JPR SYMPOSIUM Current status and future control of cesium contaminationin plants and algae in Fukushima

Radioactive pollution and accumulation of radionuclides in wildplants in Fukushima

Tetsuro Mimura • Mari Mimura • Daisuke Kobayashi •

Chiyo Komiyama • Hitoshi Sekimoto •

Masaaki Miyamoto • Akira Kitamura

Received: 29 July 2013 / Accepted: 3 September 2013 / Published online: 8 December 2013

� The Botanical Society of Japan and Springer Japan 2013

Abstract The radionuclide status of wild plants and soil

in the Fukushima area was investigated during the period

May 2011 to October 2012, using an imaging plate (auto-

radiograms) or a high purity germanium detector. Analyses

of autoradiograms showed that wild plants grown in March

2011 were strongly polluted with fallout released from the

Fukushima 1 Nuclear Power Plant. The radioactivity was

mostly due to fallout adsorbed on the surface of the plants.

On the other hand, a number of herbaceous plants were

regularly collected in the Fukushima area and their

radionuclide concentrations were measured with a high-

purity germanium detector. Plants grown in March 2011

showed very high levels of 134Cs and 137Cs, but these

radioactivity levels decreased rapidly after July 2011 and

eventually became lower than that of endogenous 40K.

During this period, the radioactivity of the soil remained

high. We therefore suppose that a significant proportion of

the radioactivity detected from plants harvested after July

2011 was most likely derived from soil dust attached on the

plant surface. Autoradiograms of rice plants were virtually

identical between plants cultivated in Fukushima and

Osaka area, reflecting the background radiation due to 40K.

Keywords Contamination of wild plants � 137Cs �134Cs � Fukushima Daiichi Nuclear Power Plant �Radionuclides

Abbreviation

F1NPP Fukushima 1 Nuclear Power Plant

Introduction

On March 11, 2011 a huge earthquake and tsunami hit the

north east of Honshu island in Japan, causing severe

damages to F1NPP. As a consequence, enormously huge

levels of radioactivity represented by 137Cs, 134Cs and 131I

were released and spread across a wide area of Fukushima

prefecture.

Following this catastrophic event, a group of Japanese

plant scientists was assembled to determine the impact of

the radioactive contamination on wild and cultivated

plants. The results of 2 years’ investigation are now

Electronic supplementary material The online version of thisarticle (doi:10.1007/s10265-013-0599-6) contains supplementarymaterial, which is available to authorized users.

‘Fukushima Daiichi Nuclear Power Station’ is cited as ‘Fukushima 1

Nuclear Power Plant’ in the present manuscript.

T. Mimura (&) � M. Mimura

Department of Biology, Graduate School of Science,

Kobe University, Rokkodai, Nada, Kobe 657-8501, Japan

e-mail: mimura@kobe-u.ac.jp

D. Kobayashi

Cellular and Integrative Physiology, School of Medicine,

Fukushima Medical University, Fukushima 960-1295, Japan

C. Komiyama � A. Kitamura

Department of Marine Engineering, Graduate School of

Maritime Sciences, Kobe University, Fukae-minami,

Higashi-nada, Kobe 658-0022, Japan

H. Sekimoto

Faculty of Agriculture, Utunomiya University,

Utunomiya 321-8505, Japan

M. Miyamoto

Radioisotope Division, Center for Supports to Research

and Educational Activities, Kobe University, Rokkodai,

Nada, Kobe 657-8501, Japan

123

J Plant Res (2014) 127:5–10

DOI 10.1007/s10265-013-0599-6

presented in this issue (JPR symposium), providing

invaluable data on the radiocontamination levels in the

environment immediately after the accident and subsequent

time-dependent changes.

The Nuclear Regulation Authority of the Japanese

government provides continuous monitoring data for

environmental radioactivity levels in both the land and the

ocean in the Fukushima area (http://radioactivity.nsr.go.jp/

ja/index.html). The Ministry of Agriculture, Forestry and

Fisheries has been publishing data on radioactivity levels in

agricultural products and foods (http://www.maff.go.jp/e/

quake/press_110312-1.html). The radioactive status of wild

plants including woods, shrubs and some herbaceous plants

in the broader ecological system has also been described by

many reports (Sakamoto et al. 2012; Smolders and Tsukada

2011; Tagami et al. 2012; Tanaka et al. 2013; Tazoe et al.

