Letters
Temporal change of Cs-137 concentration in fruit in the non-decontaminated biotope in Saitama, Japan, after the Fukushima Daiichi Nuclear Power Plant accident
2024 Volume 4 Pages 95-98
Details
2024 Volume 4 Pages 95-98
We measured Cs-137 concentrations in persimmon and yuzu (citron) fruits within a non-decontaminated biotope in Saitama Prefecture, which was affected by the Fukushima Daiichi Nuclear Power Plant (FDNPP) incident in 2011, spanning from 2011 to 2019. In 2011, the Cs-137 concentration in yuzus was five times higher than that in persimmons. In 2019, the Cs-137 concentrations in the two fruits were almost the same. Additionally, these values were nearly equal to the yuzus collected in Fukushima Prefecture before the FDNPP accident occurred. The effective half-lives of Cs-137 calculated based on the survey results were 380 days and 310 days for persimmons and yuzus, respectively.
The findings indicated that the Cs-137 concentration in fruits in non-decontaminated areas decreased faster than predicted by the physical half-life of Cs-137, which is 30 years.
The accident at the Fukushima Daiichi Nuclear Power Plant (FDNPP) caused by the Great East Japan Earthquake in March 2011 resulted in the release and diffusion of radioactive materials into the environment. Saitama Prefecture is an inland prefecture adjacent to the northern part of Tokyo and 220 km south of Fukushima Prefecture, but the effects of the FDNPP accident have been confirmed in Saitama Prefecture. Air radiation levels measured by thermoluminescence dosimeter at seven locations in Saitama Prefecture were 0.53–0.61 mGy/y in 2010 but increased to 0.67–1.00 mGy/y in 2011 (Miyake et al., 2017).
The Center for Environmental Science in Saitama(CESS), established in 2000 in Kazo City, Saitama Prefecture, has a biotope on its site. The biotope is designed to replicate a landscape typical of the region in the 1950s, containing forests, farmland, reservoirs, and grasslands. Miyake et al. (2018) measured the radioactivity concentrations in plants collected at the biotope, including yuzu (citron) and persimmon, for 2 years after the FDNPP accident; Cs-134 and Cs-137 were detected in all samples. Cs-137 concentrations varied by species. Even for the same fruit, the concentration of Cs-137 in yuzus collected in 2011 was about five times higher than that in persimmons.
Among man-made radioactive materials, Cs-137 has a long physical half-life (30 years), and there is concern that it may remain in the environment for a long period. According to the Ministry of Health, Labour and Welfare, Japan (2024), radioactive Cs concentrations exceeding 100 Bq/kg are occasionally been detected in river fish, wild animals, and wild vegetables, even though more than 10 years have passed since the FDNPP accident. Radioactive materials have been similarly detected for a long period in the case of other nuclear power plant accidents. In Croatia, which was affected by the Chernobyl nuclear power plant accident in 1986, Cs-137 concentrations were 0.4–611.5 Bq/kg in wild boars collected in 2000–2002 (Vilic et al., 2005) and 0.95–1,210 Bq/kg dry in mycorrhizal mushrooms collected in 2012 and 2016 (Tucaković et al., 2018).
In cases of extensive environmental contamination by radionuclides, decontamination efforts are impeded by the expansive contaminated area and access restrictions in heavily affected zones. About 6% of the arable land in Japan is used for growing fruit trees and tea (Ministry of Agriculture, Forestry and Fisheries, Japan, 2023). Therefore, it is important to determine the extent to which the radioactivity concentration in fruit decreases without decontamination. However, to our knowledge, there are no reports of long-term measurements of Cs-137 concentrations in fruits grown in non-decontaminated areas. Therefore, in this study, we measured Cs-137 in fruits in a non-decontaminated biotope from 2011 to 2019 and determined the effective half-lives of Cs-137. The fruits of persimmon and yuzu were studied. Persimmon is a deciduous fruit tree, and yuzu is an evergreen fruit tree.
