Relationship between particle size and radiocesium in fluvialsuspended sediment related to the Fukushima Daiichi NuclearPower Plant accident

Kazuya Tanaka • Hokuto Iwatani • Aya Sakaguchi •

Yoshio Takahashi • Yuichi Onda

Received: 2 February 2014

� Akademiai Kiado, Budapest, Hungary 2014

Abstract We collected fluvial suspended sediments in

Fukushima after the Fukushima Daiichi Nuclear Power

Plant (FDNPP) accident and analyzed the 137Cs concen-

tration in bulk and size-fractioned samples to investigate

the particle-size-dependent distribution of radiocesium.

The 137Cs concentration in bulk suspended sediments

decreased from August to December 2011, possibly

reflecting a decrease of radiocesium concentration in its

source materials. Smaller particles had higher radiocesium

concentrations, reflecting larger specific surface areas. Silt-

and sand-size fractions occupied more than 95 % of the

total 137Cs in the suspended sediments. The contribution of

clay-size fractions, which had the highest 137Cs concen-

tration, was quite small because of their low frequency. A

line of the data showed that the particle size distribution of

radiocesium was essential to evaluate the migration and

distribution of radiocesium in river systems where radioc-

esium is mainly present as particulate form after the

FDNPP accident.

Keywords Fukushima � Radiocesium � Particle size �Suspended sediment

Introduction

The distribution and migration of radionuclides, in partic-

ular radiocesium (134Cs and 137Cs), have been extensively

investigated since the Fukushima Daiichi Nuclear Power

Plant (FDNPP) accident. The initial stage of radiocesium

distribution in the environment has been gradually clarified

by the efforts of many researchers [1–7]. Radiocesium

deposited on the ground was rapidly and strongly adsorbed

by soils and sediments, and was fixed at the very surface

layers within 5 cm from the surface [8–11]. Once radioc-

esium is fixed on soil and sediment particles, it is not

readily desorbed from the solid phase [10, 12–14]. Such

radiocesium-bearing particles would be eroded and trans-

ported through river systems [15, 16]. Indeed, we observed

the migration of river particulate matter maintaining strong

radioactivity [14]. Thus, particulate matter can be the main

carrier of radiocesium rather than the dissolved form [17].

Eroded soil particles are transported, and finally deposited

on the bottom of rivers, lakes, estuaries and oceans,

depending on their particle size and flow rate of river

water. Therefore, with time, transportation of radiocesium-

bearing particles via rivers would increase the degree of

contamination in downstream areas.

It is well known that smaller particles have higher radi-

ocesium concentration, reflecting larger specific surface

areas [12, 18–20]. He and Walling [21] demonstrated that

radiocesium was distributed more in smaller particles based

on empirical observations in laboratory experiments. As

erosion and transportation are particle-size-dependent pro-

cesses, the distribution of radiocesium in different size

K. Tanaka (&)

Institute for Sustainable Sciences and Development, Hiroshima

University, 1-3-1 Kagamiyama, Higashi-Hiroshima,

Hiroshima 739-8530, Japan

e-mail: kt0830@hiroshima-u.ac.jp

H. Iwatani � A. Sakaguchi � Y. Takahashi

Department of Earth and Planetary Systems Science, Graduate

School of Science, Hiroshima University, 1-3-1 Kagamiyama,

Higashi-Hiroshima, Hiroshima 739-8526, Japan

Y. Onda

Center for Research in Isotopes and Environmental Dynamics,

University of Tsukuba, 1-1-1 Tennodai, Tsukuba,

Ibaraki 305-8572, Japan

123

J Radioanal Nucl Chem

DOI 10.1007/s10967-014-3159-1

fractions is important for evaluating the fate of radiocesium

in the environment. It is possible that fine-grained suspended

particle-associated transport dominates the land–ocean flux

of radiocesium transported via rivers. We investigated the

dependence of radiocesium distribution on particle size in

riverbed sediments in Fukushima after the FDNPP accident

[22]. Analytical results of different particle size fractions

suggested that the smaller particle size fractions having

higher radiocesium concentration were more selectively

transported downstream from upstream sites. However, such

a particle size-dependent distribution has not been investi-

gated in truly ‘‘suspended’’ sediment in a river, which is the

particulate matter in ongoing transportation in rivers.

