Online ISSN: 1884-4111 Print ISSN: 0033-8303
Radioisotopes 70(2): 63-72 (2021)


塩化カリウム施肥圃場で栽培された稲の土壌からの放射性セシウムの移行係数の経年減少(2015~2019年)Annual Reduction of Transfer Factor of Radiocesium from Soil to Rice Cultivated in a KCl Fertilized Paddy Field from 2015 to 2019

1ふくしま再生の会NPO Resurrection of Fukushima

2東京大学大学院農学生命科学研究科Graduate School of Agricultural and Life Sciences, The University of Tokyo

3福島大学食農学類Department of Agriculture, Fukushima University

4星薬科大学Hoshi University

受付日:2020年6月16日Received: June 16, 2020
受理日:2020年10月28日Accepted: October 28, 2020
発行日:2021年2月15日Published: February 15, 2021

我々は,東京電力福島第一原子力発電所事故による放射性物質汚染により避難を余儀なくされた福島県飯舘村で2012年以降連年稲の試験栽培を行ってきた。2012–2013年は圃場の土壌の除染度,塩化カリウム施肥によって玄米の放射性セシウムが減少することを示した。2015–2019年の塩化カリウム施肥圃場での結果では,土壌のセシウム137濃度はほとんど変わらなかったが,玄米,ワラのセシウム137濃度は1/5以下に減少した。土壌から玄米,ワラへのセシウム137の移行係数は玄米では0.0022(2015年)から0.0003(2019年)に,ワラでは0.0262(2015年)から0.0028(2019年)となった。これはこの間の土壌の交換性カリウムの増加による寄与もあるが主にはセシウム137のエイジングによる土壌への固定化が進み,徐々に稲に吸収されにくくなることを示唆した。この示唆は,2017, 2018, 2019年の乾燥土壌の交換性セシウム137の比較分析によっても支持された。

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 (2015–2019) showed further more than 80% reduction of 137Cs concentration in the brown rice and straw at KCl fertilized paddy soil, in spite of little reduction of 137Cs concentration of the soil. The transfer factor of 137Cs 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, 137Cs immobilization or fixation on clay minerals by ageing, not only in early years after the accident (2011–2015), but also in later years (2015–2019). This implication was supported by comparative analysis of exchangeable 137Cs of dry soil of 2017, 2018 and 2019.

Key words: radioactive fallout; soil; rice; caesium-137; transfer factor; Fukushima Dai-ichi Nuclear Power Plant Accident

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