Greenhouse experiment with Rice

Transfer factors of ¹³⁷Cs in rice grain and straw in different soils are given in Table 8. The TF values were higher to straw than to grain. The values did not differ greatly between the three different Inceptisol soils. There was a slightly higher value of TF for ¹³⁷Cs in both rice grain and straw from the soil where normally rice is grown during the monsoon season which has higher clay content than the other two soils.

TABLE 8. TRANSFER FACTOR OF ¹³⁷Cs IN RICE GRAIN AND STRAW IN THREE ALLUVIAL SOILS

Rice (2001)

Grain Straw Soil

TF x 1000 SD (±) TF x 1000 SD (±)

1 7.65 0.55 9.73 0.43

2 7.64 0.62 10.26 0.66

3 8.70 0.46 10.70 0.55

Wheat (2001–02)

Grain Straw

1 5.14 0.54 6.73 0.39

2 5.11 0.58 7.25 0.68

3 6.20 0.47 7.71 0.56

Rice (2002)

Grain Straw

1 5.45 0.55 6.98 0.40

2 5.42 0.59 7.52 0.70

3 6.53 0.48 7.99 0.57

Wheat (2002–03)

Grain Straw

1 4.03 0.55 4.55 0.38

2 4.00 0.59 5.08 0.72

3 5.12 0.48 5.56 0.58

In the following crop of wheat taken in the winter season (2001–02) the TF values in both grain and straw were lower than those found in the first rice crop. In the next monsoon season the rice crop showed slightly higher values than the previous wheat crop but they were lower than in the rice crop grown a year before, immediately after soil contamination. The fourth crop of wheat taken in the next season had still lower values of ¹³⁷Cs transfer factors. The soil having higher clay content (soil No. 3) consistently showed higher values of transfer factors to all the four crops.

4. CONCLUSIONS

Under both pot culture and field conditions, the transfer factor values for ¹³⁷Cs and ⁹⁰Srare higher for cabbage than those for wheat and maize. Also in the successive crops of cabbage the TF values are similar to those found in the first crop after contamination. The TF values for ¹³⁷Cs and ⁹⁰Sr are 50–

100 times higher than those for ¹³⁷Cs. In rice the soil to plant transfer factor for ¹³⁷Cs is similar to those observed for upland cereal crops of wheat or maize and from soil having a higher clay content the

137Cs transfer factor was greater.

REFERENCES

[1] TSUKAD, A.H., HASEGAWA, H., Soil to plant transfer of ¹³⁷Cs and other essential and trace elements in cabbage plants, J. Radioanal. Nuclear Chem. 252 (2002) 219–224.

[2] JACOBS, D.G., “The effect of collapse inducing cations on the caesium sorption properties of hydrobiotite”, International Clay Conference 1963 (Proc. Symp. Stockholm, Sweden, 12–16 August, 1963), Vol. 1 (ROSENQVIST, I.Th, GRAFF-PETERSEN, P., Eds) Pergamon Press, New York (1963) 239–248.

[3] TULLIN, S., et al., Potash fertilizer use reduces crop contamination with radioactive nuclides, International Fertilizer Correspondent 35 (1994) 4–5.

[4] SACHDEV, P., et al., Adsorption-desorption of radiocaesium (¹³⁷Cs) in semi-arid and tropical soils, J Nuclear Agric. Biol. 24 (1995) 201–209.

[5] INTERNATIONAL ATOMIC ENERGY AGENCY ‘Measurements of radionuclides in food and environment — A Guidebook’, Technical Reports Series No. 364, IAEA, Vienna (1994).

S. Uchida¹, K. Tagami¹, M. Komamura²

1 Research Center for Radiation Safety,

National Institute of Radiological Sciences, Chiba, Japan

2 National Institute for Agro-Environmental Sciences, Ibaraki, Japan

Abstract

To obtain the TFs of ¹³⁷Cs and ⁹⁰Sr from paddy soils to rice, analyses of global fallout ¹³⁷Cs and ⁹⁰Sr in both rice and the associated soils were carried out. Paddy soils were collected nationwide from 12–15 sampling sites from 1987–2001, and rice plants grown on these soils were also collected. The geometric-mean-TFs of ¹³⁷Cs were 0.0026 for brown rice and 0.0011 for polished rice, and for ⁹⁰Sr they were 0.015 and 0.0059, respectively. When the distribution ratios of ¹³⁷Cs and ⁹⁰Sr in polished rice to brown rice were calculated, the average ratios were 0.35 for ¹³⁷Cs and 0.36 for ⁹⁰Sr, indicating that these radionuclides were mainly associated with the bran. Then the TF of ¹³⁷Cs was particularly focused on in this study because the available data was sufficient for statistical analysis (i.e. the variation in the TFs for Cs from paddy soils to rice grain). The variation of TFs of ¹³⁷Cs for polished rice at each site year-to-year was very small, being within the range of two orders of magnitude, whereas the variation of ¹³⁷Cs-TFs for polished rice was somewhat greater between sample sites across Japan.

The relationships between TF ¹³⁷Cs and soil properties (pH, exchangeable-potassium, cation exchange capacity and clay content) were also investigated. No correlations were found for ¹³⁷Cs-TFs with these properties, which was difficult to explain. More data must be collected for both TF and soil properties.

1. INTRODUCTION

The ingestion of contaminated crops and livestock products is the most important pathway through which radionuclides are taken into the human body and contribute to internal radiation dose. Radiation dose estimation is usually made with the aid of mathematical models in which model parameters express transfers of radionuclides from one environmental compartment to another. These parameters are often described as the concentration ratios of radionuclides between two compartments, when the system is in equilibrium. For example, transfers of nuclides from soil to crops, from grass to milk and from grass to meat have been estimated by using the parameters TF, F_m and F_f, respectively [1].

From the viewpoint of more precise radiological assessment, it is necessary to obtain the variations of transfer parameters that are used in the models. Among the parameters, a soil-to-crop transfer factor (TF) is the key parameter that directly affects the internal dose assessment for the ingestion pathway.

Eating habits, however, differ between countries. For example, in European and North American countries, livestock products including meat, eggs and milk make a big contribution, whereas, cereals and vegetables are the major dietary components in Asian and South American countries. In Japan, rice is the most commonly consumed cereal and is therefore the most important crop for internal radiation dose assessment.

We obtained the TFs of Cs and Sr from paddy soils to rice by analyses of fallout ¹³⁷Cs and ⁹⁰Sr in both rice and the associated soils. It is illegal to use any kind of radioactive materials in open fields in Japan, hence our analyses used radioactive fallout. In this study, we particularly focused on the variation in the TFs for Cs from paddy soil to rice grains.

2. MATERIALS AND METHODS