Journal of Environmental Radioactivity 238-239 (2021) 106724

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Journal of Environmental Radioactivity
journal homepage: www.elsevier.com/locate/jenvrad

A modeling study on the oceanic dispersion and sedimentation ofradionuclides off the coast of Fukushima
Yuki Kamidaira a, *, Yusuke Uchiyama b, Hideyuki Kawamura a, Takuya Kobayashi a,
Shigeyoshi Otosaka cabc

Nuclear Science and Engineering Center, Japan Atomic Energy Agency, Tokai, Ibaraki, Japan
Department of Civil Engineering, Kobe University, Kobe, Hyogo, Japan
Atmosphere and Ocean Research Institute, The University of Tokyo, Kashiwa, Chiba, JapanA R T I C L E I N F O

A B S T R A C T

Keywords:
Fukushima Daiichi nuclear power plant
accident
Radiocesium
Suspended radionuclides
Sediment transport
ROMS

We developed a three-dimensional prognostic oceanic dispersion model that accounted for the phase transfer ofradionuclides between seawater, suspended particles, and seabed sediments with multiscale grain sizes. A
detailed hindcast of 137Cs in the seabed sediment off the Fukushima coast was conducted to investigate thetransfer mechanism of dissolved 137Cs derived from the Fukushima Daiichi Nuclear Power Plant (FNPP1) acci­
dent toward the seabed sediment. Extensive model-data comparison demonstrated that the model could satis­
factorily reproduce the oceanic structure and 137Cs concentrations in the seawater and seabed sediment. The
model successfully reproduced the major features of the observed spatial variation of the 137Cs activities in thesediment, which represented more than 90% of the sedimentary radiocesium existing in the coastal area off
Fukushima several months after the accident. Shear stress associated with the resuspension of the seabed sedi­ment was induced by waves near the shore and by current velocity offshore of the study area. The adsorption of137
Cs on the seabed sediment differed depending on the particle size, with adsorption on clay being the mostsubstantial. The distribution of 137Cs in the sediment off the Fukushima coast was formed mainly owing toadsorption from the dissolved phase by June 2011, when the impact of the direct oceanic 137Cs release fromFNPP1 was remarkable. After June 2011, seabed sediment became a source of 137Cs released to the seawaterowing to resuspension with and desorption from the sediment.

1. Introduction
The Great East Japan Earthquake occurred on March 11, 2011,
generating catastrophic tsunamis that struck the Tohoku coast. Conse­
quently, substantial amounts of radioactive substances, especially radi­
ocesium (134Cs (half-life 2.06 years) and 137Cs (half-life 30.2 years)),
were released accidently from the Fukushima Daiichi Nuclear Power
Plant (FNPP1) to Pacific coastal waters. After the FNPP1 accident,
several observational and computational studies on oceanic radionu­
clide dispersion and transport have been conducted (e.g., Masumoto
et al., 2012; Tsubono et al., 2016). Several years after the accident, thedissolved 137Cs concentrations in the coastal shelf water off Fukushima
were found to have decreased by dispersal and dilution via thesynoptic-scale Kuroshio-Oyashio current system to preaccident back­ground values (Kitamura et al., 2014; Takata et al., 2016). However,compared with the concentration of dissolved radiocesium, that in theseabed sediment has not decreased remarkably (Kusakabe et al., 2013).
Although 137Cs concentrations in the coastal shelf sediment off
Fukushima have decreased gradually (Kusakabe et al., 2017; Otosaka,
2017; Takata et al., 2017; Tateda et al., 2020), the long-retained radi­
ocesium in the seabed sediment may transfer through the benthic foodchain (Sohtome et al., 2014; Bezhenar et al., 2016; Tateda et al., 2016;
Wang et al., 2016). Another important reason for evaluating the radio­
cesium in the seabed sediment in the area is that it contributes radio­
cesium to the seawater off Fukushima. On the basis of the in situ sampled
data, Takata et al. (2016) reported that desorption/dissolution fromsediment is one of the main sources contributing 137Cs to the inventoryof radiocesium in the waters off Fukushima. These results suggest thatthe estimation of the radiocesium retention capacity of the seabedsediment and evaluation of the radiocesium transfer mechanism be­
tween the seabed sediment and water are important for marine envi­
ronmental assessment.

* Corresponding author.
E-mail address: kamidaira.yuki@jaea.go.jp (Y. Kamidaira).
https://doi.org/10.1016/j.jenvrad.2021.106724
Received 20 May 2021; Received in revised form 13 August 2021; Accepted 21 August 2021
Available online 3 September 2021
0265-931X/© 2021 Elsevier Ltd. All rights reserved.

