remaining samples of small pieces were ashed at a temperature lower than ... The dried soil (100 g) and ashed potato samples (50 g) were compressed into a ...
P-4a-242
Transfer of 137Cs, essential and trace elements from soil to potato plants in an agricultural field H. Tsukada and H. Hasegawa Institute for Environmental Sciences, Department of Radioecology, 1-7 Ienomae, Obuchi, Rokkasho-mura, Kamikita-gun, Aomori 039-3212, Japan
ABSTRACT The concentrations of 137Cs, essential and trace elements were measured in soils and potato tubers collected from 26 agricultural fields in Aomori, Japan, and soil-to-potato transfer factors were determined. The elements were divided into two groups. The first group (Cl, K, Ca, etc.) showed an inverse correlation between the transfer factors and the concentrations of the elements in the soils, while for the second group (Sc, Co, etc.) the transfer factors were independent of the soil concentrations of the elements. The transfer factors of 137Cs (0.0037-0.16), derived from global fallout, were well correlated with those of naturally stable Cs (0.000520.080). These transfer factors showed a negative correlation with the soil concentrations of K and Cs, but they were independent of the organic material contents in the soils. These results suggest that the transfer of stable Cs could serve as a natural analog to predict the behavior of radiocesium in the soil-plant pathway. The distributions of these elements were determined for the entire potato plant. The concentrations of the elements were lower in the tubers than in leaves, petioles and stems. During the harvesting of potatoes the elements in the non-edible portions of the potato plants are returned to the soil, where they may again be utilized in the soil-potato pathways. Therefore, the distributions of elements in plant components can provide useful information for understanding the transfer of radionuclides and elements from the soil to plants in agricultural fields. The concentration ratios for Sr/Ca in potato plant components showed relatively constant values while those for Cs/K varied. These findings suggest that the translocation rates of both Ca and Sr were similar within a potato plant, whereas those of K and Cs were different. Consequently, the transfers of both Ca and Sr may predict the behavior of radiostrontium. The transfer of Cs could be used to predict the behavior of radiocesium, whereas the transfer of K cannot be used to indicate the behavior of radiocesium.
INTRODUCTION Radionuclides released into the environment reach the human body through several transfer processes (1). The soil-to-plant transfer factor is one of the important parameters widely used to estimate the internal radiation dose from radionuclides through food ingestion. In general, transfer factors show a large degree of variation dependent upon several factors such as soil type, species of plants and other environmental conditions (2-4). Therefore, in order to assess more realistically the internal radiation exposure to the public around nuclear facilities, site-specific parameters should be taken into account in the area concerned. Due to a predicted long-term transfer of radionuclides in the environment, an understanding knowledge of the geochemical and ecological cycles is also needed as they relate to the behavior of not only radionuclides but also associated elements. In addition, the distribution of radionuclides in plant components is beneficial in understanding the dynamics of radionuclides in an agricultural field. Because non-edible parts of agricultural plants are returned to the soil, they may again be utilized in the soil-plant pathway. Cesium-137 is an important radionuclide for the assessment of radiation exposure to the public because of its high fission yield, relatively long half-life (30.2 years) and relatively high transferability in the environment and in the human body. The deposition of 137Cs in Japan was mainly derived from nuclear weapons tests from the 1950s to 1980s, and from the Chernobyl accident in 1986 (5-7). The transfer factor of 137 Cs increased with increased organic materials in the soil (8). It decreased with an increase in clay content (9), K content (9, 10) and pH (9, 10) of the soil. However, these relationships were not always observed by all researchers. Several studies showed that the transfer factor of 137Cs was not affected by the soil-pH (8), or by the clay content of the soil (11). Most of the transfer factors of 137Cs have been examined by radiotracer experiments using lysimeters and/or pots because of the difficulty of using radioisotopes in an open field. Even though an agricultural field is an artificial system controlled by plowing and fertilizing every year for plant growth, the conditions existing in an agricultural field are quite different from those in a laboratory experiment. The parameters and their variations observed in the agricultural fields are more realistic in estimating the transfer of radioisotopes and elements. In Japan, nearly all 134Cs was derived from the Chernobyl accident because of its short half life (2.06 years) and not present in fallout from atmospheric nuclear weapon tests (7), while most 137Cs remained as the remnant of the fallout from nuclear weapon testings. However, Tsukada et al. (12) reported that the transfer factors of 134Cs and 137Cs from substrata to mushrooms showed similar values, although the residence periods of both radionuclides were different from each other in the soil. This implies that the aging effect in artificially added 134Cs and 137Cs had disappeared several years after deposition. Some observations showed that transfer 1
P-4a-242 factors of 137Cs in plants decreased gradually with time as a result of aging, and then reached a constant level (13, 14). The transfer factors of 137Cs were still several times higher than that of stable Cs (Cs) for wild mushrooms, however, that of 137Cs in mushrooms was significantly correlated with that of Cs in a constant ratio (12, 15). Consequently, the behavior of Cs in an open field may be regarded as a useful analogy in predicting the longterm transfer of 137Cs. The present study examined the concentrations of 137Cs, essential and trace elements in soils and potato tubers collected from agricultural fields in Aomori, Japan. These measurements were used to determine their soil-to-potato transfer factors and the correlations between the concentrations of elements in soils and their transfer factors. Moreover, the distribution of essential and trace elements in potato plant components were determined.
