Radiocesium concentrations in wild mushrooms after the accident at the Fukushima Daiichi Nuclear Power Station: Follow-up study in Kawauchi village

Since the accident at the Chernobyl Nuclear Power Plant, it has become well known that radiocesium tends to concentrate in wild mushrooms. During the recovery process after the accident at the Fukushima Daiichi Nuclear Power Station (FDNPS), it is important to perform follow-up measurements of the activity concentrations of radiocesium in mushrooms. We evaluated the activity concentrations of the detected artificial radionuclides (radiocesium) in wild mushrooms collected from Kawauchi village, which is within 30 km of the FDNPS, in 2015, four years after the accident. We found that the radiocesium was determined in 147 of 159 mushroom samples (92.4%). Based on the average mushroom consumption of Japanese citizens (6.28 kg per year), we calculated committed effective doses ranging from <0.001 to 0.6 mSv. Although committed effective doses are relatively limited, even if residents have consumed mushrooms several times, continuous monitoring of the radiocesium in mushrooms in Fukushima is needed for sustained recovery from the nuclear disaster.

artificial radionuclides (radiocesium) in wild mushrooms collected in Kawauchi village (Fig. 1), located within 30 km of the FDNPS, and we found radiocesium activity concentrations exceeding 100 Bq/kg (the current regulatory limit for radiocesium for general foods) in 125 of 154 mushrooms (81.2%) collected in 2013, two years after the accident 15 . During the recovery process following the accident at the FDNPS, it is important to perform follow-up measurements of the radiocesium in mushrooms, to monitor the dynamics of radiocesium in the environment, and to minimize the internal radiation exposure of residents of Fukushima through the consumption of contaminated foods. In this study, we evaluated the radiocesium in wild mushrooms in Kawauchi village collected in 2014 and 2015, three and four years after the accident.

Results
The activity concentrations of radiocesium [the sum of individual activity concentrations ( 134 Cs + 137 Cs)] in mushrooms collected in 2015 are summarized in Fig. 2. The activity concentration of radiocesium was detected in 147 of 159 mushroom samples (92.5%). Among them, less than 99 Bq/kg of radiocesium was detected in 24 mushroom samples (15.1%), 100-999 Bq/kg was detected in 80 mushroom samples (50.3%), and more than 1,000 Bq/kg was detected in 43 mushroom samples (27.0%). On the other hand, radiocesium was not detected in 12 mushroom samples (7.5%).
The activity concentrations of radiocesium [the individual activities of 134 Cs and 137 Cs and the sum of individual activity concentrations ( 134 Cs + 137 Cs)] in each species of mushroom samples collected in 2015 are summarized in Table 1   ScIeNtIfIc REPORTS | 7: 6744 | DOI:10.1038/s41598-017-05963-0 aspratus. Radiocesium was not detected in Lyophyllum decastes (n = 4), Pholota squarrosa (n = 2), Lyophyllum shimeji (n = 1), and Grifola frondosa (n = 1) In 2014, only 81 Sarcodon aspratus were collected, and in all of them activity concentration of radiocesium was detected. The maximum activity concentration of 134 Cs in these samples was 1,500 Bq/kg, and the median and minimum activity concentrations were 230 and 30 Bq/kg, respectively; the maximum activity concentration of 137 Cs was 4,500 Bq/kg, and the median and minimum activity concentrations were 740 and 100 Bq/kg, respectively. We compared the activity concentrations of radiocesium in the Sarcodon aspratus sampling collection between 2014 (n = 81) and 2015 (n = 68) to evaluate the trend of radiocesium concentrations in the same species (Fig. 3), the concentrations of 134 Cs in 2015 were significantly lower than those in 2014 (p = 0.002), whereas there was no difference in the concentrations of 137 Cs between 2014 and 2015 (p = 0.45).
Next, we mapped the distribution of mushrooms with radiocesium collected in 2015 (Fig. 4). No clear relationship was observed between sampling spots and cesium concentrations. Finally, we calculated committed effective doses, as shown in Table 2. Among the 147 mushroom samples collected in 2015 that contained detectable activity concentrations of radiocesium, we calculated committed effective doses ranging from <0.001 to 0.6 mSv.