Y. Kamidaira et al.

Journal of Environmental Radioactivity 238-239 (2021) 106724

The grain size distribution of the seabed sediment and 137Cs activities
in the overlying bottom water are vital for representing simulated 137Cs
activities in the sediment (Otosaka and Kobayashi, 2013; Kusakabe
et al., 2013; Ambe et al., 2014; Black and Buesseler, 2014; Misumi et al.,2014; Fukuda et al., 2018; Tateda et al., 2020). The numerical hindcast
by Misumi et al. (2014) could be regarded as one of the most successfulmodeling studies on 137Cs in the seabed sediment off the Fukushimacoast. In their model, 137Cs transfer between bottom water and sedimentwith grain size distribution occurs via adsorption and desorption. Its
results were generally found consistent with in situ 137Cs measurements,
temporally and spatially, although they slightly underestimated 137Csconcentration compared with the observation because the model does
not consider 137Cs in the suspended particulate form.
Several modeling studies on oceanic radionuclide dispersion suggestthat the interactions of nonconservative radionuclides with particulatematter must be considered. For example, Kobayashi et al. (2007) con­ducted the oceanic dispersion simulation of 137Cs released from the
British Nuclear Fuels Ltd. in the Irish Sea using a numerical simulation
model system that consisted of an ocean current model and a Lagrangianparticle tracking model. Periáñez (2008) performed the oceanic
dispersion simulation of 137Cs and 239,240Pu in the Alborán Sea, western
Mediterranean, using a quasi-three-dimensional numerical model
considering sediment transport. Both models consider the exchanges ofradionuclides between the dissolved and solid phases (suspended matterparticles and seabed sediments). Consequently, their studies successfully
reproduced the main characteristics of the radionuclide migration andmovement in the study area. Choi et al. (2013) is the only modelingstudy of 137Cs off the Fukushima coast that considers the interactions of
dissolved radionuclides with particulate matter. They investigated thespatiotemporal variation of Fukushima-derived 137Cs in both seawaterand sediment several months after the accident using a coupled
Lagrangian particle tracking-ocean circulation model that considers theinteractions of dissolved radionuclides with particulate matter.
Although they did not compare the simulated results with observed data
quantitatively, they found that major adsorption on bottom sediments
occurred during the first month after the release, and a large quantity ofradionuclides settled on the sea floor at one point.
Sediment resuspension is a source of suspended particulate matter inthe ocean, and near-bottom orbital motions due to surface waves are an
important factor in resuspension. Bottom stress, which determines thesediment resuspension, depends on the combined effect of waves andcurrents. Blaas et al. (2007) analyzed suspended sediment transport
processes in the Santa Monica Bay and San Pedro Bay using a regional
sediment transport model considering the wave-enhanced bed boundary
layer. They suggested that surface waves are a dominant factor in theresuspension of bed material on the Southern Californian shelves.
Sediment resuspension is vital for a good representation of the radio­
nuclides in the ocean because the radionuclides adsorbed on sedimentare also released toward the water by resuspension. However, existingmodeling studies investigating the spatiotemporal variation of
Fukushima-derived radionuclides in the ocean do not consider the effectof waves on the resuspension.
Recently, the impact of the inflow of radioactive materials fromrivers to the coastal margin off Fukushima on the marine environmenthas become significant (Kitamura et al., 2014; Iwasaki et al., 2015;
Kakehi et al., 2016; Ikenoue et al., 2020). A portion of the radionuclides
released from FNPP1 into the atmosphere has deposited on the landaround FNPP1. These radionuclides that have been adsorbed on surface
soil particles are being transported to the ocean through the river
channel networks. Radionuclides adsorbed on soil particles have alonger-term impact on the marine environment compared with dis­solved radionuclides directly released from FNPP1 because the former
takes a longer time to be transported to the ocean due to the hydro­
logical process. In addition, these suspended particles introduced fromriver discharge and resuspended from the seafloor have different
radionuclide adsorption/desorption and transport characteristics fordifferent particle sizes, thus it will be necessary to evaluate the envi­
ronmental impact of each particle size. Therefore, a model considering
radionuclides adsorbed on multi-size class particles should be developed
to assess their impact on the environment.
In the present study, we developed an oceanic dispersion model
considering the interactions of dissolved radionuclides with suspended
particles and seabed sediment with multiscale grain sizes. A numericalexperiment of oceanic three-dimensional (3D) dispersal and initial
dilution of the 137Cs released from the FNPP1 accident was conductedusing the developed model to investigate the transfer mechanism of
137
Cs toward the seabed sediment. Our analysis is based on doublenested ocean downscaling 3D circulation modeling using the Regional
Oceanic Modeling System (ROMS; Shchepetkin and McWilliams, 2005;2008) embedded within assimilative oceanic reanalysis, afour-dimensional (4D) variational ocean reanalysis for the Western
North Pacific over 30 years (FORA-WNP30; Usui et al., 2017) productwith a lateral grid resolution of 1/10◦ and atmospheric forcing from theassimilative grid point value of the global spectral model (GPV-GSM; e.
g., Roads, 2004), and mesoscale model (MSM; e.g., Isoguchi et al., 2010)
reanalysis products.
The remainder of this paper is organized as follows. The developedmodel and modeling framework used for the hindcast experiment for ayear following the FNPP1 accident are described in Section 2. Section 3
shows the model results and discusses the sedimentary 137Cs distribution
mechanisms. Sections 3.1, 3.2, and 3.3 illustrate an extensive compari­son between the model results and field observation and satellite
altimetry data to validate the model’s capability to reproduce the Kur­oshio, 3D oceanic structure, and distribution of 137Cs in the marine
environment. Section 3.4 considers the time variation and distributionof near-bed 137Cs and sediment to investigate the transfer mechanism of
137
Cs toward the seabed sediment. A summary and the conclusions are
given in Section 4.
2. Methods
2.1. Model description
In this study, we developed a 3D oceanic dispersion model thatconsiders the interactions of dissolved radionuclides with multiclasssuspended particles and seabed sediments using ROMS. This model isbased on the nonconservative radionuclide dispersion model presented
by Periáñez (2008; 2013) and the sediment transport model presented
by Blaas et al. (2007).
The suspended particles and radionuclide transport are described by
Eulerian advection-diffusion equations, which consider the verticalsettling term and fluxes by the erosion from and deposition onto theseabed sediment. Radionuclides are assumed to exist not only as dis­solved radionuclides in the water column but also in suspended matterand the seabed sediment. Since radionuclide adsorption onto anddesorption from particles depend on the grain size, we assumed thatsuspended matter and the seabed sediment have a grain size distribu­
tion. Details of the model equations are presented in supplementarymaterials (S1 Model description).
2.1.1. Sediment transport model
The equation for the time evolution of the suspended matter con­centration in seawater with each size class j (j = 1, 2, and … N) is written
as follows:
{
}
) } ∂
∂Mj ∂ {(
∂Mjui − δi,3 ws,j Mj −(1)Ki
= Sj ,+∂xi∂t ∂xi∂xi
where Mj is the suspended matter concentration in size class j (dimen­sion (kg m− 3)); ui is the velocity component along the xi direction (ms− 1); Ki is the eddy diffusivity (m2 s− 1); ws,j is the vertical settling ve­
locity (m s− 1), which depends on the sediment grain size but is2

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Journal of Environmental Radioactivity 238-239 (2021) 106724

independent of flow conditions and concentrations; and Sj represents thesources and sinks of suspended matter (kg m− 3 s− 1). The subscript i = 1,
2, and 3 denotes the spatial coordinate (with x3 vertically upward). The
repetition of the Index i implies summation, but Eq. (1) is solved sepa­
rately for each size class j. The vertical particle settling along the x3
direction is considered with the Kronecker delta (δi,3; no unit) on theadvection term.

schematic illustration of the interaction processes between the radio­
nuclide phases in the model is presented in Fig. 1.
2.2. Model configuration
We used the state-of-the-art oceanic circulation model ROMS tosimulate the circulation off the northeastern Pacific coast of Japan on adouble-nested configuration. ROMS is a free-surface, terrain-following,primitive-equation 3D regional hydrodynamic model with a hydrostatic
assumption and Boussinesq approximation. We conducted a simulta­
neous simulation of hydrodynamics and tracer (viz., 137Cs and sedimentparticles) transport. Fig. 2 shows the numerical domains of the oceanic
downscaling model in a double-nested configuration embedded in the
FORA-WNP30 domain (not shown). Details of the configurations ofhydrodynamic model are presented in supplementary materials (S2Configurations of hydrodynamic model).
The dispersion model descried in Section 2.1 was applied to the 137Cs
derived from FNPP1 and the sediment released from the major rivers inJapan and resuspended from the seafloor. The parameters for the sedi­ment and 137Cs are summarized in Tables 1 and 2. The active layer
thickness L was assumed to be at least a few median grain diameters
thick to centimeter order in previous studies on radionuclide dispersion
or sediment transport models (e.g., Blaas et al., 2007; Kobayashi et al.,2007; Periáñez, 2013). Kobayashi et al. (2007) and Periáñez (2013) set
the active layer thickness as L = 10 cm and successfully reproduced themain characteristics of radionuclide migration and movement in theocean. In some observations, significant concentrations of 134Cs weredetected even at 10-cm sedimentary layers, especially in coastal regions(e.g., Otosaka and Kato, 2014). In this study, we assumed the thicknessof the active layer L = 10 cm and horizontally uniform. The remaining
parameters for the sediments used in Eqs. (1)–(5) were determined
experimentally and consistent with those of Blaas et al. (2007) and
Wang (2002), whereas the radionuclide parameters were consistent
with those of Periáñez (2013).
The 137Cs input into the ocean—that from atmospheric deposition
and that directly released from FNPP1—was imposed as dissolved formin the model, as in Kamidaira et al. (2018). The dissolved 137Cs point
source was introduced as directly released from FNPP1 into the ocean on
the basis of in situ 137Cs measurement (Kawamura et al., 2011). The
137
Cs point source was imposed in the grid adjacent to FNPP1 fromMarch 26, 2011 to June 30, 2011. The release rate reached 1.67 × 10− 2
PBq h− 1 on April 10, started to decrease, and reached the order of 10− 4
PBq h− 1 by the end of April.
We also adopted data on the atmospheric deposition at the sea sur­face as the area source of dissolved 137Cs; the data were from theWorldwide version of System for Prediction of Environmental

