A summary of the samples measured after the Fukushima accident and the fraction exceeding the regulatory limits is given in the Supporting Information, Table S2, which is based on the information provided by MHLW.²⁰ Naturally, the number of samples measured was greatest for Fukushima prefecture (see Supporting Information Table S2). Also the highest number of samples exceeding the regulatory limits was found there. With the increase of radioanalytical facilities, the number of samples measured increased from 21,549 samples in the first year to 34,857 in the second year and 40,759 samples in the third year after the accident. By laying focus on “suspicious” or sensitive food types, the fraction exceeding the limits in food from Fukushima increased from 3.3% in the first year to 4.0% in the second year; however, it dropped to 1.5% in the third year and 0.6% in the final period of observation (Apr. 1, 2014 until Aug. 31, 2014).

Other prefectures were significantly less affected (e.g., in Gunma 0.2% or in Ibaraki 0.6% exceeding limits in the first year), with the notable exception of Saitama prefecture south of Fukushima, which reported 3.6% of all measured samples exceeding the limits in the first year, thus a higher percentage even than for Fukushima prefecture. From a radioecological point of view, it is interesting to note that it was exclusively samples of Japanese tea that exceeded the limits in Saitama in the first year. The Japanese tea plant is known to absorb deposited cesium by foliar uptake and to translocate the cesium from older leaves to younger leaves, which are then harvested and used for the production of tea.²⁸ Since this mechanism is only relevant in the first year after the accident, the number of tea samples exceeding the limits dropped from 127 (3.6%) in the first year after the accident to 0 in all following periods of observation. The 13 samples (0.3%) from Saitama exceeding limits in the second year were mostly deer meat and mushrooms; so were the 6 (0.1%) in the third year. In the latest period of observation no exceedances were reported from Saitama. Outside the Tohoku area, only very few samples exceeded the limits, as shown in Supporting Information Table S2. In these remote prefectures, it was mostly radioecologically sensitive organisms (such as fungi) that caused the exceedances.

The database for vegetarian produce commences with monitoring data obtained on Mar. 21, 2011 in Fukushima prefecture and Mar. 17, 2011 in other prefectures outside Fukushima (Ibaraki). It is likely, though, that some scattered measurements were already conducted prior to these dates but not included into the data set. In Fukushima, exceedances of the provisional regulatory limits were reported right on March 21; in other prefectures, on March 18. Naturally, in the initial phase, ¹³¹I was the main cause for exceedances of the limit. The maximum radiocesium activity concentrations dropped within a month by more than an order of magnitude, from 82 kBq/kg on March 21 to less than 8 kBq one month later (Figure 1). However, still a significant number of samples exceeded the provisional regulatory limit of 500 Bq/kg (indicated by the magenta line in Figure 1). For comparison, also the new regulatory limit of 100 Bq/kg (valid after Apr. 1, 2012) is included in the following figures in the form of the light blue dotted line.

By the beginning of August 2011, hardly any samples violated the regulatory limit, until the trend was reversed by mid-August due to high radiocesium found primarily in mushrooms (other foods occasionally exceeding the limits were, e.g., seaweed or (citrus) fruits). This trend peaked in early September 2011, when mushrooms containing high amounts of radiocesium (28 kBq/kg in coral fungi) were reported. Such high values have been observed in non-mushroom-vegetables only until the beginning of April 2011. Later in the mushroom season, a second distinct peak was observed in November which was mainly due to dried mushrooms. Also dried tea leaves contributed to the high activity levels. After this second peak, activity concentrations dropped again, until in January 2012 a third, much less pronounced peak was observed, not involving any mushrooms but primarily dried produce (dried yacon (leaves), dried taro, but also citrus fruits (yuzu) and Japanese radish and horseradish leaves). It is obvious from Figures 1 and 2 that the measurement density was much lower over the holiday season in late December 2011, so that a gap can be observed here.

A very similar or even more pronounced picture is shown in Fukushima’s neighboring (or affected) prefectures (Figure 2).

