TRANSFER TO MUSHROOMS

Mushrooms are an important foodstuff in Japan. The total annual production of edible mushrooms is 457 000 ton/year [5.103] and the mean mushroom consumption for adults is 17.0 g/d [5.104].The collection of wild mushrooms is less important than the production of cultivated mushrooms (e.g. the annual collection of Tricholoma matsutake, the most familiar wild mushroom species in Japan, was 69 ton in 2016 [5.105]). However, wild mushrooms are more important in mountainous areas of Japan, where collecting mushrooms is an important recreational activity for local residents [5.106, 5.107].

After the FDNPP accident, radiocaesium was measured in wild mushrooms in many areas across eastern Japan. The Japanese government restricted commercial shipping of foods with radiocaesium activity concentrations exceeding 100 Bq/kg (usually FM basis). As a result of food monitoring surveys, shipment of wild mushrooms was restricted in 110 municipalities of 10 prefectures (26 June 2019) [5.108]. The restriction is now partially lifted in 12 municipalities for specific mushroom species. As part of these restrictions, mushroom shipments were restricted on a regional basis without distinguishing between species.

Radionuclide transfer from soil to mushrooms is quantified using aggregated transfer factor (𝑇_ag) values according to genus and species. 𝑇_ag values derived before the FDNPP accident are compiled in the IAEA TRS 472 [5.30] and a summary of this data is published elsewhere [5.109]. After the FDNPP accident, the 𝑇_ag values for wild mushrooms in Japan have been reported in a subsequent study [5.110] (Table 5.7).

Another approach for 𝑇_ag calculations was based on environmental monitoring data organised by prefectural governments. Even though site specific data on contamination of the soil at the sampling sites are not available, normalized concentrations (NC) for mushrooms could be calculated using estimates of the contamination obtained from airborne surveys (Table 5.8 and Appendix III). For the NC calculation, the geometric mean of radiocaesium deposition was calculated for each municipality and decay corrected to the date of the FDNPP accident (11 March 2011).

TABLE 5.7. Aggregated transfer factor 𝑇_AG (M²/KG DM) FOR ¹³⁷CS IN WILD

MUSHROOMS AT KAWAUCHI VILLAGE, 20–30KM FROM THE FDNPP, IN 2012 [5.110].

Mushrooms group GM Mean GSD ^a SD Min Max

Mycorrhizal fungal species 0.24 0.65 1.7 0.77 0.01 3.0

Saprotrophic fungal species 0.10 0.24 1.1 0.49 0.03 1.8

a unitless

FIG. 5.22. Comparison of reported Tag values in Europe after the Chernobyl accident [5.109] and NC values in Japan after the Fukushima accident derived from public data monitored by the local governments. Results for common species in the two studies were plotted.

Since the NCvalues were originally calculated on a fresh mass basis (m²/kg FM), the data given in Table 5.8 were converted to dry mass basis (m²/kg DM) using dry/wet ratios of mushroom species from published articles [5.111–5.120]. The NC values varied by about 100-fold between species. Most of the mycorrhizal species had higher NCvaluesthan other types of mushrooms, with the exception of some saprotrophic fungal species, which also had high NC values (e.g.

Pholiota lubrica,Table III.4). The NC values obtained in Fukushima studies were compared with the Tag values reported in Europe for seven common species [5.109] (Table 5.9 and Fig.

5.22). Although the number of species was small and the slope of the regression line is 0.28, the comparison indicated that the 𝑇_ag values of radiocaesium in the same species measured in Europe and Japan are within the same order of magnitude.

In Japan, sawdust mushroom cultivation is more popular for mushroom production than bed-log cultivation (Fig. 5.23). The CR value for shiitake mushrooms (Lentinula edodes) of 0.18 calculated using 64 samples with sawdust cultivation is relatively low (GSD 1.6 and range 0.062–0.63; Bq/kg FM mushroom to Bq/kg DM substrate). The radiocaesium content of such cultivated mushrooms can be further minimized by diluting the growing medium with uncontaminated sawdust.

158

TABLE 5.8. NORMALISED CONCENTRATION NC (m²/kg DM) FOR ¹³⁷CS IN WILD MUSHROOMS

TABLE 5.9. COMPARISON OF REPORTED 𝑇_AG VALUES IN EUROPE [5.36], [5.109]) AND NC VALUES DERIVED FROM THE FUKUSHIMA STUDIES

Species D/F ratio ^a N GM (dry)

a D/F ratio: dry to fresh mass ratio (kg/kg) summarized from published articles. Mean values by genus were used. Since a ratio for Kuehneromyces mutabilis was not obtained, the mean ratio of all data excluding polypore species (with high D/F ratio) was used

b no data

FIG. 5.23. Cultivated shiitake mushrooms (Lentinula edodes). Left: sawdust cultivation (Photo: Hiromi Mukai), Right: bed-log cultivation (Photo: Japan Non-Wood Forest Products Promotion Association).

The annual production in Japan of shiitake mushrooms using bed-logs (usually oak trees) is 25 000 ton/year [5.105]. Bed-log cultivation is normally performed outdoors on the forest floor using wood logs taken from nearby forests. The geometric mean of the CR values for bed-log mushrooms of 0.43 is higher than that for sawdust cultivation (n = 48, GSD = 2.49, min = 0.037, max = 2.8, [5.121]). Consequently, bed-log shiitake mushrooms with radiocaesium activity concentrations exceeding regulatory limit values were reported in many municipalities during the first few years after the FDNPP accident. As part of the remediation strategy, the Japanese government provided guidelines for safe production such as restricting the use of contaminated logs and outdoor cultivation. The number of municipalities with shipping restrictions for bed-log shiitake mushrooms has decreased with time [5.122].

Overall, the ratios between ¹³⁷Cs uptake of different mushroom species observed in the areas affected by the FDNPP accident are similar to those observed for the same mushroom species obtained after the Chernobyl accident. Some differences may be explained by the different time periods for which the CR values were derived. Monitoring for the Chernobyl data began after a longer time period following the release of radionuclides to the environment compared to data obtained for Fukushima.