Transfer factors of radioactive Cs, Sr, Mn, Co and Zn from Japanese soils to root and leaf of radish

Journal of Environmental Radioactivity 63 (2002) 251–264 www.elsevier.com/locate/jenvrad

Transfer factors of radioactive Cs, Sr, Mn, Co and Zn from Japanese soils to root and leaf of radish T. Ban-nai ∗, Y. Muramatsu Environmental and Toxicological Sciences Research Group, National Institute of Radiological Sciences, 4-9-1 Anagawa, Inage-Ku, Chiba 263-8555, Japan Received 31 August 2001; received in revised form 18 February 2002; accepted 1 March 2002

Abstract Transfer factors (TFs) of some selected radionuclides from ten different Japanese soils to radish have been studied by radiotracer experiments. The geometric mean values of TFs (on a wet weight basis) of radioactive Cs, Sr, Co, Mn and Zn for edible parts of radish (tuber) were 0.0090, 0.029, 0.00094, 0.0034 and 0.067, respectively. TFs for leaf were higher than those for tuber. The geometric mean values of leaf/tuber ratios were 4.1 for Cs, 4.9 for Sr, 1.6 for Co, 11 for Mn and 1.9 for Zn. Most of the Cs TFs obtained for andosol, which is the most common arable soil in Japan, were higher than those for the other soils. This might be due to the high concentrations of organic matter and alophen in andosol. The obtained TFs were compared to reference values of IAEA Technical Report 364.  2002 Elsevier Science Ltd. All rights reserved. Keywords: Transfer factor; Radionuclides; Cesium; Soil; Radish

1. Introduction The soil-to-plant transfer factor (TF) is regarded as one of the most important parameters in environmental safety assessment needed for nuclear facilities (IAEA, 1994). This parameter is necessary for environmental transfer models, which are

∗

Corresponding author. Tel.: +81-43-206-3255; fax: +81-43-206-3267. E-mail address: [email protected] (T. Ban-nai).

0265-931X/02/$ - see front matter  2002 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 5 - 9 3 1 X ( 0 2 ) 0 0 0 3 2 - 2

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useful in the prediction of the radionuclide concentrations in agricultural crops for estimating dose impacts to man. The International Atomic Energy Agency (IAEA) has compiled TFs (or concentration ratios) of some radionuclides (or elements) from the soil to the edible part of plants. These data were mainly derived from studies using European or North American soils. TFs are expected to be influenced by soil characteristics. Therefore, it is necessary to obtain regional specific values of TFs for estimating radionuclide transfers. Andosol is one of the most common soil types in Japan. According to Oba (1984), andosol represents about 47% of Japanese upland soil. Therefore, in our previous study we used this soil type. We have already reported on the transfer of radioactive Cs, Sr, Co, Mn and Zn from andosol to leaf vegetables (Ban-nai, Muramatsu, & Yanagisawa, 1995) and root vegetables (Ban-nai, Muramatsu, & Yanagisawa, 1999). We found the Cs TFs for leaf vegetables were higher than the IAEA value (IAEA, 1982). However, data are still lacking to establish regional specific values for Japan. Therefore, it is necessary to obtain more data on TFs for agricultural crops, with a special reference to soil types (andosol and other soil types). In this study, we selected radioactive Cs, Sr, Mn, Co and Zn for the following reasons. Cesium-137 and 90Sr are regarded as important radionuclides in radioecology, because of their relatively high fission yields and influence on human health. Manganese-54, 60Co and 65Zn are also important, because they are produced in nuclear facilities as activation products. Since Mn and Zn are essential elements for plants and Cs and Sr are known as analogues of K and Ca, respectively, their studies are also worthwhile from the viewpoint of plant physiology. We selected a small species of radish (Raphanus sativus L.) to study for the following reasons. (1) Cultivation of this plant is relatively easy for most soils and its growth period is short (about 30 days). (2) The size is small and suitable for laboratory pot experiments. (3) Radish is commonly eaten in Japan and other countries. (4) Both parts, tubers and leaves, can be used in the experiments to obtain information on the differences of TFs to root and leaf vegetables. We carried out radiotracer experiments using soils collected from ten different places in Japan.

