Depth distribution of radioactive caesium in soil after cultivation and the difference by year of the uptake of radioactive caesium in rice in Fukushima Prefecture after the 2011 nuclear accident

An investigation concerning the transfer of radioactive caesium from soil to ears of rice in the paddy fields of Fukushima after the Fukushima Dai-ichi Nuclear Power Plant accident has been carried out. Because the investigation in 2013 revealed relatively higher concentrations of radioactive ¹³⁷Cs and ¹³⁴Cs in the rice ears from one of the neighboring four contiguous fields, the aim of the present research carried out in 2014 and 2015 was to investigate the features of the paddy field where the contaminated rice ears were produced. It was revealed that the concentrations of radioactive ¹³⁷Cs and ¹³⁴Cs in the soil decreased with increasing depth, despite the fact that the cultivation was carried out in the preceding year. The Cs depth dependency was different among the fields. The ¹³⁷Cs and ¹³⁴Cs penetrated more deeply in the field where a relatively higher radioactive caesium concentration in ears of rice was observed. The difference depended on the variation of grain size distribution of soil, which affected the uptake of caesium in the rice ears. Four fields were cultivated in 2013 and 2014, although the upper two fields were not cultivated in 2015. Some locations showed relatively higher concentrations of radioactive ¹³⁷Cs and ¹³⁴Cs for rice ears in the two bottom fields in 2015, compared to the concentrations in 2013 and 2014, suggesting the possibility of a change of grain size distribution of soil. (author)