2.1.2. Radionuclide transport model
The equation for the time evolution of the radionuclide concentra­tion in the dissolved phase is written as follows:
(

)

N (
)
∑
∂Cd ∂(ui Cd ) ∂∂Cdm− k1,jKi=−
Cd + k2 Cs,j − λCd + Sd ,
+
∂xi
∂t
∂xi
∂xi
j=1

(2)

where Cd is the radionuclide concentration in the dissolved phase (Bqm− 3); Cs,j is the radionuclide concentration in the suspended matter insize class j (Bq m− 3); km1,j and k2 are kinetic coefficients for adsorption
onto and desorption from the suspended matter (s− 1), respectively; λ isthe decay constant of the radionuclides (s− 1); and Sd represents thesources or sinks of dissolved radionuclides (Bq m− 3 s− 1). The second andthird terms on the left side imply the summation of the terms over all thevalues of the Index i.
The equation for the time evolution of the radionuclide concentra­tion in the suspended matter in each size class j is written as follows:
(
)
)}
{ (
) } ∂
∂ Cs, j∂ Cs, j∂ {(mui − δi,3 ws,j Cs, j −Cd − k2 Cs, j − λCs, j +Ss,j ,
Ki
=k1,j
+
∂xi
∂xi
∂t
∂xi
(3)
where Ss,j represents the sources and sinks of radionuclides in the sus­pended matter (Bq m− 3 s− 1). The index summation is the same as that in
Eq. (1).
The seabed sediment is modeled as a two-layer system similar to
those of Reed et al. (1999) and Blaas et al. (2007). The two-layer system
has an active layer and a substrate. As is the case for the sediment intheir studies, we assume that the radionuclides in the active layerinteract with both the water column and substrate, whereas the radio­
nuclides in the substrate do not touch the water column; they onlyinteract with the upper active layer according to the sediment erosionand deposition. The radionuclides in the bottom sediment phase areassumed to be not advected horizontally. The equation for the time
evolution of the radionuclide concentration in the seabed sedimentactive layer in each size class j is written as follows:⃒
ws,j Cs,j ⃒x3 =− h − Ej As,j
∂As,j s (Hz Cd )|x3 =− h()()− k2 ϕAs,j +− λ1,j Ab,j − λ2,j As,j − λAs,j ,= k1,j∂t
Lρs,j 1− pj fj
Lρs,j 1− pj fj(4)where the first and second terms at the right-hand side represent theinteraction between the sediment and the dissolved phase, whereas thethird term at the right-hand side represent the interaction between thesediment and the suspended phase; ks1,j is the kinetic coefficient for
adsorption onto the seabed sediment (s− 1); ϕ is a correction factor (nounit); Ab,j is the radionuclide concentration in the seabed sediment
substrate (Bq kg− 1); λ1,j and λ2,j are the kinetic coefficients (s− 1) for theadsorption and desorption between the active layer and the substrate,
respectively; these coefficients depend on the net flux into the seabedsediment from the water (viz., erosion and deposition of the sediment).
The equation for the time evolution of the radionuclide concentra­tion in the seabed sediment substrate in each size class j is written as
follows:
)
∂Ab,j
Lfj (
− λ1,j Ab,j − λ2,j As,j − λAb, j ,= −∂t
Lb fb,j(5)
Fig. 1. Schematic of the interaction processes between the radionuclide phasesin the model.

where Lb is the thickness of the seabed sediment substrate (m). A
3

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Journal of Environmental Radioactivity 238-239 (2021) 106724

Fig. 2. Double-nested ROMS model domains forthe parent ROMS-L1 (left plot) and its enlarged
view (right)—the child ROMS-L2 model domain is
illustrated using black solid boxes in both panels;
bathymetry down to the depth of 2000 m is shownusing black contours (m); the red star indicates thelocation of Fukushima Daiichi Nuclear Power Plant(FNPP1); the thick black line identifies the transect
for the cross-sectional plots in Figs. 13 and 14; andthe inventory of sedimentary radiocesium in
Fig. 12 was estimated for the area inside themagenta line. (For interpretation of the references
to color in this figure legend, the reader is referredto the Web version of this article.)

The suspended matter in this study was considered to be the landderived sediment particles introduced from river discharge and resus­
pended from the seafloor. We took into account three size classes: sand
(d1 = 125 μm), silt (d2 = 24 μm), and clay (d3 = 4 μm). We assumed thatall sediment fractions were non-cohesive. The initial distribution of thesediment bed composition of the ROMS-L1 model was obtained from thespatiotemporally interpolated fraction fields of dbSEABED (Jenkins,
1997, 2002), which is a database based on laboratory measurements anddescriptive observations. Fig. 3 shows the initial condition of the sedi­ment fractions imposed on the ROMS-L2 domain. Sand is mainly
distributed in the coastal region, whereas the other fine sediments are
distributed offshore.
The suspended matter flux from atmospheric deposition was notconsidered in our application (Fatmj = 0). The sediment volume fluxes
from rivers have a strong correlation with river discharge (e.g., Tani­
guchi et al., 2019). Uchiyama et al. (2014) estimated the averagecross-sectional concentration at the mouth of a river using an equationthat relates the river discharge to the river basin area. This equation isbased on the empirical L-Q (discharge to sediment flux) relation pro­
posed by Takekawa and Nihei (2013). The cross-sectional averaged
sediment concentration (c; mg l− 1) is obtained as follows:

Table 1
Parameters for each grain size using in sediment transport model. dj: thediameter of the particles, ρs,j: the particle dry weight density, ws,j: the verticalsettling velocity, Eu,j: the entrainment rate, τcr,j: the critical bed shear stress.
Class
name
Sand
Silt
Clay

dj
(μm)
125
24
4

ρs,j (kg
m− 3)

2650
2650
2650

ws,j (mms− 1 )
9.4
0.4
0.1

Eu,j (kg m− 2s)
− 3

2.5 × 10
1.0 × 10− 4
1.0 × 10− 4τcr,j (N
m− 2)

0.15
0.07
0.02

Table 2
Sediment and137Cs parameters using in sediment and radionuclide transportmodel.
Parameter description

Value

Thickness of the active layer
Initial thickness of the substrate
Porosity
Kinetic coefficient for desorption from suspended matter
Correction factor
Exchange velocity

L = 10 cm
Lb = 1 m
p = 0.4
k2 = 1.16 × 10− 5 s− 1
ϕ = 0.01
χ = 2.1 × 10− 8 ms− 1c=aEmergency Dose Information version II (WSPEEDI-II; Terada and Chino,2008). The 137Cs release amount from March 12, 2011 to April 30, 2011
estimated by Katata et al. (2015) was applied as the atmospheric release
rate for WSPEEDI-II. The WSPEEDI-II model domain encompasses the
ROMS-L1 domain, with a horizontal resolution of 10 km in both thelatitudinal and longitudinal directions. The three-hourly deposition
amount of the airborne radioactive 137Cs predicted by WSPEEDI-II was
assumed to have deposited in the dissolved form at the sea surface fromMarch 12, 2011 to May 31, 2011.
On the other hand, the 137Cs input from rivers were not considered.
Based on the Universal Soil Loss Equation (USLE), Kitamura et al. (2014)
estimated annual amount of 137Cs discharge to the ocean from the 14
river basins in 2011 within the ROMS-L2 domain to be 8.4TBq, whereas
the amount of direct oceanic discharge from FNPP1 was about 3.5PBq.
Therefore, the influence of 137Cs input from rivers during the analysis
period is considered to be small.
The 137Cs released into the ocean as particulate matter was notconsidered (Ss,j = 0; Fatms,j = 0) because its quantity was considerably
smaller than that of the dissolved 137Cs released from FNPP1 within the
study period. In this study, we only considered the 137Cs derived fromFNPP1 to understand its dynamics in the ocean, although 137Cs had been
existing in the marine environment due to a global fallout before theaccident (1–2 Bq m− 3 in seawater; 1–2 Bq kg− 1 in seabed sediment; e.g.,
Kusakabe et al., 2013). Thus, the initial conditions of 137Cs in all phasesof the ROMS-L1 and ROMS-L2 models were set to zero.