In the early aftermath of the accident, mainly samples from Ibaraki (and some from Tochigi as well as one from Gunma) exceeded the regulatory limits; however, radiocesium concentrations were significantly lower than what was observed in Fukushima. After mid-April only very few samples (parsley from Ibaraki in mid-May) exceeded the limits, but from September 2011 on many violations occurred again. In this case, the maximum contamination levels in this period were even significantly higher than those found in the early period after the accident. Again, this peak was mainly due to (dried) mushrooms (e.g., Shiitake) and lasted until the end of March 2012. It is interesting to note that although mostly samples from Ibaraki caused the majority of exceedances in the early period, it was mostly samples from Tochigi that were responsible for some of the high activity concentrations (>1 kBq/kg) in the fall–winter period of 2011. However, also samples from Gunma, Miyagi, Chiba, and Ibaraki had relatively high contamination levels. Yamagata and Niigata were less affected.

According to the database (Supporting Information Table S3), the first two samples of beef from Fukushima prefecture were taken already on Mar. 15, 2011, but they did not reveal any detectable activities. The main monitoring campaign of meat/eggs from Fukushima, however, started on Mar. 26, 2011 and revealed detectable ¹³¹I in chicken eggs right on the very first day. One day later, radiocesium was detected in chicken eggs. In contrast to vegetarian produce, the peak activity concentration was not observed in the very beginning with meat/eggs. After a constant increase and buildup of radiocesium activity concentrations, the provisional regulatory limits were exceeded for the first time on Jun. 10, 2011 in beef with a total radiocesium activity of almost 2 kBq/kg (Figure 3). At the end of June/beginning of July several beef samples clearly exceeded the regulatory limits; then the maximum levels dropped again. The maximum activity concentrations, however, were observed in boar meat on Sep. 5, 2011 and Dec. 26, 2011 (14600 and 13300 Bq/kg, respectively). Although the “summer peak” that was well-observed with vegetables (mainly due to the fact that mushrooms are remarkable cesium accumulators²⁹⁻³¹) was not as clearly pronounced with meat (see Figure 3), the mechanisms for these peak activities are similar. Boars are well-known for feeding on mushrooms and other hyperaccumulators, thus accumulating high activities of radiocesium.³²

Similarly, the specific diet of deer and Asian black bears (including berries and lichen) also leads to higher activity concentrations in the meat of bears (e.g., Oct. 13, 2011) and deer (e.g., Dec. 26, 2011) (see Supporting Information Table S3).^33,34 Although the category of beef dominated the violations of the provisional regulatory limit in the early phase after the accident, with very few exceptions, most cattle did not exhibit high activities from late summer 2011 onward. The violations of the limit were clearly dominated by boar meat. Also in the meat/eggs monitoring campaign, a significant gap can be observed during the holiday season in December 2011.

In Japanese prefectures other than Fukushima, monitoring of meat/eggs started with a significant delay (Figure 4). Although some samples were taken and measured as soon as March 20, the systematic survey of meat/eggs started only by the middle/end of July 2011. Due to the delay it seems likely that some above-limit meat/eggs may have made it into the markets and may have been consumed. The first sample we are aware of that exceeded the radiocesium regulatory limit was deer meat from Tochigi (Jul. 3, 2011; 1069 Bq/kg). Soon, boar meat from Tochigi also exceeded the limit (Jul. 16, 2011; 990 Bq/kg). Beef also caused several exceedances, with the highest value found in Iwate (not shown in Figure 4) with 2430 Bq/kg on Aug. 18, 2011. Several beef samples from Miyagi also exceeded the regulatory limits in late summer and fall 2011. Again, it was primarily boar, beef, and deer meat that caused the violations in the affected prefectures around Fukushima. From December 2011 to February 2012, it was mainly boar and deer meat in Tochigi and Ibaraki that exceeded the radiocesium limits.

Relatively little has been published in English scientific literature about radionuclide contamination levels in potable water³⁵⁻³⁸ and its treatment in response to the Fukushima accident.³⁹ The databases,²²⁻²⁴ however, allow an assessment of the activity concentrations of ¹³¹I and radiocesium (^134+137Cs) in tap water (see Figure 5).

It is interesting to note that in tap water ¹³¹I activity concentrations (Figure 5a) were not only much higher than the respective radiocesium concentrations but also that the maximum contamination levels were roughly in the same order of magnitude in all 4 of the most affected prefectures (Fukushima, Chiba, Ibaraki, Tokyo). The levels dropped quickly after the accident: after Mar. 23, 2011, our databases did not report any exceedances of the limits of ¹³¹I (300 Bq/kg). Restrictions for tap water were canceled by Apr. 1, 2011. Figure 5a also shows that the ¹³¹I levels dropped faster than just due to physical decay (gray diagonal lines), suggesting a shorter effective half-life in tap water than 8 days. Data on radiocesium in tap water are rather sparse (Figure 5b). The data compiled in our databases did not indicate any exceedances of the early regulatory limit for radiocesium (200 Bq/kg). Since later monitoring (late summer of 2011) did not show any detectable radiocesium in tap water,²² it appears unlikely that the new regulatory limit of 10 Bq/kg (blue dotted line) was exceeded.