MATERIALS AND METHODS Sample collection and pretreatment Soil and potato tuber samples were collected from 26 agricultural fields throughout Aomori Prefecture in Japan during the period from 1991 to 1994, as shown in Figure 1. A plastic core sampler was used to collect soil cores 15 cm in diameter and 5 cm in depth (approximately 1 kg) at 8 sites evenly distributed in each sampling field. Potato tuber samples (20 kg) were collected by hand in the same field. The soil core samples collected from each field were dried at 60°C and then passed through a 2 mm sieve. The 8 sieved soil samples in each field were thoroughly mixed and pulverized with an agate ball mill. Potato tuber samples were washed, peeled, and then the edible parts were cut into small pieces. Each 500 g sample was dried at 70°C and then pulverized in a stainless steel cutter blender for analysis of stable elements. For the analysis of 137Cs, the remaining samples of small pieces were ashed at a temperature lower than 450°C to avoid loss of alkali elements by dry ashing. In addition, component samples on a potato plant were collected from an experimental field. They were freeze-dried and then pulverized in a stainless steel cutter blender.
Sample analysis The dried soil (100 g) and ashed potato samples (50 g) were compressed into a plastic bottle (45 mm diameter and 50 mm height), and then the concentrations of 137Cs were determined with a high purity Ge gamma-ray detector connected to a multichannel analyzer system by counting for 30000-300000 s and 300001000000 s, respectively. The detection efficiency of 137Cs depended on the sample thickness and was obtained by using mixed radionuclide standard reference materials for 5 different thicknesses. Counting errors of 137Cs in soil were 2-10 %, while those in potato were 2-20 % due to the low concentrations of 137Cs. The concentrations of essential and trace elements in soil and potato tuber samples were determined by a neutron activation analysis. Approximately 10-100 mg of dried soil and potato samples were sealed separately in small polyethylene bags. Several bags were placed together into a polyethylene capsule and
Aomori Prefecture
140 142°E
42°E 40°E
Japan
Figure 1.
Sampling sites for paired soil and potato samples in Aomori Prefecture. 2
P-4a-242 Table 1
Arithmetic mean and one standard deviation of 137Cs (Bq kg-1 dry wt.), essential and trace elemental concentrations (mg kg-1 dry wt.) in soil and potato tuber
137
Cs
Potato tuber Arithmetic mean 1σ 5.5E-1 ± 5.1E-1
Br Ca Cl Co Cr Cs Fe I K Mn Mo Na Sc Zn
1.1E+1 2.0E+2 2.6E+3 6.8E-2 4.8E-1 4.4E-2 2.3E+1 6.2E-2 2.0E+4 6.7E+0 1.9E-1 4.3E+1 7.2E-4 1.6E+1
± ± ± ± ± ± ± ± ± ± ± ± ± ±
8.5E+0 8.8E+1 1.2E+3 5.4E-2 8.6E-1 3.6E-2 7.6E+0 6.0E-2 3.3E+3 3.2E+0 1.2E-1 3.3E+1 3.0E-4 4.6E+0
Soil Arithmetic mean 1.6E+1 ± 9.4E+1 1.5E+4 3.0E+2 1.6E+1 8.2E+1 5.5E+0 5.0E+4 2.1E+1 1.2E+4 1.1E+3 5.0E+0 1.1E+4 2.0E+1 1.2E+2
± ± ± ± ± ± ± ± ± ± ± ± ± ±
1σ 1.0E+1 5.5E+1 8.6E+3 1.4E+2 6.1E+0 5.8E+1 3.2E+0 8.9E+3 1.9E+1 4.1E+3 2.8E+2 1.8E+0 3.8E+3 6.3E+0 4.6E+1
irradiated in the Rikkyo University TRIGA Ⅱ reactor (neutron flux: 5.5 × 1011 n cm-2 s-1) and/or the Japan Atomic Energy Research Institute JRR-4 reactor (neutron flux: 3.4 × 1013 and/or 6.0 × 1013 n cm-2 s-1). The irradiated samples were cooled and then counted by a Ge gamma-ray detector connected to a multichannel analyzer, as described in a previous paper (16). Otherwise, the concentrations of stable elements in potato plant component samples were determined by means of inductively coupled plasma-mass spectrometry (VG, PQΩ) and/or atomic adsorption spectrometry (HITACHI, Z-8200). The dried sample (500 mg) was put in a TeflonTM PFA pressure decomposition vessel with mixed acid (HNO3 + HClO4 + HF), and then decomposed with a microwave digester (SPEX, CDS7000). The organic content of soils was estimated from the loss of weight after ashing at 450°C for 3 h.
