久保田富次郎，申文浩，濵田康治，人見忠良, 阿武隈高地に立地するNため池における水中の放射性セシウムのモニタリング, 農村工学研究所技報, 217, 85-100, 2015.

久保田富次郎，申文浩，濵田康治，人見忠良, 阿武隈高地に立地するNため池における水中の放射性セシウムのモニタリング, 農村工学研究所技報, 217, 85-100, 2015.

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東日本大震災に起因する東京電力福島第一原子力発電所の事故により放出され福島県を中心に広く沈着した放射性物 質は，営農再開に向けた懸念要因になっており，放射性物質を蓄積しやすいため池における放射性物質の動態把握も課 題のひとつである。本報では，阿武隈高地に立地する典型的な谷型ため池を対象に，2013 年度より着手した水中の放射 性セシウムの動態調査の一環として測定された放射性セシウム濃度の状況とその規定要因について検討を行った。その 結果，ため池への流入水および流出水ともに，溶存態放射性セシウム濃度は 0.02 ~ 0.11Bq/kg と低かった。また，ため池 への流入水，流出水の全放射性セシウム濃度の値は，夏期におよそ1~ 3 Bq/kg 程度とやや高く，10 月以降には濃度は 0.2 ~ 1.5Bq/kg と全般的に低下する傾向が認められ，それは，ほぼ懸濁態の放射性セシウム濃度の値で説明できることが わかった。

キーワード:ため池，放射性セシウム，農業用水

Radioactive materials were released over wide areas around the Tokyo Power Fukushima Daiichi Nuclear Power Plant after the accident caused by the earthquake and tsunami on March 11, 2011. Radioactive cesium pollution in agricultural reservoirs that supply irrigation water in this region was a concern for the restart of farming. Therefore, it is important to determine the movement characteristics of radioactive cesium around agricultural reservoirs. In this ongoing study since June 2013, we measured the concentrations of radioactive cesium in inflow, outflow, and pooled water in a small agricultural reservoir located in the Abukuma Mountains. The area of the reservoir was 0.37 ha, and its pondage was 7500 m3. The concentration of total radioactive cesium in the water was relatively high, around 1 – 3 Bq/kg, during the summer, but decreased to 0.2 – 1.5 Bq/kg after October. However, the concentration of total radioactive cesium of the outflow temporarily increased to 2.9 – 4.0 Bq/kg, when the water level decreased by 1.2 m in the middle of September. The increase in radioactivity was believed to be due to disturbance of the bottom water and sediment caused by relatively fast water flow. The dissolved radioactive cesium concentration in the inflow was 0.02– 0.07 Bq/kg, whereas that in the outflow was 0.02 – 0.11 Bq/kg; both were considerably lower than the total radioactive cesium concentration. The ratios of dissolved radioactive cesium to total radioactive cesium were 1 – 5% in the inflow and 1 – 3% in the outflow. Thus, our results showed a positive correlation between the total radioactive cesium concentration in stored water and that in precipitation that occurred over the previous 5 days.

Key words: agricultural reservoir, radioactive cesium, agricultural water