at SciVerse ScienceDirect

Journal of Environmental Radioactivity 116 (2013) 59e64

Contents lists available

Journal of Environmental Radioactivity

journal homepage: www.elsevier .com/locate/ jenvrad

Paddy-field contamination with 134Cs and 137Cs due to Fukushima Dai-ichiNuclear Power Plant accident and soil-to-rice transfer coefficients

Satoru Endo*, Tsuyoshi Kajimoto, Kiyoshi ShizumaQuantum Energy Applications, Graduate School of Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi-Hiroshima 739-8527, Japan

a r t i c l e i n f o

Article history:Received 16 March 2012Received in revised form21 August 2012Accepted 29 August 2012Available online 24 October 2012

Keywords:Fukushima Dai-ichi Nuclear Power PlantaccidentTransfer coefficientSoil to rice134Cs137Cs

* Corresponding author. Tel.: þ81 82 424 7612; faxE-mail address: endos@hiroshima-u.ac.jp (S. Endo

0265-931X/$ e see front matter � 2012 Elsevier Ltd.http://dx.doi.org/10.1016/j.jenvrad.2012.08.018

a b s t r a c t

The transfer coefficient (TF) from soil to rice plants of 134Cs and 137Cs in the form of radioactive depo-sition from the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident in March 2011 was investi-gated in three rice paddy fields in Minami-Soma City. Rice crops were planted in the following May andharvested at the end of September. Soil cores of 30-cm depth were sampled from rice-planted paddyfields to measure 134Cs and 137Cs radioactivity at 5-cm intervals. 134Cs and 137Cs radioactivity was alsomeasured in rice ears (rice with chaff), straws and roots. The rice ears were subdivided into chaff, brownrice, polished rice and rice bran, and the 134Cs and 137Cs radioactivity concentration of each plant partwas measured to calculate the respective TF from the soil. The TF of roots was highest at 0.48 � 0.10 inthe field where the 40K concentration in the soil core was relatively low, in comparison with TF values of0.31 and 0.38 in other fields. Similar trends could be found for the TF of whole rice plants, excludingroots. The TF of rice ears was relatively low at 0.019e0.026. The TF of chaff, rice bran, brown rice andpolished rice was estimated to be 0.049, 0.10e0.16, 0.013e0.017 and 0.005e0.013, respectively.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

The nuclear accident at the Fukushima Dai-ichi Nuclear PowerPlant (FDNPP) occurred as a consequence of the massive earth-quake and associated tsunami that struck the Tohoku and northernKanto regions of Japan on March 11, 2011. The released radioactivenuclides were deposited over a wide area of the Tohoku and Kantoregions. Minami-Soma City in Fukushima Prefecture is locatedabout 10e40 km northenorthwest of the FDNPP and covers about398.5 km2. About three-quarters (288 km2) of the south side ofMinami-Soma is located within 30 km of the FDNPP, and the areawithin 20 km (107 km2) from the FDNPP is still designated anevacuation zone. The designation of ‘emergency evacuation prep-aration areas’ was removed for the region located within the 20e30-km belt (181 km2) on September 30, 2011.

At the time of the FDNPP accident, radionuclide deposition of129mTe, 129Te, 131I, 132Te, 132I, 134Cs, 136Cs, 137Cs, 140Ba and 140La wasidentified in the soil of Fukushima Prefecture (Endo et al., 2012);however, the short-lived radionuclides such as 129mTe, 129Te, 131I,132Te, 132I, 136Cs, 140Ba and 140La decayed for several monthsfollowing their deposition. However, the long-lived radionuclides,

: þ81 82 424 2453.).

All rights reserved.

134Cs (half-life: 2.06 years) and 137Cs (half-life: 30.2 years), area critical factor for the internal and external exposure of humans toradiation for many months following such an accident. In partic-ular, 134Cs and 137Cs lead to internal exposure through consumptionof crops since cesium and potassium are both group 1 metals andare thus easily taken up by plants.

The transfer coefficient (TF) from soil to rice plants in Japan wasevaluated by Tsukada et al. (2002a, 2002b) using the so-calledglobal fallout 137Cs derived mainly from nuclear weapons testsfrom the 1950s to the 1980s and also from the Chernobyl accidentin 1986 (Hirose et al., 2008). The TF from soil to polished rice wasestimated to be quite low (0.0016). However, it is not clear whetherthe TF of radionuclides, which originated from extensive depositiondue to the FDNPP accident, is the same as that of global fallout longafter its deposition.

