Keywords

16.1 Introduction

Radiocesium released from the Fukushima Daiichi nuclear power plant (FDNPP) accident on March 11, 2011, spread over a wide area of East Japan. Wild mushrooms often contain a high level of radiocesium even in areas with lower levels of contamination. The University of Tokyo has seven research forests located in East Japan, 250–660 km from FDNPP, where radiocesium contamination is relatively low. Some varieties of mushrooms collected there, however, contained radiocesium over the regulatory level of 100 Bq/kg in 2011 over the following years. Surveys of radiocesium contamination of wild mushrooms were conducted in the University of Tokyo Forests (UTFs), because fungi, including mushrooms, are a major components of the forest ecosystem.

Mushrooms are known to accumulate radiocesium (Byrne 1988; Kammerer et al. 1994; Mascanzoni 1987; Muramatsu et al. 1991; Sugiyama et al. 1990, 1994). However, the radiocesium concentration ratio in mushrooms relative to the soil was rather low (Heinrich 1992). This trend was also observed in a previous study (Yamada 2019). Further, a considerable proportion of 137Cs in forest soil is retained by the fungal mycelia, and fungi are thought to prevent the elimination of radiocesium from ecosystems (Brückmann and Wolters 1994; Guillitte et al. 1994; Vinichuk and Johanson 2003; Vinichuk et al. 2005). Thus, fungal activity is likely to contribute substantially to the long-term retention of radiocesium in the organic layers of forest soil by recycling and retaining radiocesium between fungal mycelia and soil (Muramatsu and Yoshida 1997; Steiner et al. 2002; Yoshida and Muramatsu 1994, 1996).

The dynamics of radiocesium in the ecosystem can be inferred by comparing the natural decay according to the physical half-life of radiocesium with actual changes, that is, the changes owing to the biological or ecological half-life considering migration, absorption, and excretion. According to analyses from that point of view, the ecological half-life of radiocesium in mushrooms has been reported to be longer than that of plants (Kiefer et al. 1996; Zibold et al. 2001; Strandberg 2004; Fielitz et al. 2009).

Previous studies (Yamada 2013, 2019) have summarized radiocesium contamination of wild mushrooms in UTFs after the Fukushima accident. We found rapid uptake of radiocesium in one species of mushroom after the Fukushima accident. In addition, we found residual contamination from the global fallout of atmospheric nuclear weapons tests or the Chernobyl accident. We also attempted to analyze the factors that determine the changes in radiocesium concentration (Yamada 2018, 2019). In the current study, the dynamics of radiocesium were surveyed over a 10 year period, and features of the time course transition in mushroom contamination were summarized.

16.2 Research Sites and Sampling

Mushrooms that appeared from the ground every Autumn from 2011 to 2020 were collected. In addition, their presumptive soil substrates, that is, the O horizon (organic litter layer, called the A0 horizon in Japan), the A horizon (mineral layer and accumulated organic matter), and the C/O horizon [mineral layer with a small quantity of organic matter, which is relatively unaffected by pedogenic processes (Soil Survey Staff 2014)], were also collected. Collected mushroom species were shown in Yamada et al. (2018) and Yamada (2019). Samples from the following three research forests were used for the current analyses.

Chichibu: The University of Tokyo Chichibu Forest (UTCF)

Fuji: Fuji Iyashinomori Woodland Study Center (FIWSC)

Chiba: The University of Tokyo Chiba Forest (UTCBF)

The location of each research forest, examples of mushrooms, the appearance of the environment where samples were collected, and sample preparation for radioactivity measurement can be seen in Yamada (2019). The radioactivity concentrations of 134Cs and 137Cs were determined using a germanium semiconductor detector. The distribution of radiocesium deposition and γ-ray air dose rate in East Japan in 2011 was presented in the previous studies (Yamada 2013, 2019).