Qb− 1
,
Ab− 1

(6)

where Q is the river flow flux (m3 s− 1); A is the river basin area (km2);
and a and b are constant coefficients (a = 160.0, b = 1.62). Q and A are
estimated from the monthly climatology data of the major river dis­charges compiled by the Japan River Association. The fractions of thethree sediment classes in the riverine discharge are determined empir­
ically on the basis of the outcome of the USLE-based river sedimentmodeling conducted by the JAEA. The sediment flux ratio is assumed tobe 5:4:2 (sand:silt:clay) uniformly for all Japan rivers in reference to theriver simulation result obtained using the USLE-based model in Yama­guchi et al. (2014).
We conducted a simultaneous simulation of the hydrodynamics andtracers in the ROMS-L1 and ROMS-L2 models. The tracer simulation
results from the ROMS-L1 model were set as the boundary and initialconditions for the innermost ROMS-L2 model. In this manner, the effectof the 137Cs deposited outside the ROMS-L2 region and re-influx oftracers discharged outside the ROMS-L2 region were regarded as theboundary conditions of ROMS-L2.
3. Results and discussions
3.1. Model validation
We compared the model results with satellite and in situ observations
to validate the reproducibility of the hydrodynamics. The activity of4

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Journal of Environmental Radioactivity 238-239 (2021) 106724

Fig. 3. Initial condition of the sediment proportions with bathymetry down to 2000-m depth in black contours (m).

dissolved 137Cs is crucial for better reproducibility of 137Cs activity inseafloor sediment. Thus, we verified the modeled dissolved 137Cs con­centrations, in addition to the oceanic flow field, against data observed
from in situ water sampling.
Fig. 4 shows a comparison between the surface velocity (horizontal
velocity of the topmost grid cells) magnitude from the ROMS-L2 model
results and that from the Aviso delayed-time mapped absolute dynamic
topography from multiple satellite altimetry data (gridded daily com­posite at 1/4◦ resolution; e.g., Le Traon et al., 1998). Overall, the model
results agreed with the Aviso data in terms of both magnitude andspatial patterns, including the Kuroshio meandering through March­–June 2011, which have a significant effect on the initial dilution of the
dissolved 137Cs. For instance, both the model results and the satellite
altimetry data showed the existence of a mesoscale eddy at around 39◦ N
and the Kuroshio meandering at around 144◦ E, confirming the repro­
ducibility of mesoscale variability.
Fig. 5a shows a scatter diagram of the modeled and measured nearbed temperatures in the coastal areas off Fukushima. The observed data
are based on CTD measurements from vessels at sampling stations(Oikawa et al., 2013), as shown by the circles in Fig. 7. The temperatureof the bottom-most grid cells from ROMS-L2 agreed well with theobservational data, with a correlation coefficient R of 0.9485, root meansquare error (RMSE) of 1.432, and model skill score of 0.9735 (a model
skill score of one indicates perfect agreement, and zero means complete
disagreement; Wilmott, 1981). These results suggest that ROMS-L2 hadsatisfactory reproducibility in terms of synoptic dynamics, including theKuroshio path and the oceanic flow field near FNPP1.
Observations by the Ka’imikai-o-Kanaloa (KOK) cruise were regar­
ded as an offshore 137Cs dataset for June 2011 (Buesseler et al., 2012) for
the 3D spatial distribution. The modeled dissolved 137Cs activity in thesurface water (topmost grid cells) was compared with the activity
measured on June 06–18, 2011 by the KOK cruise; these activities aresuperposed as colored circles in Fig. 6. Overall, the ROMS-L2 model
reproduced the observations reasonably well, presenting high concen­
trations near the shore and decreasing trends in the Kuroshio down­
stream region. A scatter diagram of the modeled versus measured
dissolved 137Cs is shown in Fig. 5b (the available observational data of
Buesseler et al. (2012) from the surface to 1000-m depth were used). The
reproducibility of radionuclide concentration by the oceanic dispersion
model is evaluated by an index, factor 10, which is the ratio of themodeled values to the observed values by a factor of 1/10 to 10 (e.g.,
Kamidaira et al., 2018). A large proportion of the modeled 137Cs waswithin the range of a factor 10, as indicated by the dotted lines, relative
to the measured 137Cs with 81.8%, with an apparent improvement

Fig. 4. Plan view plots of time-averaged surface velocity magnitude—the upper plots are the ROMS-L2 result, whereas the lower plots are based on Aviso satel­
lite altimetry.
5

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Journal of Environmental Radioactivity 238-239 (2021) 106724

Fig. 5. Scatter diagrams of the various quantities from the ROMS-L2 results—(a) temperature against the in-situ sampled data (Oikawa et al., 2013); (b) dissolved
137
Cs concentrations against the in-situ sampled data (Buesseler et al., 2012); (c) near-bed dissolved 137Cs concentrations against the in-situ sampled data (Oikawa
et al., 2013); (d) 137Cs concentrations in the seabed sediment against the in-situ sampled data (Kusakabe et al., 2013); the solid line represents the 1:1 correlation; it isbounded using two dashed lines indicating deviations with the factors of 1/10 and 10 in Fig. 5b–d; and the white, red, and brown circles are the data from stations at
the water depths of <200, 200–400, and >400 m, respectively. The red dotted lines are the orders of magnitude of preaccident background values of the 137Csconcentrations. The in-situ data of Fig. 5a, c, and d are measured in May, June, July, September, October, December 2011, and February 2012. The in-situ data ofFig. 5b are measured in June 2011. R, correlation coefficient, RMSE, root mean square error, and skill, model skill score. (For interpretation of the references to color
in this figure legend, the reader is referred to the Web version of this article.)

