Sludge samples taken at the mouths of two major rivers emptying into Tokyo Bay showed radioactive cesium contamination linked to the Fukushima No. 1 nuclear plant crisis grew by 1.5 to 13 times since August, a researcher at Kinki University said Monday.
The contamination poses no immediate health risk since no seafood from Tokyo Bay has seen contamination levels exceed the government-set threshold. But close, long-term monitoring of the seabed mud is needed, said Hideo Yamazaki, professor at Kinki University’s Research Institute for Science and Technology.
“Contamination is flowing into the bay from rivers, including the Edogawa River, where cities with high radiation levels like Kashiwa (in Chiba Prefecture) are located upstream,” Yamazaki told The Japan Times.
“Contaminated sludge appears to be . . . accumulating on the bottom at the mouth of the rivers,” he added.
Yamazaki, an expert on how radiation and chemical substances impact the environment, and his team took the samples at three locations at the mouths of the Arakawa and Edogawa rivers on April 2 following studies carried out in August.
Samples of mud pulled from 1 meter below the seabed at the sites turned up cesium contamination ranging from 7,305 to 27,213 becquerels per square meter. The August readings were between 578 and 18,242 becquerels per square meter.
Yamazaki noted a thirteenfold rise was detected in a spot where the August readings were relatively low. He said, however, the contamination does not pose a health threat, even if a child were to play in the water.
Although radioactive mud will continue to flow into the bay, the peak contamination concentrations should be within the next couple of years, considering that the half-life of cesium-134 is about two years, Yamazaki said.
“If the contamination were to spread to fish, it is possible that radioactive isotopes could accumulate when bigger fish feed on smaller ones,” he said. “We’re scheduled to continue our monitoring in the following years” to study such cases.