Most of the radioactive cesium that spewed from the Fukushima No. 1 nuclear plant and fell in broad-leaf forests remained near the surface and likely did not spread to groundwater, researchers said.
The researchers at the government-affiliated Japan Atomic Energy Agency began a study in May 2011 to monitor how cesium migrates in the ground below deciduous forests in Ibaraki Prefecture.
The forests were about 65 kilometers southwest from the crippled plant, which sits on the Pacific Coast.
When the disaster unfolded at the nuclear plant in March 2011, huge amounts of radioactive cesium landed on woodlands in a vast area around the plant.
Early readings in the study showed an average of 20 kilobecquerels of cesium per square meter in the surveyed area. About 70 percent of the cesium was present in a layer of fallen leaves.
Seven months later, the research team found that readings were down to one-fourth of the initial level.
In contrast, cesium levels tripled in the soil up to 5 centimeters deep after most of the cesium on the leaves seeped into the earth.
When the researchers measured cesium levels in the same soil in August 2012, they discovered that most of the cesium had remained there.
They also monitored how much of cesium had descended to 10 cm from 5 cm in the soil. The results showed only about 0.2 percent of the cesium moved in fiscal 2011, while the figure for fiscal 2012 was about 0.1 percent.
By autumn 2011, most of the cesium on the leaves had been washed into the soil by rainfall. The researchers also believe that rising temperatures accelerated the decomposition of the fallen leaves, resulting in more cesium sinking into the soil.
But after that, there was little movement in the cesium.
“In a future study, we want to look at cesium in the soil of needleleaf forests and forecast the impact on the nearby environment after monitoring the cesium’s movements to forestry products and areas beyond woodlands,” said Takahiro Nakanishi, a specialist of geoenvironmental science at the agency.
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