concentration derived from FNPP1. This is consistent with the proposalof Tsumune et al. (2013), who also considered the inflow of 137Cs
outside the model domain into the regional model.
Oikawa et al. (2013) observed dissolved 137Cs activities at the same
monitoring stations as those of the temperatures in Fig. 5a. The modelednear-bed dissolved 137Cs activity concentrations (i.e., concentrations of
the bottom-most grid cells) were compared with those measured by
Oikawa et al. (2013), and the two sets of findings are superposed ascolored circles in Fig. 7. The ROMS-L2 model exhibited a high concen­
tration in Sendai Bay, Japan (around 38◦ N), in May and a southward
along-shelf transport of 137Cs after June. Overall, the concentrations of
ROMS-L2 were comparable with the observations; high concentrations
appeared in shallow near-shore areas. A scatter diagram of the modeled
versus measured near-bed dissolved 137Cs is shown in Fig. 5c. A large
proportion of the modeled 137Cs was within an order of magnitude of themeasured 137Cs, with a factor 10 of 72.2%, but it was slightly over­estimated at the stations shallower than 200 m and underestimated at
the stations deeper than 400 m. The modeled dissolved 137Cs dispersal in
the coastal area, which occurred several months after the accident, isshown in Fig. 8. Until the middle of March, dissolved 137Cs from at­
mospheric deposition contaminated shallow coastal areas and reacheddepths reaching 100 m. Afterward, the direct release of dissolved 137Csfrom FNPP1 started, and 137Cs was transported to Sendai Bay. The
concentration in Sendai Bay was high until the end of May. Then, theradionuclides were transported southward along the shelf due to a
reduction in the mesoscale eddy (centered at 36.5◦ N, 141.4◦ E) associ­ated with an approaching tropical cyclone. On the basis of anFig. 6. Modeled dissolved 137Cs concentrations (indicated by colors) from
ROMS-L2 at the surface—the colored circles (using the same color scale) indi­cate the instantaneous in-situ concentrations measured in June 2011 (Buesseler
et al., 2012). (For interpretation of the references to color in this figure legend,
the reader is referred to the Web version of this article.)compared with the result of Kamidaira et al. (2018), whose regionalmodel underestimated the measured 137Cs considerably at the hori­zontal resolution of 1 km. They did not consider the inflow of 137Cs fromoutside the model domain as a boundary condition of their innermost
model. This result suggests that the parent model results should be taken
into account as recurring influxes through boundaries to enhance
reproductivity in terms of the regional modeling of the dissolved 137Cs
6

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Journal of Environmental Radioactivity 238-239 (2021) 106724

Fig. 7. Modeled near-bed dissolved 137Cs concentrations (indicated by colors) from ROMS-L2 with bathymetry in black contours (m)—the colored circles (using thesame color scale) indicate the instantaneous in-situ concentrations measured in May, June, July, September, October, December 2011, and February 2012 (Oikawa
et al., 2013). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)observational approach, Aoyama et al. (2012) reported the same 137Cs
transport mechanism by the mesoscale eddy. It was considered that theoverestimation of the simulated dissolved 137Cs concentration on the
coast, despite the overall reproducibility of the flow field, was due to thesmoothing of the short time scale coastal flow by TS-nudging. In addi­tion, the observed dissolved 137Cs concentrations at the stations deeper
than 400 m were 0.1–2 Bq m− 3; these were near the background value,
which was not considered in the present study. Thus, ROMS-L2 suc­cessfully reproduced the 3D distribution and dynamics of 137Cs derived
from FNPP1 along the coast off Fukushima.
In summary, the double-nested ROMS model in this study is satis­
factorily capable of reproducing the behavior of the Kuroshio path and3D oceanic structure in the coastal area off Fukushima, while themodeled dissolved 137Cs reasonably agreed with observations.

the simulated concentrations were within one order of magnitude of theobserved values, with a factor 10 of 77.60% (Fig. 5d). ROMS reproduced
the observations reasonably well, especially at the stations shallower
than 200 m, but it tended to underestimate at the stations deeper than
200 m. In particular, ROMS did not reproduce most of the low observed
137
Cs concentrations at the stations deeper than 400 m. These low 137Cs
activities observed in deeper areas were 2–60 Bq kg− 1, which werehigher than the preaccident background value, suggesting that they
were derived from FNPP1.
As shown in Section 2, 137Cs may have transferred to the seabedsediment from the seawater via adsorption (in the dissolved phase) anddeposition (in the suspended phase). Time series of near-bed dissolved
137
Cs concentration and 137Cs concentration in the sediment are shownin Fig. S1 (supplementary materials). The simulated 137Cs concentra­
tions in the sediment were comparable to observations at the stations
shallower than 200 m despite overestimation of coastal bottom water
dissolved concentrations. The numerical hindcast by Misumi et al.
(2014), which did not consider the suspended matters, showed the same
trend as this study. They suggested that because of the slow adsorp­
tion/desorption timescale, the simulated temporal variation of 137Cs insediments is smoothed relative to that in the overlying bottom water.
Although our model takes into account the suspended matters, it can still
be regarded as a result of the predominance of adsorption from thedissolved form in the formation of the initial distribution of sedimental
137
Cs in the coastal area. On the other hand, ROMS underestimated theobserved concentrations of the near-bed dissolved 137Cs at depths
greater than 400 m (Figs. 5c and 7, and S1). However, the observed
dissolved 137Cs concentrations at depths greater than 400 m were about
the same as the preaccident background value; thus, they could not be
attributed to the sedimentary 137Cs distribution formation. Therefore,
these low 137Cs concentrations in the sediment in the deeper areas may
have transferred in the suspended phase.
Otosaka and Kato (2014) showed that the dominant process of

3.2. 137Cs in seabed sediment
We verified the modeled 137Cs concentrations in the seabed sediment
against observed data obtained from in situ sampling. Kusakabe et al.
(2013) investigated 137Cs concentrations in the seabed sediment off thecoast of FNPP1 from the same monitoring cruise as that in Oikawa et al.
(2013). Fig. 9 shows a comparison between the modeled and observed
137
Cs concentrations in the seabed sediment, which are superposed ascolored circles. The results from ROMS were the average values for threeparticle sizes (i.e., sum of the concentrations of the three particle grain
sizes per unit weight of the particles). The high 137Cs concentrations inthe sediment were distributed along the shore, mainly near FNPP1.
Similar to the distribution of the near-bed dissolved 137Cs (Figs. 7), 137Csconcentration increased along the shore from 36◦ N to 37◦ N after June.
These trends were consistent with the distribution of the observed
values.
However, ROMS underestimated the observed values on the shelfslope at depths deeper than 200 m. The scatter plot shows that most of7

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Y. Kamidaira et al.

Fig. 8. Daily-averaged spatial distributions of surface dissolved 137Cs concentrations from ROMS-L2 at the surface (upper plots) and of the bottom-most grid cells(lower plots) in March 16, May 21, and May 31, 2011—the black vectors represent the daily-averaged velocity.

incorporating radiocesium into sediments differs at a depth of about
200 m from the relationship between the inventory of radiocesium insediments and bottom depth. Specifically, it is inferred that dissolved
radiocesium in seawater is predominantly adsorbed on sediments in the
coastal areas, and scavenging of radiocesium from the surface layer ispredominant in the offshore areas. In addition, Otosaka et al. (2014)
installed a sediment trap at a station at 873-m depth about 100-km eastof FNPP1 over two years after the FNPP1 accident and performed
time-series sampling of sinking particles. They observed 137Cs concen­
trations reaching 400 Bq kg− 1 in the suspended matter. According totheir study, the primary mechanisms for the accumulation of 137Cs in theoffshore seabed within five months after the accident were adsorption of137
Cs onto particles (mainly organic or biological) in the surface layer
and rapid sinking. They also reported the proportion of these suspended
particles that contributed to the formation of the 137Cs distribution in
the sediment. About 60% or more of these particles were of biologicaland organic origin, and rather low lithogenic contents. Fig. 10 shows the137
Cs concentration in the near-bed suspended matter (averaged values
for sand, silt, and clay) produced by the model. The distribution pattern
was similar to those of the dissolved 137Cs and the 137Cs in the seabedsediment. There was no dominant suspended 137Cs on the shelf slope at
depths below 200 m, while the high concentrations observed in Otosaka
et al. (2014) were not reproduced by the model.
Fig. 11 shows the simulated near-bed suspended sediment concen­
tration (sum of the weight concentrations per unit volume of the threeparticle grain sizes). High sediment concentrations were mainly

