Japan Atomic Energy Agency's researchers say radioactive materials that remained on the top 5 centimeters of the soil for the first 3 months of the nuclear accident may have already migrated down to as deep as 30 centimeters.
It may be difficult to decontaminate, says the agency who happens to be in charge of the government pilot project to decontaminate within the no-entry zone and planned evacuation zone in Fukushima, using joint ventures by the largest construction companies in Japan. (For more on the confused state of this pilot program, see my post on NY Times article.)
But on the other hand, it's been too late anyway in Fukushima, Miyagi, and areas in Kanto with significant amounts of radioactive fallout (except for Miyagi, whose data is still not disclosed, if it exist), where farmers tilled the land in spring last year to grow vegetables and rice upon encouragement from the government.
From Kyodo News (3/14/2012):
Radioactive materials may have migrated to 30 centimeter deep in the soil, may affect decontamination efforts
Radioactive materials deposited on the ground after the Fukushima I Nuclear Power Plant accident stayed within 5 centimeters from the soil surface in June last year, 3 months after the accident started, says a research team at Japan Atomic Energy Agency. However, the group says there is a possibility that radioactive materials have now migrated to between 10 and 30 centimeters from the surface, one year after the accident.
The team thinks rainwater carries radioactive materials down the soil. Haruo Sato, a researcher in the Horonobe Underground Research Center of JAEA (in Horonobe-cho in Hokkaido) warns, "The longer decontamination takes, the deeper radioactive materials migrate, making the decontamination effort more difficult."
Oops... The government and many researchers have said that radioactive materials remain on the shallow surface of the soil, citing the case of Chernobyl. I guess they didn't realize the warm, rainy climate of Japan was different from Chernobyl. The annual average rainfall in Fukushima is 1,300 to 1,600 millimeters, whereas in Chernobyl it is 300 millimeters.
Looking at blog posts and tweets from last summer, many in Japan were hoping the large amount of rain that Japan gets would wash away the radioactive materials to the rivers and to the sea quickly. Instead, it drove them down the soil, according to JAEA.