentrations of 137Cs
in the samples were decay-corrected to the sampling date.
The 137Cs flux was obtained by multiplying the mass fluxof the sinking particles by the 137Cs concentration.

10/13
10/14
10/15
10/16
10/17
10/18

Number of samples
7
7
7
7
7
7
6
7

09/22
09/23
09/24
09/25
09/26
09/27
09/28

Sampling period
2017/2/4 – 2/10
2017/3/8 – 3/14
2017/6/6 – 6/12
2017/8/22 – 8/28
2017/8/31 – 9/6
2017/9/22 – 9/28
2017/10/13 – 10/18
2017/10/27 – 11/2

141

0

Fig. 6 Mass flux and 137Cs concentration data gathered in this study. Sediment trap observations were conducted
eight times in 2017. The black points show the 137Cs concentration (dry). The bars show the mass flux.

T. MISONOU et al.

large fluctuation range of 0.68–850 g m−2 day−1 and tended
to be high from winter to spring (February to June). Highmass fluxes were seen in September, which were thought
to be due to typhoons (Sanvu) and low pressure. The 137Csconcentrations varied from 91 to 7,900 Bq kg−1 andtended to increase from summer to autumn (August to
November).
3.2.1 Differences in 137Cs Flux between Coastal andOffshore Regions
137
Cs flux was calculated by multiplying the massflux by the 137Cs concentration. Figure 7 shows the sum of
the 137Cs fluxes for each observation period. The 137Cs
fluxes varied from 18 to 550 Bq m−2 day−1 and had large
fluctuations during each observation period. The 137Cs
flux of the sinking particles was compared to theminimum–maximum values reported in offshore regions(depth: 873 m) 100 km away from the FDNPP. The
offshore results indicated that the 137Cs flux ranged from
9.4 × 10−4 to 9.8 × 10−2 Bq m−2 day−1 (Otosaka et al., 2014).
There was a difference of several orders of magnitude
between the coastal and offshore 137Cs fluxes. Even
though these results cannot be unequivocally comparedbecause of the different collection periods and mooring
positions of the sediment trap, it is thought that the 137Cs
flux near the coast is higher than that offshore.
3.2.2 Estimation of the Origin of the Sinking 137Cs Flux
We examined variations in mass flux and 137Csconcentrations to estimate the origin of 137Cs fluxes. The
relationship between mass flux and 137Cs concentration
was found by linear regression analysis. The correlation
coefficient and P-value were found to be 0.01 and 0.43,
respectively (Fig. 8). The quantitative correlation between
the mass flux and the concentration of 137Cs is considered
to be low. On the other hand, Spearman’s rank ordercorrelation coefficient (Hogg & Craig, 1978) was − 0.71.
Therefore, it is thought that mass flux and radioactive Csconcentration tend to have a negative relationship. As
described in Section 3.2, the 137Cs concentration ofsinking particles tended to increase from summer toautumn. In particular, the maximum 137Cs concentration

137Cs flux (Bq m-2 day-1)

600
500
400
300
200
100
0

was observed in November. According to a reportpresenting measured particulate 137Cs concentrations in
the Ukedo River on October 27, 2017, the 137Csconcentration was 24.7 Bq g−1-dry. This was under highflow conditions (Nakanishi & Sakuma, 2019). Therefore,
it is suggested that the river flow increases from summer
to autumn (August to November), and the amount ofradioactive Cs inflow from the land area also increases.
In contrast to river discharge, the mass flux in thesediment trap experiment tended to be high from winterto spring (February to June) when the 137Cs concentration
was low. In winter, sediment discharge from the Ukedo
River tended to decrease (Sakuma et al., 2019b). It has
been reported that seabed sediments containing
radioactive cesium are resuspended in winter andtransported offshore (Otosaka et al., 2014). Moreover,resuspension due to waves has been reported in a coastal
area (Yagi et al., 2015). Therefore, it is thought that
seabed sediments with low 137Cs concentrations werecollected in the sediment trap due to resuspension caused
by waves in winter. The 137Cs flux obtained from theobservations was divided into winter to spring (February
to June) and summer to autumn (August to November).
As a result, the 137Cs flux in winter was found to be about
1.5 times that in summer. The contribution of winter 137Cs
flux to the annual 137Cs flux was 60%, suggesting that
resuspension had a significant impact in coastal areas.
3.2.3 Estimation of Annual Cs Deposition
The annual total deposition of 137Cs estimated fromthe observation results of 137Cs flux was 11.4 kBq m−2.
The 137Cs inventory of the seabed sediments sampled in2014 was 72 kBq m−2 (Tsuruta et al., 2017). The
contribution of 137Cs flux of sinking particles to the seabedsediments was about 16% (including river inflow andseabed sediment resuspension). It has been reported that
rates of desorption and resuspension of seabed sediments
range from 0 –29 % (Otosaka, 2017), and the results ofthis study are also within that range.
It will be necessary to consider diffusion into
R2: 0.01
P-value: 0.43
y=0.015x