Agriculture is a key industry inMinami-Soma City, and rice is thestaple food of the Japanese people. Therefore, the radionuclideconcentration in rice is an important factor for not only the internalexposure of people to radiation but also the economy in Minami-Soma City. To estimate the TF from contaminated paddy fields torice plants, soil cores of 30-cm depth were sampled and their 134Csand 137Cs radioactivity was measured using a Ge-detector. In addi-tion, the 134Cs and 137Cs concentration in not only whole rice plantsbut also rice ears (rice with chaff), straws and roots harvested fromthe contaminated fields was obtained. This paper presents our

S. Endo et al. / Journal of Environmental Radioactivity 116 (2013) 59e6460

findings on the measured radioactivity of soil cores and rice plantsand discusses the TF of radioactivity from the soil to rice plants.

Fig. 2. Photograph of (a) a paddy field, (b) rice plant and (c) divided rice plant sample.

2. Materials and methods

2.1. Test paddy field location

The test paddy fields were located in Minami-Soma City, whichis 22.5 km northenorthwest of the FDNPP, as shown in Fig. 1. Threeadjacent lowland paddy fields (F1, F2 and F3) were selected as testfields with the cooperation of their owners. The soil type of the testfields was wet Andosol of the Okuma soil series, according to thesoil map issued by the Ministry of Land, Infrastructure, Transportand Tourism, Japan MLIT (MLIT, 2011). The GPS coordinates of thetest fields are 37�36056.700 N, 140�56017.100 E. The area of F1, F2 andF3 is about 1490, 1490 and 1980 m2, respectively; typical dimen-sions of Japanese paddy fields. Photographs of the test paddy fieldsof F1eF3 and rice plant samples are shown in Fig. 2(a)e(c).

Rice plants (Oryza sativa cv. Koshihikari) were planted in May2011 following the FDNPP accident. The fields were maintained byan organic farming method (without agricultural chemicals), withwell groundwater for F1 and F2 and dam water from Yokogawadam located upstream of the Ohta River for F3. The rice plants wereharvested at the end of September. Five sheaves of rice weresampled from each paddy field. After harvesting, and after checkingair dose rates at 1 m above the ground surface, which ranged from0.74 to 1.1 mSv/h, soil core sampling was conducted on October 14,2011. Soil cores of 30-cm depth were sampled from the paddy fieldsand designated C1, C2 and C3 for F1, F2 and F3, respectively. Twocores from each field were taken and designated as “-1” and “-2”(e.g., C1-1 and C1-2). As a reference, one core of 20-cm depth ofuncultivated paddy field soil was sampled from a farm locatedabout 4 km north of the test fields.

Since the ear samples were too small for threshing, additionalbrown rice, polished rice and rice bran samples of 1 kg each fromeach test field were also provided by the field owner after thresh-ing, A rice chaff sample was collected from a mixture of chaff fromrice ears harvested from each of the three test paddy fields.

2.2. Sample preparation and radioactivity measurement

The 30-cm depth soil cores (4.7 cm f) were subdivided into 5e7.5 cm sections. Since soil cores were compressed to 20 cm, theireffective section increment was calculated by multiplying by about

Fig. 1. Location of the test paddy fields in Minami-Soma City, Fukushima Prefectu

2/3. Samples were oven-dried at 80 �C. for 8e24 h. The driedsamples were sieved through a 2-mmmesh to remove pebbles andlarge pieces of organic content. Each soil sample (40 g) was packedinto a polystyrene container (4.8 cm f � 3 cm height) afterhomogenization in a polyethylene bag. Gamma-rays from the soilsample were measured using a low background Ge-detector (GMX-30200-P, ORTEC). The detection efficiency of the Ge detectors wasdetermined with less than 5% uncertainty using nine nuclide mixedactivity standard volume sources (MX-033, Japan Isotope Associa-tion) containing radionuclides of 109Cd, 57Co, 139Ce, 51Cr, 85Sr, 137Cs,54Mn, 88Y and 60Co. The sum effect of gamma-rays was estimatedfor 134Cs using the ratio of 605 keV-134Cs gamma-ray counting rateto 662 keV-137Cs gamma-ray by increasing the distances (20 cm)between the detector and the soil sample. The sum correctionfactor was estimated to be about 5.6%.

re, 22.5 km north-northwest of the Fukushima Dai-ichi Nuclear Power Plant.

a b

c d

e

g

f

Fig. 3. Depth profiles of radioactivity concentration in paddy field soil. (a) F1-1, (b) F1-2, (c) F2-1, (d) F2-2, (e) F3-1 and (f) F3-2, respectively. For comparison, the depth profile of anuncultivated soil is shown in (g). The solid line is a visual guide.