2. Materials and methods The surface soils used in the experiments were collected in several areas in Japan. Soil types and sampling areas are shown in Table 1 and Fig. 1 and their chemical properties are shown in Table 2. Concentrations of relevant stable elements were determined by ICP-MS and ICP-AES (Yoshida, Muramatsu, Tagami, & Uchida, 1998). A quantity of each surface soil sample (610 g dry weight), radiotracers (137CsCl, 85SrCl2, 60CoCl2, 54MnCl2, 65ZnCl2), and mixed chemical fertilizer (2 g pot⫺1; N:P:K=14:10:13) were thoroughly mixed in a closed container. Each soil mixture was then used to fill two pots (surface area, 80 cm2; volume, 1.4 l). The amounts of radiotracers added to each pot are shown in the footnote of Table 1. The pots were placed in a growth chamber (Koitotoron, Koito-kogyo Co. Ltd).

2 2 1 2 1 1 1 1 2 2

F-10 F-55 F-49 F-64 F-68 F-70 F-60 P-54 P-62 P-48

Andosol Andosol Andosol Thick high-humic andosols Yellow soils Dark red soils Gray upland soils Gray lowland soils Gravelly gray lowland soils Fine-textured, strong-gley soils

Soil types (Amano, 1994)

Humic andosol Humic andosol Humic andosol Humic andosol – – Gleysol Eutric fluvisol Gleysol Eutric fluvisol

Soil types (FAO/UNESCO)

Tokai-mura, Ibaraki Prefecture Tokai-mura, Ibaraki Prefecture Rokkasho-mura, Aomori Prefecture Kawasaki-shi, Kanagawa Prefecture Takayama-shi, Gifu Prefecture Niimi-shi, Okayama Prefecture Mombetsu-shi, Hokkaido Prefecture Ureshino-machi, Mie Prefecture Toyama-shi, Toyama Prefecture Yamagata-shi, Yamagata Prefecture

Collection areas

a Run no. 1: added radiotracers (137CsCl: 2500 Bq pot⫺1; 85SrCl2: 1400 Bq pot⫺1; 60CoCl2: 15,000 Bq pot⫺1; 54MnCl2: 10,000 Bq pot⫺1; 65ZnCl2: 10,000 Bq pot⫺1). Run no. 2: both run 1 and run 2 follow the same procedures except regarding concentrations of radiotracers. Added radiotracers (137CsCl: 200 kBq pot⫺1; 85SrCl2: 120 kBq pot⫺1; 60CoCl2: 3300 kBq pot⫺1; 54MnCl2: 630 kBq pot⫺1; 65ZnCl2: 490 kBq pot⫺1).

Run no.a

Sample no.

Table 1 Soil types and collection areas

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Fig. 1.

Soil collection areas on Japanese map.

One week after mixing each soil with the radiotracers, two seeds were sowed in each pot and cultivated until harvest, i.e. about one month. The chamber was maintained at 24–28 °C during the daytime (12 h) and at 20–25 °C during the night (12 h). The light intensity at the plant level in the chamber was about 30,000 lux. Radish samples cultivated in soils F-64 (thick high-humic andosol) and P-48 (finetextured, strong gley soils) did not grow well and the sizes of their tubers and leaves were considerably smaller than those of the others. The wet weight of radish tuber per one plant ranged from 1.4 to 14 g and those of radish leaves per one plant ranged from 2.5 to 18 g except for F-64 and P-48. After the plants were harvested, they were divided into organ parts (e.g. tubers and leaves). The tuber (root) parts were washed in water and peeled in order to avoid direct contamination from the soil. Samples were placed in polyethylene vials to measure radioactivities with a Ge-detector. When the samples were too big to be placed in vials, they were divided into several portions. The weighted mean values of TFs of radionuclides for leaves and roots were calculated from the individual measurements. Errors due to counting statistics (1s) were less than 15%. After measurement of radioactivities, the samples were freeze-dried to calculate the dry/wet ratios. The radionuclide concentrations in the soil samples were also measured. Decay corrections were made at the beginning of cultivation. The soil-to-plant TF of a radionuclide is defined as “concentration of the radio-

K (mBq g⫺1)

148 187 182 135 265 279 374 403 523 337

F-10 F-55 F-49 F-64 F-68 F-70 F-60 P-54 P-62 P-48

16.6 13.6 16.7 16.6 7.6 8.0 18.4 7.2 6.9 22.6

1.76×103 2.11×103 2.95×103 9.83×102 3.45×102 8.59×102 6.19×102 1.44×103 2.08×103 1.37×103