Certain background levels of radiocesium exist in Japan due to the fallout from atmospheric nuclear explosions of the 20th century. Currently, the impact of the Fukushima accident can easily be distinguished from the background by the presence of the relatively short-lived reactor nuclide ¹³⁴Cs.⁴⁰ The average ¹³⁴Cs/¹³⁷Cs activity ratio at the time of the accident was 0.98 in food.²¹ After some years of decay, however, it will become increasingly difficult to determine residual activities of ¹³⁴Cs. As an alternative, the ratio of ¹³⁷Cs to long-lived ¹³⁵Cs (T_1/2 = 2 × 10⁶ a) can be used instead;⁴¹⁻⁴⁴ however, this is laborious and requires special instrumentation. Cesium-135, therefore, is unlikely to become a tracer for routine monitoring campaigns. Instead, the contribution of the pre-Fukushima background can only be estimated based on previous monitoring data (see Table S4 in the Supporting Information). See Supporting Information Figure S2 for the temporal evolution of the ¹³⁴Cs/¹³⁷Cs activity ratio in food over the first year.

The ¹³⁷Cs background data (Figure 6) from Japan reveal that most samples were rather low in radiocesium, most of which with less than 0.5 Bq/kg (with the exception the 1992 samples of mutton and chocolate). Generally, more samples of meat/eggs than vegetarian produce exhibited detectable ¹³⁷Cs activities. Interestingly, several samples of meat/eggs from Fukushima prefecture were higher in ¹³⁷Cs activities than samples from other prefectures (Figure 6). It is unlikely, however, that the background will contribute significantly to the total post-Fukushima contamination of foods.

In contrast to ¹³⁷Cs, more vegetarian produce revealed detectable ⁹⁰Sr activity concentrations than animal product samples (Figure 7). This is probably due to the fact that ⁹⁰Sr is a bone-seeking radionuclide, so that any ⁹⁰Sr taken up by animals is hardly transferred to the muscle but to the bones instead. A notable exception may be milk, which is naturally rich in Ca and thus a good carrier for ⁹⁰Sr. Note that only one sample of (condensed) milk was included in the data set of the ⁹⁰Sr background. In any case, activity concentrations usually did not exceed 0.5 Bq/kg (with the exception of one green pepper sample from 2004). Samples from Fukushima prefecture proved to exhibit similar ⁹⁰Sr activity concentrations like other prefectures.

Determination of radiostrontium is rather laborious, making it one of the understudied radionuclides after the Fukushima nuclear accident.⁴⁵ In order to account for the environmental presence of ⁹⁰Sr, Japanese authorities assumed a constant ratio between ⁹⁰Sr and ¹³⁷Cs, the latter of which can be measured rapidly using γ-spectrometry.^8,13 According to this assumption, the ⁹⁰Sr activity concentration should not exceed 10% (March 2011 to April 2012) or 0.3% (after April 2012), respectively, of the respective ¹³⁷Cs activity concentration. This assumption is routed in observations following the Chernobyl accident and atmospheric nuclear explosion fallout, e.g., ref (17). Since the Fukushima nuclear accident was a much more powerful source of radiocesium than of less volatile radiostrontium than Chernobyl or the nuclear weapons fallout,^3,46 it was a reasonable and conservative approach to implement the same ratio as the maximum ⁹⁰Sr content in Fukushima’s contaminations.

Our analysis of the background activities, however, shows that this assumption is at risk (Figure 8). The (rather low) number of samples that exhibited detectable activities of both ¹³⁷Cs and ⁹⁰Sr showed that the vast majority of the food samples exceeded the ⁹⁰Sr/¹³⁷Cs ratio of 0.1, and all were higher than 0.003. Most samples even showed a ratio of ≥2. Only the one sample of mutton that was discussed previously with its unusually high ¹³⁷Cs activity concentration had a ⁹⁰Sr/¹³⁷Cs ratio of <0.1. Generally, meat/eggs proved to show a slightly lower ⁹⁰Sr/¹³⁷Cs ratio than vegetarian produce, owing to the generally greater activity of ¹³⁷Cs in meat/eggs compared with ⁹⁰Sr.