distributed in the shallow areas 100 m or less; they hardly appeared on
the shelf slopes between 100 and 600 m. This was attributed to the
distance from rivers, which are less affected by sediment inflow, andweak erosion on the shelf slope. Therefore, suspended sediment was notconsidered to be involved in the formation of sedimental 137Cs in
offshore regions deeper than 400 m. These results suggest that theomission of biological and organic particles from the model may havecaused the underestimation of the 137Cs concentration in the deep
seabed sediment. Consideration of the process of formation of organicand biological suspended particles at the surface is potentially the key toimproving the reproducibility of offshore sedimental 137Cs distributions.
For instance, as planktons near the surface of the ocean die and decay,
they fall toward the seafloor as particulate matter (called marine snow),
adsorbing trace elements and their isotopes (e.g., Decho and Gutierrez,2017).
In addition, the thickness of the seabed active layer, where 137Cs
adsorbs on and desorbs from the seabed sediment, was assumed to be
horizontally uniform in this study. Otosaka (2017) suggested that theactive layer thickness off the Fukushima coast has a horizontal distri­bution; it seems relatively thin (less than 10 cm) in deeper areas. Thisimplies that the proposed model may overestimate the 137Cs desorption
from the seabed sediment at deeper depths. Future studies should use amodel that includes these biogeochemical processes and considers thehorizontal distribution of the active layer thickness.
Otosaka and Kato (2014) estimated the cumulative inventory ofradiocesium derived from FNPP1 in the seabed sediment from8

Y. Kamidaira et al.

Journal of Environmental Radioactivity 238-239 (2021) 106724

Fig. 9. Modeled 137Cs concentrations in seabed sediment (sum of the concentrations of three particle grain sizes per unit weight of the particles) from ROMS-L2, withbathymetry in black contours (m)—the colored circles indicate the instantaneous in-situ concentrations measured in May, June, July, September, October, December
2011, and February 2012 (Kusakabe et al., 2013).

Fig. 10. Modeled near-bed 137Cs concentration in the suspended matter (sum of the concentrations of three particle grain sizes per unit weight of the particles) from
ROMS-L2, with bathymetry in black contours (m).

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Journal of Environmental Radioactivity 238-239 (2021) 106724

Fig. 11. Modeled suspended sediment concentration (sum of the weight concentration per unit volume of three particle grain sizes) in the bottom water from ROMSL2, with bathymetry in black contours (m).

observational data in the domain shown in Fig. 2. They estimated theinventory using 134Cs, as 137Cs was released in almost the same amountas that of 134Cs during the FNPP1 accident (e.g., Buesseler et al., 2017).
According to their estimate, 200 ± 60 TBq of 134Cs was present in theseabed sediment in the domain in October 2011 (decay corrected toMarch 11, 2011). The amount of 137Cs in the seabed sediment by ourmodel was estimated to be 450 TBq, which was comparable in orders ofmagnitude to the observed amount.
Fig. 12a shows the proportion of radiocesium inventory in the sedi­ment, integrated for eight different depth ranges to the total inventory
for the depth range of 0–1500 m in October 2011. The inventories fromROMS were calculated in the active layer only (upper 10 cm of seabedsediment). As mentioned earlier, ROMS may have underestimated 137Cs
deposition to the seabed sediment via the suspended phase on theoffshore shelf slope. Indeed, the inventory at depths below 200 m wasless than 1% in the model; the observed inventory was 6.0%. Moreover,most of the modeled radiocesium was in regions shallower than 200 m.

This pattern was consistent with the observations. In the model, 88.8%(10.7%) of the radiocesium was at depths of 0–100 m (100–200 m); theobserved estimate was 81.0% (13.0%). Given that the inventory esti­
mations were calculated using 10%–30% of the uncertainty of theobserved data, the model inventory can be deemed consistent with theobservation. From the above results, we conclude that the sedimentary137
Cs from ROMS overall agreed with the observation, which represents
more than 90% of the radiocesium existing in the seabed sediment in the
coastal area off Fukushima. On the other hand, the 137Cs distribution on
the seabed sediment was underestimated below 400 m mainly due to theomission of biological and organic particles from the model.
3.3. Radiocesium flux from seawater to seabed sediment
In order to evaluate underestimation of sedimental 137Cs concen­
tration by ROMS-L2 due to the omission of biological and organic par­ticles from the model, we estimated vertical radiocesium flux from
Fig. 12. (a) Proportion of radiocesium inventory inthe sediment, integrated for different depth ranges
and to the total inventory for depth range of
0–1500 m in October 2011. (b) Area and time
averaged vertical radiocesium flux (MFCs). The in­ventory and MFCs were estimated for the area inside
the magenta line in Fig. 2. Blue and yellow bars areamount estimated with 137Cs from ROMS-L2, and134
Cs by Otosaka and Kato (2014), respectively.
(For interpretation of the references to color in thisfigure legend, the reader is referred to the Web
version of this article.)10

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Journal of Environmental Radioactivity 238-239 (2021) 106724

seawater to seabed sediment in the domain shown in Fig. 2. Area andtime averaged vertical radiocesium flux (MFCs; mBq m− 2 day− 1) isdefined by Eq. (7),MFCs =

Invcs
Area T

range of 0–1500 m. The simulated MFCs showed the same orders ofmagnitude as observed values in regions shallower than 400 m. Below
the 400 m, the simulated MFCs were two to three orders of magnitude
smaller than the observed ones. As discussed in section 3.2, it isconsidered that the rapid sinking of radiocesium-bound particles atsurface observed by Otosaka et al. (2014) needs to be taken into accountin the model to improve the underestimation of offshore MFCs. Accord­
ing to their results, surface particulate fluxes of 137Cs at 100 km offshore
from FNPP1 in March 2011 was in the orders of magnitude of 103 mBq
m− 2 day− 1, which decreased exponentially to 101 mBq m− 2 day− 1 in
December 2011. On the other hand, simulated MFCs in the offshore area
were in the orders of magnitude of 100 mBq m− 2 day− 1. Thus, scav­
enging of radiocesium from the surface layer was considered to bemainly due to biological and organic particles. A more sophisticatedmodel that considers biological processes should be developed to(7)

where InvCs is increase in radiocesium derived from FNPP1 for different
depth ranges until October 31, 2011(Bq), Area is area for different depth
ranges (m2), and T is the number of days from March 11, 2011 to
October 31, 2011 (234 days). Here, we assumed InvCs as inventory in thesediment in October 31, 2011(137Cs from ROMS-L2 and 134Cs from
Otosaka and Kato, 2014). MFCs is a quantitative representation of thetransfer of dissolved and suspended radiocesium in seawater to seabedsediment.
Fig. 12b show MFCs for nine different depth ranges for the depth

Fig. 13. Hovmöller diagrams across the transect of the various quantities from the ROMS-L2 results, with bathymetry in black and white contours (m)—(a) bottom
stress; (b) emerging positions of exceeding the critical bottom stress for sand (red), silt (green), and clay (gray); (c) bottom orbital velocity due to waves; (d) bottom
velocity due to mean flow; and (e) near-bed dissolved 137Cs concentration in the bottom water. (For interpretation of the references to color in this figure legend, thereader is referred to the Web version of this article.)11