RESULTS AND DISCUSSION
Annual deposition of
137
Cs (Bq m -2 )
Concentrations of 137Cs, essential and trace elements in soils and potato tubers 1.0E+4 1.0E+3 1.0E+2 1.0E+1 1.0E+0 1.0E-1 1960
1965
1970
1975 1980 Year
1985
1990
Figure 2. Annual deposition of 137Cs in Aomori, Japan during the period from 1960 to 1994. The 137Cs depositions during the period from 1960 to 1976 were estimated from those in Tokyo (5) by means of the 137Cs deposition ratio of 1.4 times, which was that in Aomori (6) to Tokyo (7).
3
Root uptake of radionuclides and elements by plants depends upon their vertical distribution in the actual rooting zone in soil. Cesium-137 derived from fallout has been deposited in the surface soil, whereas stable elements originally existed in soil materials and/or were enriched by application of fertilizers necessary for plant growth. Typical examples of vertical 137Cs concentrations in the upper 20 cm of soil had a similar value within 2σ counting errors in each field because soils in farm fields are generally cultivated every year. In addition, the concentrations of essential and trace elements in a soil were also uniformly distributed throughout the layer to the depth of 30 cm. Consequently, the vertical
P-4a-242 distributions of 137Cs, essential and trace elemental concentrations were homogeneously mixed at least in the soil layer to the depth of 20 cm where the roots of arable crops generally develop. The mean concentrations and their standard deviations (1σ) of 137Cs, essential and trace elements in soil and potato tuber samples collected from 26 agricultural fields are shown in Table 1. Past atmospheric nuclear explosion tests and the Chernobyl accident accounted for the presence of 137Cs in soil. The contribution of 137Cs derived from the Chernobyl accident was estimated to be about 10 % of the total amount of 137Cs in the forest soil in Rokkasho, Aomori (12). Most of the 137Cs in the soil had accumulated by the early 1980s, when Japan received most of the fallout from weapons testing (Figure 2). The arithmetic mean of 137Cs concentrations in soils was 16 Bq kg-1 (Table 1), which was of a similar order of magnitude as for other Japanese soils (6). The arithmetic mean concentrations of essential and trace elements in soils for potatoes were within the order of magnitudes in the cultivated soils of other kinds of plants in Aomori (16). The content of organic materials in soils was 2.5-18 %. The mean concentration of 137Cs in potato tubers was 0.55 Bq kg-1 dry wt. (0.11 Bq kg-1 fresh wt.), which was several times higher than other agricultural crops, such as 0.032 (Japanese radish) and 0.063 Bq kg-1 fresh wt. (cabbage), observed in Aomori (6). The specific activity of 137Cs (the ratio of the activity of 137Cs to that of Cs) in soils, 4.2 ± 4.3 Bq mg-1, was lower than that in potatoes of 18 ± 15 Bq mg-1. The ranges of most elemental concentrations in soils and potato tubers were within one order of magnitude except for Cr and I in soils, and Br, Co, Cr Cs, I and Na in potato tubers.