S. Endo et al. / Journal of Environmental Radioactivity 116 (2013) 59e64 61

The pH of soil was found to be 6.3, 6.0 and 5.8 for F1, F2 and F3,respectively by using a pH meter (HORIBA B-211).

Rice plants fromthe threepaddyfieldsweredried separately for 1monthoutdoors under a roof andwere divided into roots, straws andrice ears (rice with chaff) as shown in Fig. 2(c). Total mass of eachsample was determined by an electronic balance. Straws were sub-divided into four parts of about 20 cm each. Roots and straws werecut into small pieces of about 5 mm in length. Cut roots, straws andrice ears were then packed into the polystyrene containers and

submitted for gamma-ray spectrometry to assess radioactivityconcentration. Brown rice and polished rice samples (40 g), rice bransamples (20 g) and a rice chaff sample (10 g) were separately placedinto polystyrene containers and measured with the Ge-detector.

2.3. Transfer coefficient from the soil to rice plants

The TF, which was first introduced by Myttenaere (1972), isa useful tool for estimating the radioactivity concentration in plants

Table 2Radioactivity concentration in dried rice plants. The averaged values of five sheavesof sampled rice plants are presented and the decay is corrected to September 29,2011. Errors are the standard deviation of counting statistics.

Fieldid

Sampleid

Component Total driedmass (g)

Measureddriedmass (g)

134Cs (Bq/kg) 137Cs (Bq/kg)

F1 P1-1 Root 16.09 9.14 534 � 74 716 � 142P1-2-1 Straw 15.24 6.69 137 � 52 188 � 58P1-2-2 Straw 10.31 6.19 90 � 26 149 � 41P1-2-3 Straw 5.69 5.40 118 � 49 156 � 64P1-2-4 Straw 3.43 3.47 70 � 10 113 � 24

S. Endo et al. / Journal of Environmental Radioactivity 116 (2013) 59e6462

to determine the contamination density in the soil. The TF from thesoil to rice plants is defined as the ratio of radioactivity concen-tration (Bq/kg) of the dried plant (or each part of the plant) to theaverage radioactivity concentration (Bq/kg) of the dried soil fromthe surface to a depth of 15 cm as:

TF ¼

�134;137Cs ðBq=kg-dry weightÞ

�plant�

134;137Cs ðBq=kg-dry weightÞ�soil

(1)

Inour investigation,we calculated theTF fromthe soil to rice roots,rice straws and rice ears separately. Similarly, the TF was calculatedseparately for the chaff, brown rice, polished rice and rice bran.

3. Results and discussions

3.1. Soil radioactivity concentration

Results of 134Cs and 137Cs concentration decay-corrected forSeptember 29, 2011 are shown in Fig. 3 (a)e(f). 134Cs and 137Csconcentrations in the paddy field soil were not very depth depen-dent, whereas those of uncultivated soil from Minami-Soma Citydecreased exponentially with depth, as shown in Fig. 3 (g). Thedepth profile of the uncultivated soil shows that radionuclidesremain predominantly within 5 cm of the surface. In contrast, theradioactivity concentration of the tested paddy field soils isapproximately uniform due to cultivation (i.e., tilling).

Samples C3-1 and C3-2 show a low concentration below a depthof approximately 15 cm, suggesting that the soil of F3 was notsufficiently mixed. The averaged radioactivity concentration(mean � SD) in C1, C2 and C3 from surface to approximately 15 cmdepth is shown in Table 1. The radiocesium concentration of F3 isabout 1.7-fold higher than that of F1 and F2. This discrepancy couldbe due to the difference of irrigation water: well groundwater vsdam water. The radiocesium concentration in a water sample fromYokogawa dam on 16 September 2011 was reported as “notdetected” (ND) to 2 Bq/L for 134Cs and ND to 3 Bq/L for 137Cs by theMinistry of Education, Culture, Sports, Science and Technology,Japan (MEXT) and Fukushima Prefecture (Fukushima Prefectureweb site 2012). The MEXT also reported the radiocesium concen-tration in a sludge sample from Yokogawa dam on 28 November2011 as 10 kBq/kg for 134Cs and 13 kBq/kg for 137Cs (2011). Sincea high radiocesium concentration (1.4e33 kBq/kg for 137Cs) ina sediment sample from the Ohta River was reported (MEXT, 2011),irrigationwater from the dammight have contained small amountsof contaminated mud.