1000

Mass flux (g m-2 day-1)

142

100
10
1
0.1

10

100

1000

10000

137Cs concentration (Bq kg-1)

Fig. 7 Total value of 137Cs flux for each sampling period.

Fig. 8 
Relationship between mass flux and 137Cs concentration(dry). The R2, P-values and regression equations were
determined by linear regression analysis.

Summary of radioactive Cs dynamics studies in coastal areas and assessment of river impacts
seawater and the possibility of leaching of radioactive Cs
by seawater in future studies. It will also be necessary toconsider the balance of radioactive Cs supplied via rivers,and determining the contribution of sediment deposition
and resuspension by acquiring sinking particles and dataon seabed sediments and sea conditions will be important.

4. Summary
We obtained the following conclusions from a
review of previous studies in the Ukedo River area andthe results of measuring radioactive Cs concentrations ofsinking particles caught in a sediment trap.
• The radioactive Cs concentration in seabed sedimentsshows a decreasing trend. However, the location of
the peak value of the radioactive Cs concentration
tends to change. In addition, the results of the 137Cs
inventory during each observation period wereconsistent with the physical decay of 137Cs, suggestingthat the radioactive Cs flowing in from the river ismoving locally near the estuary (Sanada et al., 2018).
137• The Cs flux in winter to spring (February to June)is 60% of the annual flux, suggesting that
resuspension plays a significant role in coastal areas.
137• The total annual deposition of Cs in 2017 isestimated to have been 11.4 kBq m−2. The 137Cs
inventory of seafloor sediments sampled in 2014 was
72 kBq m−2 (Tsuruta et al., 2017), and thecontribution of subsiding particles to the seafloor
sediments was about 16%.
Acknowledgments
The authors wish to express sincere thanks to Sanyo
Techno Marine Co., Ltd. for providing measurement data.
The Soma Futaba Fisheries Cooperative is thanked forproviding a fishing boat. Sincere thanks are extended toanonymous reviewers for their valuable and constructive
comments, which helped improve the clarity of themanuscript.
References
Ambe, D., Kaeriyama, H., Shigenobu, Y., Fujimoto, K., Ono, T.,
Sawada, H., Saito, H., Miki, S., Setou, T., Morita, T. and
Watanabe, T. (2014) Five-minute resolved spatial distribution ofradiocesium in sea sediment derived from the Fukushima
Daiichi nuclear power plant. Journal of Environmental
Radioactivity, 138, 264–275. Retrieved from https://doi.org/
10.1016/j.jenvrad.2014.09.007
Arita, K., Yabe, T. and Hayashi, S. (2014) Actual situation ofconcentration and inventory of radioactive cesium in
Matsukawaura lagoon sediment, Fukushima prefecture. Journalof Japan Society of Civil Engineers, seriesG, Environmental
Research, 70(7), III_225–III_231. (in Japanese with English
abstract)
Black, E.E. and Buesseler, K.O. (2014) Spatial variability and thefate of cesium in coastal sediments near Fukushima, Japan.
Biogeosciences, 11, 5123–5137. Retrieved from https://doi.org/

