Radioactive cesium likely from the crippled Fukushima No. 1 nuclear power plant has sunk more than 20 centimeters deep in the mud of the seabed in Tokyo Bay, according to a scientist.
Radioactive cesium likely from the crippled Fukushima No. 1 nuclear power plant has sunk more than 20 centimeters deep in the mud of the seabed in Tokyo Bay, according to a scientist.
While it is known that more than 90 percent of cesium deposits remain in the uppermost 5-cm layer of soil on land, Hideo Yamazaki, a professor of environmental analysis at Kinki University, said he found that seabed cesium concentrations peaked at much greater depths at one location.
"The accelerated sinking has favorable effects on maritime contamination," Yamazaki said.
Last August, Yamazaki sampled seabed mud at four locations near the mouth of Arakawa river in Tokyo Bay. He said his analysis confirmed the presence of radioactive cesium in depths of 24-26 cm at one location.
The cesium concentrations peaked at depths of 12-14 cm at another location.
The cesium deposit had a maximum density of 18,242 becquerels per square meter. That is 25 times greater than the peak cesium density in the mud in Lake Biwako, Shiga Prefecture, due to fallout from atmospheric nuclear tests in the past.
The maximum cesium density in Tokyo Bay is slightly less than 30 percent of the equivalent of 0.23 microsievert per hour, a radiation level used as a threshold for decontamination efforts on the ground, Yamazaki said.
Tokyo Bay has a large population of benthos, or organisms living in the seabed mud. Yamazaki hypothesizes that cesium sank deep because the benthos ate mud on the seabed surface and discharged excrement deeper in the mud.
It would normally take 10-20 years for contaminants to sink more than 20 cm because mud builds at an annual rate of 1-2 cm at the measurement locations.
Large amounts of cesium flow in from rivers and accumulate on the seabed. The rate of that process will peak 1-2 years from now, Yamazaki said.