Soil-to-potato transfer factors The soil-to-plant transfer factor is one of the important parameters used to estimate the concentrations of radionuclides and elements in plants according to a transfer model. The soil-to-potato transfer factors for 137 Cs and elements are defined as follows: Concentration in potato (Bq kg-1 dry wt. for 137Cs or mg kg-1 dry wt. for element) Transfer factor (TF) =
(1) Concentration in soil (Bq kg-1 dry wt. for 137Cs or mg kg-1 dry wt. for element)
Transfer factor
In the present study, the concentrations of 137Cs and elements in the upper 5 cm soil layer were regarded as representative of those throughout the soil layer, because of their homogeneous distribution in the actual rooting zone for arable crops, up to 20 cm in depth. The transfer factor generally shows a very wide range of variation dependent on soil types and other environmental conditions. 1.0E+2 Therefore, the parameter used for Br Ca the estimation of the transfer of Cl Co the nuclides should be evaluated 1.0E+1 Cr Cs under site-specific conditions. Fe I The relationship K Mn between concentrations of 1.0E+0 Mo Na essential and trace elements in Sc Zn soils, and their soil-to-potato transfer factors were indicated in 1.0E-1 Figure 3. Several of these elements were divided into two groups, each having different 1.0E-2 transfer factor characteristics. In the first group of elements, such as Cl, K, Ca, etc., an inverse correlation existed 1.0E-3 between the transfer factors and their concentrations in soils. However, in the second group, 1.0E-4 the transfer factors of elements including Co, Sc, etc. were independent of their 1.0E-5 concentrations in soils. Similar 1.0E-1 1.0E+0 1.0E+1 1.0E+2 1.0E+3 1.0E+4 1.0E+5 observations were reported for natural radionuclides in a wet Concentration in soil (µg g-1) marshland (17). Figure 3. Relationship between concentrations of elements in soils and The soil-to-potato their transfer factors. transfer factors of 137Cs and Cs 4
P-4a-242 obtained in this study were in the range of 0.00370.16 and 0.00052-0.080, respectively. All of the transfer factors of 137Cs obtained were higher than those of Cs, showing a good correlation (plotted with log-log) as shown in Figure 4. This implies 1.0E-1 that artificially added 137Cs is still more mobile in the soil and more easily absorbed by plants than Cs, even though most of the 137Cs had been deposited 1.0E-2 on the soil more than 10 years ago. Similar results on the transfer factors of 137Cs and Cs were previously reported in wild mushrooms (12, 15). Several researchers (18, 19) reported properties for 1.0E-3 137 Cs and Cs in soils using a sequential extraction method. Most of 137Cs and Cs appeared in strongly bounded fractions such as interlayer 1.0E-4 exchange sites in clay mineral lattices, however, the fractions of extractable 137Cs and 137Cs combined 1.0E-4 1.0E-3 1.0E-2 1.0E-1 1.0E+0 with organic material were time-dependent. The Transfer factor of Cs transfer factors of both 137Cs and Cs decreased with increasing K concentrations in the soil, showing a Figure 4. Comparison of soil-to-potato transfer relatively good correlation. Similar observations factors between 137Cs and stable Cs. reported that the transfer factor of 137Cs decreased with increasing concentrations of K in soils (9, 10). In general, the concentrations of K in potato tubers were remarkably higher than other elements (Table 1). Potassium is one of the important essential elements (N, P and K) from the viewpoint of plant physiology. In addition, both K and Cs are alkali elements. The concentration of K in potato tubers was relatively constant (the range of 13000-28000 mg kg-1), independent of that in the soil. This suggests that the excessive amount of K applied as fertilizers (availability of K in plants) for plant growth may have inhibited the root-uptake of 137Cs by plants. The ratio of 137Cs concentrations between plants and soil solutions was significantly dependent on K concentration in the soil solution (11, 20). These observations indicate that the transfer of 137Cs from soil to plant may be affected by the concentration of mobile K in the soil. The relationship between the transfer factors of both 137Cs and Cs, and K concentration in soils in this study would be expressed in a similar non-linear observation for the 137Cs-K system (9, 10). The relationship is described as: Transfer factor of
137
Cs
1.0E+0
logTF = a - blog[K]
(2)
where [K] is the concentration of K in soils. The best fit for the gradient b in the equation (2) was estimated as approximately 1.6 for both 137Cs (|r| = 0.65) and Cs (|r| = 0.74). In addition, the transfer factor of 137Cs showed a similar negative correlation with Cs concentration in soils as previously described for that of Cs (Figure 3). Observations on the relationship among the transfers of 137Cs and Cs, and K concentration might be useful for preliminary estimations of the transfer parameters of the nuclides for specific sites. In other words, these field studies on Cs and K could be used to determine the long-term transfer of 137Cs in the environment. Bergeijk et al., (8) reported that the transfer factor of 137Cs increased as the concentration of organic materials increased in soils. However, in this study, no clear correlation was found