Concerns about radiation in Japan have now spread to the soil surrounding the crippled Fukushima Daiichi nuclear reactor. One level that was reported this week was high enough to suggest people in that area should be evacuated, an expert says. But he cautions that it's hard to draw conclusions about these spot measurements without more data.
Today, Japanese officials told the population living up to 30 kilometers from the plant that they should consider leaving the area, expanding the previous 20-kilometer radius evacuation zone. But according to news reports, the advice stems from difficulties in supplying the region with food and water, not radiation levels.
Meanwhile, on Wednesday the Japanese science ministry began to report measurements of cesium-137 in upland soil around the plant. The levels are highest from two points northeast of the plant, ranging from 8690 becquerels/kilogram to a high of 163,000 Bq/kg measured on 20 March from a point in Iitate about 40 kilometers northwest of the Fukushima plant.
The soil measurements are more significant for evacuation purposes than radioactivity in the air, says nuclear engineer Shih-Yew Chen of Argonne National Laboratory in Illinois, because cesium dust stays underfoot while air is transient. Levels of cesium-137 are also more important than soil readings of iodine-131, which is short-lived and more of a concern in milk and vegetables. "It's the cesium that would prompt an evacuation," says Chen.