6.42×103 6.09×103 3.37×103 8.62×103 5.28×103 4.13×103 2.39×103 3.94×103 3.25×103 6.93×103

14.0 76.4 17.6 55.9 49.9 31.6 24.9 10.2 9.0 8.8

Exchangeable K2O (mg/dry soil 100 g)

Ca (mg/dry soil 100 g)∗

6.69 0.54 0.05 0.12 0.08 0.14 0.08 0.33 0.25 0.06

AEC (cmol kg⫺1)∗∗

Fe (mg/dry soil 100 g)∗

CEC (cmol kg⫺1)

1.78×102 7.54×102 6.66×102 4.34×102 8.74×102 9.01×102 1.30×103 1.10×103 1.67×103 1.14×103

K (mg/dry soil 100 g)∗

67.7 222 134 547 278 336 392 274 106 455

Exchangeable CaO (mg/dry soil 100 g)

3.04 2.29 1.41 – 4.46 5.99 8.94 3.34 3.35 1.22

Cs (ppm)

107 137 197 – 36.4 115 73.2 152 259 95.8

25.1 20.2 10.6 – 16.8 20.7 4.47 14.3 10.4 13.1

– 146 99.1 175 88.7 140 33.0 71.6 68.8 91.3

101 144 85.2 – 109 111 38.1 92.6 103 114

Sr (ppm) Co (ppm) Mn (ppm) Zn (ppm)

0.04 0.21 0.55 0.36 0.25 0.23 0.05 0.12 0.38 0.06

0.32 0.29 0.59 0.34 0.29 0.14 0.14 0.15 0.16 0.21

4.27 3.98 3.20 3.40 1.06 1.32 2.43 1.64 1.36 2.84

4.31 4.19 3.75 3.76 1.31 1.55 2.48 1.76 1.74 2.90

5.46×103 4.06×103 1.20×103 5.10×103 1.08×103 5.94×102 2.42×102 2.26×102 4.48×102 2.40×102

3.06×103 1.72×103 6.81×102 7.63×103 1.36×103 1.10×103 2.19×102 3.28×102 3.51×102 1.81×103

Organic Total carbon, dry N, dry soil (%) soil (%)

Active Al Active Total Inorganic (mg/dry Fe (mg/dry carbon, dry carbon, dry soil 100 g) soil 100 g) soil (%) soil (%)∗

Coefficient of variances of duplicate determinations were mostly less than 10%, except for the values with the following marks: ∗coefficient of variance was not calculated since measurement was carried out once; ∗∗coefficient of variance was more than 10% and less than 25%.

40

5.27 6.27 5.82 6.71 6.51 6.29 5.72 6.55 6.15 5.98

F-10 F-55 F-49 F-64 F-68 F-70 F-60 P-54 P-62 P-48

Sample no.

pH∗

Sample no.

Table 2 Chemical properties of soils studied

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nuclide per unit weight (dry or wet) of the plant organ at the time of harvest (Bq g⫺1)” divided by “concentration of the radionuclide per unit weight of dry soil (Bq g⫺1) at the time of sowing”. The concentration of the radionuclides in soils at the time of harvest was calculated on the basis of the vegetable weights and vegetable concentrations. As a result, the decrease of concentrations of the radionuclides in soils between sowing and harvest was below 1%. We measured, in principle, four sets of leaf samples and four sets of tuber samples from plants grown in each soil. Every set of tuber samples was measured one by one. In some cases, more than four leaf samples (e.g. individual leaves of a radish sample) for one soil type were used for the measurement. To estimate TFs for the leaf part, both concentrations and weights of individual samples were considered and the weighted mean values of TFs were calculated.

