No abstract

We performed consecutive field trials of rice cultivation to reduce radiocesium ( 134 Cs and 137 Cs) absorption by rice in a partially decontaminated paddy soil in the Iitate Village in Fukushima prefecture, Japan. People had evacuated this area because of the high levels of radioactive contamination caused by the nuclear disaster in 2011 at the Fukushima Daiichi Nuclear Power Station, Tokyo Electric Power. The radiocesium concentrations were measured for paddy soil and for lowland rice grown on variously decontaminated paddy soil in 2012 and 2013. The results show that the radiocesium concentration in the brown rice cultured in the fields of Sasu and Maeda with 2000-6000 Bq/kg dry weight (0-15 cm average soil depth) was below 40 Bq/kg, which is below the Japanese new standard for food (100 Bq/kg). In addition, the radiocesium concentration in the brown rice depended on the decontamination level of the paddy soil. In addition, the radiocesium concentration in the rice was reduced depending on the exchangeable K content of the soil, which plateaued around 20 mg K 2 O per 100 g dry soil. However, in 2013, in a test field of Komiya where the radiocesium concentrations were higher than 8000 Bq/kg dry weight, brown rice with more than 100 Bq/kg was harvested, indicating the need for further decontamination. Overall, our results show that decontamination and additional K fertilization can reduce the radiocesium concentration in rice to less than the new standard, and that we could resume rice cultivation in the Iitate village by rather practical way.

We performed consecutive field trials of rice cultivation to monitor radiocesium contamination in harvested rice from 2012, in the Iitate Village in Fukushima Prefecture, where people were forced to be evacuated due to high level of radioactive contamination caused by the disaster at the Fukushima Dai-ichi Nuclear Power Plant of Tokyo Electric Power. The early year results (2012-2013) 1,2) showed the radiocesium concentration in the brown rice was reduced depending on the decontaminated level of paddy soil and on the exchangeable K content of the soil. This report of later year results (2015)(2016)(2017)(2018)(2019) showed further more than 80% reduction of 137 Cs concentration in the brown rice and straw at KCl fertilized paddy soil, in spite of little reduction of 137 Cs concentration of the soil. The transfer factor of 137 Cs from soil to brown rice reduced from 0.0022 in 2015 to 0.0003 in 2019 and that to straw reduced from 0.0262 in 2015 to 0.0028 in 2019, respectively. Exchangeable positive ions of the soil were also analyzed. Multiple regression analyses of all data of transfer factor in 2015 to 2019 to year (ageing) and exchangeable K ion as variables shows that the main causal factor is year (ageing) with some supportive effect of increase of exchangeable K ion. This implicates that radiocesium in soil was gradually transformed to a form more difficult to be absorbed by rice, that is, 137 Cs immobilization or fixation on clay minerals by ageing, not only in early years after the accident (