Obtaining the distribution of radiocesium with particle size

information is of fundamental importance for studies

concerned with the transportation of radiocesium-bearing

particles. In this study, therefore, we analyzed radiocesium in

fluvial suspended sediments after separation into different

particle size fractions.

Samples and methods

We collected fluvial suspended sediments at upstream and

downstream sites, labeled KUS and KDS, in the Kuchibuto

River (Fig. 1). The Kuchibuto River is one of tributaries of

the Abukuma River, which is the largest river in Fuku-

shima Prefecture. Relatively large amounts of samples

were necessary to measure the radiocesium concentration

in each fraction after size separation. Therefore, the

Fig. 1 Sampling locations of

fluvial suspended sediment in

the Kuchibuto River. Enlarged

view showing the sampling

points in the Kuchibuto River as

cited from the Digital Japan

Web System [34]. A and

B indicate the upstream (KUS)

and downstream sites (KDS),

respectively

J Radioanal Nucl Chem

123

suspended sediment samples were collected using a time-

integrated suspended sediment sampler [23]. Time-inte-

grated samples were collected during two different periods

at both KUS and KDS (Table 1). Table 1 shows only the

amounts of suspended sediments used in this study that

were part of the collected samples. The remaining sedi-

ments were stored as archives.

We separated suspended sediments into different parti-

cle size fractions by two methods. One was filtration for the

KUS samples, and the other was sedimentation for the

KDS samples based on Stokes’ law [18, 19, 24]. Before

separation by filtration or sedimentation, the samples were

sufficiently suspended in water. First, for both the KUS and

KDS samples, particles larger than silt size (i.e. [63 lm)

were separated by sieving. In particular, two-step sieving

was carried out to separate fragments of plants larger than

125 lm for the KUS samples. Finally, the KUS samples

were divided into five fractions of particles with [125,

63–125, 20–63, 3–20 and 0.2–3 lm by sieving and filtra-

tion. The KDS samples were separated into six fractions,

corresponding to [63, 40–63, 20–40, 10–20, 2–10 and

\2 lm size fractions, using sieving and sedimentation.

Separation was performed by the sedimentation method

based on Stokes’ law, assuming an average density of

2.6 g/mL (= kaolinite) and a spherical shape for all parti-

cles. It should be noted that we did not remove organic

materials for the size separation to avoid any changes in the

original samples. Furthermore, we did not use a dispersing

agent for aggregates. Raw samples without any treatment

may affect the precise separation of the particles. The

specific surface areas of separated particles were deter-

mined with a Brunauer-Emmett-Teller (BET) analyzer

(BELSORP-mini, BEL Japan, Inc). Also, X-ray diffraction

(XRD) patterns for separated fractions of fluvial suspended

sediments were measured with a powder X-ray diffrac-

tometer (MultiFlex, Rigaku Co.) using CuKa radiation at

40 mA and 40 kV. All the samples were scanned over

4�–70� at a rate of 1.0� min-1 with a step interval of 0.02�.

After the size separation, homogenized samples of each

fraction were loaded into a cylindrical polystyrene container

with inner diameter of 2.0 cm and height of 4.5 cm. The

amounts of 0.2–3 lm size fractions for the KUS samples

were fairly small (Table 1), and therefore the filters on which

the particles were collected were folded and packed into a

plastic bag (4.5 9 4.5 cm2). Each sample was placed on a

planar-type Ge semiconductor detector (GC4018/7915-30/

ULB-GC, CANBERRA) to determine the count rate of c-

rays emitted from 137Cs (662 keV). The detection efficiency

on the geometry of the samples was determined using the

International Atomic Energy Agency (IAEA) reference

material IAEA-444 [25]. Count rates were converted to

radioactivity, and then the radiocesium concentration was

calculated in units of Bq/g. All the samples except one

(KUS_Dec_3) were measured within a 5 % error (1r stan-

dard deviation from counting statistics). All the activities

were corrected to the corresponding sampling dates. The

radiocesium concentration in bulk (not separated) samples

was also determined in the same way. More detailed infor-

mation on the measurement is given in Tanaka et al. [25].