3. Results and discussion The TFs of the radionuclides obtained in this study for ten different soils are given in Table 3 (tubers and leaves). The data are the mean values of four samples (two samples×two pots) from each soil except for soils F-68 and F-64 (mean of three samples). The standard deviations of the values within the same soil samples were, in most cases, less than 50%. However, the variations of TFs between the soil types were very large, e.g. up to two orders of magnitude. 3.1. Transfer factors of Cs The TFs of Cs for tubers (peeled) and leaves of the radish samples (on a wet weight basis) obtained from the ten different soils were in the range of 0.0014–0.064 and 0.0091–0.35, respectively. The geometric mean values of TFs obtained for tubers and leaves were 0.0090 and 0.035, respectively. The values for leaves were much higher than those for tubers. Cesium is known to behave like K in plants. An important function of K is to adjust osmotic pressure of cells. In storage organs, K also controls the adjustment of osmotic pressure of cells in the early stage of growth, then the role of carbohydrates in the adjustment becomes more important with lapse of time (Chino & Obata, 1991). Therefore, concentrations of K in the tubers are expected to drop with growth of the radish, and Cs should follow the same tendency. On the other hand, the concentration of K in leaves does not decrease; it should be constant to maintain the enzyme activity of photosynthesis (Chino & Obata, 1991). This may cause a high concentration of Cs in leaves. The TFs of Cs for both tubers and leaves in andosols (F-10, F-55 and F-49) were higher than those in other soils except for P-62 (gravelly gray lowland soils). It is well known that andosol contains organic matter and alophen in high concentrations (Shoji, Nanzyo, & Dahlgren, 1993). Since the sorption of Cs by soil organic matter and alophen is not so high, it is expected that plants take up the nuclide easily from andosol. These characteristics of andosol might be related to the high TF values for Cs. In contrast to andosol, Cs should be closely associated with clay minerals in

20.2±0.6 22.1±0.4 17.3±0.9 22.5±0.4 17.0±0.6 12.8±0.2 12.2±0.6 22.4±1.0 19.1±0.9 20.8±1.4

0.030±0.004 0.016±0.005 0.039±0.026 0.0028±0.0015 0.0054±0.0032 0.0014±0.0005 0.0023±0.0007 0.0080±0.0026 0.064±0.022 0.0074±0.0023 0.018±0.021 0.0090

Cs

Sr

0.13±0.07 0.065±0.012 0.012±0.006 0.082±0.014 0.0032±0.0021 0.0078±0.0016 0.0091±0.0030 0.0082±0.0037 0.17±0.05 0.18±0.09 0.066±0.070 0.029

85

0.0024±0.0013 0.00075±0.00029 0.0018±0.0011 0.0026±0.0005 0.00018±0.00011 0.00024±0.00007 0.00092±0.00021 0.00017±0.00008 0.0031±0.0019 0.0031±0.0032 0.0015±0.0013 0.00094

Co

60

F-10 andosol F-55 andosol F-49 andosol F-64 thick high-humic andosols F-68 yellow soils F-70 dark red soils F-60 gray upland soils P-54 gray lowland soils P-62 gravelly gray lowland soils P-48 fine-textured, strong-gley soils Arithmetic mean Geometric mean

13.4±0.1 13.1±0.1 7.8±0.1 11.9±0.3 15.1±1.3 12.4±0.4 10.5±0.2 10.5±0.2 11.5±0.7 9.1±0.5

Cs

0.10±0.04 0.058±0.017 0.25±0.20 0.0091±0.0014 0.019±0.012 0.0039±0.0002 0.0072±0.0026 0.032±0.011 0.35±0.12 0.039±0.016 0.087±0.12 0.035

137

Sr

0.94±0.25 0.48±0.12 0.064±0.042 0.36±0.12 0.013±0.003 0.020±0.003 0.030±0.009 0.046±0.012 0.93±0.12 0.58±0.08 0.35±0.38 0.14

85

0.0040±0.0020 0.0017±0.0007 0.0028±0.0018 0.0032±0.0005 0.00026± 0.0011 0.00019±0.00005 0.00082±0.00029 0.00026±0.00010 0.0065±0.0035 0.0084±0.0041 0.0028±0.0029 0.0014

Co

60

Dry TF/wet TF ratio Transfer factor for leaves (on a wet weight basis)

F-10 andosol F-55 andosol F-49 andosol F-64 thick high-humic andosols F-68 yellow soils F-70 dark red soils F-60 gray upland soils P-54 gray lowland soils P-62 gravelly gray lowland soils P-48 fine-textured, strong-gley soils Arithmetic mean Geometric mean

137

Dry TF/wet TF ratio Transfer factor for tubers (on a wet weight basis)

Table 3 Transfer factors of radionuclides from soils to radish tubers

Mn

Mn 0.21±0.08 0.073±0.016 0.13±0.10 0.066±0.011 0.0071±0.0032 0.0029±0.0002 0.015±0.004 0.0078±0.0016 0.29±0.06 0.047±0.009 0.085±0.097 0.037