The deposition density (kBq/m2) of 134Cs and 137Cs was calcu-lated as the sumof the total radioactivity in a soil core divided by thesampling area (17.35 cm2), and decay was corrected for March 15,2011, when the main deposition occurred (Endo et al., 2012). Thedeposition of 134Cs and 137Cs on the test fields is estimated to have

Table 1The averaged radioactivity concentrations (mean� SD) in C1, C2 and C3 from surfaceto approximately 15 cm depth. And also, the deposition density of 134Cs and 137Cs.Statistical errors of initial deposition are <1%.

Field id Core no.a Radioactive concentration (Bq/kg) Depositiondensity (kBq/m2)

134Cs 137Cs 40K 134Cs 137Cs

F1 C1-1 1331 � 19 1606 � 11 332 � 26 210 240C1-2 1183 � 11 1408 � 9 367 � 20 140 160

F2 C2-1 1423 � 7 1764 � 8 383 � 23 150 150C2-2 1452 � 7 1943 � 11 420 � 39 290 330

F3 C3-1 2381 � 16 3110 � 17 430 � 55 300 340C3-2 2381 � 17 3096 � 18 395 � 49 263 296

a Two soil cores (-1 and -2) were taken from each of the three test paddy fields.

been 140e300 kBq/m2 and 150e340 kBq/m2, respectively. Theradiation dose rate at 1 m above the ground on March 15, 2011 wasevaluated and found to have a deposition density of 129,129m,132Teand 131,132I, which were the main radionuclides at that time, asestimated using the ratio of their radioactivity to that of the 137Csreported previously (Endo et al., 2012). The dose rate, including thatfrom radiocesium on March 15, 2011, was 8.3e18.5 mGy/h, as eval-uated using the conversion factor from contamination density to airdose rate by Beck (1980). Furthermore, the dose rate on October 14,2011 for the cultivated ground was estimated to be 0.62e1.5 mGy/husing an averaged conversion factor, based on Beck’s conversionfactor (Beck, 1980) with decay-correction, from the surface toa depth of 5 cm. The dose rate on October 14, 2011 is consistentwithmeasured in situ values of 0.74e1.1 mSv/h.

3.2. Radioactivity concentration in rice plants

Sample mass and 134Cs and 137Cs concentration decay-correctedfor September 29, 2011 are shown separately for rice roots, strawsand ears inTable 2. The averagedrymass of rice roots, straws andearswas 13.3 g, 30.1 g and 43.0 g, respectively. The area of one sheaf isapproximately 0.06 m2 (20 cm � 30 cm). Yields per square meterwere estimated as 220 g, 500 g and 720 g for roots, straws and ears,respectively. The averagedmass fraction of each part of strawwas 9%,17%, 31% and 43% from top to bottom. Fractional dry mass distribu-tion in rice plants is 15% for roots, 50% for ears and 35% for straws.

Our results are slightly different from those reported by Tsukadaet al. (2002b) for O. sativa cv. Mutsuhomare (roots, 4%; straws, 50%;ears, 46% (polished rice, 34%; bran, 4%; chaff, 8%)). Such a differencemight be due to differences in cultivar and cultivation and samplingconditions. In particular, some root loss might have occurred. If thefractional mass distribution of our samples is scaled to the results ofTsukada et al, dry mass yields of chaff, brown rice, polished rice andrice bran per square meter are estimated to be 120 g, 590 g, 520 gand 62 g, respectively.

