The amount of radioactive cesium that flowed into the sea through a river running across central Fukushima Prefecture when a powerful typhoon hit in September 2011 totaled 6.2 trillion becquerels, about 60 percent of the total for a 12-month period, researchers say.
The number was measured from Sept. 19 to Sept. 27, 2011, when Typhoon No. 15 swept through the region surrounding the Abukumagawa river.
Heavy rainfall and strong winds contributed to the high volume of cesium during the nine-day period, researchers say.
Cesium released from the crippled Fukushima No. 1 nuclear power plant in a triple meltdown in March 2011 landed in the soil in the region, which measures some 5,000 square kilometers wide. The fallout is gradually finding its way into the river through storm-water runoff.
A team of scientists from Kyoto University, the Meteorological Research Institute and other research bodies monitored water levels, depth of rainfall, the turbidity of the water and the concentration of cesium at six sites along the river from June 2011 to May 2012.
Based on the results, the researchers estimated the volume of cesium that flowed into the sea at the mouth of the Abukumagawa river in Iwanuma, Miyagi Prefecture.
In the 12-month period, the amount of cesium that reached the ocean was about 10 trillion becquerels, almost the same as the figure for cesium that directly flowed into the ocean from the crippled plant during the same period.
“Most of the cesium was attached to fine sediments in the river,” said Yosuke Yamashiki, associate professor of environmental engineering at Kyoto University. “It was not absorbed by living things, meaning the cesium is not significantly impacting marine life.”
He added that monitoring should be stepped up during typhoon season.
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