2012; Yoshihara et al. 2013). Monitoring of ocean water

and plankton has also been published (Buesselera et al.

2012; Inoue et al. 2012; Yoshida and Kanda 2012). While

radioactive contamination of agricultural products is

extensively monitored and strictly regulated for food

safety, much less attention has paid to wild herbaceous

plants (Furuta 2012; Higaki et al. 2012; Sakamoto et al.

2012). In the present study, we have measured radioactive

contamination of individual herbaceous plants and trees

using imaging plate autoradiograms and gamma ray mea-

surement with a high-purity germanium detector. Samples

were collected from the Fukushima area every few months

and the changes in radioactivity levels in herbaceous

plants, trees and soil were measured. Quantitative com-

parison was also made between the F1NPP-derived radio

Cs and the naturally occurring radionuclide 40K.

Following the Chernobyl accident in 1986, a multitude

of publications has reported the status of wild plants in

radiopolluted area. These include some pioneering resear-

ches on the uptake and distribution of radionuclides by

various plant species as well as their physiological

responses. Nevertheless, our descriptions of the levels and

changes of radiopolluted wild plants in Fukushima region

provide important information toward better management

of radioactive contamination in this region.

Materials and methods

Plant materials

Wild plants and soils were periodically collected in

Fukushima prefecture from May 2011 to October 2012.

Two sites were selected: Site 1, in Fukushima city

(37�390N, 140�320E, c.a. 50 km from the F1NPP); and Site

2, in Motomiya city (37�300N, 140�280E, c.a. 50 km from

the F1NPP). There are paddy rice fields in both sites. Most

plants collected for this experiment were growing on the

ridges around rice fields or the edges of vegetable fields. As

much soil as possible was removed from plants by hand,

Fig. 1 A photograph (a) and autoradiograms (b and c) of a Sasa plant (Sasa palmate) collected at Site 2 on Sept 7, 2011. Autoradiograms

b before or c after sample washing

6 J Plant Res (2014) 127:5–10

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but the samples were not rinsed with water. Soils were

collected from the ground surface using a garden trowel

and usually contained plant roots and humus. Rice samples

were cultivated either in Iwaki city in Fukushima prefec-

ture (37�100N, 140�430E, c.a. 40 km from the F1NPP) or in

Yao city in Osaka prefecture (34�370N, 135�380E, c.a.

570 km from the F1NPP), and were harvested in October

2011 at respective sites. All samples collected in the

Fukushima area were sent to Kobe and subjected to

radioactivity measurements within a few days.

Autoradiography of radionuclides

Fresh samples collected from the Fukushima area were

wrapped with polyvinyl film, then photographed. In some

plants, samples were washed in running water and wiped

with towel paper before imaging. Each sample was set on

an imaging plate (FUJIFILM, Tokyo, Japan) and exposed

for about five days, and the image was taken and analyzed

at a spatial resolution of 50 lm, using an imaging analyzer

(Typhoon 9400, GE Healthcare Bio-Sciences Corp., NJ,

USA). Individual radionuclides could not be distinguished

by this analysis.

Measurement of radioactivity levels

Gamma radiation activities of samples collected from the

Fukushima area were measured with a high-purity germa-

nium detector at Kobe University. Details of analyses are

described in this issue (Mimura et al. 2014).

Results and discussion

Autoradiograms of wild plants in Fukushima

Figure 1 shows a photograph (a) and autoradiograms (b

and c) of a Sasa plant (Sasa palmate) collected at Site 2 on

Sept 7, 2011. Figure 1b shows an autoradiogram before

Fig. 2 Photographs (a and c) and autoradiograms (b and d) of woody

plants Taxus cuspidate (a and b) and Quercus serrata (c and

d) collected at Site 2 on June 1, 2011. The gray arrow in (b) shows

leaves that grew in the previous year (2010) and already existed when

the F1NPP accident occurred. The black arrow in (b) points to leaves

that appeared in 2011 after the accident

Fig. 3 Autoradiogram of fallen leaves picked up from the ground at

Site 2. The upper two leaves had fallen in 2010 before the F1NPP

accident, and the lower three leaves were newly fallen in autumn

2011, after the accident. All leaves were collected at Site 2 on Oct. 4,

2011

J Plant Res (2014) 127:5–10 7

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sample washing, whereas Fig. 1c shows the same sample

after washing. Dots in the images represent fallout (dan-

gerous radioactive dust that is in the air after nuclear

explosion)-derived radionuclides released from F1NPP and

were similarly distributed before and after washing. Dots

represented radionuclides and were unable to wash off by

our hands. Because this sample was collected six months

after the F1NPP accident, a portion of the radionuclides

attached on the surface radionuclides could had been

washed off by rain. Nevertheless, radionuclide dots were

still visible on the sample surface.