The radioactivity concentration of the whole rice plant,excluding the roots, was 113e186 Bq/kg for 134Cs and 160e197 Bq/kg for 137Cs, respectively. Radiocesium concentrations in F3 ricesamples were about 1e1.6-fold higher than those of F1 and F2. A

P1-3 Ear 45.39 20.4 30 � 9 39 � 8PT-1 Total 96.2 145 � 22 197 � 9

F2 P2-1 Root 12.63 7.49 449 � 70 611 � 45P2-2-1 Straw 13.00 6.34 126 � 50 204 � 86P2-2-2 Straw 9.18 5.83 110 � 38 160 � 49P2-2-3 Straw 4.89 5.23 106 � 35 156 � 55P2-2-4 Straw 2.58 3.32 130 � 46 195 � 61P2-3 Ear 42.8 21.8 35 � 12 46 � 18PT-2 Total 85.0 113 � 31 160 � 46

F3 P3-1 Root 11.3 8.12 815 � 360 1105 � 360P3-2-1 Straw 10.7 6.98 223 � 26 334 � 77P3-2-2 Straw 8.30 4.95 151 � 26 206 � 17P3-2-3 Straw 4.60 5.02 146 � 20 201 � 42P3-2-4 Straw 2.34 2.94 168 � 61 235 � 98P3-3 Ear 40.7 21.0 46 � 4 59 � 10PT-3 Total 77.9 186 � 53 179 � 43

Table 3Radioactivity concentration in brown rice, polished rice, and rice bran. Errors are thestandard deviation of counting statistics.

Fieldid

Sampleid

Component Measureddried samplemass (g)

134Cs(Bq/kg)

137Cs(Bq/kg)

40K(Bq/kg)

F1 R1-1 Brown rice 40 15.5 � 0.8 22.5 � 0.5 58.6 � 3.9R1-2 Polished rice 40 13.9 � 0.4 19.4 � 0.4 25.9 � 2.0R1-3 Rice bran 20 162 � 5 232 � 5 247 � 32

F2 R2-1 Brown rice 40 18.5 � 0.5 24.1 � 0.5 63.3 � 3.2R2-2 Polished rice 40 7.5 � 0.4 10.9 � 0.4 23.0 � 1.9R2-3 Rice bran 20 231 � 5.7 276 � 6.0 488 � 32

F3 R3-1 Brown rice 40 36.4 � 1.0 52.0 � 1.0 69.1 � 3.3R3-2 Polished rice 40 17.9 � 0.5 24.1 � 0.5 19.2 � 1.8R3-3 Rice bran 20 244 � 6 342 � 6 370 � 29

F1e3a R4 Chuffa 10 56.3 � 1.6 87.7 � 1.6 110 � 2

a Mixture of rice chaff samples taken from all three test paddy fields.

Table 5TF expressed as (Bq/kg-dry weight)/(Bq/kg-dry weight) of each part of rice plant.

Field id Sample id Component Measureddried samplemass (g)

TF

134Cs 137Cs

F1 R1-1 Brown rice 40 0.013 � 0.001 0.015 � 0.001R1-2 Polished rice 40 0.011 � 0.001 0.013 � 0.001R1-3 Rice bran 20 0.13 � 0.004 0.16 � 0.003

F2 R2-1 Brown rice 40 0.013 � 0.001 0.013 � 0.001R2-2 Polished rice 40 0.005 � 0.0003 0.006 � 0.0002R2-3 Rice bran 20 0.16 � 0.004 0.15 � 0.003

F3 R3-1 Brown rice 40 0.015 � 0.001 0.017 � 0.001R3-2 Polished rice 40 0.008 � 0.0002 0.008 � 0.0002R3-3 Rice bran 20 0.10 � 0.003 0.11 � 0.002

F1e3a R4 Chaffa 10 0.043 � 0.002 0.049 � 0.005

a Mixture of rice chaff samples taken from all three test paddy fields.

S. Endo et al. / Journal of Environmental Radioactivity 116 (2013) 59e64 63

similar trend existed for soil concentrations. The radioactivityconcentration was highest in roots and lowest in ears.

The 134Cs and 137Cs concentration in rice ears, subdivided intopolished rice, bran and chaff is listed in Table 3. The concentrationof radiocesium in rice bran was higher than that in polished riceand chaff. For example, in F1, the 134Cs and 137Cs concentration ofrice ears was 30 � 10 and 43 � 14 Bq/kg, respectively. This showsthat the radiocesium was present in slightly higher concentrationsin the rice parts close to the surface. The same trends could be seenfor F2 and F3. The 137Cs concentration of ears (43 � 14 Bq/kg)corresponded to the sum of chaff (15.7 � 5 Bq/kg-ear), rice bran(20.2 � 6.6 Bq/kg-ear) and polished rice (14.3 � 4.7 Bq/kg-ear)(chaff þ branþ polished rice ¼ 50.2 Bq/kg-ear; slightly higher thanthe measured value), assuming the fractional mass distribution byTsukada et al. (2002b). We then assessed the changes of the radi-ocesium concentration in polished rice by washing samples R1 andR2 with water and found an 11 � 5% reduction.

