A REVIEW OF 137Cs TRANSFER TO FUNGI AND CONSEQUENCES FOR MODELLING ENVIRONMENTAL TRANSFER

GILLETT*, A.G. and CROUT, N.M.J

Environmental Science Division, School of Biological Sciences, University of Nottingham, LE12 5RD, UK

* To whom all correspondence should be addressed.

Email: andy.gillett@nottingham.ac.uk

ABSTRACT 1

A review of the published literature describing 137Cs transfer to fungi was carried out, 2

summarising the collated data to determine factors controlling transfer and identify an 3

appropriate modelling approach to predict future contamination. 4 137Cs transfer ratios (TR) are derived for fungi species collected within Europe and the CIS. 5

Considerable variability in TRs is demonstrated, with TRs varying between 10 m2 kg-1 across all species and over three orders of magnitude for individual species (e.g. 7

Boletus badius). Generally, meta-information (such as habitat and soil attributes) is poorly 8

reported in the literature so that classification of the TR is limited to the effect of nutritional type 9

(P saprophytic parasitic. Analysis of the literature data set 10

(a heterogeneous source) suggests that there is no statistical evidence to indicate a decrease in 11

TRs for 10 years after the Chernobyl accident. 12

Spatial analysis of a data set for Belgium indicates variability in 137Cs transfer within a 13

sampling location, such that fruitbodies collected over a scale of approximately 5km would show 14

activities as variable as those collected over a much larger scale ( or > 50km). Therefore, it is 15

proposed that the collated data sets for individual species can be used to derive best estimates 16

for the parameters describing the distribution of TRs. These can then be used to estimate an 17

effective TR, which, when combined with local soil deposition level and frequency and effect of 18

culinary practices, can give an estimate of the activity of fungi consumed by the general 19

population. 20

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INTRODUCTION 25

The importance of the consumption of fungal fruitbodies (i.e. sporocarps) by some animal 26

species, such as roe deer and sheep, as a source of 137Cs intake has been discussed by numerous 27

authors (Hove et al., 1990; Johanson et al., 1994 and Kiefer et al., 1996). The intake of fungi 28

(term used by the authors to indicate fungi sporocarps in the subsequent text) by humans has 29

been shown to be a major factor in autumnal increases of radiocaesium activity of rural 30

populations in Russia (Skuterud et al., 1997a). Urban populations have also been found to have 31

significant radiocaesium intake due to fungi (Mehli and Strand, 1998). Ban-nai et al. (1997) 32

estimated fungi consumption could account for 32% (6 Bq year-1) of the total annual dietary 33

intake of radiocaesium within Japan. Higher potential annual intakes of 137Cs (based on the 34

measured daily dietary activities of potato, vegetable, beef, milk and cranberry collected between 35

September and October 1994) of 4380 Bq per person (adult males) have been calculated for the 36

Chernobyl affected Rovno and Volynsky regions of the Ukraine (Shiraishi et al., 1997). Shutov 37

et al. (1996) estimated fungi could contribute up to 60-70 % of dietary 137Cs intake of those 38

adults collecting fungi and berries from forests within Russia. 39

Although, fungal sporocarps may only account for 0.5% of the overall inventory of radiocaesium 40

(ignoring the fungal mycelium) within a forest ecosystem (Seminat, 1998) their high 41

contamination compared to other plant species (Bakken and Olsen, 1990), long ecological half-42

life (Jacob and Likhtarev, 1996) and dietary importance in some populations, especially within 43

the CIS (Skuterud et al., 1997a), requires their attention in models estimating dose to human 44

populations (Howard and Howard, 1996). 45

Fungi fruitbodies have been known to have high activity concentrations of 137Cs relative to 46

higher agricultural plants (Tsukada et al., 1998; Bakken and Olsen, 1990) since the 1960s and 47

1970s (Kiefer et al., 1965; Haselwandter et al., 1988) and elevated contamination levels have 48

been measured worldwide (e.g. Elstner et al., 1987; Horyna and Randa, 1988; Teherani, 1988; 49

Gaso et al., 1996; Garner and Jenkins, 1991; Sugiyama et al., 1994 and Yoshida et al., 1994). 50

