Variation of ¹³⁷Cs activity concentration in flood and pore water in paddy fields and its transfer to rice

Caesium-137 (¹³⁷Cs) is a major anthropogenic radionuclide released into the environment as a result of the TEPCO Fukushima Daiichi Nuclear Reactor Station accident (occurring on March 11, 2011). Rice, being a staple food in Asian countries, including Japan, and is predominantly cultivated in paddy fields. Consequently, ¹³⁷Cs present in rice is absorbed from both soil and irrigation water, making it the most important crop for estimating internal radiation doses. In this study, over the 2018-2022 cultivation periods, flood water and pore water samples were collected biweekly from paddy fields. These samples were analyzed to measure the ¹³⁷Cs activity concentration, as well as the potassium (K⁺) and ammonium (NH₄⁺) concentrations. Under anaerobic conditions, the ¹³⁷Cs ⁺ activity concentration in pore water increased markedly to reach a value 20-fold higher than that in flood water, correlating with NH₄⁺ concentration. However, despite the release of ¹³⁷Cs ⁺ caused by increased NH₄⁺ concentrations in pore water due to reduction processes, the ¹³⁷Cs⁺/K⁺ ratio did not increase, which was attributed to the simultaneous release of K⁺. The competition between ¹³⁷Cs⁺ and K⁺ uptake by plants indicates that rice uptake of ¹³⁷Cs is not necessarily enhanced during the waterlogging period.