54

0.015±0.003 0.0037±0.0004 0.012±0.009 0.0050±0.0025 0.00072±0.00066 0.00047±0.00019 0.0021±0.0008 0.00080±0.00031 0.032±0.006 0.0038±0.0022 0.0075±0.0098 0.0034

54

Zn

Zn 0.18±0.04 0.43±0.19 0.067±0.052 0.11±0.03 0.022±0.019 0.027±0.003 0.054±0.021 0.053±0.017 0.41±0.17 0.87±0.46 0.22±0.28 0.11

65

0.095±0.008 0.18±0.06 0.024±0.012 0.086±0.014 0.013±0.009 0.038±0.013 0.055±0.015 0.035±0.014 0.15±0.04 0.35±0.07 0.10±0.11 0.067

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most of the other soils, which might result in lower transfer of Cs from soil to plants. We examined whether there was any correlation between the TFs of Cs and stable Cs contents. However, no significant correlation was found. Since most stable Cs should be fixed in the soil solid phase, the uptake of 137Cs might not directly be affected by stable Cs concentrations in soil. According to Roca and Vallejo (1995), Cs uptake into plants was suppressed by K supply. However, in our experiment there was no correlation between the TFs of Cs and the K contents in soils even if the effect of chemical fertilizer was taken into consideration. Malm, Uusi-Rauva, and Paakkola (1991) reported that addition of K did not have any clear effect on the transfer factor. Statistical analyses by Nisbet and Woodman (2000) showed that transfer factors for radiocesium could not generally be predicted as soil characteristics (e.g. exchangeable potassium in soils). There should be two effects of K, as mentioned in the UNSCEAR report (UNSCEAR, 2000), i.e. K may dilute Cs ions, which decreases Cs uptake, but K may also act to desorb the fixed Cs, which increases uptake. The lack of a clear correlation between K and Cs observed in our study suggests the occurrence of the above-mentioned effects in our soils. No correlation was observed between the TFs of Cs and the contents of Rb and NH4 in the soil. 3.2. Transfer factors of Sr The TFs of Sr for tubers and leaves of the radish (on a wet weight basis) were in the ranges of 0.0032–0.18 and 0.013–0.94, respectively. The geometric mean values of TFs of tubers and leaves were 0.029 and 0.14, respectively. In our previous study (Ban-nai et al., 1999), we also observed that the concentrations of Sr in the leaves of root vegetables (e.g. radish, carrot and turnip) were much higher than in their root parts. It is known that Ca concentration in phloem sap was lower than that in xylem sap (Obata & Chino, 1991). This suggests that Ca and possibly also Sr (as an analogue of Ca) would be accumulated in leaves. Bukovac and Wittwer (1957) carried out experiments on absorption and mobility of foliar applied radioactive isotopes and reported that Sr and Ca were not exported from the leaf, rather the leaf absorbed them. The reason for the highest concentration of Sr being in the leaf should be its immobile character there. 3.3. Transfer factors of Co, Mn and Zn The TFs of Co, Mn and Zn for tubers of radish (on a wet weight basis) were in the ranges of 0.00018–0.0031, 0.00047–0.032 and 0.013–0.35, respectively. The TFs of Co, Mn and Zn for leaf parts (on a wet weight basis) were 0.00019–0.0084, 0.0029–0.29 and 0.022–0.87, respectively. The variations were very large (up to two orders of magnitude). The geometric mean values of TFs obtained for tubers were 0.00094 for Co, 0.0034 for Mn and 0.067 for Zn and those for leaves were 0.0014 for Co, 0.037 for Mn and 0.11 for Zn. The TFs of Co and Zn for leaf parts of radish cultivated in soils F-70 and F-60 were somewhat lower than those for root parts. The TFs of Mn for leaf parts in andosols (F-10, F-55, F-49 and F-64) were higher

