Radioactive cesium contamination levels in a river near the Fukushima No. 1 nuclear plant rise in the spring and fall in the autumn, a new study shows.
The researchers believe the rise is attributable to very large numbers of leaves containing radioactive substances falling into rivers in the spring. In one year, the radioactive cesium level in the river in springtime was up to five times that in autumn.
Hirokazu Ozaki, research team leader and assistant professor at Tokyo University of Agriculture and Technology, said, “There is a possibility that radioactive substances are concentrated in the bodies of fish through the food chain, so it’s important to grasp what’s happening in the rivers. This study is unprecedented, and we’d like to continue.”
A group of Tokyo University of Agriculture and Technology researchers analyzed sediment samples taken at 35 locations along the middle reaches of the Abukuma River in Fukushima Prefecture, 40-50 kilometers from the atomic power station, in spring and autumn from 2012 to 2014.
The average density of radioactive cesium-137 per kilogram of sediment was 1,450 becquerels in spring 2012, 1,270 becquerels in autumn 2012, 2,700 becquerels in spring 2013, 451 becquerels in autumn 2013, 1,080 becquerels in spring 2014 and 600 becquerels in autumn 2014.
The highest level was 22,800 becquerels at one location in spring 2013, and there is a wide variation from location to location.
According to researchers, fallen leaves and carcasses of animals containing concentrated radioactive materials fall into the river in spring, increasing the amount of radioactive cesium in the river. Then the rainy season from June to mid-July, along with the typhoons that tend to strike during summer and early autumn, causes the amount of water in the river to surge, sweeping sediment to the river’s lower reaches and decreasing cesium levels in the fall, they say.