Due to very similar half-lives of both ⁹⁰Sr and ¹³⁷Cs, any of these ratio anomalies cannot be due to physical decay. Instead, analysis of 1959–1995 data on ⁹⁰Sr and ¹³⁷Cs in rice²⁶ and wheat²⁷ reveals that a low ⁹⁰Sr/¹³⁷Cs activity ratio is justified only for the initial period of a couple of years, as the ratio rises over the years (Figure 9). The analysis shows that wheat has a higher ratio than rice, but both rise significantly over the following years and decades following the period of the main nuclear fallout in the 1960s. Only the Chernobyl accident caused a singular outlier in the series of wheat measurements. The Chernobyl accident was a significant source of ⁹⁰Sr only in the closer vicinity of the destroyed NPP, but in Japan only significant amounts of airborne radiocesium were observed.⁴⁷

This analysis reveals that ⁹⁰Sr exhibits a higher mobility and bioavailability than radiocesium, whereas ¹³⁷Cs is more readily adsorbed and immobilized on clay minerals, thus causing the distortion of the initial activity ratio in food. One can speculate that the data presented in Figure 8 represent the plateau of this distortion where the ⁹⁰Sr/¹³⁷Cs activity ratio of most foodstuffs remains at a rather constant level between 1 and 10. The reason for the slower increase of the ratio in rice is most probably due to a radioecological anomaly of paddy-cultivated rice. The paddy cultivation causes the formation of ammonia from putrefaction. The NH₃ dissolves in water and forms NH₄⁺ ions which are very efficient ion exchangers for adsorbed Cs⁺ ions on clay minerals. Compared with conventional cultivation methods, the paddy cultivation of rice thereby increases the mobility of Cs and hence makes ¹³⁷Cs more bioavailable.

The increasing ⁹⁰Sr/¹³⁷Cs activity ratio and its effects on the regulatory limit must be taken into account for the Fukushima nuclear accident and future radioecological considerations with respect to food safety and monitoring. The current assumption of the maximum ⁹⁰Sr/¹³⁷Cs activity ratio in food will be no longer true within a few years after the accident.

The impact of this erroneous assumption of a constant ⁹⁰Sr/¹³⁷Cs activity ratio can be quantified as shown in Figure 10. For this figure, a daily consumption of 1.7 kg of (solid) foods was assumed (National Nutrition Survey of Japan⁴⁸). Also we assume no contribution from short-lived ¹³⁴Cs, because the effects will become critical after several years only. Consumption over 1 year of foods contaminated with 100 Bq/kg ¹³⁷Cs and 10 Bq/kg ⁹⁰Sr (authority-assumed ratio of ⁹⁰Sr/¹³⁷Cs of 0.1) will cause a committed dose of 1 mSv. With a higher ⁹⁰Sr/¹³⁷Cs ratio, the received dose increases as shown in Figure 10. In this figure, any white areas are “covered” by the Japanese regulations: foods with ¹³⁷Cs contaminations > 100 Bq/kg will be “rightfully” banned; foods with ¹³⁷Cs contaminations < 100 Bq/kg and low ⁹⁰Sr/¹³⁷Cs ratios will be “rightfully” permitted. However, any colored areas in Figure 10 are “blind spots” that remain uncovered by the regulations: Foods with activities falling into this area will be falsely permitted although their consumption may be critical. For example, when foods contaminated with 23 Bq/kg ¹³⁷Cs are consumed over the period of a year (clearly below the current limit of 100 Bq/kg), a ⁹⁰Sr/¹³⁷Cs ratio of 2 will already cause a committed dose of 1 mSv. At this activity ratio, a contamination level of 46 Bq/kg ¹³⁷Cs (less than half of the current limit) will deliver 2 mSv. If we keep in mind that the pre-Fukushima samples often had ⁹⁰Sr/¹³⁷Cs activity ratios > 2 (up to 10), this scenario illustrates the potentially severe impact of this erroneous assumption of a constant ratio at 0.1 (or even below).

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 radiocesium 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 in vegetable dropped quickly, but peaked again due to mushrooms and 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 September 2011. This time, it was mainly boar meat that was highly contaminated. Iodine-131 in tap water exhibited high levels shortly 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 Japanese authorities 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 and ¹³⁷Cs 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 both ¹³⁷Cs and ⁹⁰Sr; otherwise the ⁹⁰Sr content of food will soon be underestimated.