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.
Based on a rough estimate, a person standing on soil with 163,000 Bq/kg of cesium-137 would receive about 150 millisieverts per year of radiation, says Chen. This is well above the U.S. Environmental Protection Agency standard of 50 millisieverts per year for an evacuation. (Per day, it's 0.41 millisieverts, which is equivalent to four chest x-rays.) But Chen adds, "one point [of data] doesn't mean that much."
The hot spot is similar to levels found in some areas affected by the 1986 Chernobyl nuclear reactor accident in the former Soviet Union. Assuming the radiation is no more than 2 centimeters deep, Chen calculates that 163,000 Bq/kg is roughly equivalent to 8 million Bq/m2. The highest cesium-137 levels in some villages near Chernobyl were 5 million Bq/m2.