The government is trying to get a clearer picture of radioactive cesium accumulations at the bottom of Tokyo Bay, which derive from the crippled Fukushima No. 1 nuclear power plant.
Hideo Yamazaki, a professor of environmental analysis at Kinki University in Osaka Prefecture, sampled mud at four locations near the mouth of the Arakawa river last August.
At one location, cesium was detected at a depth of more than 20 centimeters.
He measured cesium concentrations of several hundred becquerels per kilogram of mud, roughly the same level as found dozens of kilometers from the Fukushima nuclear plant.
However, almost no cesium has been found in fish in Tokyo Bay.
Cesium binds itself with clay, when mixed in mud, and does not seep into the water. For this reason, it is believed that cesium is not easily absorbed by plants or transferred to creatures that live in the sea.
Waters off Fukushima Prefecture became contaminated with cesium in two ways: either as a result of contaminated water flowing into the sea or because the radioactive substance became airborne.
Scientists said marine creatures in the prefecture have absorbed cesium that was dissolved in seawater in large amounts.
Cesium carried by air currents has fallen over the Kanto Plain, 200 kilometers from the Fukushima No. 1 plant. Rain has swept cesium, together with mud, into rivers and eventually into the sea.
Mud will continue to flow into Tokyo Bay, burying cesium deep under the seabed mud.
It is estimated that cesium concentrations of mud flowing into the bay will peak a year or two after the nuclear disaster.
The natural environment has extremely limited amounts of radioactive cesium. There are two types of radioactive cesium: one remains for more than 20 years and the other disappears very quickly.
Cesium sampled in Tokyo Bay includes the latter type, which has led scientists to believe that it did not derive from the Chernobyl nuclear disaster in 1986.