16.3 Gamma Ray Air Dose Rate at the Mushroom Collection Sites (Fig. 16.1)

Gamma ray air dose rate (μSv/h) 1 m above ground level was measured with a dose rate meter using a CsI (Tl) scintillation detector (Yamada 2019). Although considerable variation in dose rate was observed among UTFs due to environmental variation such as geological features and contamination level, trends of changes and levels in dose rate were similar within each UTF. The air dose rate in Hokkaido (The University of Tokyo Hokkaido Forest, UTHF), where no contamination derived from the Fukushima accident was recognized, is also shown in Fig. 16.1 as a control. Similar levels of pre-Fukushima contamination from global fallout were estimated from 137Cs/134Cs ratio in soils in Chiba (UTCBF) and Fuji (FIWSC). The air dose rate before the Fukushima accident in Fuji, however, was lower than in Chiba, probably owing to geological features. The dose rate slightly decreased in Chiba with time, whereas the decrease was not clear in Fuji. Since 2015, the dose rate in Fuji has become almost similar to the dose rate recorded in Chiba. Although contamination due to the Fukushima accident did not reach Hokkaido, the dose rate was higher in Hokkaido (UTHF) than that in Fuji and Chiba. One year after the Fukushima accident, the dose rate in Chichibu (UTCF) was higher than that in other UTFs, and was over 0.1 μSv/h particularly in high mountainous areas, and then gradually reduced by approximately half by 2015. Although it seems that some hotspots remain in Chichibu, the dose rate has dropped to approximately 0.05 μSv/h since 2016. The dose rates at all UTFs sites are considered to be stable at almost preaccident values.

Fig. 16.1
A line graph of change in air dose rate. The U T C B F line of Chiba follows a declining trend. The lines of U TH F, U T C F, and F I W S C fluctuate constantly and remain almost at the same level.

Change in air dose rate 1 m above ground at each University of Tokyo Forest site. S spring, A autumn

Full size image

16.4 Dynamics of Radiocesium Contamination in the University of Tokyo Forests

16.4.1 Overall Trends (Fig. 16.2)

There were some patterns in the changes in 137Cs concentration (Yamada 2019; Yamada et al. 2019). In some cases, such as in Fuji’s saprotrophic fungus Pholiota lubrica, the 137Cs concentration tended to decrease clearly and consistently. In contrast, it has been reported in Europe that the concentration of radiocesium in mushrooms increased for a few years after the Chernobyl accident (Borio et al. 1991; Smith and Beresford 2005). A similar pattern was seen in several cases, such as mycorrhizal fungus Suillus grevillea, showing a tendency to increase from 2011 to 2012 and then decrease. However, the 137Cs concentration often did not clearly change. In this way, the changes in 137Cs concentration in mushrooms were roughly seen to follow three distinct patterns. In the O horizon, there were many sites where the 137Cs concentration gradually decreased, but there were also sites where the decreasing tendency was not clear. In the C/O horizon observed in Fuji or A horizon, the 137Cs concentration increased once, but in many cases no significant change was observed. The changes up to 2020 are the same as the previously reported trends up to 2015 and 2017, and the 137Cs concentration tends to gradually decreases in the O horizon and tends to be retained in the A or C/O horizon. Mushrooms showed a variety of tendencies in the middle of litter and soil.

Fig. 16.2
3 line graphs of 137 C s dynamics in mushrooms, an o horizon, and an a horizon. A cluster of lines fluctuates constantly in all 3 graphs.

137Cs dynamics in mushrooms and soils collected from three research forests

Full size image

16.4.2 Trend at the Same Sampling Sites (Fig. 16.3)

Yearly changes in 137Cs concentration were compared between the mushrooms, O horizon, A (or C/O) horizon. In the O horizon, there was a clear tendency for the 137Cs concentration to decrease over time. It decreased in any period even if it was divided into 1–4, 4–7, and 7–10 years after the accident. A large difference in the temporal change of 137Cs concentration among mushrooms was observed, and a variety of patterns were seen, such as those that decreased, those that did not decrease, and those that increased temporarily. In general, the mushrooms tended to retain 137Cs stably from the beginning, or alternatively to decrease partially followed by retention. In the A or C/O horizon, an increase or decrease of radiocesium was observed, but there was no clear continuous increase or decrease. It seems that part of the 137Cs in the O horizon has moved to the A or C/O horizon. These trends in mushrooms and soil continued for 10 years after the accident.

Fig. 16.3
4 scatterplots of cesium 137 changes between 2011 and 2020. Mushroom, o horizon, and A horizon are scattered at equal levels in all 4 plots.

