The study, published in the Proceedings of the National Academy of Sciences journal, found that Fukushima prefecture was “highly contaminated” after the Fukushima Daiichi nuclear power plant was damaged in the March 11 earthquake and tsunami.
The level of radioactive material found in neighboring prefectures, such as Miyagi, Tochigi and Ibaraki, was lower but could still pose a threat to food production in some areas, the researchers said.
The study, led by Teppei Yasunari of the Universities Space Research Assn. in Maryland, looked at levels of cesium-137, which is of particular concern because it takes decades to decay.
The researchers used daily measurements collected in most prefectures along with computer-generated models of particle dispersion based on weather patterns to estimate the level of contamination across Japan.
The legal limit in Japan for concentrations of cesium-137 and cesium-134 in farm soil is 5,000 becquerels per kilogram (2.2 pounds). Cesium-137 makes up about half of the total for the concentrations, which are produced together.
The study found that the level of contamination in east Fukushima exceeded the safety limits. Results from neighboring prefectures were within the legal limits, but the researchers advised local authorities to conduct supplementary soil sampling.
-- Alexandra Zavis in Los Angeles
Photo: Masayuki Tsuchiya disposes of contaminated milk at a dairy farm in Kawamata city in Japan's Fukushima prefecture on March 23, 2011. Credit: European Pressphoto Agency