3.3. Transfer coefficient from soil to rice plants

The TF of 134Cs and 137Cs is listed in Table 4. The TF of radio-cesium (134Cs or 137Cs) from the soil to rice roots was 0.31e0.36 forF2 and F3 and 0.44e0.48 for F1, and that of the whole rice plant,excluding roots, was similar at 0.078e0.88 for F2 and F3 and 0.12e0.13 for F1. In contrast, the estimated TF of radioactive cesium to

Table 4Transfer coefficient (TF) expressed as (Bq/kg-dry weight)/(Bq/kg-dry weight) of eachpart of rice plant.

Field id Part TF

134Cs 137Cs

F1 Root 0.44 � 0.06 0.48 � 0.10Straw 0.11 � 0.04 0.13 � 0.04Straw 0.074 � 0.022 0.10 � 0.028Straw 0.097 � 0.040 0.105 � 0.043Straw 0.058 � 0.008 0.076 � 0.016Ear 0.025 � 0.007 0.026 � 0.005Total 0.12 � 0.02 0.13 � 0.01

F2 Root 0.31 � 0.05 0.33 � 0.02Straw 0.088 � 0.035 0.112 � 0.047Straw 0.076 � 0.027 0.088 � 0.027Straw 0.074 � 0.024 0.086 � 0.030Straw 0.090 � 0.032 0.11 � 0.03Ear 0.025 � 0.009 0.025 � 0.010Total 0.079 � 0.021 0.088 � 0.025

F3 Root 0.34 � 0.15 0.36 � 0.12Straw 0.094 � 0.011 0.108 � 0.025Straw 0.064 � 0.011 0.066 � 0.005Straw 0.061 � 0.008 0.065 � 0.014Straw 0.071 � 0.026 0.076 � 0.031Ear 0.019 � 0.001 0.019 � 0.003Total 0.078 � 0.022 0.083 � 0.022

rice ears was lower at 0.019e0.026 in comparison with those forother parts. A higher concentration in roots compared with shootswas reported for rice plants by Myttenaere (1972). Our results forroots show a similar trend with Myttenaere (1972). Although thesoil surrounding the roots was washed off with water, smallamounts of soil might have remained.

Of the rice ear parts, the TF was highest for rice bran (0.10e0.16)followed by brown rice (0.013e0.017) and then polished rice(0.005e0.013) as listed in Table 5. In polished rice, the 137Cs TF of F2and F3 was 0.006 and 0.008, respectively, which was smaller thanthe 0.013 of F1. The trend where the TF of F1 was relatively higherthan that of F2 and F3 for both whole rice plants and rice parts maybe correlated with the concentration of 40K in F2 and F3 soils. Theother possibility is the depth profile because the TF might haveaffected the uptake of radioactive cesium by the roots.

The TF from soil to polished rice as estimated by Tsukada et al.was relatively low at 0.0016 (Tsukada et al., 2002a,b). Choi et al.(2005) reported the TF from the soil to hull seeds and whereasfor the straw were estimated to be 1.4 � 10�4 and 3.2 � 10�4,respectively. Our TF values of 0.005e0.017 for the brown and pol-ished rice are high compared with Choi’s results (2005).

Studies of the Chernobyl accident revealed that the 137Cs contentin exchangeable forms in the soil varied from 15% to 55% (IAEA,2001). Due to sorption processes of soil particles, the exchange-able forms of 137Cs in normal soils develop after a fewyears,which is3e5-fold lower than that immediately after the Chernobyl accident,and the content of such exchangeable forms in normal soils is 1e4-fold lower (IAEA, 2001). The TF of 137Cs from soil to plant is expectedto decrease with time and shows a tendency to stabilize (IAEA,2001). In particular, the TF of polished rice will decrease to around0.0001e0.0003within a few years. Therefore, the TF of radioactivityare valid only for the first few years. Moreover, paddy fields areirrigated via canals, which could contain contaminated water. Suchactivity can affect radiocesium concentration.

