Tuesday, August 2, 2011

Soil contamination update

In an article on the attempt of a concerned citizen in Japan to make her own measurements of radioactivity contamination from the Fukushima Daiichi accident, The New York Times (August 1, 2011 pg. A1, A3) has provided the first isometric sketch of accident-derived radiocesium soil contamination that we have seen. The New York Times article republished a May 26, 2011 compilation of soil contamination levels of Cs-134 and Cs-137, in which it cites the US Department of Energy and the Japanese Ministry of Education, Culture, Sports, Science, and Technology as being the source of the data.

An anomalous characteristic of this article is the use of soil contamination reporting units expressed in becquerels per square foot instead of per square meter. One square meter contains 10.76 square feet. To compare the data in this article with the hundreds of reports of Chernobyl soil contamination, multiply the Bq/sq m by 10. Also consult the extensive Chernobyl fallout database republished in the Fukushima Daiichi: Nuclear Information Handbook.

The isometric sketch reprinted in this article shows an extensive area of contamination “279,000 or more Bq/sq ft” extending to the northwest of the accident site beyond the 18 mile radius (+/- 21 miles) towards the town of Iidate. This means that soil contamination in this area is approximately 3 million or more Bq/sq m. Maximum radiocesium contamination levels from the Chernobyl accident were +/- 5 million Bq/sq m, with contamination areas above 100,000 Bq/sq m occurring thousands of kilometers away from the accident site. The isometric map also shows contamination in the range of 93,000 to 279,000 Bq/sq ft extending +/- 26 miles to the northwest as well as approximately 6 miles to the southwest towards Shidamyo. These levels of soil contamination resulted from a brief shift in the prevailing trans-Pacific wind direction, which brought accident-derived emissions inland.

A startling aspect of this report is the use of white, rather than grays, to signify areas with contamination “less than 28,000 Bq/sq ft”. The use of the color white implies the areas outside of the gray were un-impacted, when, in fact, large areas away from the accident site have soil contamination, which may approach 300,000 Bq/sq m. During the 1960s, cumulative weapons-testing-derived radiocesium contamination peaked at less than 5,000 Bq/sq m, yet there was worldwide concern about the health physics impact of fallout in this range of soil contamination. This isometric map inadvertently graphically illustrates the need for a much more comprehensive survey of soil contamination levels throughout Japan. The Fukushima Daiichi accident scenario is complicated by the fact that undocumented emissions are still occurring from all seven accident sites. Changes in prevailing wind directions due to the advent of the rainy season also raise the possibility of additional contamination occurring throughout Japan from both ongoing emissions and re-suspended contamination.

The article in The New York Times provides the first hint that detailed soil contamination data is available and can be easily converted into isometric maps that the general public can understand. The use of 300,000 Bq/sq m (28,000 Bq/sq ft) cutoff for reporting radiocesium soil contamination is absolutely unacceptable. As a matter of both honor and practicality, a much more detailed post-rainy season survey of accident-derived contamination should be the demand of all Japanese citizens. The extensive documentation of the many radioisotopes released by the Chernobyl accident, numerous useful definitions, and a survey of protection action guidelines is easily accessible for any concerned citizen interested in the impact and evaluation of nuclear accidents in Fukushima Daiichi: Nuclear Information Handbook. This Handbook, sponsored by the Davistown Museum may be purchased through Amazon.com or ordered from the Davistown Museum.

The New York Times article focuses on the attempts of Japanese private citizen Kiyoko Okoshi to monitor radiation in the area of her home in Iwaki Town in response to the lack of accurate data from the Japanese government. This article may serve the useful purpose of alerting all concerned citizens to the failure of the Japanese government and TEPCO to execute comprehensive monitoring of accident emissions in the form of a survey of soil contamination levels by the indicator nuclides Cs-137 and Cs-134. The lack of this easily compiled data also emphasizes the lack of public disclosure of real time monitoring of ongoing accident emissions, which continue to occur in both the water washout pathway and as volatile airborne emissions.

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