Fruit samples were collected at the biotope of the CESS as in Miyake et al. (2018). The concentrations of radioactive materials in fallout have been measured monthly in Saitama Prefecture. The Cs-137 concentration in the fallout sample collected in March 2011 was 5,300 MBq/km2, whereas that collected in June 2011 was 18 MBq/km2. The Cs-137 concentration decreased to less than 0.5% during the 3 months. Since we started the sample collection at the biotope in October 2011, it is assumed that most of the radioactive materials derived from the FDNPP accident had fallen to the ground surface before the survey. No decontamination work was conducted in the biotope.
SAMPLINGPersimmon and yuzu fruits grown in the biotope were collected annually from 2011 to 2019, with the exception of 2017. The fruits were washed well with water before pretreatment. Normally, only the flesh portion of the persimmon is consumed, while both the peel and the flesh are consumed for yuzu. Therefore, the flesh portion without the skin and seeds was used to analyze persimmon, and the whole fruit was used for yuzu. As a rule, at least 1 kg of fruit was used for each sample. In years of good harvest, multiple samples were collected.
Soil samples around the fruit trees were also collected from 2011 to 2019. Soil samples were collected from the ground surface to a depth of 20 cm at 3–7 points each year; these were mixed and treated as one sample.
MEASUREMENTSample preparation and measurements were performed according to the Series of Environmental Radioactivity Measuring Methods established by the Ministry of Education, Culture, Sports, Science and Technology, Japan (1982, 1983) and the Nuclear Regulation Authority, Japan (2020). Fruit samples were dried at 105°C and then ashed at 450°C for 24 h. The ash from the fruit samples was ground, passed through a 355 μm mesh sieve, and then filled into a 100 mL polypropylene container (U-8 container). Soil was dried at 105°C and foreign matter such as pebbles and plant fragments was removed. The soil clods were ground, passed through a 2-mm mesh sieve, and filled into a U-8 container.
Gamma-ray spectrometry was performed using a high-purity germanium detector with a wave height analyzer (Mirion Technologies). The measurement time was 79,200–259,200 s. Cs-137 was quantified using the peak at 661.64 keV. The radioactivity concentrations were decay-corrected to the sampling day. Measurements from 2011 to 2013 were based on previously reported values (Miyake et al., 2018).
CALCULATION OF EFFECTIVE HALF-LIFEThe effective half-lives were calculated based on the measured data of persimmons and yuzus. The radioactivity concentration due to radioactive decay over time is expressed by the following equation (1).
| (1) |
Where At is the Cs-137 concentration at the time t; A0 is the Cs-137 concentration in 2011; λ is the effective decay constant of Cs-137, t is the elapsed time since sampling day in 2011. Equation (2) expressed equation (1) as a logarithm.
| (2) |
Using the Cs-137 concentrations in fruit, A first-order approximation line for each fruit was created. The intercept was fixed to LnA0. The value of λ was obtained from the slope of the first-order approximation line. Based on the obtained λ, the effective half-life Teff was calculated by equation (3).
| (3) |
The Cs-137 concentrations in persimmon and yuzu are shown in Figs. 1 and 2, respectively. The concentration of Cs-137 in persimmons collected in 2011 was 2.3 Bq/kg fresh, whereas that collected in 2019 was 0.025–0.058 Bq/kg fresh. The concentration decreased by 97%–99% during this period. The Cs-137 concentration of yuzus collected in 2011 was 12 Bq/kg fresh, whereas that collected in 2019 was 0.032–0.048 Bq/kg fresh. The concentration decreased by more than 99%. The Cs-137 concentrations of persimmons and yuzus collected in the biotope in 2019 were comparable to those of yuzus collected in Fukushima Prefecture between 2005 and 2010 before the FDNPP accident (not detected or 0.04 Bq/kg fresh; JCAC, 2024).
The concentration of Cs-137 in yuzus collected in 2011 was about five times higher than that in persimmons. One possible reason for this is the effect of pretreatment. Because persimmons were analyzed without peeling, the Cs-137 concentration may have been lower in persimmons than in yuzus, which included the peel. However, since the pretreatment of each fruit was the same during this study, there should be no problem in evaluating the temporal variation of each fruit. Another possible reason is the nature of the fruit tree. Yuzu is an evergreen fruit tree that grows leaves throughout the year in contrast to persimmon, a deciduous fruit tree; in March, when the FDNPP accident occurred, persimmons had not yet put on leaves. The radioactive materials from the FDNPP accident that fell on the yuzu trees adhered to and accumulated on the leaves as well as the trunks and branches, suggesting that more radioactive materials were transferred to the fruits of the yuzu trees.