Figure 2 shows photographs (a and c) and autoradio-

grams (b and d) of woody plants, Taxus cuspidate (a and b)

and Quercus serrata (c and d) collected at Site 2 on June 1,

2011. The autoradiogram of Taxus (Fig. 2b) shows radio-

activity accumulated in the lower (older) part of the plant

(gray arrow) that existed before the F1NPP accident, but

little or no activity in the upper (younger) part (black arrow)

that grew after the accident. The gamma radiation activities

were measured with a high-purity germanium detector.

The lower part showed 13,011.0 ± 113.7 Bq kg DW-1

for 134Cs and 15,060.0 ± 176.0 Bq kg DW-1 for 137Cs.

By comparison, the upper part recorded only 349.0 ±

39.4 Bq kg DW-1 for 134Cs and 296.5 ± 43.2 Bq kg

DW-1 for 137Cs. Thus, most of the radioactivity seemed to

originate from fallout-derived radionuclides. In the upper

part, some radioactivity was seen in the autoradiograms, but

this may have derived from adhering soil dust or contami-

nation from other older leaves.

In the Quercus plant, where all of the leaves developed

after the accident (Fig. 2c), only a small amount of radio-

activity was evident from the autoradiograms (Fig. 2d).

A few dots were dispersed in different parts of the leaf.

Gamma radiation measured by the germanium detector

gave activities of 1,653.3 ± 71.2 Bq kg DW-1 for 134Cs

and 1,532.5 ± 88.4 Bq kg DW-1 for 137Cs. In this auto-

radiogram, we could not distinguish radiation from dif-

ferent radionuclides. The slight general exposure of the

image in Fig. 2d may occur partly from radioactive Cs, but

partly from 40K. In the case of radioactive Cs, two possible

routes cause radioactive contamination: translocation of Cs

from other plant parts, i.e., roots or old leaves, and con-

tamination caused by very fine soil dust particles, which

did not appear as strong dots in the autoradiogram as

shown in Figs. 1, 2, 3.

Figure 3 shows an autoradiogram of fallen leaves picked

up from the ground at Site 2. In the images, the upper two

leaves had fallen in 2010 or earlier, and the lower three

leaves were newly fallen in autumn 2011. The old fallen

leaves showed radioactivity of 106,560.0 ± 544.7 Bq kg

DW-1 for 134Cs and 123,650.0 ± 808.1 Bq kg DW-1 for137Cs. On the other hand, the lower new fallen leaves

showed only 1,833.8 ± 183.7 Bq kg DW-1 for 134Cs and

2,025.1 ± 233.9 Bq kg DW-1 for 137Cs. When the fallout

Fig. 4 Time-dependent changes in the concentrations of the radio-

nuclide 134Cs (light gray), 137Cs (black), and 40K (dots) in Trifolium

spp. sampled from June 2011 to October 2012. An inset shows an

autoradiogram of the same plant species collected at Site 2 on 23 July

2011

Fig. 5 Time-dependent changes in the concentrations of the radio-

nuclides 134Cs (light gray), 137Cs (black), and 40K (dots) in Equisetum

arvense sampled from June 2011 to October 2012. An inset shows an

autoradiogram of the same plant species collected at Site 1 on 1 June

2011

8 J Plant Res (2014) 127:5–10

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was released from F1NPP, the old fallen leaves had already

been on the ground and the fallout-derived radionuclides

must have adhered on their surfaces. The slight activity in

the leaves that fell in the autumn of 2011 may have been

derived from contamination from the soil after falling, or

directly from the fallout adsorbed on the leaf surface prior

to leaf falling.

Changes in gamma radiation activities of wild plants

and soils

Together with the above observations, we periodically

collected other wild plants and measured their levels of

radioactive contamination. We collected Trifolium spp.