Scatter diagram indicating 137Cs (Bq/kg DW) changes from 2011 to 2020 in mushrooms and soils from the same sampling sites. The oblique solid line (Y = X) indicates the same 137Cs concentration between 2011 and 2020

Full size image

16.5 Examples of Radiocesium Transfer at the Same Sampling Sites (Fig. 16.4)

The transition of radiocesium derived from the Fukushima accident can be elucidated when decay correction is performed for 134Cs. However, there are drawbacks to this method, such as falling below the detection limit and large error due to the rapid decrease in its concentration. Therefore, we determined the changes in 137Cs decay corrected for the date of the Fukushima accident, March 11, 2011. Here, as typical examples of changes observed, trends in 137Cs concentrations at one site in Chichibu and two sites in Fuji are shown.

Fig. 16.4
3 connected line graphs of cesium 137 versus years. In Chichibu, the lines of R e, O horizon, and A horizon follow a decreasing trend. In Fuji, the third, the lines of P I, L I, O horizon, and C or O horizon fluctuate with ups and downs. In Fuji the second, the S I, L h, O horizon, and C or O horizon lines fluctuate constantly.

Examples of dynamics of 137Cs, corrected for March 11, 2011, at the same sampling site. Re Russula emetica, Pl Pholiota lubrica, Ll Lactarius laeticolor, Sl Suillus luteus, Lh Lactarius hatsudake

Full size image

At the site that seems to be a local hotspot in Chichibu, the concentration of 137Cs in the O horizon was relatively high, but the subsequent decrease was also remarkable. It is thought that temporary imports of radiocesium into the A horizon occurred; however, the export became dominant after that. The 137Cs in the mycorrhizal Russula emetica in this location fluctuates according to the concentration in the O horizon and the A horizon.

Contamination from Fukushima was slight in Fuji. However, the saprotrophic fungus P. lubrica, which is said to have a superficial hyphal layer, had already absorbed a large amount of radiocesium by the fall of 2011 and showed a tendency of a gradual decrease in the radiocesium level. Radiocesium did not change significantly in the mycorrhizal fungus Lactarius laeticolor and it continued to retain these levels. Mycorrhizal Lactarius hatsudake and Suillus luteus tended to predominantly discharge radiocesium after initial absorption. The absolute amount of contamination in the O horizon was not large in Fuji, but there was a tendency for export of radiocesium from the O horizon, and a tendency for retention or import to be greater in the C/O horizon.

16.6 Transfer of Radiocesium—Changes in Decay Corrected 137Cs/134Cs Radioactivity Ratio in Each University Forest (Fig. 16.5)

Changes in 134Cs concentration decay corrected for Mar. 11, 2011, indicate contamination only from Fukushima. To match the transfer of radiocesium from global fallout and Fukushima, a simple comparison of the decay corrected 134Cs and 137Cs values is difficult to interpret. Therefore, decay correction was performed and changes in 137Cs/134Cs were compared (Rühm et al. 1997).

Fig. 16.5
9 line graphs of the changes in 134 and 134 Cs for the years from 2011 through 2020. The lines in the graph fluctuate.

Changes in 137Cs/134Cs ratio, corrected for March 11, 2011, in each of three UTFs

Full size image

High values of 137Cs/134Cs were observed in mycorrhizal mushrooms in 2011, which indicates that radiocesium from before the Fukushima accident remained and accumulated in the fungal mycelium (Yamada 2013, 2019). After that, this value was maintained in some cases, but also decreased in many cases. Many of the imports and exports were balanced, but imports exceeded exports in some cases. Therefore, this decrease of 137Cs/134Cs was considered to be caused by the absorption of radiocesium derived from the Fukushima accident. For saprotrophic mushrooms, it was initially low at almost 1.0, but then gradually increased. This suggested that although the amount of radiocesium remaining in the mycelia before the Fukushima accident was small, radiocesium derived from Fukushima was taken up by the time of collection (7 months after the accident) and then released and transferred to the outside. The value in the O horizon ranged from 1 to 2, indicating that 137Cs before Fukushima remained in the surface layer. Since 134Cs was scarcely present in the C/O horizon, changes in 137Cs/134Cs could not be tracked. However, changes in 137Cs in the previous section suggested the export or import of radiocesium depending on the location.