4. Conclusion

In summary, the TF of 134Cs and 137Cs in rice plants harvestedfrom paddy fields contaminated by the FDNPP accident in 2011 wasdetermined by measuring soil core and rice plant samples. Theradiocesium concentration in soil cores and rice plants frompaddies irrigated with dam water was 1e1.7-fold higher than thatby well groundwater. The dam water might have contained smallamounts of contaminated mud. The radiocesium concentration inthe soil and/or plants in the paddy fields could have been affectedby irrigation with dam water, which might have contained moreradiocesium than the groundwater.

The TF of roots was highest at 0.48 � 0.10 in F1, where the 40Kconcentration in the soil core was relatively low, compared with TF

S. Endo et al. / Journal of Environmental Radioactivity 116 (2013) 59e6464

values of 0.31e0.38 in F2 and F3. Similar trends could be found forthe TF of whole rice plants, excluding roots (0.13 � 0.01 in F1,0.088� 0.025 in F2 and 0.083� 0.022 in F3). The TF of rice ears wasrelatively low at 0.019e0.026. For the TF of chaff, rice bran, brownrice and polished rice was estimated to be 0.049, 0.10e0.16, 0.013e0.017 and 0.005e0.013, respectively.

Acknowledgment

The authors are grateful to Mr. A. Yasukawa for making hispaddy fields available for study and for providing rice plantsamples.We are also grateful toMr. Y. Sakurai at Nikken Co. andMs.N. Ando at Nagoya Broadcasting Network Co. Ltd. for their guidanceon sampling points.

References

Beck, H.L., 1980. Exposure Rate Conversion Factors for Radionuclides Deposited onthe Ground. U.S. Department of Commerce, Springfield, Virginia 22161. EML-378.

Choi, Y.H., Lim, K.M., Park, H.G., Park, D.W., Kang, H.S., Lee, H.S., 2005. Transfer of137Cs to rice plants from various paddy soils contaminated under floodedconditions at different growth stages. J. Environ. Radioact. 80 (1), 45e58.

Endo, S., Kimura, S., Takatsuji, T., Nanasawa, K., Imanaka, T., Shizuma, K., 2012.Measurement of soil contamination by radionuclides due to Fukushima Dai-ichiNuclear Power Plant accident and associated cumulative external dose esti-mation. J. Environm. Radioact. 111, 18e27.

Fukushima Prefecture, 2012. http://www.pref.fukushima.jp/j/koukyouyousuiiki0217.pdf (accessed 27.06.12.) (in Japanese).

Hirose, K., Igarashi, Y., Aoyama, M., 2008. Analysis of the 50-years records of theatmospheric deposition of long lived radionuclides in Japan. Appl. Radiat. Isot.66, 1675e1678.

IAEA, 2001. Present and Future Environmental Impact of the Chernobyl Accident,Study Monitored by an International Advisory Committee under the ProjectManagement of the Institut de protection et de sûreté nucléaire (IPSN), France.International Atomic Energy Agency, Vienna, Austria. IAEA-TECDOC-1240.

Ministry of Education, Culture, Sports, Science and Technology (MEXT), 2011.http://radioactivity.mext.go.jp/ja/contents/1000/139/24/5410_021714_2.pdf/(accessed 29.5.12.), (in Japanese).

Ministry of Land Infrastructure Transport and Tourism (MLIT), Japan, Soil Map ofJapan, http://tochi.mlit.go.jp/tockok/tochimizu/F3/ZOOMA/0719/index.html,15thDecember 2011.

Myttenaere, C., 1972. Absorption of radiocaesium by flooded rice: relative impor-tance of roots and shoot base in the transfer of radioactivity. Plant. Soil. 36 (1),215e218.

Tsukada, H., Hasegawa, H., Hisamatsu, S., Yamasaki, S., 2002a. Transfer of 137Cs andstable Cs from paddy soil to polished rice in Aomori, Japan. J. Environ. Radioact.59, 351e363.

Tsukada, H., Hasegawa, H., Hisamatsu, S., Yamasaki, S., 2002b. Rice uptake anddistributions of radioactive 137Cs, stable 133Cs and K from soil. Environ. Pollut.117, 403e409.