Discussion
Recently, we evaluated the radiocesium in wild mushroom samples collected in Kawauchi village and found that radiocesium exceeding 100 Bq/kg was detected in 125 of 154 mushroom samples (81.2%) collected in 2013, two years after the FDNPS accident 15 . In our current study, radiocesium concentrations were detected in 123 of the 159 mushroom samples (77.4%) collected in 2015, four years after the accident. These results suggest that the portion of mushroom samples with radiocesium concentrations above 100 Bq/kg did not dramatically change in Fukushima over two years. In accordance with the shorter half-life of 134 Cs, the activity concentrations of 134 Cs (half-life = 2.1 years) in 2015 were significantly lower than those found in 2014, whereas there was no difference in the activity concentrations of 137 Cs (half-life = 30.1 years) between 2014 and 2015. Although the residential houses have been extensively decontaminated since the accident, the forests of Fukushima Prefecture have not been decontaminated yet 15 . As our results suggest that it takes time to observe a decrease in the radiocesium in wild mushrooms, careful discussion will be needed among stakeholders to determine the necessity of decontaminating the forests in Fukushima.
On the other hand, we calculated committed effective doses ranging from <0.001 to 0.6 mSv based on the average annual intake of mushrooms by Japanese citizens. Previously, we calculated effective doses ranging from 0.1-1.60 mSv in 2013 15 . These results suggest that internal radiation exposure due to the consumption of wild mushrooms remained relatively limited in Kawauchi village. We have evaluated the activity concentrations of n *1 134 Cs Median (minimummaximum) (Bq/kg)  16 ; this confirms that the internal radiation doses are acceptably low compared to the public dose limit of 1 mSv/year 16,17 . Although residents who returned to the village may have a higher chance of consuming locally produced foods, it does not increase the meaningful of internal doses. Kawauchi village is the first local authority to return to its hometown following the evacuation after the accident 18 . Currently, almost 68% of residents returned to the village and restarted their lives. Before the accident, the village was famous for its wild mushrooms, including Sarcodon aspratus and Tricholoma matsutake. Because the collection and consumption of wild mushrooms is a part of the culture of this village, residents are keenly interested in the radiocesium in the wild mushrooms. In 2013, we began collaborating with residents to prepare  a "mushroom map" that includes information about radiocesium concentrations in the mushrooms collected in the village 11 . During the recovery phase from the nuclear disaster, the engagement of stakeholders, including residents, local authorities, and scientists, is important to deciding the future direction of the community. The International Commission on Radiation Protection (ICRP) emphasizes that stakeholder engagement is key to the development and implementation of radiological protection strategies for most existing exposure situations, and as experience in stakeholder engagement has grown, it has become possible to use many of the lessons learned as a basis for the development of best practices among the radiation protection community 19 . We believe our collaboration in Kawauchi village will contribute to the development of such practices.
Our study has several limitations. We could not evaluate the relationship between radiocesium activity concentrations in mushroom samples and the concentrations in soil due to insufficient soil samples. Further comprehensive studies are necessary to evaluate the activity concentrations of radiocesium in mushroom samples in Fukushima after the accident. Additional analytical uncertainties arise because the committed effective doses from dietary intake of mushrooms cannot measure day-to-day variations in individuals. Further, we did not evaluate the potential loss of radiocesium upon cooking in mushrooms. The influence of eating habits, including cooking methods, must be considered. In this study, we evaluated internal doses from the ingestion of mushrooms, but we did not evaluate the external doses received from being in contaminated forest areas while collecting mushrooms. Further comprehensive analyses with detailed reports on all areas around FDNPP are needed.
In conclusion, we evaluated the activity concentrations of radiocesium in wild mushrooms in Kawauchi village collected in 2014 and 2015, three and four years after the accident, and we confirmed that radiocesium was still detected in most samples. We explained our current results to the residents in the village. Although committed effective doses are relatively limited, we believe that continuous monitoring of the activity concentrations of radiocesium in mushrooms and risk communication with residents in Fukushima is needed for sustained recovery from the nuclear disaster.

