137Cs in the meat of wild boars: a comparison of the impacts of Chernobyl and Fukushima
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The impact of Chernobyl on the 137Cs activities found in wild boars in Europe, even in remote locations from the NPP, has been much greater than the impact of Fukushima on boars in Japan. Although there is great variability within the 137Cs concentrations throughout the wild boar populations, some boars in southern Germany in recent years exhibit higher activity concentrations (up to 10,000 Bq/kg and higher) than the highest 137Cs levels found in boars in the governmental food monitoring campaign (7900 Bq/kg) in Fukushima prefecture in Japan. The levels of radiocesium in boar appear to be more persistent than would be indicated by the constantly decreasing 137Cs inventory observed in the soil which points to a food source that is highly retentive to 137Cs contamination or to other radioecological anomalies that are not yet fully understood.
KeywordsFukushima Chernobyl Sus scrofa Foodstuff Food safety 137Cs Ecological half-life
In the course of the Chernobyl nuclear accident (April 26, 1986) and the Fukushima nuclear accident (March 11, 2011), large amounts of radionuclides have been released and deposited in the environment [1, 2]. The majority of the released activity was due to volatile radionuclides such as 131I, 132Te, 134Cs, and 137Cs. Following the Chernobyl nuclear accident, the importance of both the short and long-term health effects of releases of short-lived 131I into the environment has been recognized . Longer-lived 134Cs and 137Cs, together with other long-lived, more refractory fission products such as 90Sr or actinides such as plutonium remain in the environment for a very long time after a nuclear accident. However, their emissions from Fukushima [4, 5, 6, 7, 8, 9] did not compare nearly to the releases from Chernobyl [2, 10, 11]. Radiocesium exhibits a potential health threat, especially upon intake with contaminated food [12, 13, 14, 15, 16, 17].
Food has been identified as a major contributor to the total radiation exposure after the nuclear accidents at Chernobyl and Fukushima [18, 19]. In our previous analysis of food monitoring data , we have identified wild boars (Sus scrofa) as hyperaccumulators of radiocesium from Fukushima. The purpose of this study is to compare published data from Europe and Japan for a juxtaposition of the impacts of Chernobyl and Fukushima on contamination levels in wild boars as well as to discuss possible radioecological implications of these data.
Materials and methods
Existing data from food (or environmental) monitoring programs were used for this study. For the assessment of the Fukushima impact, MHLW data [19, 20] were used. Data for the Ukraine were taken from Gulakov . For Germany, data were taken from LFU  and Semizhon et al. . Austrian Data were taken from Sontag et al.  and AGES . Data from Croatia were obtained from Sprem et al. .
Results and discussion
Comparison of activity concentrations
This is the reason why a comparison of the compliance with the regulatory limits in Fig. 1 is difficult, because for Chernobyl-affected samples will only contain 137Cs, whereas Fukushima-affected samples will contain both 134Cs and 137Cs.
After the Chernobyl accident, boar meet from the vicinity of the NPP was contaminated to a much higher extent. Gulakov  reported on the maximum activity concentration amongst 3 wild boars from the “alienation zone” in 1996 of 661,000 Bq/kg, hence almost two orders of magnitude higher than levels found in Fukushima prefecture. It is remarkable, however, that this extraordinarily high value was found 10 years after the accident and not earlier. The levels observed in wild boars from the highly contaminated areas of Europe remained high for many years: even in the 2010s, wild boar meat from southern Germany (more than 1400 km away from the Chernobyl NPP site) occasionally exhibits 137Cs activity concentrations that are higher than those observed in Japan (see Fig. 1). Environmental agencies frequently report on boar meat in Central Europe the exhibits 137Cs activity concentrations that are in the range of 20 kBq/kg, which is a factor of 2–3 higher than the maximum values reported after Fukushima. It should also be noted that only selected boars among the wild boar population exhibit exorbitant activities, while others remain rather close to the detection limit. Hence there is a great variability which makes a complete assessment based on a limited number of samples difficult or impossible.
Data from independent researchers  studying wild boars from Japan and the distribution of radiocesium in their organs were largely in line with the results of the governmental food monitoring campaign. They found maximum radiocesium activity concentrations of 15,000 Bq/kg (approximately half of which is 137Cs). They also found the highest activity concentrations, as expected, in muscle tissue, but also in kidneys, tongue and heart tissue.
Effective and ecological half-lives
Based on the Fukushima data set we used for this study (comprising the first year after the accident), a calculation of the ecological half-life, therefore, is not feasible. It is unclear whether radiocesium activities have reached their maximum from where an effective half-life could be calculated. For relatively long-lived radionuclides, such as 137Cs a larger time-frame would be necessary to observe the decline in activity over several years.
However, the data for the Chernobyl accident allow for an estimation of the ecological half-lives of 137Cs observed in Europe. Based on data from southern Germany, Semizhon et al.  have concluded that the 137Cs levels found in wild boars have remained almost constant over the decade from 1998 to 2008. Indeed, we also observed constant activity concentrations for boar from southern Germany in the period of observation 2012–2015. In fact, the activity concentrations even exhibited a slight increase rather than a decline that would have been expected due to physical decay and environmental processes leading to a decline of activity.
Lastly, the question remains what the reason for this discrepancy is—long persistence in the “sink” (boar), though comparably short persistence in the primary “reservoir” (soil) in the environment. If the source gets constantly weaker, how can the sink retain its high levels over long periods of time? This is an obvious oxymoron to the expected decline of a radionuclide’s activity in both soil as well as boar due to decay and migration. One, it has to be acknowledged that soil is not the only reservoir, as there are intermediary organisms such as lichen or fungi which may act as soil-independent sources (secondary reservoirs) for the boars. Two, we hypothesize that the apparent time constancy of 137Cs in boar meat may, in part, be due to chemical changes in the deposited radiocesium. In particular, a partial transition of granular (thus insoluble) radiocesium to ionic and water soluble radiocesium may increase the bioavailability to fungi and fodder organisms of the boars and hence counterbalance the decline in absolute numbers of radiocesium atoms in the reservoir.
The impact of the Chernobyl nuclear accident on the 137Cs activities found in wild boars in Europe has been much higher than the impact of the Fukushima nuclear accident on wild boars in Japan. Although there is great variability within the 137Cs concentrations throughout the wild boar populations, some boars in southern Germany exhibit higher activity concentrations than the highest levels found in boars in Fukushima prefecture in Japan. Although we could prove that the radiocesium inventory in soil in southern Germany decreases constantly according to its effective half-live, the levels in boars appear to be much more persistent, if not constant. Currently, this oxymoron has not been fully resolved.
We thank the governmental organizations in Japan, Germany and Austria for providing open and transparent access to their data. This work was supported by CDC NIOSH Mountain and Plains Education and Research Center (Grant Number T42OH009229-07), and the US Nuclear Regulatory Commission (NRC) (Grant Number NRC-HQ-12-G-38-0044). Its contents are solely the responsibility of the authors and do not necessarily represent the official views of any of the funding agencies.
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