1 明治大学理工学部応用化学科Department of Applied Chemistry, Meiji University
2 明治大学大学院理工学研究科Graduate School of Science and Technology, Meiji University
3 日本原子力研究開発機構福島環境安全センターFukushima Environmental Safety Center, Japan Atomic Energy Agency
多摩川中流域で定点観測を実施し、河川水及び底質中の放射性セシウムの挙動を2012年から2016年にかけて調査した。河川水及び底質中の放射性セシウム濃度は、時間とともに減少した。これらの放射性セシウムの滞留半減期は 134Cs で 0.778–1.50 年、137Cs で 1.22–2.17 年であり、放射性セシウムの風化現象が確認された。雨により水中が懸濁すると、懸濁態放射性セシウム濃度が一時的に増加した。一方、溶存態放射性セシウムはこの影響を受けなかった。放射性セシウム濃度は河川水よりも底質の方がかなり高く、多摩川の放射性セシウムの多くが底質に存在していた。底質に関して逐次抽出、元素および結晶相分析を行い、放射性セシウムの化学状態と底質への吸着メカニズムを調査した。底質中の放射性セシウムは安定した化学形態で存在しており、バイオタイトが放射性セシウムを取り込んでいる可能性があった。
Behavior of radioactive caesium, derived from Fukushima Dai-ichi Nuclear Power Plant Accident, in river water and sediment investigated during 2012–2016. Concentrations of radioactive caesium in river water and sediment were decreased with time exponentially. The residence half-life of radioactive caesium in the river water and sediment were 0.788–1.50 year for 134Cs, 1.22–2.17 year for 137Cs. Any decrease in radioactive caesium concentration in the Tama river is because of weathering effect than radioactive decay. Concentrations of suspended radioactive caesium temporarily increased when sediments were resuspended due to rain. On the other hand, dissolved radioactive caesium is not easily impacted by this factor. Radioactive caesium concentration in sediments was considerably higher than that in river water. It indicated that much of the radioactive caesium in the Tama river existed in the sediments. Sequential extraction, elemental and crystal phase analysis were performed on the sediments and examined the chemical state of radioactive caesium as well as the adsorption mechanism. Radioactive caesium in sediment was present in a stable chemical form, and there is possibility that radioactive caesium was incorporated in biotite.
Key words: radioactive caesium; river water; sediment; fixed point observation; biotite; Fukushima Dai-ichi Nuclear Power Plant Accident
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