Concentrations of radioactive cesium on the seafloor after the 2011 Fukushima nuclear disaster were discovered to be higher in muddy depressions than on the rocky seabed, maritime researchers said.
"We are learning that the nuclear accident didn't contaminate the entire ocean, but created spots that tend to have higher radioactive levels than others," said Blair Thornton, a researcher from the University of Tokyo's Institute of Industrial Science. "We want to continue investigating."
At the request of the Nuclear Regulation Authority, a team of researchers from the University of Tokyo, the National Maritime Research Institute and Kanazawa University charted a distance of more than 1,000 kilometers. Sites measured ran 50 km north to south and 25 km west to east off the coast of Fukushima Prefecture, where the stricken Fukushima No. 1 nuclear power plant is located.
They used a ship equipped with an instrument specially developed to measure seafloor gamma ray levels while simultaneously surveying the underwater landscape using sonar.
Per every kilogram of seabed soil, the average concentration of cesium-137 was 90 becquerels. But of the locations charted 4 km offshore, 20 sites measured higher than 1,000 becquerels and some locations 6 km offshore registered levels as high as 2,000 becquerels.
According Thornton, the locations with higher concentration of radioactive materials were found in depressions where more mud tends to accumulate than in rocky seabeds. The cesium atoms likely bond with small particles in the mud and create highly concentrated residues of radioactive materials.
The survey also found that the seabed south of the crippled Fukushima plant had high readings, most likely due to the current of the ocean at the time of the March 11, 2011, accident.
Some locations 1.6 km and 2.5 km off the mouth of the Abukumagawa river in Miyagi Prefecture also had high readings, 1,300 becquerels and 2,700 becquerels per kilogram, respectively. These high levels of radioactivity presumably come from the radioactive cesium carried by the river within the soil, flowing northward through Fukushima Prefecture.
Little change was observed in the seabed soil even after the typhoon season, suggesting that fine mud on the ocean floor is rarely dispersed once it settles.
The team also conducted experiments to see how much cesium in the seabed soil is dissolving into the water and found levels of between 1 and 2 percent. This is significantly less than the 10 percent recorded immediately after the accident, and suggests contamination of seawater from the ocean floor is diminishing.
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