Results and discussion

Distribution of particle size in suspended sediment

Particles larger than 2 mm were not found in fluvial sus-

pended sediment samples. The frequency of size distribu-

tion shows that the sand-size fraction (i.e. [63 lm)

occupied 40–60 % of weight in the KUS and KDS_Dec

samples, but they were 10 % in the KDS_Aug sample

(Table 1; Fig. 2). The frequencies of the clay-size (i.e.

0.2–3 or\2 lm) fractions for all the samples were less than

1 % in weight. Thus, the corresponding silt-size fractions

were 40–60 % in the KUS and KDS_Dec samples, but

90 % in the KDS_Aug sample. The December samples

(KUS_Dec and KDS_Dec) contained larger amounts of

sand-size fractions ([63 lm) than the August samples

(KUS_Aug and KDS_Aug) possibly because the December

samples were integrated for longer time, reflecting heavy

rainfall events, for example, on September 21, 2011 [26],

when larger particles were transported more efficiently.

Mineralogy of suspended sediment

XRD patterns for separated fractions are shown in Fig. 3.

Two large peaks around 27� and 28� are attributed to quartz

and plagioclase, respectively. Considering the geology of

the sampling areas, i.e. granitic rocks [27], the appearance

of the two peaks, as well as other sharp peaks is quite

reasonable. The particle size fractions that were smaller

than 63 lm had similar XRD characteristics to those

observed in previously reported river sediments and sus-

pended particulate matters in Fukushima [14, 28].

Although the strong intensity of quartz and plagioclase

suppressed that of other minerals, XRD patterns for the silt-

size fractions showed peaks around 8.8�, corresponding to

smectite, mica and/or illite. The XRD patterns indicated

qualitatively that micaceous clay minerals were present

more in the silt-size fractions than in the sand-size frac-

tions. Two peaks around 12.4� and 25� corresponded to

chlorite and/or kaolinite.

137Cs concentration in bulk suspended sediment

The analytical results of 137Cs are tabulated in Table 1. The137Cs concentrations were 12.8 ± 0.2 and 6.21 ± 0.21 Bq/

J Radioanal Nucl Chem

123

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J Radioanal Nucl Chem

123

g in the bulk KUS_Aug and KUS_Dec samples, respec-

tively, indicating a decrease in the 137Cs concentration

from August to December. Similarly, the bulk 137Cs con-

centration decreased from 18.0 ± 0.4 to 2.62 ± 0.06 Bq/g

in the KDS samples (KDS_Aug and KDS_Dec). Fallout

radiocesium has been fixed at the very surface of soil layers

[8–11]. The strong fixation of radiocesium by soils is

attributed to the adsorption on phyllosilicate minerals such

as clay minerals [14, 28–30]. Surface soil particles with

high radiocesium contents would be eroded into rivers by

surface runoff, and then less contaminated soils in lower

layers would be exposed to the surface. As a result, the

radiocesium concentration in exposed surface soil at hin-

terlands would decrease with time. Possibly, the

radiocesium concentration in the fluvial suspended sedi-

ments reflects the decrease in radiocesium in source

materials [31].

Particle size distribution of 137Cs in suspended

sediment

The 137Cs concentration in separated particles is plotted

against particle size in Fig. 4. Smaller particle size frac-

tions have higher radiocesium concentrations. The higher137Cs concentration in smaller particles reflects their larger

specific surface areas, giving more sorption sites for radi-

ocesium, as well as the mineralogy (Fig. 3). Overall, the137Cs concentration increased with increasing specific

surface area (Fig. 5), which is consistent with previous

studies that emphasized the importance of the relationship

between particle size and radiocesium concentration [12,

18–21]. However, our results could not be well fitted by a

power function such as the data by He and Walling [21].