between the transfer factor of 137Cs and the content of organic materials in soils. This may be attributed to the fact that variable organic contents in all agricultural soils are actually due to the addition of artificial fertilizers and the recycling of crop residues, depending on each field. The geometric means and the 95% confidence intervals of the transfer factors of both 137Cs and Cs in the present study are indicated in Table 2, together with the reported values for clay and loam by IAEA (4). The 95% confidence intervals of the transfer factors were Table 2 Soil-to-potato transfer factors in the present work and IAEA. within 2 orders of magnitude. Concentrations of 137Cs and stable Cs in potato tubers and those in soil were The geometric mean of the based on dry weight. Range showed 95% confidence ranges transfer factor of 137Cs was Transfer factor Reference 0.030, which was 4 times higher Geometric mean Range than that of Cs at 0.0075. On 137 the other hand, that of 137Cs 3.0E-2 5.0E-3 1.8E-1 This work Cs reported by IAEA (4), Cs 7.5E-3 6.6E-4 - 8.4E-2 This work determined mostly by lysimeter 137 7.0E-2 7.0E-3 - 7.0E-1 IAEA (4) Cs and/or pot experiments using 5
P-4a-242 radiocesium, was 2.3 times higher than that in this study. The difference could be attributed to the difference between experimental and environmental conditions. The soil-to-potato transfer factor of Cs in the present study was approximately one order of magnitude lower than the IAEA's value (4). 1.0E+0
1.0E-1
1.0E-2 1.0E+4
1.0E+5
-1
-1
Concentration of Cs (µg g )
Leaf Petiole Stem Tuber
Concentration of Sr (µg g )
1.0E+3
(a) K vs. Cs
1.0E+2
Leaf Petiole Stem Tuber
(b) Ca vs. Sr
1.0E+1
1.0E+0
1.0E-1 1.0E+1 1.0E+2 1.0E+3 1.0E+4 1.0E+5 Concentration of Ca (µg g-1)
1.0E+6
Concentration of K (µg g-1)
Figure 5. Relationship between concentrations of (a) K and Cs, and (b) Ca and Sr in potato plant components. The samples were collected on June 21, July 18 and 30, and August 14, 1996.
Distribution of essential and trace elements in potato plant components During the harvesting of potatoes, the elements in the non-edible portions of the potato plants are returned to the soil, where they may again be utilized in the soil-plant pathways in agricultural fields. Therefore, the distributions of elements in plant components can give us useful information for understanding the transfer of radionuclides and elements. The concentrations of most elements in potato plants were lower in the tubers than in leaves, petioles and stems, however, those of alkali elements showed a relatively equal distribution throughout the plant for each element. Green et al. (21) also reported that the distribution of 137Cs was essentially uniformly distributed throughout a tuber. In addition, the concentrations for Cs vs. K and Sr vs. Ca in potato plant components were plotted in Figure 5. The concentrations of both K and Cs in each component were within approximately one order of magnitude for each element, although the concentration ratios of Cs/K varied. The concentrations of Ca and Sr in tuber were about two orders of magnitude lower than in leaf, petiole and stem parts, while the concentration ratios of Sr/Ca indicated a constant value. These findings suggest that the translocation rates of both Ca and Sr were similar within a potato plant, whereas those of K and Cs were different. Consequently, the transfers of both Ca and Sr may predict the behavior of radiostrontium. The transfer of Cs could be used to predict the behavior of radiocesium, whereas the transfer of K cannot be used to indicate the behavior of radiocesium.
CONCLUSIONS The transfer factors of essential and trace elements have two groups on the basis of their characteristics. The transfer factor of 137Cs obtained in agricultural fields was in the range of 0.0037-0.16, which was higher than that of Cs. This implies that the fraction of transferable components of 137Cs is still larger than that of Cs in the soil, although most of 137Cs fallout had been deposited prior to the 1980s. The transfer factors of 137Cs and Cs were negatively correlated with K concentration in the soil. Therefore, the relationship between K concentration and the transfer factor of Cs in agricultural fields may be useful for estimating the transfer factor of 137Cs without radiotracer experiments. The translocation rates of Ca and Sr were similar in a potato plant, whereas those of K and Cs were different. Due to a predicted long-term transfer of radionuclides in the environment, distributions of not only radionuclides but also related stable elements provide beneficial information. For a better understanding of transfer processes of radionuclides, more studies on the transfer of related elements in the environment will be required.
ACKNOWLEDGEMENTS This work was supported by a grant from the Science and Technology Agency, Japan. We are grateful to Professor S. Yamasaki (Tohoku University), Drs. Y. Nakamura (National Institute of Radiological Sciences) and P. T. Lattimore (The University of Maryland University College) for their useful discussions and comments. We would also like to thank Mr. T. Iyogi (the Institute for Environmental Sciences) for his help in 6
P-4a-242 sample collection.
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