Monthly Change in Radioactivity Concentration of ¹³⁷Cs, ¹³⁴Cs, and ⁴⁰K of Paddy Soil and Rice Plants in Fukushima Prefecture

Abstract

 Monthly fieldwork was conducted in the paddies of Fukushima Prefecture in 2016, to obtain samples of the paddy soil and rice plants. The monthly change in the radioactivity concentrations of ¹³⁷Cs, ¹³⁴Cs, and ⁴⁰K of the samples was investigated, using a germanium semiconductor detector. Three-phase transfer factors (TFs) of ¹³⁷Cs from the paddy soil to the roots (TF₁), from the roots to the leaves (TF₂), and from the leaves to the ears (TF₃) of rice plants were calculated. The results showed that the radioactivity concentration of ¹³⁷Cs and ¹³⁴Cs in the paddy soil varied seasonally, while the concentration of ⁴⁰K showed an almost opposite seasonal change compared to ¹³⁷Cs and ¹³⁴Cs. The radioactivity concentration of ¹³⁷Cs and ¹³⁴Cs in the roots increased 60 days after planting, while the concentration of ⁴⁰K decreased. Furthermore, the radioactivity concentration of ¹³⁷Cs, ¹³⁴Cs, and ⁴⁰K in the leaves and ears decreased over time. Correlations of the TFs of ¹³⁷Cs with the radioactivity concentration of ⁴⁰K suggested that ¹³⁷Cs and ⁴⁰K were competitively absorbed by the roots, however, they were transported to the leaves and ears in the same manner. In conclusion, the transportation of ¹³⁷Cs (TF₂ and TF₃) in rice plants was high despite the low absorption of ¹³⁷Cs (TF₁) in the early stage of rice growth. Therefore, it is recommended that the potassium concentration in the paddy should be high during the early stage of growth to prevent radioactive cesium contamination.