High concentrations of radioactive cesium have been accumulating at the bottom of 10 major dams within a 50-kilometer radius from the disaster-stricken Fukushima No. 1 Nuclear Power Plant, a survey by the Environment Ministry has found.
Radioactive cesium emanating from the 2011 nuclear disaster is pooling at those dams, which are used to hold drinking water and for agricultural use, after the substances flew into there from mountains, forests and rivers. The radiation levels at the bottom of those dams top those set for designated waste at over 8,000 becquerels per kilogram.
While the Environment Ministry plans to monitor the situation without decontaminating the dams on the grounds that radiation levels in dam water is not high enough to affect human health, experts are calling for the ministry to look into measures to counter any future risks.
The ministry began a monitoring survey on those dams and rivers downstream in September 2011 to grasp the moves of radioactive substances flowing into them from mountains and forests that are not subject to decontamination work. The survey samples water at 73 dams in Tokyo, Iwate and seven other prefectures about once every several months.
Among them, there were 10 dams in Fukushima Prefecture where the average concentration of cesium in the surface layer of bottom soil measured between fiscal 2011 and 2015 topped the regulated levels for designated waste. Those dams include Ganbe Dam in the village of Iitate with 64,439 becquerels per kilogram of cesium, Yokokawa Dam in the city of Minamisoma with 27,533 becquerels, and Mano Dam in Iitate with 26,859 becquerels.
Meanwhile, the surface water at those 10 dams contained no significant amount of cesium, meaning the level was well below the drinking water criteria at 10 becquerels.
While the total amount of cesium deposited at the bottom of those dams is unknown from the environment ministry's survey, a separate study conducted at Ogaki Dam in the town of Namie by the Ministry of Agriculture, Forestry and Fisheries' Tohoku Regional Agricultural Administration Office estimated in December 2013 that there was a combined 8 trillion becquerels of cesium 134 and cesium 137 at the dam. The figure came about after estimating the amount of accumulated cesium every 10-meter-square area based on cesium levels in sedimentary soil sampled at 110 locations at the bottom of the dam, which is for agricultural use.
The National Institute for Environmental Studies in Tsukuba, Ibaraki Prefecture, will shortly begin a full-scale survey on cesium concentrations at several dams.
"At the moment, it is best to contain cesium at those dams. If we dredge it, the substance could curl up and could contaminate rivers downstream," said an Environment Ministry official.
A previous version of this article incorrectly stated that surface water at 10 dams contained 1-2 becquerels of cesium per liter. This has been corrected to state that the surface water contained no significant amount of cesium.