Materials and Methods
Sampling of mushrooms. All wild mushrooms were collected in Kawauchi Village (the public office, N37°  in December 2011 18 . The Evacuation Order in its entirety (within a 20 km radius of the FDNPP) was lifted in June 2016, at which time all of residents in the village could return to their hometowns. Mushrooms have been sampled every year during mushroom season (summer-autumn) since 2013. We asked residents of the village to collect mushrooms and to show the location of each mushroom. We collected 81 Sarcodon aspratus mushroom samples from September to November of 2014, and 159 mushroom samples of 23 species from September to November of 2015. Among the 159 mushroom samples collected in 2015, 68 (43.0%) were Sarcodon aspratus, 17 (10.7%) were Hygrophorus russula, and 10 (6.3%) were Albatrellus confluens ( Table 1). The variety of collected mushrooms is quite wide, but some of these are not well known or easy to find, so they were not included in the sampling as they are less likely to be collected for ingestion. All mushroom samples collected were classified according to type into their typical categories of saprophytic or symbiotic, as shown in Table 1.
After collection, all samples were washed by water to remove the soil, and broken into smaller pieces using a mixer machine. The samples, approximately 41 g each wet weight (median), were enclosed in 100-mL plastic containers made of polypropylene (inner diameter, 50 mm; height, 14-42 mm) for radionuclide measurements.
All samples were measured fresh and analyzed with a high-purity germanium detector (ORTEC ® , GMX30-70, Ortec International Inc., Oak Ridge, TN, USA) coupled with a multi-channel analyzer (MCA7600, Seiko EG&G Co., Ltd., Chiba, Japan) for 3,600 s. The measuring time was set to detect the objective radionuclide, and the gamma-ray peaks used for the measurements were 604.66 keV for 134 Cs (2.1 y) and 661.64 keV for 137 Cs (30 y). Decay corrections were made based on the sampling date, and detector efficiency calibration was performed for different measurement geometries using mixed-activity standard volume sources (Japan Radioisotope Association, Tokyo, Japan). Activity concentrations of radiocesium were automatically adjusted based on the date of collection, and the data were defined as the activity concentrations at the collection date. The relative efficiency was 31%, and energy resolution of the spectrometer was 1.85 keV for 60 Co. The detection limit was 11.5 Bq/kg for 134 Cs and 9.2 Bq/kg for 137 Cs (median), and counting errors were ±7.3 Bq/kg for 134 Cs and ±14.1 Bq/kg for 137 Cs, respectively. Sample collection, processing, and analysis were executed in accordance with standard methods of radioactivity measurement authorized by the Ministry of Education, Culture, Sports, Science, and Technology, Japan 21 . All measurements were performed at Nagasaki University (Nagasaki, Japan). The sum of 134 Cs and 137 Cs concentrations was indicated as "activity concentration of radiocesium" in order to aid comparison with the current regulatory limit for radiocesium (100 Bq/kg for general foods), which is determined by the central governments of Japan. In most samples, mushrooms contained 134 Cs and 137 Cs. However, in some samples, only 137 Cs was detected, because 134 Cs concentrations were below the detection limit. For such samples, concentrations of 137 Cs were indicated as "concentrations of radiocesium. " Committed effective doses. The committed effective dose from 137 Cs and 134 Cs due to mushroom consumption was calculated using the following formula: where C is the activity concentration of the detected artificial radionuclide (radiocesium) (Bq/kg), D int is the dose conversion coefficient for adult intake (age 20 and older, 1.9 × 10 −5 mSv/Bq for 134 Cs and 1.3 × 10 −5 mSv/Bq for 137 Cs), and e is the estimated value of annual intake from the latest statistical data issued by the Ministry of Health, Labour, and Welfare, Japan in 2015 22,23 . From this report, annual intakes of mushrooms were estimated at 6.28 kg/ year in Japanese citizens, which is based on mean values of adults ( > 20 y). We assumed that annual intake could be attributed to each species.

Statistical analysis.
Values of activity concentrations of radiocesium are presented as median, minimum and maximum. The number of samples of each type of mushroom are listed in Table 1. Analysis of variance (ANOVA) was performed to compare radiocesium activity concentrations in mushroom samples collected in 2014 with radiocesium activity concentrations in mushrooms collected in 2015. Probability values less than 0.05 were considered statistically significant. All statistical analysis was performed using SPSS statistics 22.0 (SPSS Japan, Tokyo, Japan).