(Fig. 4) and Equisetum arvense (Fig. 5) and measured the

radioactivity of the shoot with a high-purity germanium

detector after as much soil as possible and root tissues were

removed by hand. In both plants, when the first samples

were collected prior to June, 2011, some plants had already

begun to grow at the end of March and might have been

polluted with the fallout released from F1NPP. Those

samples showed very high radioactivity from 134Cs and137Cs. In summer, new shoots of these plants began to grow

and since they were not exposed to the main initial fallout,

radioactivity was quite low, and was similar in the fol-

lowing year. In Figs. 4 and 5, sample numbers are quite

low (n = 2 or 3), because of limited collection and

machine time. One sample was made for a set of several

plants, and this might have caused a problem because if

some plant contained much higher radioactivity than oth-

ers, it should distort the average value. Samples in 2012

always showed low levels of radioactivity from radioactive

Cs. One possibility may be uptake of radioactive Cs from

the soil, or alternatively, surface contamination by wind-

blown soil particles. In fact, several dots were seen in

autoradiograms of these plant samples (Figs. 4, 5 insets).

But in both plants the radioactivity from 40K was higher

than that from radioactive cesium in late 2011 and 2012.

The background exposure on the autoradiograms is most

probably due to 40K.

In preliminary measurements, we also determined the

radioactivity of soil samples (Supplementary Table 1). For

these measurements, soil was collected from the surface

and only measured once. Soil radioactivity in the first

samples was between about 3,000 and 6,000 Bq kg DW-1

and even in late 2012, soil samples showed several thou-

sand Bq kg DW-1. Hence, if particles from these soils

became attached to leaf surfaces, the plant radioactivity

would be increased.

To compare contamination of grass and trees, we also

measured the radioactivity of leaves and twigs of a plum

tree at Site 1, although these are only single measurements

(Supplementary Table 2). As shown in Supplementary

Table 2, twigs that existed in the spring of 2011 showed a

high level of radioactivity, but twigs that appeared in 2012

showed very low activities. Leaves in early 2011 were

more radioactive than those that grew in 2012. It is

unknown why the radioactivity of leaves in 2012 was

Fig. 6 Comparison of autoradiograms of rice plants grown and

harvested at Iwaki city in Fukushima prefecture (a and b) or at Yao

city in Osaka prefecture (c and d)

Table 1 Radioactivity levels of leaves of rice plants cultivated in

Iwaki and Osaka

Cultivation

place

134Cs 137Cs 40K

(Bq kg DW-1) (Bq kg DW-1) (Bq kg DW-1)

Iwaki 18.23 ± 1.28 19.87 ± 1.47 642.33 ± 68.27

Osaka N.D. N.D. 256.13 ± 88.37

N.D. no radioactivity was detected

J Plant Res (2014) 127:5–10 9

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higher than that of new twigs. Possibly, leaves with broad

surfaces retain more soil dust. Alternatively, radioactive Cs

accumulated more in leaves than in twigs.

Comparison of gamma radiation activities of samples

between Fukushima and Osaka

Figure 6 shows autoradiograms of harvested rice plants.

Rice plants in Fig. 6a and b were cultivated at Iwaki city in

Fukushima prefecture and those in Fig. 6c and d were

cultivated at Yao city in Osaka prefecture. The autoradio-

grams of plants from these regions were similar. It is

unlikely that rice plants from Osaka would receive signif-

icant amounts of radionuclides released from F1NPP, given

that the distance between Osaka and Fukushima is at the

minimum around 570 km.

Table 1 shows the activities of radionuclides in leaves of

rice plants from the two regions. Rice plants cultivated in

Fukushima contained detectable levels of 134Cs and 137Cs

as well as the naturally occurring radionuclide 40K,

whereas rice plants cultivated in Osaka contained only 40K

and no detectable Cs radioisotopes. Thus, the slight expo-

sure of the autoradiogram of rice plants grown in Osaka

must be due only to gamma radiation of 40K.

Acknowledgments We thank Mr. Toshiyasu Okochi, Family Ouchi

and Ms. Kazumi Tsutsui for their kind permission to collect plant and

soil samples from their fields. We are grateful to Dr. Robert Reid, The

University of Adelaide, Australia, for his kind assistance with the

English text. The present work was supported in part by a grant from

the Mitsui & Co., Ltd. Environment Fund, a grant from Japan Society

for the Promotion of Science and a grant from the Ministry of Edu-

cation, Culture, Sports, Science and Technology of Japan (Grant-in-

Aid no. 24110007).

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