Asahi Shimbun Japan
February 24, 2012
By TAKASHI SUGIMOTO / Staff Writer
Concentrations of radioactive cesium at the bottoms of rivers and lakes vary widely, but include some with the potential to affect ecosystems, according to Environment Ministry studies on water systems.
The ministry released the full results on the studies on radioactive fallout from the Fukushima No. 1 nuclear power plant on surrounding water systems. The surveys were conducted between August and January at 725 locations in Fukushima, Miyagi, Yamagata, Gunma, Tochigi, Ibaraki, Chiba and Iwate prefectures.
The results showed concentrations of waterborne radioactive cesium mostly remained below 1 becquerel per liter, with a maximum of 7 becquerels per liter detected in a river in Fukushima Prefecture.
But the sand beneath the water contained considerably varying concentrations of cesium, including levels potentially harmful to food chains.
In Miyagi Prefecture, cesium concentrations measured tens of becquerels per kilogram of dry sand in most rivers and marshes, but the reading was 11,100 becquerels per kilogram along the Nanakitagawa river in Sendai. Other locations in excess of 1,000 becquerels per kilogram also dotted the prefecture.
The distribution of cesium depends not only on the amount of radioactive fallout but also on topography. The geography may have caused cesium to accumulate from across the entire river basin of the Nanakitagawa, sources said.
Some rivers in Ibaraki and Chiba prefectures also showed high readings. They mostly coincide with locations that came under heavy nuclear fallout from the Fukushima No. 1 plant, the sources said.
Sand beneath ponds and lakes in some mountainous areas of Gunma and Tochigi prefectures produced relatively high readings. Cesium likely flowed in from the surrounding woods, where radiation levels are high.
The Environment Ministry plans to conduct another round of surveys because riverbed sand can move around during heavy rains.
To search for the survey results, type the name of a prefecture into the ministry’s Japanese website: (http://www.env.go.jp/press/index.php).