Removing fallen leaves can substantially reduce radioactive contamination of soil in forests, researchers said in a finding that could help determine priority areas for clean-up measures in Fukushima Prefecture.
Removing fallen leaves can substantially reduce radioactive contamination of soil in forests, researchers said in a finding that could help determine priority areas for clean-up measures in Fukushima Prefecture.
A team of scientists, including Tsukuba University professor Yuichi Onda and researchers at the Meteorological Research Institute, surveyed soil contamination in three forests in the Yamakiya district of Kawamata, which lies in the planned evacuation zone near the Fukushima No. 1 nuclear power plant, between June and August.
In a broadleaf forest and a young coniferous forest, 90 percent of cesium concentrations were accumulated in fallen leaves and only 10 percent in the soil, according to the team's report released on Sept. 13.
In an old coniferous forest, which is less dense than the others, about 50 percent of cesium concentrations were found in the soil.
Decontamination of forests holds a key to removing large amounts of radioactive materials released from the nuclear plant because more than 70 percent of Fukushima Prefecture is covered by forests.
The researchers said the finding can also be used to efficiently remove radioactive materials from forest parks and other wooded areas frequented by people.
The survey was funded by the science ministry.
Concentrations of cesium-134 and cesium-137 on soil were 710,000 becquerels per square meter in the forest of broadleaf trees, such as Oriental white oak, according to the survey.
An 18-year-old forest of Japanese cedars had concentrations of 470,000 becquerels, while 910,000 becquerels were recorded in a 40- to 50-year-old cedar forest.
In the broadleaf forest and the young Japanese cedar forest, about 90 percent of these cesium concentrations were accumulated in fallen leaves.
After the Chernobyl accident, an area contaminated with cesium levels of 555,000 becquerels per square meter or more was designated as a forced migration zone.
The researchers also found that large amounts of cesium attached to leaves or branches fell to the ground when it rained.
Unlike broadleaf forests, coniferous forests already had many leaves when the Fukushima crisis started in March.
The rainwater that fell on coniferous forests contained up to 806 becquerels of cesium per liter, exceeding the safety standard of 200 becquerels for potable water, while the rainwater outside the forests contained 1 becquerel or less.
Onda said many residents near the surveyed area own forests near their homes.
"If radioactive materials are removed only from homes and their surroundings, radiation levels could be affected by concentrations in forests," he said.
Onda suggested that fallen leaves, as well as leaves and branches of coniferous trees, be removed to lower cesium levels in forests.
(This article was written by Akiko Okazaki and Hisae Sato.)