Evaluation of ~(137)Cs ageing by dynamics of ~(137)Cs/~(133)Cs ratio in Andosol paddy fields with/without potassium fertilizer application

摘要

The mobility of Cs-137 in soil decreases with time owing to fixation by micaceous minerals. Such ageing is a critical parameter for estimating and predicting annual change in Cs-137 contamination risk of agricultural products. The decrease in the exchangeable fraction of Cs-137 has traditionally been used as an index of the Cs-137 ageing. Under field conditions, however, exchangeable Cs-137 is influenced by several environmental factors. In this study, we propose a new index to evaluate the Cs-137 ageing with minimum influence of environmental factors. The ratio of the exchangeable Cs-137 fraction to exchangeable fraction of Cs-133 (((137) Cs/Cs-133)(exch)) eliminates the influence of environmental factors on exchangeable Cs-137. We assessed the applicability of the (Cs-137/Cs-133) exch index, using a four-year field study of a rice paddy in allophanic Andosol, starting 200 days after the Fukushima Dai-ichi Nuclear Power Plant accident. The influence of K fertilization was also investigated. The Cs-137 and Cs-133 exchangeable fractions varied together, indicating that both were similarly controlled by environmental factors. The values of ((137) Cs/Cs-133)(exch) decreased with time, reflecting( )(137)Cs fixation by the ageing. The half-time of the (Cs-137/Cs-133)(exch) decline was 6.6-17.7 years. Relative to K fertilization, the lack of K fertilization seemed to affect the Cs-137 ageing in two ways: the early (137)Cs( )fixation progressed more rapidly, probably because fewer competing K+ ions were present, and the long-term ageing process was occasionally hampered, probably by the release of reserve K from micaceous minerals. The (Cs-137/Cs-133)(exch) values were similar to the ratio of the Cs-137 to Cs-133 transfer factor of the rice straw. Thus, we conclude that the (Cs-137/Cs-133)(exch) index is reliable for evaluating the Cs-137 ageing, decrease in Cs-137 mobility caused by the diffusion into micaceous mineral interlayer, in the field.