Conclusion of an article recently published in

 Environmental Science & technology, 2015, 49, 2875-2885

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Analysis of Japanese Radionuclide Monitoring Data of Food Before and After the Fukushima Nuclear Accident

Stefan Merz, Katsumi Shozugawa,*‡and Georg Steinhauser*,§,⊥

†Atominstitut, Vienna University of Technology, Stadionallee 2, 1020 Vienna, Austria

‡Graduate School of Arts and Sciences, The University of Tokyo, Meguro-ku, Tokyo 153-8902, Japan

§Environmental and Radiological Health Sciences, Colorado State University, Fort Collins, Colorado 80523, United States

⊥Institute of Environmental Radioactivity, Fukushima University, Fukushima 960-1296, Japan

http://pubs.acs.org/doi/pdf/10.1021/es5057648

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Lessons Learned.

An ample set of food monitoring data allows for the observation of general radioecological trends, such as the mobility and bioavailability of radionuclides. In this particular case, the long series of pre-Fukushima monitoring data teach us that the 90Sr/137Cs activity ratio is not constant in foodstuffs but constantly increases with time, thus causing an underestimation of the internal exposure as long as a constant (and low) ratio is assumed by the regulatory bodies. Data from pre-Fukushima monitoring campaigns revealed that animal products tend to be higher contaminated with radiocesium, whereas vegetarian produce exhibited higher activity concentrations in radiostrontium. The stunning amount of post-Fukushima food monitoring data clearly allows the identification of radioecologically“sensitive”foodstuffs. For the vegetarian food sector, these sentinels are primarily mushrooms and to a lower extent yuzu (citrus fruits), berries and Japanese radish. Due to its special diet, the boar is a suitable sentinel in the animal product sector as it feeds on mushrooms and other hyperaccumulators.

In summary, the Fukushima nuclear accident triggered an unprecedented monitoring campaign for radionuclides in food.Vegetables from Fukushima prefecture exhibited high radio-cesium activity concentrations soon after the accident.

However, by late summer of 2011, it was mostly mushrooms or dried vegetarian foodstuffs that exceeded the provisional regulatory limit. A similar picture was observed in other affected prefectures: after an initial high, activity concentrations invegetable dropped quickly, but peaked again due to mushroomsand dried vegetables. This confirms the necessity to monitor mushrooms as sentinel species for radiocesium [30].

Monitoring of meat/eggs started with significant delay after the accident, especially in prefectures other than Fukushima. Due to the constant intake of contaminated pasture, radiocesium concentrations in animal products from Fukushima built up relatively

slowly and peaked for the first time in early July 2011. In this initial period, it was mainly beef responsible for exceedances of the provisional regulatory limits. After the peak, activity concentrations dropped again to rise back from September2011. This time, it was mainly boar meat that was highlycontaminated. Iodine-131 in tap water exhibited high levelsshortly after the accident in several affected prefectures, but no exceedances of the limit were observed after March 2011.

Radiocesium levels in tap water were rather low. Given the high monitoring density, the mostly rapid response of Japaneseauthorities and the rapid decrease of very high initial contamination levels of the most common foods, it seems very unlikely that more than very few members of the public in Japan exceeded the maximum permissible internal exposure of 1 mSv/year. This observation is in agreement with the results of previous studies. [9−11, 21, 49−51]

A key finding of this study is that the correlation between 90Sr and137Cs may soon no longer follow the assumption of a maximum 90Sr/137Cs activity ratio of 0.1 or even 0.003 in food. Background data from Japan suggested that after several years following the release into the environment, the 90Sr/137Cs activity ratio observed in food rises significantly (most of the samples showing a ratio > 2). This calls for an adaption of the current policy and also increased monitoring efforts with respect to 90Sr. The diminution of the regulatory limit (90Sr/137Cs = 0.003) as of April 2012 was an adaption into the wrong direction. The Japanese authorities are urged to reimplement the“old”limit (90Sr/137Cs = 0.1), which probably will have to be raised further in the future. This observation fosters the need for continuous monitoring of both137Cs and 90Sr; otherwise the 90Sr content of food will soon be underestimated.