Observed contamination levels of 137Cs, even within the same species, show both high spatial 51

and temporal variability (Fraiture, 1992). Several factors have been implicated : mycelium 52

habitat and depth (Giovani et al., 1990; Guillitte et al., 1994; Rhm et al., 1997); forest type-53

fruitbody location (Andolina and Guillitte, 1990; Fraiture, 1992); sampling strategy (Andolina 54

and Guillitte, 1990); soil clay content (Fraiture et al., 1990); pH (Bakken and Olsen, 1990); soil 55

moisture and/or microclimate (Tsvetnova and Shcheglov, 1994; Jacob and Likhtarev, 1996). 56

It is not presently possible to estimate generic effective ecological half-lives across fungi species 57

because species with superficial mycelium (Collybia and Clitocybe sp.) will attain highest 58

contamination within a few months of fallout whilst other deeper penetrating species (such as 59

Boletus edulis) will achieve contamination peaks several years after deposition (Fraiture et al., 60

1990). Therefore, ecological half-lives can be deduced but may be site-specific and will be 61

closely controlled by forest-type and litterfall (due to the effects on the weathering and recycling 62

of radionuclides), soil properties and seasonal fluctuations in microclimate (Rhm et al., 1998). 63

Amundsen et al. (1996) observed ecological half-lives for transfer factors of between 2 and 6 64

years in Norway for different fungi species (though standard errors were up to 8 years) by 65

sampling soil to a 5 cm depth, whilst Rhm et al. (1998) derived ecological half-lives of between 66

2.8 and 7.7 years for the different horizons within a Bavarian forest utilised by the mycelia of 67

different species. Conversely, using Russian data Jacob and Likhtarev (1996) found no 68

significant time dependency in 137Cs transfer. It is apparent further detailed study is required to 69

clarify any time dependency. 70

Information on the spatial scale over which mushroom contamination varies is generally lacking 71

from the literature with some notable exceptions (Dahlberg et al., 1997). This is a serious gap in 72

knowledge from a modelling perspective because if most of the variation occurs over very small 73

scales (i.e. metres) it will be difficult to predict differences in uptake. The objective of this paper 74

is to review and summarise the data collated for radiocaesium transfer to fungi and to identify an 75

appropriate modelling approach to predict food chain contamination. 76

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SOURCES OF DATA 78

Two data sets have been used in the analysis : a survey of the published literature and a large 79

scale study carried out in 1986 and 1987 in Belgium (Fraiture et al., 1989). The data and 80

methodology are described below. 81

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Literature data set 83

A general review of the (primarily) post Chernobyl literature on radiocaesium (137Cs) transfer 84

from soil to fungi fruitbodies has been carried out for the period 1986-1997. Transfer has 85

generally been summarised in the literature as the aggregated Transfer Coefficient commonly 86

referred to as the Tag (Skuterud et al., 1997b) or occasionally as the ATC (Gaso et al., 1996). 87

This is defined as the ratio between fungi activity (at time t) and the initial deposit of 88

radiocaesium (at time t=0, assumed to occur at 1st May 1986). Consequently, the variation in 89

Tags over a period of time (as in this analysis) will include a systematic bias due to the physical 90

decay of 137Cs. To account for this, in this paper, the initial soil deposit has been decay corrected 91

(to the time of fungi sampling) and we shall term the ratio used as the Transfer Ratio or TR 92

(defined as the ratio of fungi activity to soil deposit, both at time t). In practice, the difference 93

between the two transfer terms (TR and Tag) will be relatively small compared to variability that 94

is generally reported within and between species due to other factors. 95

A total of 558 TRs have been found from the 27 literature sources shown in Table 1 (referred to 96

in this paper as the NU97 data set) comprising samples collected from at least 13 countries 97

within Europe and the CIS at 95 different sites. The number of TRs observed for each country 98

was as follows : Ukraine (91); Germany (87); Denmark (54); Italy (47); Finland (45); Sweden 99

(43); Poland (42); Croatia (36); Austria (35); Czech Republic (32); Russia (20); Norway (15); 100

Slovenia (6) and unspecified (5). The largest number of TRs observed at one site (for a number 101

of species) is 54 at Tisvilde Hegn (Denmark), only 15 sites had recorded > 10 TRs. It should be 102

stressed that this review generally uses TRs as summarised by the authors (i.e. arithmetic mean) 103

and, therefore, does not represent the entire population of individual TRs which will consist of 104

many thousands. 105

The TR values have either been direct