The Transition of Radiocesium in Peach Trees After the Fukushima Nuclear Accident

Fukushima Prefecture has 6820 ha of orchards in 2016 and it is one of the top fruit producing areas in Japan (National Statistics Center 2016). When looking at each kind of fruit, the area of the land under cultivation with peaches is the second largest in Japan (1810 ha), and this means that the prefecture is one of the major producers of fruit in Japan even after the Fukushima Daiichi nuclear accident. With regards to changes to the retail price of fruit, Komatsu (2014) and the author’s previous study (Takata 2016), show that most fruits have returned to their pre-accident prices, yet the situation is not the same as before.

Six years after the accident, the concentration of ¹³⁴Cs is thought to be in low in fruit because its half-life is about 2 years. On the other hand, ¹³⁷Cs, whose half-life is longer than ¹³⁴Cs can still be detected. However, previous studies found that radiocesium in fruit decreased more quickly than its actual half-life; for example, the concentration of ¹³⁷Cs decreased by one third in the second year, and by another one third in the third year after the accident (Takata et al. 2014, 2016). These results correspond to what was found in several types of fruit after the Chernobyl nuclear power plant accident (Antonopoulos-Domis et al. 1990; Madoz-Escande et al. 2004). However, there are few reports which traced and investigated products in the same region to examine a long-term transition of the concentration of radiocesium in fruit trees, and the future transition is uncertain. To understand this point, it is necessary to investigate the concentration of radiocesium not only in fruit continuously but also in trees which are the source of Cs in fruit. The author’s previous study found that radiocesium in soil has almost no relationship with Cs in fruit in most cases. This is because (1) the radiocesium transfer factor (TF) ratio for fruit is lower than annual crops in Japan, (2) trees are no longer affected by fallout which fell directly on bark, and largely contributed to the concentration of radiocesium in the fruit (Takata et al. 2012b, 2013a), (3) since radiocesium in the trees remain in situ year after year, this has become a source of radiocesium in fruit (Takata et al. 2012d, 2013c), and (4) most kinds of fruit in Fukushima Prefecture (except a few kinds such as blueberries) do not form their rooting zones within 5 cm from surface horizon and thus they do not absorb radiocesium, which was distributed only on the surface horizon unevenly (Takata et al. 2013b). The fourth reason is also related to the fact that potassium fertilizer has no effect in most cases. Therefore, it is necessary to investigate trees continuously to understand the source of radiocesium in fruit.

In this chapter, new research findings, after the author’s previous two reports stated above, are reported. Especially, understanding the dynamics of radiocesium in fruit and redistribution of ¹³⁷Cs in trees 6 years after the accident is the focus.

After the nuclear power plant accident, one variety of peach “Akatsuki” was investigated continuously regarding the concentration of radiocesium in the fruit in Shimo-oguni district, Ryozen-machi, Date City from harvest in 2001 to 2016. In 2011, the concentration of radiocesium in the harvested fruit was measured. From 2012 to 2016, the concentration of radiocesium and ⁴⁰K was measured throughout fruit development: from Day 15 after the full bloom to harvest (approximately Day 103).

Figure 9.1 shows the transition of radiocesium (calculated using dry mass) in peach fruit. The red arrows indicate the concentration in the harvested fruit. As shown in the author’s previous research, the concentration decreased approximately by one third from 2011 to 2012. It also decreased by one third from 2012 to 2013. Though it was expected that the year-over-year decrease would continue in the same way, the concentration in the fruits in 2014 showed a slightly higher value than the level in the prior year. In 2015 and 2016, the concentration of radiocesium in the harvested fruit decreased slightly, but this decrease was not significant, and the transition after 2013 tended to be a gradual decrease or almost no change. This fact has both good and concerning aspects. The positive aspect is that it is possible to continue to produce fruit without concerns when the concentration of radiocesium is lower than the current standard value of 100 Bq/FW. It is expected that fruit will not require dedicated countermeasures, whereas some other crops such as rice needs continuous countermeasures (e.g., application of potassium fertilizer) to lower the radiocesium concentration. Applying potassium fertilizer and absorption of radiocesium from soil will not be elaborated on in this chapter because it has already been covered in the author’s previous articles. On the other hand, the concerning points are regional problems and issues related to sales methods. Regional problems refer to the fruit production in regions showing a high concentration of radionuclides, such as Hama-dori region. Hama-dori region represented by Namie-machi is famous for Asian pear production, and the findings presented above does not instill confidence to resume production there. Yet, it is unrealistic to reuse the same trees to resume fruit production in these regions because the trees have gone without pruning and pest control, and the trellis are likely to have deteriorated. Also, the soils were contaminated with a large amount of radionuclides and it would be necessary to replace the soil and to plant new trees. Therefore, the data shown in this study has no negative effect on fruit production, and what should be considered first is not issues of cultivation but financial aspects such as compensation for farmers during the time without income, the replacement and planting of new trees, and decontamination of surface soil. On the other hand, Japanese persimmon produced mainly in Date City can carry with it a severe problem because of how this fruit is sold. In Date City, persimmons are processed and sold as Anpo-gaki. It is a kind of Hoshi-gaki (dried persimmon) and the fruits are semi-dried. In Japan, because a standard value of radiocesium is determined in products at the point of sale, the radiocesium concentration is measured in the dried persimmon to compare it with the standard value (100 Bq/kg). Therefore, we need to be careful in producing raw materials for dried persimmon because radiocesium in persimmon is concentrated after drying. Although, Anpo-gaki started to be sold again in 2013, it is necessary for producers whose fruits show radiocesium concentrations close to the standard value to carry out drastic measures such as replanting to ensure the concentration in the fruit is below the standard value. The green arrows in Fig. 9.1 show the concentration of radiocesium in fruit 15 days after the full bloom. It has been decreasing year-over-year until 2016, while the concentration at the time of harvest stopped decreasing. The concentration of radiocesium in trees may contribute to the different trends between 15-day-old fruit and harvested fruit. Young organs in peach trees grow using their nutrient reserves for 3–4 weeks after flowering, and inadvertently this results in the transfer of radiocesium to young organs. Young fruit show high a concentration of radiocesium because they rely on the tree’s nutrient reserves. As the fruit matures and nutrients in the soil contribute more, the radiocesium concentration is believed to decrease. The decrease of the concentration following maturity of fruits is common with the transition of ⁴⁰K in fruits shown in Fig. 9.2. Though the ratio of the transition is different between these two elements, they show a similar tendency. On the other hand, the year-over-year transition of ⁴⁰K could not be seen. This may be because the source of ⁴⁰K and ¹³⁷Cs transferred into fruit is different.