In Chichibu’s mycorrhizal mushrooms, this 137Cs/134Cs value was low, that is, the radiocesium derived from Fukushima was absorbed quickly, and the contribution of the previous radiocesium was small. This value was not high even in the O and A horizon, suggesting the relatively high contamination from the Fukushima accident. The cases studied from Chiba were also of mycorrhizal mushrooms. One of them had a large value, and the value was maintained every year throughout the period. In the A horizon of Chiba, the value was initially higher than 1. This showed that 137Cs before Fukushima remained approximately the same as 137Cs derived from Fukushima, and it is presumed that it was incorporated into mushrooms.

16.7 Transfer of Radiocesium (137Cs) Derived from Fukushima Accident Compared with Pre-Fukushima 137Cs (Fig. 16.6)

To understand the biological and ecological dynamics such as migration and retention of radiocesium, the contribution of the Fukushima accident and global fallout due to past nuclear weapons tests or the Chernobyl accident was evaluated from the ratio of 134Cs and 137Cs. Furthermore, since the proportion of 137Cs derived from Fukushima is considered to be greatly affected by the degree of contamination, the transition of the relationship between the concentration of 137Cs and the proportion of 137Cs before Fukushima and derived from Fukushima was investigated for each sample collected at the same sites (Fig. 16.6).

Fig. 16.6
5 connected line graphs. The peaks are as follows. Chichibu Russula emetica 70 in 2016, Fuji Pholiota lubrica 250 in 2018, Fuji Lactarius hatsudake 100 in 2018, Fuji O horizon 148 in 2020, and Chichibu A horizon 100 in 2016. Values are approximated.

Examples of changes in the ratio of 137Cs derived from the Fukushima accident compared with pre-Fukushima 137Cs

Full size image

In Fig. 16.6, the change to the lower right indicates that 137Cs derived from Fukushima is taken in, the change to the upper left indicates that 137Cs derived from Fukushima is preferentially discharged. The change to the left horizontal direction shows that 137Cs from before Fukushima and 137Cs from Fukushima are discharged in the same way.

Various patterns of changes were seen in the mushrooms, but many of them, such as L. hatsudake and S. luteus, changed from the upper left to the lower right and then changed to the upper left again. Another pattern that changed from the right to the left was seen in R. emetica in Chichibu and P. lubrica in Fuji. Intermediate forms between the two were also seen relatively frequently. In the case of L. hatsudake and S. luteus, it is speculated that 137Cs derived from Fukushima was gradually absorbed after the accident, and then as the 137Cs from Fukushima in soil such as the O horizon decreased, the emission of 137Cs from Fukushima exceeded the absorption. In the case of P. lubrica, a large amount of 137Cs derived from Fukushima was already absorbed in 2011, but the subsequent changes in 137Cs seen in this species are similar to those seen in mycorrhizal mushrooms.

In the O horizon, the pattern that changed from the lower right to the upper left is conspicuous, indicating that the Fukushima-derived 137Cs preferentially migrated to the outside. Changes in the A or C/O horizon often did not show any particular pattern.

These changes were not only because radiocesium is well retained in mushrooms, but the radiocesium released from the Fukushima accident was exported from the O horizon relatively quickly, whereas the radiocesium from before Fukushima generally remained in the O horizon. In other words, the radiocesium remaining in the O horizon is not adsorbed by clay minerals and should be able to easily migrate, but most of it tends to be retained in the material cycle system of the soil surface layer. Although it cannot be evaluated quantitatively, we presume that the retention of radiocesium in the hyphae is one factor for this. However, as in the case of R. emetica and P. lubrica, the proportion of 137Cs derived from the Fukushima accident has tended to decrease in mushrooms in recent years. The radiocesium value in the ecosystem from Fukushima may have now attained values similar to those seen before the Fukushima accident.

16.8 Concluding Remarks

We have measured the radioactivity concentrations of 134Cs and 137Cs over a monitoring period of 10 years, and analyzed and clarified the characteristics of radioactivity in wild mushrooms found in the University of Tokyo Forests. Monitoring data of 134Cs and 137Cs concentration provided the time course patterns of radiocesium accumulation, transfer, and selective retention of absorbed radiocesium in mushrooms. Further long-term monitoring of radiocesium is necessary to more precisely determine the dynamics of the contamination, through analyses comparing the levels of 134Cs and 137Cs; this is generally considered difficult because of the short half-life of 134Cs.