Online ISSN: 1884-4111 Print ISSN: 0033-8303
Radioisotopes 65(5): 221-235 (2016)


千葉県柏市内の調整池における水・底泥中放射性セシウムの分布状況と環境動態Distribution and Behavior of Radiocaesium in Water and Sediment of Retention Basins at Kashiwa City, Chiba Prefecture

千葉県環境研究センターChiba Prefectural Environmental Research Center ◇ 290–0046 千葉県市原市岩崎西1–8–8 ◇ 1–8–8 Iwasakinishi, Ichihara-shi, Chiba Pref. 290–0046, Japan

受付日:2015年9月24日Received: September 24, 2015
受理日:2016年3月9日Accepted: March 9, 2016
発行日:2016年5月15日Published: May 15, 2016

手賀沼を流末とする柏市内の調整池の水中ならびに底泥中における放射性セシウムの分布状況と環境動態を検証した。福島第一原子力発電所事故から約4年が経過した時期の底泥から10 kBq/kg以上の放射性セシウムが検出され,調整池は下流域に対する放射性セシウムの供給源の1つであることが裏付けられた。台風などの豪雨時において,水中放射性セシウムの懸濁態の存在割合が平水時と比較して上昇することから,豪雨は放射性セシウムの下流域への流出に関与していることが示唆された。

According to the research conducted by the Government of Japan and Chiba Prefectural Government, the sediments of Lake Tega and its tributaries are still highly contaminated by radiocaesium (over 1,000 Bq/kg) derived from the Fukushima Daiichi Nuclear Power Plant. To investigate the possible sources that would transport and redistribute radiocaesium to the Lake Tega system, which is urbanized compared to Fukushima Prefecture, we have selected three retention basins, corresponding to upstream of Lake Tega, located in Kashiwa City, Chiba Prefecture. Temporal changes in concentrations of radiocaesium in water and sediment samples were determined during 2013–2014. The concentrations of radiocaesium in both samples tended to decrease during the sampling period, although some of the sediment samples were still high radiocaesium concentrations exceeding 10 kBq/kg. These observations suggest that the radiocaesium in the retention basins, which play an important role in supplying radiocaesium to the downstream area, is highly contaminated compared to the downstream area. Increments of the concentrations of radiocaesium in particulate form were observed from the water samples collected immediately after the passage of large typhoon Wipha. This result implies that heavy rainfall events may be playing an important role in the migration of existing radiocaesium from the catchment areas to the Lake Tega system.

Key words: Fukushima Daiichi Nuclear Power Plant; retention basins; radiocaesium in water; radiocaesium in sediment; Kashiwa City; Chiba Prefecture

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