143

10.5194/bg-11-5123-2014
Bostick, B.C., Vairavamurthy, M.A., Karthikeyan, K.G. andChorover, J. (2002) Cesium adsorption on clay minerals: An
EXAFS spectroscopic investigation. Environmental Science &Technology, 36, 2670–2676. Retrieved from https://doi.org/
10.1021/es0156892
Hirose, K. (2016) Fukushima Daiichi Nuclear Plant accident:
Atmospheric and oceanic impacts over the five years. Journal ofEnvironmental Radioactivity, 157, 113–130. Retrieved fromhttps://doi.org/10.1016/j.jenvrad.2016.01.011
Hogg, R. and Craig, A. (1978) Introduction to Mathematical
Statistics, Ch.9, 338–339, Macmillian Publishing Co., Inc.
Katata, G., Chino, M., Kobayashi, T., Terada, H., Ota, M., Nagai,
H., Kajino, M., Draxler, R., Hort, M.C., Malo, A., Torii, T. andSanada, Y. (2015) Detailed source term estimation of theatmospheric release for the Fukushima Daiichi Nuclear Power
Station accident by coupling simulations of an atmosphericdispersion model with an improved deposition scheme andoceanic dispersion model. Atmospheric Chemistry and Physics,
15, 1029–1070. Retrieved from https://doi.org/10.5194/acp-151029-2015
Kusakabe, M., Oikawa, S., Takata, H. and Misonoo, J. (2013)
Spatiotemporal distributions of Fukushima-derived radionuclides
in nearby marine surface sediments. Biogeosciences, 10, 5019–
5030. Retrieved from https://doi:10.5194/bg-10-5019-2013
Ministry of Education, Culture, Sports, Science and Technology(MEXT) (2011) Result of the forth Airborne Monitoring Survey
by MEXT. Retrieved from https://radioactivity.nsr.go.jp/en/
contents/ 4000/3179/24/1270_1216.pdf (Accessed 25 Nov 2020)
Mishra, S., Kasar, S., Takamasa, A., Veerasamy, N. and Sahoo, S.K.
(2019) Measurement of uranium distribution coefficient and235U/238U ratio in soils affected by Fukushima Dai-ichi nuclear
power plant accident. Journal of Environmental Radioactivity,
198, 36–42. Retrieved from https://doi.org/10.1016/j.jenvrad.
2018. 12. 019
Nakanishi, T. and Sakuma, K. (2019) Trend of 137Cs concentrationin river water in the medium term and future following the
Fukushima nuclear accident. Chemosphere, 215, 272–279.
Retrieved from https://doi.org/10.1016/j.chemosphere.2018.10.
017
Nuclear Regulatory Commission (NRC) (2018) Site of past data
page. Digital Japan. Retrieved from https://radioactivity.nsr.
go.jp/ja/list/512/list-1.html (Accessed 17 July 2020)
Otosaka, S. and Kobayashi, T. (2013) Discharge and distribution ofradionuclides in the marine environment. Journal of Japan
Society on Water Environment, 36 (3), 95–98. (in Japanese)
Otosaka, S. and Kato, Y. (2014) Radiocesium derived from the
Fukushima Daiichi nuclear power plant accident in seabedsediments: initial deposition and inventories. Environmental
Science: Process & Impacts, 16 (5), 978–990. Retrieved fromhttps://doi.org/10.1039/c4em00016a
Otosaka, S., Nakanishi, T., Suzuki, T., Satoh, Y. and Narita, H.
(2014) Vertical and lateral transport of particulate radiocesium
off Fukushima. Environmental Science & Technology, 48,
12595–12602. Retrieved from https://doi.org/10.1021/
es503736d
Otosaka, S. (2017) Processes affecting long-term changes in 137Csconcentration in surface sediments off Fukushima. Journal of
Oceanography, 73(5), 559–570. Retrieved from https://doi.
org/10.1007/s10872-017-0421-5
Rathour, S.S., Kato, N., Tanabe, N., Senga, H., Hirai, Y., Yoshie,
M. and Tanaka, T. (2015) Spilled oil autonomous tracking using
autonomous sea surface vehicle. Marine Technology Society,49(3), 102–116. Retrieved from https://doi.org/10.4031/
MTSJ.49.3.15