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259

than those in all other soils except for P-62 (gravelly gray lowland soil). However, no correlations were observed between the TFs of Mn and the Mn contents. The TFs for tubers of radish (mean value) were in the order Zn⬎Sr⬎Cs⬎Mn⬎Co. The TFs for leaves (mean value) were in the order Sr⬎Zn⬎Cs and Mn⬎Co. 3.4. Leaf/tuber ratio The leaf/tuber ratio was defined in this study as “concentration of a nuclide in leaf part” divided by “concentration of a nuclide in tuber part”. They were calculated from Table 3 and are shown in Fig. 2. The leaf/tuber ratios of Cs, Sr, Co, Mn and Zn were in the range of 2.7–6.6, 2.6–7.3, 0.79–2.7, 6.1–20 and 0.71–2.8, respectively. The ratios of maximum value/minimum value of leaf/tuber ratios were less than 14, which were markedly smaller than the ratios of maximum value/minimum value of each soil type (up to two orders of magnitude). This suggested that the leaf/tuber ratios were rather constant for all soils used in this study and the types of soils did not influence the transportation rates of nuclides from tuber to leaves. The geometric mean values of the leaf/tuber ratio were in the order Mn⬎Sr⬎Cs⬎Co and Zn. The leaf/tuber ratios of Sr had a positive correlation with those of Mn (correlation coefficient r2=0.52). Previously, we found that the distribution patterns of Mn in different parts of leaf vegetables had a similar tendency to those of Sr (Ban-nai et al., 1995). According to the experiments conducted by Bukovac and Wittwer (1957) on absorption and mobility of foliar applied radioactive isotopes, the mobility of Mn from leaves was lower than that of Zn and K. Due to the immobile character of Mn, it is expected that Mn accumulates with time in leaves. The similar trends of leaf/tuber ratios of Mn and Sr were explained by the above-mentioned phenomena. Obata and Kitagishi (1982) studied the temporary change of the Zn concentration with growth in the leaf of rice plants. The Zn concentration in the leaf increased with time and the highest value was observed at the end of leaf growth after which

Fig. 2.

Leaf/tuber ratios calculated from TFs (on a wet weight basis) of radish.

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the concentration decreased. This indicated that Zn is translocated from older leaves to other parts, in which cell divisions are frequent, such as new leaves and root. Then no particular accumulation of this element is expected in leaves with time. The low leaf/tuber ratios observed for Zn might reflect the above-mentioned character. We previously also found that Zn was concentrated in growing parts (e.g. new leaves, root) (Ban-nai et al., 1995). The leaf/tuber ratios of Zn had a positive correlation with those of Co except for F-49. We have been unable to explain this yet. The leaf/tuber ratio of Zn obtained for radish cultivated in F-70 (dark red soil) had the lowest values of all the elements. There was no correlation between leaf/tuber ratio and weights of leaf and root. 3.5. Comparison with reference values reported by IAEA Two types of reference values for TFs have been reported by the IAEA (1982, 1994). The former are default values of concentration factors, which are identical to the TFs (on a wet weight basis). These values are a kind of reference (or a representative) value for common (unidentified) food crops. Relevant values are shown in Figs. 3(a) and (b) for comparison with the data obtained in the present study. The latter reference values, described in technical report 364 (IAEA, 1994) (TR-364 hereafter) are TF values (on a dry weight basis) for edible parts of different food crops (radish, cabbage, spinach, etc.) and/or crop types (root crops, mixed green vegetables etc.). The values are mainly derived from an IUR (International Union of Radioecology) project in which several laboratories from different countries participated, although the amount of data for many elements is still not sufficient. In this report, expected values and 95% confidence ranges (upper and lower values) are also given for several elements. Our results obtained in the present experiments are compared with these IAEA values and they are shown in Figs. 4(a) and (b). For the comparison we selected relevant values from TR-364 in consideration of crop name and types. (Information on the crop names or types used in Figs. 4(a) and (b) are given in Table 4.) The presently obtained geometric mean values of TFs (on a wet weight basis) of leaves for Sr, Co, Mn and Zn were considerably lower than IAEA values (1982), whereas the values for Cs were higher. We have also observed only the Cs TFs for leaf vegetables were higher than the IAEA values (1982) for these five nuclides (Ban-nai et al., 1995). In the case of tubers, TFs (on a dry weight basis) for Cs in some samples were higher than the upper value of TR-364 values, whereas some values for Sr and Co were lower than the lower values. In particular, TFs of Co for all soils in this study were lower than the expected TR-364 value. 3.6. Comparison with our previous data (including other vegetables) We have reported the TFs of radioactive Cs, Sr, Co, Mn and Zn from andosol (F-10) to different leaf vegetables (lettuce, spinach, cabbage, Chinese cabbage and