Also, some data for KDS_Aug were much lower than that

expected from the trend against the specific surface area

(Fig. 5b). One of the reasons for the unfitness and deviation

of the data could be that we did not remove organic

materials from the samples. It is possible that organic

materials such as humic substances cover the surface of

clay minerals, which interrupts adsorption of radiocesium

on clay minerals [32]. Such interruption by organic mate-

rials can explain the deviation of the lower 137Cs concen-

trations observed in the data for KDS_Aug (Fig. 5b). From

another point of view, we should consider the possibility

that the suspended sediment comprised particles originat-

ing from different sources with different radiocesium

contents. Input of particles with lower radiocesium con-

centration from different sources may have caused the

deviation of the data.

The contribution of each particle size fraction to the

total 137Cs was calculated from the frequency and the 137Cs

Fig. 2 The size frequency

distribution of fluvial suspended

sediment. a KUS and b KDS

Fig. 3 XRD patterns for the silt- (KDS_Dec_40–63 and KUS_

Dec_20–63) and sand-size (KDS_Dec_63 and KUS_Dec_63–125)

fractions of suspended sediment

J Radioanal Nucl Chem

123

concentration (Table 1). The silt-size fractions occupied

more than 55 % of 137Cs in the KUS and KDS samples.

Sand-size fractions contributed to most of the remaining137Cs (9–45 %). The ratio of 137Cs in the clay-size fraction

(0.2–3 lm) for the KUS samples was negligible. The

contribution of clay-size fractions (\2 lm) in the KDS

samples was about 3–4 %, but still small. The contribution

of clay-size fractions did not significantly affect the total

radiocesium content because of their low frequency,

although they have the highest concentration in each sus-

pended sediment sample (Figs. 3, 4; Table 1).

In this way, the contribution of each particle size frac-

tion to the total radiocesium was determined by the weight

frequency of each particle size fraction, as well as by the

radiocesium concentration. As noted above, it is possible

that fluvial suspended sediment is composed of particles

with different sources, where the degrees of contamination

are different because of inventories of radionuclides orig-

inating from the FDNPP accident. It is not easy to under-

stand the distribution of radiocesium in suspended

sediment only from bulk sample analysis. Variations in

radiocesium concentration of particulate matters should be

considered with information on particle size distribution

and their radiocesium concentration, supporting the eval-

uation and prediction for the migration and distribution of

radiocesium in the environment.

It should be also noted that small size fractions of fluvial

suspended sediment are not necessarily transported as

discrete particles, but as aggregates of small mineral par-

ticles and/or organic materials in the environment [23, 33].

This means that it could be possible for smaller particles to

be contained in larger particle size fractions, because, as

described above, we did not use a dispersing agent in order

to keep samples in the original conditions in the environ-

ment. Thus, 137Cs concentrations in larger particle size

fractions could be apparently higher because of the con-

tribution of smaller particles in aggregates and/or aggre-

gation of smaller particles with organic materials. The low

contribution of the clay-size fraction to the total 137Cs may

be partly attributed to incorporation in larger particle size

fractions (Table 1). The aggregation of particles could have

an effect on the apparent relationship between particle size

Fig. 4 137Cs concentration of

each separated fraction plotted

against particle size. The

median of range for separated

particles was plotted as

representative of the particle

size

Fig. 5 137Cs concentration of

each separated fraction plotted

against specific surface area

determined by BET analysis

J Radioanal Nucl Chem

123

and radiocesium concentration. Considering the actual

forms of particles such as aggregates transported, the par-

ticle-size-dependent data of radiocesium without any

chemical treatments for separation of aggregates may be

better to evaluate the migration of radiocesium in the

environment.

Conclusions

The particle size-dependent distribution of radiocesium

was examined in fluvial suspended sediment collected after

the FDNPP accident. Bulk samples showed decrease in137Cs concentration in the suspended sediment from

August to December 2011. Smaller particle size fractions

contained higher 137Cs concentration. While keeping the

existence of aggregates in mind, analysis of radiocesium in

each particle size fraction as well as the bulk sample is

essential to evaluate the migration and distribution of ra-

diocesium in the environment through particulate matter.

Acknowledgments This work was supported by a Grant-in-Aid for

Scientific Research on Innovative Areas Grant Number 24110008.

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