Sunflowers next to useless for nuclear decontamination
Sunflowers in an untended rice paddy in the village of Iitate, Fukushima Prefecture, in July (The Asahi Shimbun)
Farm ministry research has dashed hopes that sunflower seeds planted in areas around the Fukushima No. 1 nuclear power plant would help clean up radioactive contamination.
The seeds were sown within the evacuation areas in the belief they would soak up radioactive materials, but the Ministry of Agriculture, Forestry and Fisheries reported Sept. 14 that sunflowers only absorbed about 0.05 percent of the radioactive cesium in contaminated ground.
The laborious process of scraping off surface soil was far more effective.
Since late May, the ministry has been studying farmland decontamination methods at six locations in Iitate village and Kawamata town.
The ministry found that removing 3 centimeters of topsoil together with shallow-rooted grass was the most effective method of decontamination, reducing radioactive cesium by 97 percent.
Scraping off about 4 cm of surface soil when shallow-rooted grass was not present removed about 75 percent of the radioactive cesium.
When the surface soil was removed after applying a solidifying agent, 82 percent of the radioactive cesium was removed.
Other methods tested included filling rice paddies with water, then tilling and stirring the soil and draining the water. That reduced cesium by 36 percent.
Sunflower planting was highly ineffective by comparison, absorbing only one-2,000th of the cesium.
According to the ministry, 95 percent of the cesium is concentrated in the top layer of soil, less than 2.5 cm from the surface. Sunflower roots grow more than 1 meter below the surface, making it difficult for them to absorb cesium near the surface.
"There is no alternative plant that has a higher rate of absorption (than sunflowers)," a ministry official said. "From a practical point of view, we cannot rely on plants for decontamination."
Over time, cesium bonds strongly with minerals in clay soil. This makes it very hard for plants to absorb the cesium. The ministry said the most effective decontamination method was removing the soil with the cesium.
(This article was compiled from reports by Keiichiro Inoue and Takashi Sugimoto.)
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