Open image in new window

Open image in new window

Radiocesium was detected mainly in organs exposed to the atmosphere such as bark, because fruits trees are perennial plants and grew in the fields even at the time of the accident (in winter) (Takata et al. 2012a, d). Of course, fruit also absorb radiocesium from the soil through their roots, but it has already been confirmed that when considering the source of Cs in fruit, the ratio of radiocesium due to the fallout that adhered to the bark and was transferred to fruits is much higher than absorption through the roots. Therefore, it is important to grasp the transition of the concentration of radiocesium not only in the soil but also in the trees to understand the concentration in fruits. As stated in an earlier section, the concentration of radiocesium in peach fruit decreased as they matured and decreased over 3 years after the accident. It is important to understand the amount of radiocesium in trees to consider the cause of rapid decrease of the concentration over the 3 years after the accident; there is no report which investigated the amount of radiocesium in trees year-over-year. We introduce a study (Takata et al. 2016) which dug up trees after leaf fall to investigate the distribution of ¹³⁷Cs and compare the dynamic of ¹³⁷Cs in trees with that of ⁴⁰K.

We used another variety of peach “Yuzora” planted in the fruit research center of Fukushima Agricultural Technology Center. The individual parts of peach trees of a similar age and planted in the same year were separated for the investigation from 2012 to 2014. Stems and branches were collected, and the branches were sorted according to their age. The roots were collected and sorted according to their thickness. The concentration of ¹³⁷Cs and ⁴⁰K in each part were measured and calculated using the dry mass of each part to convert the measured values into the quantities (expressed by Bq) of each organ.

Figure 9.3 shows that the weight of the trees increased year-over-year. The concentration of ¹³⁷Cs in the soil was the highest in the surface horizon (0–3 cm) and decreased in the deeper horizons. This result corresponded to the investigation in orchards in Fukushima after the nuclear power plant accident (Takata et al. 2012c). On the other hand, the concentration of ⁴⁰K in the soil was almost the same across all horizons depths. Figures 9.4a and 9.5a shows that the amount of ¹³⁷Cs and ⁴⁰K, respectively, in each organ calculated by multiplying the weight of each organ and the concentration of the radionuclides together; Figs. 9.4b and 9.5b shows the distribution which converts the whole amount of the radionuclides during each crop period into 100. The distribution of ¹³⁷Cs in trees after 10 months, which is equivalent to one crop period was almost similar with what was reported for peach trees from Tokyo 5 months after the accident (Takata et al. 2012d). The percentage of total radiocesium in fruits and leaves in peach trees from Tokyo 5 months after the accident was about 20% (Takata et al. 2012d), while that value was 13% in peach trees from Fukushima 10 months after the accident in the present study (Fig. 9.4b). Though the sites of those two investigations were inconsistent with the amount of radiocesium fallout and the difference in the amount of fallout should have affected the distribution manner within the tree, the difference of the amount of ¹³⁷Cs in fruits and leaves between the fifth and tenth month may have been due to the effect of translocation of Cs. Tagami and Uchida (2016) suggested that ¹³⁷Cs in leaves is translocated to the tree body just before leaf fall in a study on cherry trees, which belongs to the same plant family as peach trees. Therefore, it is considered that cesium in leaves is translocated to the tree body just before leaf fall also in peach trees.