Cs-137 CONCENTRATION IN SOILThe Cs-137 concentrations in soil samples collected between 2011 and 2019 ranged from 17 to 34 Bq/kg dry, with no clear trend observed during this period (Fig. 3). Since the Cs-137 concentrations differed by a factor of up to two, even among soils in the same year, it is assumed that the variation in the measurement results is due to differences in the horizontal distribution. It is known that radioactive Cs is strongly adsorbed on soil particles (Tsukada et al., 2008). In 2019, the Japan Atomic Energy Agency surveyed soil at 85 points within approximately 100 km from the FDNPP and reported that radioactive Cs in soil mostly stayed from the ground surface to a depth of 4.6 cm (JAEA., 2020). Therefore, the concentrations of Cs-137 in the soil did not decrease significantly as they did in the fruit.
Based on the measurement results, we obtained the effective half-lives of Cs-137 in fruits collected in the biotope. The effective half-lives of the persimmon and yuzu fruits were 380 and 310 days, respectively; those are about one-thirtieth of the physical half-life of Cs-137. There are three Cs-137 transfer pathways in agricultural products such as vegetables and fruits (Ohno, 2015): (1) direct deposition from the atmosphere, (2) translocation from contaminated leaves and tree trunks, and (3) root absorption from the soil. A liquid containing radioactive Cs was sprayed on peach trees before germination, and radioactive Cs was detected in the leaves and fruit of the trees (Sato, 2018). Takada et al. (2012) reported that approximately 20% of the radioactive Cs in peach trees, which are deciduous fruit trees similar to persimmon, is transported out of the tree by the fruit and leaves. The amount of Cs-137 transferred from tree to fruit was considered to have decreased from year to year because Cs-137 accumulated in the tree decreased due to the translocation of Cs-137 to the fruit.
Tagami and Uchida (2015) reported that Cs-137 concentrations were measured separately for new branches, leaves, fruit fleshes, seeds, and peels of persimmons collected from 2011 to 2013, and the effective half-lives were calculated to be similar, with an average of 229±11 days. Hence, we do not think it is necessary to consider the effects of peeling persimmons. The effective half-lives in this study were longer than those reported by Tagami and Uchida (2015). Therefore, we recalculated the effective half-lives in this study based on the values obtained in the same period as Tagami and Uchida (2015) and found that the effective half-lives were 220 days and 190 days for the persimmon and yuzu fruits, respectively. These values were almost the same as those for persimmon reported by Tagami and Uchida (2015) −229±11 days. Tagami and Uchida (2015) calculated the effective half-lives using the period during which the decrease in radioactivity concentration is most active; thus, this approach is considered suitable for short-term concentration estimation. However, their approach does not consider long-term concentration changes and may overestimate the concentration decline over longer forecast periods. Our estimated effective half-lives are based on longer-term observations and are expected to reflect the current situation.
We measured Cs-137 concentrations in fruits collected in a non-decontaminated biotope in Saitama from 2011, immediately after the FNDPP accident, to 2019. Cs-137 concentrations of both persimmon and yuzu fruits decreased over time. The Cs-137 concentrations of fruits collected in 2019 were equivalent to those of yuzus collected in Fukushima Prefecture before the FDNPP accident. Effective half-lives calculated based on the Cs-137 concentrations in fruits collected from 2011 to 2019 were almost a year. These times represent one-thirtieth of the physical half-life of Cs-137 (30 years). This indicates that even in non-decontaminated areas, the Cs-137 concentrations in the fruits grown there are decreasing more rapidly than expected. In addition, the effective half-lives obtained in this study contribute to the long-term prediction of Cs-137 concentration in fruit.
The authors thank the staff members of the Center for Environmental Science in Saitama and the Saitama Prefectural Institute of Public Health for supporting this study.
