福島第一原子力発電所事故により赤城大沼一帯に降下した放射性セシウム  [in Japanese] Radioactive Cesium Precipitated in Lake Onuma of Mt. Akagi Emitted by the Fukushima Daiichi Nuclear Power Plant Accident  [in Japanese]

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Author(s)

    • 森 勝伸 MORI Masanobu
    • 群馬大学大学院理工学府環境創生部門 Department of Environmental Engineering Science, Graduate School of Science and Technology, Gunma University
    • 小池 優子 KOIKE Yuko
    • 群馬大学大学院理工学府分子科学部門 Department of Chemistry and Chemical Biology, Graduate School of Science and Technology, Gunma University
    • 角田 欣一 TSUNODA Kin-ichi
    • 群馬大学大学院理工学府分子科学部門 Department of Chemistry and Chemical Biology, Graduate School of Science and Technology, Gunma University

Abstract

赤城大沼湖心部の堆積物では放射性セシウムは湖底下5 cm以内の表層にとどまるのに対し,水深の浅い流入部や流出部では15 cmから20 cm付近まで放射性セシウムが含まれていた.流入部や流出部では,粒度の粗い堆積物粒子間への湖水の浸透あるいは湖底での水の流れによる表層堆積物の撹乱が下方まで放射性セシウムが取り込まれた原因と考えられる.湖底堆積物の主な構成鉱物はクリストバル石,石英,斜長石であり,そのほか比較的多量の非晶質物質が含まれる.これらの非晶質物質が放射性セシウムの保持に係わっている可能性がある.周辺土壌の多くは500 Bq kg<sup>−1</sup>から5000 Bq kg<sup>−1</sup>の放射性セシウム含有量であり,放射性セシウムの分布について湖周辺で地域的な顕著な偏りは見られなかった.湖底堆積物及び周辺土壌の放射性セシウム含有量から赤城山一帯に降下した放射性セシウムの降下量を見積もったが,その数値は文部科学省が航空機モニタリングで求めた赤城山一帯の沈着量に近い値だった.

In August and September, 2011, Wakasagi (<i>Hypomesus nipponensis</i>) that was captured in Lake Onuma on the top of Mt. Akagi of Gunma Prefecture contained higher levels (640 Bq kg<sup>−1</sup> and 650 Bq kg<sup>−1</sup>) of radioactive cesium than the temporal regulatory limit for food (500 Bq kg<sup>−1</sup>). We have investigated the distribution of radioactive cesium (Cs-134 and Cs-137) in deposits of Lake Onuma and Lake Konuma, as well as soils in the inner region of the somma rim of Mt. Akagi to elucidate the contamination mechanisms of the radioactive cesium. Radioactive cesium in the center site of Lake Onuma had remained within the uppermost layer (<i>ca.</i> 5 cm) of the lake deposits. On the other hand, those in the inlet and outlet sites of the lake were found in layers of 15–20 cm depth. This difference might be caused by that in grain sizes (silt) of the lake deposits, <i>i.e.</i>, smaller grain sizes in the center site, and/or the disturbance of lake deposits due to the movement of bottom water of these sites of the lake. The main minerals of lake deposits were cristobalite, quarts and plagioclase with large amounts of non-crystalline amorphous silica and aluminosilicates. The non-crystalline materials may be related to the retention of radioactive cesium in the upper layer of the deposits. Most of the soil samples collected from the inner regions of Mt. Akagi had contained radioactive cesium, ranging from 500 Bq kg<sup>−1</sup> to 5000 Bq kg<sup>−1</sup>. These values are at the same level as those of soils in mountainous areas of Gunma Pref. The estimated inventories (December 20, 2011) of radioactive cesium in the inner regions of Mt. Akagi were calculated to be 33 – 104 kBq m<sup>−2</sup>, which were close to the inventory values (30 – 100 kBq m<sup>−2</sup>) around Mt. Akagi published by the MEXT Aircraft Monitoring (November 5, 2011).

Journal

  • BUNSEKI KAGAKU

    BUNSEKI KAGAKU 66(4), 271-279, 2017

    The Japan Society for Analytical Chemistry

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