Y. Kamidaira et al.

Journal of Environmental Radioactivity 238-239 (2021) 106724

account for the process of 137Cs supply from the surface layer by thesesinking particles in the future.

averaged bottom stress exceeding the critical bottom stress of any
sediment size class, we colored the plot red for sand, green for silt, andgray for clay. Sand was resuspended only in coastal areas shallower than
100 m, while clay and silt could be resuspended even offshore. The
bottom orbital velocity due to waves (Fig. 13c) was high only near the
shore in coastal regions shallower than 100 m, while the bottom velocity
due to mean flow (Fig. 13d) was high mainly in offshore regions deeper
than 400 m, with the southward mean flow of the Oyashio current. The
strong shear stress distribution near the shore coincides with the area ofdevelopment of bottom orbital velocity, while strong shear stress dis­tribution offshore coincides with the area of development of bottom
current velocity. As mentioned in supplementary materials (S1 Modeldescription), the bottom shear stress was evaluated as a function of thebottom friction of waves (due to the orbital current velocity) and thebottom friction of currents (due to the bottom mean flow), indicating

3.4. Time variation and distribution of 137Cs in bottom water andsediment
Hovmöller diagrams of the variables in the bottom-most grid cells
and seabed sediment across the transect, as shown by the black line in
Fig. 2, are plotted in Figs. 13 and 14 to investigate the transfer mecha­
nism of 137Cs toward the seabed sediment.
Large bottom stresses exceeding the critical shear stress for clay
(0.02 N m− 2; Table 1) frequently emerged in coastal regions shallower
than 100 m (Fig. 13a). The bottom stresses were weak in the depth rangeof 100–400 m but occasionally strong in the depth range of 400–1000 m.
Fig. 13b shows whether sedimental resuspension occurred. For daily-

Fig. 14. Same as Fig. 13, but near-bed suspended sediment concentration of (a) sand, (b) silt, and (c) clay; near-bed 137Cs concentration in the suspended matter of(d) sand, (e) silt, and (f) clay; 137Cs concentration in the active layer of the seabed sediment of (g) sand, (h) silt, and (i) clay; and 137Cs concentration in the substrateof the seabed sediment of (j) sand, (k) silt, and (l) clay.
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Journal of Environmental Radioactivity 238-239 (2021) 106724

accident, clay at a depth of 400 m had FNPP1-derived 137C whose
concentrations were at the same level as the preaccident background(Fig. 14i). Significant concentrations were limited to coastal areas
shallower than 200 m; the concentrations increased until around June
but then started to decrease. This suggests that the sediments acted as asink of 137Cs until June 2011 and subsequently became a source of the137
Cs released to the seawater after that month via desorption andresuspension with particles.
The concentrations of 137Cs in the sediment substrate increased
constantly during the study period for all particle grain sizes (Fig. 14j–l).
The largest concentrations were found in sand, but they were limited tocoastal areas shallower than 100 m, while small concentrations in clay
and silt permeated offshore. As shown in Eq. (5), the concentration of137
Cs in the substrate was determined by sediment deposition andresuspension, which increased with deposition. However, since thedifference in 137Cs concentration between the active layer and substrate
was several orders of magnitude until at least a year after the accident,
the influence of the influx due to deposition to the substrate was more
substantial than that of the outflow from the substrate due to resus­
pension. Thus, the 137Cs concentration in the substrate tended to in­crease independently by sediment deposition and resuspension. It is
expected that 137Cs that has reached deep layers will remain in thesediment for a long time. Since active resuspension occurs along thecoast, the 137Cs concentration in the substrate may decrease as the or­ders of concentration of the active layer and the substrate become moresimilar.

that the shear stress was induced by waves near the shore and by thecurrent velocity offshore on the transect. At depths of 100–400 m on the
shelf slope, both orbital and bottom velocities were small, and shear
stress did not develop. This condition enabled the fine sediment particles
dispersed from the river and shallow water areas to be easily deposited.
The distribution of the near-bed suspended sediments in Fig. 14a–c
corresponded to the variation of the bottom stress, indicating that the
sediment particles were constantly resuspended in areas with highbottom stress. The distribution of suspended sand grains was limited toshallow coastal areas (100 m or less), which typically have high bottom
stresses and large sand fractions (Fig. 3). This finding suggests that sandwas transported locally, not horizontally, as vertical transport associatedwith sedimentation and resuspension. Furthermore, low concentrationsof silt and clay were distributed at depths of 100–400 m on the shelfslope offshore, where resuspension barely occurs. We assumed thatthese fine-grained sediments on the slopes were not produced by local
resuspension; instead, they were transported from other sources, e.g.,rivers and coastal areas. In regard to the dissolved 137Cs (Fig. 13e), highconcentrations reaching depths of 200 m were distributed immediately
after the accident until around June 2011, after which they became
diluted. Until around June, the influence of 137Cs influx to the ocean
through direct release and atmospheric deposition was strong enough toincrease the concentration, and the relationship between the concen­
tration fluctuation and bottom stress was unclear. When the 137Cs influx
to the ocean from FNPP1 decreased after June, there were increases indissolved 137Cs concentration associated with strong bottom stresses anda correspondence with fluctuations in suspended 137Cs and sediments(Fig. 14a–f). Since the kinetic coefficient for desorption from suspended
matter k2 = 1.16 × 10− 5 s− 1, 137Cs in suspended sediment would desorb
into the dissolved form in about one day. Therefore, the concentration
fluctuation of the dissolved form after June was considered to be due to137
Cs desorbed from the resuspended sediment into seawater.
The distributions of the near-bed 137Cs in the suspended sedimentwere limited to the vicinity of the shore for all particle sizes, while theyhardly existed offshore despite the resuspension of fine particles(Fig. 13b). The 137Cs attached to clay was the most widespread offshoreamong all the size classes of particulate grains, but its concentration was
several orders of magnitude lower than that of the dissolved phase(Fig. 14f). The distribution of the 137Cs attached to sand was limitednear-shore areas shallower than 100 m, but its concentration was high(Fig. 14d). The fluctuation of the 137Cs attached to sand coincided withthe appearance of critical shear stress (Fig. 13b), suggesting the domi­nance of 137Cs transfer to seawater due to resuspension, not desorption
from the seabed sand. The concentration of the 137Cs attached to clay
increased with resuspension after June. By contrast, it increased with orwithout resuspension before June, when the influence of 137Cs influx tothe ocean from FNPP1 was strong enough to increase the concentration,suggesting that dissolved 137Cs released from FNPP1 was adsorbed on
the clay that was advected and dispersed from the rivers and othershallow areas.
Simulated 137Cs in the active sediment layer (Fig. 14g–i) fluctuatedless than did the 137Cs in the bottom seawater, i.e., the near-bed dis­solved and suspended phases represented in Fig. 13e and Fig. 14d–f,
respectively This suggests that the fluctuation time scale of the 137Cs in
the active layer of the sediment was larger than that of seawater
advection-diffusion and sediment resuspension. Since the near-bed
concentrations of the suspended phases were considerably lower thanthose of the dissolved phases for all particle grain sizes and varied
greatly due to resuspension, adsorption via the dissolved phase wasconsidered the main source of 137Cs in the active layer of the sediment.
The patterns of 137Cs distribution in the active layer for each particle
grain size were similar, and the difference in concentration between
each particle grain size was mainly determined by the difference in the
dissolved adsorption kinetic coefficient (ks1,j) for each particle size. The
137
Cs in the seabed sediment was mainly distributed in clay anddecreased with time while spreading offshore. One year after the

