The science ministry released maps on Nov. 11 showing aerially measured accumulations of radioactive cesium from the Fukushima No. 1 nuclear plant in 18 prefectures.
Measurements were taken for the first time in six prefectures--Iwate, Toyama, Yamanashi, Nagano, Gifu and Shizuoka--in addition to the previously available contamination maps of 12 prefectures, providing an almost complete picture of eastern Japan.
Radioactive cesium has contaminated areas as far west as the border between Gunma and Nagano prefectures, and as far north as the southern part of Iwate Prefecture, according to officials.
The combined concentration of cesium-134 and cesium-137 exceeded 30,000 becquerels per square meter in certain areas of four municipalities in southern Iwate Prefecture--Oshu, Hiraizumi, Ichinoseki and Fujisawa--and parts of four municipalities in eastern Nagano Prefecture--Karuizawa, Miyota, Saku and Sakuho.
A concentration exceeding 60,000 becquerels per square meter was found near the border of Oshu and Ichinoseki cities and near the border of Saku city and Sakuho town.
The high cesium concentration levels in southern Iwate Prefecture and northern Miyagi Prefecture are said to have formed due to rainfall after a radioactive plume spread from the plant following the nuclear accident. Eastern areas of Nagano Prefecture may have been contaminated by a plume that moved southward from Gunma Prefecture.
In the latest round of measurements, the science ministry fine-tuned its methods by subtracting levels of naturally existing background radiation. The adjustments have led to considerably reduced areas in Niigata Prefecture, where the concentrations exceeded 10,000 becquerels per square meter, but high concentrations were still found in parts of Uonoma city and neighboring areas close to the border with Fukushima Prefecture.
The science ministry defines places with a concentration of more than 10,000 becquerels per square meter as "areas affected by the nuclear accident." No contamination was detected to the west of areas near the Gunma-Nagano border.
"It is possible the plume did not reach the other side of the mountains," a ministry official said.
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