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261

Fig. 3. The TFs of (a) tubers and (b) leaves (on a wet weight basis) of radish and concentration factors in food crops recommended by the IAEA.

komatsuna) (Ban-nai et al., 1995) and root vegetables (carrot, turnip and radish) (Ban-nai et al., 1999). The TF values of these five elements obtained in the present study for radish (both tubers and leaves) grown on andosol collected from the same place (F-10) were similar to those of the earlier values. Thus, there was reasonable agreement between the TF values for radish grown on the same soil type, even though the radish samples were cultivated in different years. The TF values of Cs, Sr and Zn for radish leaves were comparable to those of the other vegetables mentioned above, while the values of Co and Mn tended to be lower for radish. Among the vegetables used in our previous study (Ban-nai et al., 1995), spinach leaves showed the highest TF values for Co, Mn and Zn. In root vegetables, the TF values of radish, turnip and carrot were similar, except for the high Mn value of carrot. TF values of radionuclides are thought to be influenced mainly by two factors,

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Fig. 4. The TFs of (a) tubers and (b) leaves (on a dry weight basis) of radish and TFs in food crops recommended by the IAEA.

i.e. soil types and plant species. In this study, we obtained TF values of radish grown in 10 different soil types (variation of soil types). These data (for leaf parts) were compared with TF values obtained in our previous paper (Ban-nai et al., 1995), in which we studied TF values for five different leaf vegetables grown in the same soil type, andosol (variation of plant species). Using these data, we have examined statistically the frequency distribution to compare the variation ranges of TFs due to soil types and plant species. Results from the frequency distribution for radish leaves (on a dry weight basis) showed the influence of soil types for Sr and Cs was larger than that of vegetable types (on a dry weight basis), while the influence for Co and Mn by soil types was lower than that by vegetable types. However, there are still not enough data to generalize this tendency.

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Table 4 Information on the values selected from IAEA technical report 364 for comparison with our data which is also shown in Fig. 4 Transfer factor

Root Cs Sr Co Mn Leaf Cs Sr Co Mn

Large values

Small values

Expected values

Root crops (clay, loam) Root crops (sand) Radish –

Root crops (sand) Root crops (clay, loam) Radish –

– – Radish Radish

Mixed green vegetables (sand) Mixed green vegetables (clay, loam) – Green vegetables (peat) Green vegetables (sand) – Mixed green vegetables Mixed green vegetables Mixed green vegetables Lettuce Cabbage –

For comparison of root parts (tubers) of radish, we selected the data of “radish” from TR-364. If they were not available, we selected the data of “root crops”. It has been reported in TR-364 that radish is included in “root crops”. For comparison of leaf parts of radish, we selected the data of “mix green vegetable”. If they are not in the list of TR-364 we selected other leaf crops (“cabbage”, “lettuce”, etc. except for spinach which showed high TF). When two or three data with “95% confidence ranges of TFs” were available in TR-364 (due to the different soil types), the largest and the smallest values were selected from them.

4. Conclusions The following statements summarize our present results. 1. The soil-to-plant TFs of some selected radionuclides were obtained for radish (tuber) grown on ten different Japanese soils. The geometric mean values (on a wet weight basis) were 0.0090 for Cs, 0.029 for Sr, 0.00094 for Co, 0.0034 for Mn, and 0.067 for Zn, although the variations were very large. 2. The TFs of these nuclides for leaves of radish were much higher than those for tubers. The geometric means for the leaf/tuber ratios (on a wet weight basis) were 4.1 for Cs, 4.8 for Sr, 1.6 for Co, 11 for Mn and 1.9 for Zn. 3. The TFs of Cs for tubers and leaves in andosols tended to be higher than those in other soil types. This might be due to the high concentrations of organic matter and alophen in andosol. There was no correlation between the TFs of Cs and the K contents in soils. 4. Compared to established reference values, most of the TFs were within the 95% confidence ranges reported in IAEA technical report 364 (IAEA, 1994). However, TFs of Cs for some soils were higher than the range and those of Co were lower than the range of this report.

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Acknowledgements We wish to express our thanks to Dr K. Yanagisawa, Dr S. Uchida and Dr S. Yoshida (National Institute of Radiological Sciences) for their valuable comments and to Mr K. Ishida and Mr A. Tanaka (Kaken Co.) for their technical assistance.

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