4. Summary and conclusions
We conducted a detailed hindcast of 137Cs in the seabed sediment off
the Fukushima coast using a newly developed model that considers theradionuclide interactions between the dissolved and solid phases withmultiscale grain sizes using a double-nested ROMS model embedded
within assimilative oceanic reanalysis (FORA-WNP30). We performed
extensive model-data comparisons against satellite altimetry data andfield observations to demonstrate that the model can satisfactorily
reproduce 137Cs concentrations and oceanic structure. The model suc­cessfully reproduced the major features of the observed spatial variationof 137Cs activities in the sediment, which represent more than 90% of theradiocesium existing in the coastal area. Hovmöller diagrams across thetransect off FNPP1 were used to examine the time variation and distri­bution of 137Cs in the sediment and the variables that determine the
distribution of 137Cs in the sediment.
The results suggested that (1) shear stress was induced by waves near
the shore and by current velocity offshore on the transect off the coast of
Fukushima; (2) there was the difference in 137Cs adsorption on thesediment per particle grain size, in which adsorption on clay was themost significant; (3) 137Cs permeated into the substrate may be retainedin the seabed sediment for a long time; and (4) these 137Cs distributionsin the sediment were formed mainly due to adsorption via the dissolved
phase by June 2011, when the impact of the direct oceanic release of137
Cs from FNPP1 was significant, while seabed sediment became asource of 137Cs released to the seawater after June 2011, where the 137Cs
in the seabed sediment was transferred to the seawater by both resus­
pension with and desorption from the sediment over a long timescale.
However, the presented model underestimated the 137Cs concen­
tration in the sediment deeper than the depth of 400-m offshore. The low
137
Cs distribution observed in the offshore sediment in a previous study
suggested a strong involvement of suspended particles of biological andorganic origin (Otosaka et al., 2014). Therefore, a more sophisticatedmodel that considers biological processes should be developed in the
future.
The parameters for the interactions of 137Cs between the solid phases
and sediment movement in this study are the same as those used inprevious studies (without any optimization) for the ocean off the coast of
Fukushima. Their optimization may improve model accuracy. For
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Journal of Environmental Radioactivity 238-239 (2021) 106724

that 137Cs with suspended matter will be released to the seawater fromthe seabed sediment. Thus, researchers should continue to focus on theeffects of the seabed sediment on the marine environment as a source of137
Cs released to the seawater. Finally, the proposed radionuclide
dispersion model, which accounts for the interaction with solid phases,
should be improved and further utilized.

example, the thickness of the active layer in this study was set to beuniform, but observation studies along the Fukushima coast suggest apotential distribution of the 137Cs penetrating the thickness that accords
with the topographic slope (Otosaka, 2017; Tsuruta et al., 2017). Thereis also a lack of literature on radionuclide adsorption/desorption co­efficients for the solid phases. Further observations and laboratory ex­periments are needed to estimate these parameters.
This study showed a small influence of the 137Cs transfer via thesuspended phase on the formation of the 137Cs concentration distribu­
tion in the sediment off the coast of Fukushima for one year after theaccident. However, these effects on the marine environment may haveincreased several years after the accident. For example, studies indicatethat 137Cs deposited on land was released to the ocean in the suspended
phase with sedimental matter from rivers several years after the accident(e.g., Kitamura et al., 2014; Iwasaki et al., 2015; Kakehi et al., 2016;
Ikenoue et al., 2020). In addition, because of the frequent resuspensionof sediment particles along the coast, there is a considerable possibility

Declaration of competing interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Acknowledgments
This study was conducted using the supercomputer HPE SGI 8600 in
the Japan Atomic Energy Agency.

Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.org/10.1016/j.jenvrad.2021.106724.
Appendix. Table of notations
Notation Definition
δi,3
Kronecker delta (no unit)
λdecay constant of the radionuclides (s− 1)
λ1,jkinetic coefficients for the adsorption between the active layer and the substrate (s− 1)
λ2,jkinetic coefficients for the desorption between the active layer and the substrate (s− 1)λEstraining parameter (no unit)
ρb,j
dry bulk density(kg m− 3)
ρs,jparticle density (kg m− 3)
σϕstandard deviation of the sediment distribution on the sedimentological ϕ-scaleτbmagnitude of the shear stress on the grains (N m− 2)
τcr,jcritical shear stress for class j (N m− 2)
ϕcorrection factor (no unit)χ
exchange velocity (m s− 1)A
the river basin area (km2)radionuclide concentration in the seabed sediment substrate (Bq kg− 1)
Ab,j
As,j
radionuclide concentration in the seabed sediment active layer (Bq kg− 1)
Areaarea for different depth ranges (m2)Cdradionuclide concentration in the dissolved phase (Bq m− 3)Cs,jradionuclide concentration in the suspended matter in size class j (Bq m− 3)
Djdeposition flux onto the seabed sediment active layer (kg m− 2 s− 1)
E0, jempirical entrainment rate (kg m− 2 s− 1)Ejerosion flux from the seabed sediment active layer (kg m− 2 s− 1)
Fatmd
dissolved radionuclide flux from atmospheric deposition (Bq m− 2 s− 1)
atmsuspended matter flux from atmospheric deposition (kg m− 2 s− 1)
F jFatms,jsuspended matter radionuclide flux from atmospheric deposition (Bq m− 2 s− 1)
thickness of the bottom water layer (m)Hz
InvCs
increase in radiocesium derived from FNPP1 until October 31, 2011(Bq)Kieddy diffusivity (m2 s− 1)Lthickness of the seabed sediment active layer (m)thickness of the seabed sediment substrate (m)LbMjsuspended matter concentration in size class j (kg m− 3)
MFCs
area and time averaged vertical radiocesium flux (mBq m− 2 day− 1)Qriver flow flux (m3 s− 1)
Rjradius of the suspended matter in size class j (m)Sdsources or sinks of dissolved radionuclides (Bq m− 3 s− 1)
Sjsources and sinks of suspended matter (kg m− 3 s− 1)
Ss,jsources and sinks of radionuclides in the suspended matter (Bq m− 3 s− 1)Tnumber of days from March 11, 2011 to October 31, 2011 (234 days)aconstant coefficientbconstant coefficient
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Y. Kamidaira et al.

c
d50
dj
fb,j
fj
k1,j
km1,j
ks1,j
k2
pj
sm,j
ss,j
ws,j

river mouth cross-sectional averaged sediment concentration (mg l− 1)
median diameter of the particle size distribution (m)diameter of the particles in size class j (m)seabed sediment fraction in the substrate (no unit)
seabed sediment fraction in the active layer (no unit)
The kinetic coefficient (s− 1)
kinetic coefficients for adsorption onto the suspended matter (s− 1)
kinetic coefficient for adsorption onto the seabed sediment (s− 1)
kinetic coefficients for desorption from the suspended matter (s− 1)
seabed sediment porosity (no unit)
exchange surfaces for suspended matter in size class j (m− 1)
exchange surfaces for seabed sediment particles in size class j (m− 1)
settling velocity (m s− 1)

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