A significant quantity of radioactive cesium, likely from the crippled Fukushima No. 1 nuclear power plant, has turned up in subsea mud about 200 kilometers away, near the mouth of the Shinanogawa River on Japan's northwestern coast.
Scientists said samples taken in 2011 at Nagaoka, Niigata Prefecture, contained concentrations of up to 460 becquerels per kilogram of dry mud, a level comparable to that detected at a river mouth in Tokyo Bay last year.
Some isotopes of cesium are heavily radioactive. They are produced in uranium fission and deposits are often closely associated with nuclear accidents and atomic weapons tests.
A team sampled coastal seabed mud last August around the mouth of the river's Okozu diversion canal, which discharges into the Sea of Japan. The team was led by Hideo Yamazaki, a professor of environmental analysis at Kinki University.
The sample sites lay beneath 15, 20 and 30 meters of water. Scientists took mud from those depths, and analyzed cesium concentrations at intervals of 1 centimeter.
The highest concentration was 2-3 cm below the mud surface at a water depth of 30 m. That reading of 460 becquerels per kg compares to samples of over 400 becquerels around the mouth of the Arakawa river in Tokyo Bay in August 2011.
Both readings are dozens of times higher than contamination detected after past atmospheric nuclear tests.
At a depth of 20 m the maximum concentration was 318 becquerels per kg, while at 15 m it was 255 becquerels.
The research results will be published at the fall meeting of the Oceanographic Society of Japan, which opens in Shizuoka on Sept. 13.
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