Open image in new window

Open image in new window

Open image in new window

Figure 9.4a shows that the total amount of ¹³⁷Cs in trees was highest after 1 year and decreased in subsequent years. When looking at the year-over-year transition of each organ, ¹³⁷Cs in five and seven-year-old branches and stems decreased. The five-year-old branches were the youngest branches which received fallout, and the seven-year-old branches were the main branches at the time of the accident. There was no year-over-year transition of the distribution of ¹³⁷Cs in organs which did not receive the fallout: that includes branches younger than 4 years old, fruit and leaves (Fig. 9.4b), though the percentage of ¹³⁷Cs which was transferred to fruit and leaves was 13.1% in the first year and decreased to 7.5% and 3.9% over the subsequent years. On the other hand, the distribution of ¹³⁷Cs increased year-over-year in the four-year-old branches, which developed in the year of the accident. This could be because branches accumulated radiocesium as they grew, while new leaves and fruits were produced every year. Also, it is considered that the source of Cs in new-grown organs were the stems, five-year-old and seven-year-old branches which decreased the amount of ¹³⁷Cs in them. However, five-year-old branches were pruned during winter and the cause of the decrease could be either the pruning or redistribution. Therefore, it is estimated that the source of ¹³⁷Cs in new-grown organs were the stems and seven-year-old branches, which were not pruned.

Although there was no general tendency in the transition of the amount of ¹³⁷Cs in the roots; the roots thinner than 5 mm and rootlets had higher amounts of ¹³⁷Cs year- over-year (Fig. 9.4c). Two reasons can be considered to explain this phenomenon: (1) the absorption from the soil and (2) translocation from thick roots and terrestrial organs. However, as explained in an earlier section, radiocesium existed mainly in the surface horizon, especially to a depth of 5 cm rather than the lower horizons. Because the main root area of peach trees is deeper than the horizon, it is unrealistic that ¹³⁷Cs can be absorbed by the roots. Therefore, it is considered that translocation from thicker roots contributes to the increase in the amount of ¹³⁷Cs in medium and fine roots. The amount of ¹³⁷Cs in the trees decreased more than the estimated quantity removed by fruit, leaves and pruned branches (Fig. 9.3a). It is necessary to consider other possibilities to explain this result such as ¹³⁷Cs exudation from the surface of branches or roots and decay of rootlets. As mentioned earlier, a proportion of ¹³⁷Cs is translocated to newly grown fine roots, and these new roots often decay in a single season. Therefore, ¹³⁷Cs transferred to the soil by falling out or decay is not possibly negligible.

On the other hand, the total ⁴⁰K in trees was smallest in the first year and increased with years; the increase in the fruit and leaves were considerable (Fig. 9.5). This result is closely related to the increase of weight of the trees (Fig. 9.3), and completely opposite to the year-over-year transition of ¹³⁷Cs which was mentioned earlier.

During the 4 years of the investigation after the accident, it was found that the absorption of radiocesium from soil was much smaller than derived from the tree. The source of ¹³⁷Cs in the fruits could be the stems and main branches, and the results show that ¹³⁷Cs transferred from the stems or branches to fruits was not supplemented by the soil. On the other hand, ⁴⁰K is transported from the soil through the stems and main branches. It suggests that the dynamics of Cs and K in fruits trees is different.

It is important to investigate the movement of ¹³⁷Cs in trees, but its concentration will become too low to investigate like ¹³⁴Cs when it reaches its half-life in the future. The dynamics of Cs need to be clarified in case a similar nuclear accident was to occur elsewhere in the world in the future. Although the dynamics of K, which is in the same group as Cs, is sometimes compared to understand the dynamic of Cs, it has been found that those dynamics are partially different in fruit trees. Therefore, the difference in the distribution of Cs and K in soil, absorption through roots, the transition from roots to terrestrial parts, and the transition rate from old to new organs can be obstacles to understanding the dynamics of Cs. Though comparing stable isotope ¹³³Cs is another method, there are several problems to its use as an alternative to ¹³⁷Cs. The analysis using high precision equipment is necessary to measure ¹³³Cs in the natural environment though it is more abundant than ¹³⁷Cs. Even if ¹³³Cs is added to the environment to make the measurement easier, its dynamics and concentration may be different than ¹³⁷Cs. Also, even if ¹³³Cs was not added, the natural ¹³³Cs may show a different dynamic from ¹³⁷Cs because the natural ¹³³Cs concentration is much higher than ¹³⁷Cs. A simultaneous measurement of ¹³³Cs and ¹³⁷Cs is essential to solve these problems and it should be performed before the concentration of ¹³⁷Cs becomes too low to measure. Now we found several results on the similarity and difference of dynamics and abundance between ¹³³Cs and ¹³⁷Cs. For example, the transfer factor ratio of ¹³³Cs and ¹³⁷Cs to fruit is likely to be similar when they are applied at the same time to blueberries. Other experiments such as spraying with ¹³³Cs have been performed together with investigations using ¹³⁷Cs. Also, the difference of dynamics of Cs which stored in terrestrial and subterranean tree parts has been analyzed.