Researchers measuring fallout from the Fukushima nuclear disaster say radioactive cesium deposits in Tokyo Bay have increased by up to 70 percent since last summer.
However, they say there is no particular cause for concern because the deposits mostly lie at least 10 centimeters below the mud.
The cesium made its way into streams and tributaries and eventually flowed into the bay by way of rivers.
A team led by Hideo Yamazaki, a professor of environmental analysis at Kinki University, found that cesium deposits in some locations of Tokyo Bay had surged 1.7 times levels recorded in a similar survey last August.
At one site that had the highest cesium deposits in the previous survey, the cesium content increased from 18,200 becquerels per square meter to 27,200 becquerels per square meter.
The latest monitoring was conducted at five sites in Tokyo Bay around the mouth of Arakawa river on April 2. Researchers dug up mud and measured cesium levels at various depths to compare with the results in the previous survey.
The team found that the overall cesium level remained almost the same as last year, but that high levels were found at greater depths due to new deposits of dirt.
At a new monitoring spot, cesium had accumulated more than 30 centimeters under the mud.
At depths of between 14 and 16 cm, the cesium level was equivalent to about 2,100 becquerels per kilogram contained in dried dirt.
The cesium content was 117,000 becquerels per square meter at the new site.
The number is on par with that detected in soil in some locations in Tokyo and surrounding areas where high cesium content has been observed.
According to researchers, the impact on the environment will likely be minimal as more than 70 percent of the cesium is at least 10 cm below the mud.
In contrast, more than 90 percent of the cesium on land is detected with 5 cm of the land surface.
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