144

T. MISONOU et al.

Sakuma, K., Nakanishi, T., Yoshimura, K., Kurikami, H., Nanba, K.
and Zheleznyak, M. (2019a) A modeling approach to estimate the137
Cs discharge in rivers from immediately after the Fukushima
accident until 2017. Journal of Environmental Radioactivity,
208–209. Retrieved from https://doi.org/10.1016/j.jenvrad.2019.
106041
Sakuma, K., Hideki, T., Hayashi, S., Funaki, H., Malins, A.,
Yoshimura, K., Kurikami, H., Kitamura, A., Iijima, K. and
Hosomi, M. (2019b) Applicability of Kd for modelling dissolved
137
Cs concentrations in Fukushima river water: Case study of the
upstream Ota River. Journal of Environmental Radioactivity,
210. Retrieved from https://doi.org/10.1016/j.jenvrad.2018.09.
013
Sanada, Y., Miyamoto, K., Ochi, K., Matsuzaki, Y., Ogawa, T. andSenga, Y. (2018) Development of the unmanned surface vehicle
for radiation monitoring at seabed: Observation of temporalchange of radiocesium concentration in the sea bottom sedimentat coastal zone of Fukushima. Journal of Advanced Marine
Science and Technology Society, 24 (2), 9–18. (in Japanese withEnglish abstract)
Thornton, B., Ohnishi, S., Ura, T., Odano, N., Sasaki, S., Fujita, T.,
Watanabe, T., Nakata, K., Ono, T. and Ambe, D. (2013)
Distribution of local Cs-137 anomalies on the seafloor near the
Fukushima Dai-ichi nuclear power plant. Marine Pollution
Bulletin, 74(1), 344–350. Retrieved from https://doi.org/
10.1016/j.marpolbul. 2013.06.031
Tsuruta, T., Harada, H., Misonou, T., Matsuoka, T. and Hodotsuka,
Y. (2017) Horizontal and vertical distribution of 137Cs in seabedsediments around the river mouth near Fukushima Daiichi
nuclear power plant. Journal of Oceanography, 73, 547–558.
Retrieved from https://doi.org/10.1007/s10872-017-0439-8
Yagi, H., Sugimatsu, K., Nishi, Y., Kawamata, S., Nakayama, A.,
Udagawa, T and Suzuki, A. (2013) Field measurements ofbottom boundary layer and suspended particle materials on
Jyoban coast in Japan. Journal of Japan Society of Civil
Engineers, Series B2, Coastal Engineering, Vol.69(2),I_1046-I_1050. (in Japanese with English abstract)
Yagi, H., Sugimatsu, K., Kawamata, S., Nakayama A. andUdagawa, T. (2015) Bottom Turbidity, boundary layer dynamics,
and associated transport of suspended particulate materials off
the Fukushima coast. Impacts of the Fukushima Nuclear
Accident on Fish and Fishing Grounds, 77–89.

Toshiharu MISONOU

since 2014.

Toshiharu Misonou is a researcher at the
Fukushima Environmental Monitoring Division,
Japan Atomic Energy Agency (JAEA). His
research field is environmental radioactivity incoastal area. After earning his PhD in Civil
Engineering, he has been engaged in the studyof environmental radioactivity in coastal areas

Tadahiko TSURUTA
After majoring in geology at his university,
Tadahiko Tsuruta worked in uranium resource
exploration, focusing on R&D related togeological disposal of high-level radioactive
waste. He participated in R&D for environmental
resilience in Fukushima after the Fukushima
nuclear power plant accident. In his R&D in
Fukushima, He was in charge of research on the mobilization behavior ofradioactive cesium in coastal areas, especially for promoting seabedsediments and suspended particle sampling to estimate the future
distribution of radioactive cesium in seabed sediments.

Takahiro NAKANISHI
Takahiro Nakanishi is an assistant principal
scientist at the Fukushima Environmental
Monitoring Division, JAEA. He has been engaged
in the study of environmental radioactivity,
mainly in oceans and forests. His current researchinterests include the assessment of radiocesiumtransport behavior on the catchment scale. He is
also interested in the global carbon cycle.

Yukihisa SANADA
After obtaining his PhD in Radiation
Measurement, Dr. Sanada’s research has been
primarily related to radiation management, theuse of gamma spectrometry systems to measureradionuclide activity concentrations andworkplace dose rates at nuclear facilities. Since
the Fukushima Daiichi NPP accident in 2011, hehas been performing remote radiation surveys such as aerial radiation
monitoring using manned or unmanned helicopters. He is now developing a
new type of radiation detector to study decontamination effects, andresearching migration of radioactive substances. He is a group leader at the
Sector of Fukushima Research and Development, Japan Atomic Energy
Agency (JAEA).
(Received 24 August 2020, Accepted 23 December 2020)