Radioactive Materials in Soil May Already Have Reached 30 Centimeters Below Surface, JAEA Says

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):

地中３０センチにまで浸透か　放射性物質、除染に影響も

Radioactive materials may have migrated to 30 centimeter deep in the soil, may affect decontamination efforts

　東京電力福島第１原発事故で放出され地面に降り積もった放射性物質について、事故から３カ月後の昨年６月にはほとんどが地表から５センチまでの浅い場所にとどまっていたが、１年後の現在では１０～３０センチの深さまで浸透している可能性があるとの推定を、日本原子力研究開発機構のチームが１４日までにまとめた。

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.

　雨水がしみ込む際に一緒に運ばれるとみられる。同機構幌延深地層研究ユニット（北海道幌延町）の佐藤治夫研究員は「除染活動が遅れるほど放射性物質は深く移動し、作業の負担を大きくする恐れがある」と警告している。

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.

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USGS Paper: "Wet Deposition of Fission-Product Isotopes to North America from the Fukushima Dai-ichi Incident, March 2011"

The paper by Wetherbee, Gregory A. et al was published online on February 22, 2012 ahead of the print, on Environmental Science and Technology.

Abstract, from Environmental Science and Technology (emphasis is mine):

Using the infrastructure of the National Atmospheric Deposition Program (NADP), numerous measurements of radionuclide wet deposition over North America were made for 167 NADP sites before and after the Fukushima Dai-ichi Nuclear Power Station incident of March 12, 2011. For the period from March 8 through April 5, 2011, wet-only precipitation samples were collected by NADP and analyzed for fission-product isotopes within whole-water and filterable solid samples by the United States Geological Survey using gamma spectrometry.

Variable amounts of ¹³¹I, ¹³⁴Cs, or ¹³⁷Cs were measured at approximately 21% of sampled NADP sites distributed widely across the contiguous United States and Alaska. Calculated 1- to 2-week individual radionuclide deposition fluxes ranged from 0.47 to 5100 Becquerels per square meter during the sampling period. Wet deposition activity was small compared to measured activity already present in U.S. soil. NADP networks responded to this complex disaster, and provided scientifically valid measurements that are comparable and complementary to other networks in North America and Europe.

The map above the abstract at Environmental Science and Technology indicates cesium-137 deposition between 0.8 to 240 becquerels/square meter. But without seeing the paper I have no idea how the numbers on the map are related to the numbers in the abstract. (My guess is that the numbers in the abstract, particularly the high number, include iodine-131.)

For more about the paper, here's USGS webpage on the NADP.

USGS press release on February 22, 2012 regarding the paper is here.

Open-File Report detailing the results and methodology is here.

Here's an interesting map at USGS page on the NADP. Green dots represent the NADP sites, and "Dot size represents relative deposition amounts. Fallout amounts measured in precipitation by USGS provide a clearer picture of fission-product wet deposition across the USA."

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