Saturday, 15 July 2006

Soil -Plant Transfer of Radiocaesium (Cs-137) and Dependance on Soil Properties.

Sadhasivam Meena, Tamil Nadu Agricultural Univ, Dept of Soil Science and Agricultural Chemistry, Coimbatore, India and A. RajaRajan, Regional Research Station, Tamil Nadu Agricultural Univ, Vridhachalam, India.

Over the years there is a massive increase in the construction of nuclear reactors. All reactors discharge small quantity of radioactivity into the environment during the normal course of operation. In addition, serious accident like the one that occurred at Chernobyl, then USSR in 1986 also results in the emission of larger quantity of radioactivity in to the environment. Accident in Chernobyl in 1986 showed that agricultural practices can be affected hundreds and even thousands of kilometers from the accidental site. Therefore in depth knowledge of the behaviour of radionuclides is necessary even for countries with no nuclear power of their own and more so for country like India with potential nuclear power. Of the various radionuclides, 137Cs is one of the major long living radionuclide (t˝ = 30.2 years). This has been identified as one of the most important radionuclide with respect to food chain contamination. Soil–plant transfer factor of this element is an important step that adds to the radiation dose to man after a nuclear accident. The extent to which the plant product becomes contaminated is commonly quoted using the concentration ratio or transfer coefficient which is defined as the ratio between the radioactivity/unit dry weight of the dry plant sample divided by the radioactivity/unit dry weight of the soil. The observed variability of the experimentally determined transfer coefficient of the different food and forage plants complicates the use of a generalized concentration factor value to be employed in the prediction of transfer factor of radionuclides in different food and forage plants. As these factors vary significantly with changes in climate each country has to generate its own data base. Local empirically determined transfer factor would be necessary to make reliable predictions. To study the transfer factor of caesium –137 from soil to two forage crops namely sorghum (Sorghum biclor L.) and lucerne (Medicago sativa L.) , a green house experiment was conducted in three important soil series (Palathurai, Irugur and Madukkur) of Tamil Nadu, India. The Palathurai soil was clay loam in texture whereas Irugur and Madukkur soils were clay loam and sandy loam in texture. The pH of the Palathurai soil was 8.1, Irugur -7.5 and Madukkur-7.4. The organic carbon content for Palathurai, Irugur and Madukkur soils were 1.11, 0.43 and 0.26 % respectively. The different factitious levels of 137 Cs tried in the experiment were 20, 40, 60 and 80 k Bq kg -1 soil. The two crops differed with respect to the transfer factor. At all levels of 137 Cs applied, the transfer factor values were higher in sorghum (0.208) than in lucerne (0.084). Soils did bring about appreciable change in the transfer factor in both lucerne and sorghum. The transfer factor values recorded at all levels of 137 Cs were highest in Madukkur soil series followed by Irugur and Palathurai series in both the crops. A negative relationship was recorded between the soil to plant transfer factor and the level of 137Cs applied in both the crops. The transfer factor values decreased from 0.127 to 0.063 in lucerne and from 0.308 to 0.154 in sorghum when the 137 Cs applied increased from 20 to 80 kBq kg-1

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