Evaluation of ¹³⁷Cs ageing by dynamics of ¹³⁷Cs/¹³³Cs ratio in Andosol paddy fields with/without potassium fertilizer...

The mobility of ¹³⁷Cs in soil decreases with time owing to fixation by micaceous minerals. Such ageing is a critical parameter for estimating and predicting annual change in ¹³⁷Cs contamination risk of agricultural products. The decrease in the exchangeable fraction of ¹³⁷Cs has traditionally been used as an index of the ¹³⁷Cs ageing. Under field conditions, however, exchangeable ¹³⁷Cs is influenced by several environmental factors. In this study, we propose a new index to evaluate the ¹³⁷Cs ageing with minimum influence of environmental factors. The ratio of the exchangeable ¹³⁷Cs fraction to exchangeable fraction of ¹³³Cs ((¹³⁷Cs/¹³³Cs)_exch) eliminates the influence of environmental factors on exchangeable ¹³⁷Cs. We assessed the applicability of the (¹³⁷Cs/¹³³Cs)_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 ¹³⁷Cs and ¹³³Cs exchangeable fractions varied together, indicating that both were similarly controlled by environmental factors. The values of (¹³⁷Cs/¹³³Cs)_exch decreased with time, reflecting ¹³⁷Cs fixation by the ageing. The half-time of the (¹³⁷Cs/¹³³Cs)_exch decline was 6.6-17.7 years. Relative to K fertilization, the lack of K fertilization seemed to affect the ¹³⁷Cs ageing in two ways: the early ¹³⁷Cs 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 (¹³⁷Cs/¹³³Cs)_exch values were similar to the ratio of the ¹³⁷Cs to ¹³³Cs transfer factor of the rice straw. Thus, we conclude that the (¹³⁷Cs/¹³³Cs)_exch index is reliable for evaluating the ¹³⁷Cs ageing, decrease in ¹³⁷Cs mobility caused by the diffusion into micaceous mineral interlayer, in the field.