THE FOOD CHAIN MODULE
and also any influence of crop types and varieties, agri- percentile is close to unity, the 50th percentile values
cultural practices and crop yields. Within the animal are close to 0.4, and the 5th percentile values close to
processes panel, the experts were asked to consider a 0.06. The aggregated resuspension factors give rise to
number of factors which may affect the consumption large uncertainty bands with a range factor of about
rate of feeds by animals and the subsequent transfer of 10,000, with the 50th percentile relatively close to the
activity to the animal products such as the age and 5th percentile. For the retention times, range factors of
weight of animals, milk yields and stages of lactation, the order of 20 are found for all crops. The aggregated
quality of animal feeds, physical form of the deposited data for the concentrations in grain at harvest show
material and soil types. The experts were then allowed similar results for both strontium and caesium with
to use any information sources to assist them in range factors varying from 90 to 500. The smaller range
developing their distributions, such as experimental factors are seen for deposition at 30 days and 60 days
data, published literature and analytical techniques.
before harvest for caesium and at 15 days and 30 days
The experts used a wide range of information, draw- before harvest for strontium. For concentrations in root
ing on an international pool of published data on trans- crops at harvest the range factors are much larger than
fer in terrestrial food chains, their own experimental those seen for grain, with the range factors for strontium
data and personal experience. In general, the experts did significantly smaller than those observed for caesium;
not use models to obtain the uncertainty distributions, the range factors are smallest for deposition occurring
with the exception of those on soil migration. Here, a 60 days and 90 days before harvest for both elements.
number of analytical approaches were adopted based on
For the animals’ consumption rates, many of the
published methods or those developed by the experts experts provided uncertainty estimates based on experi-
based on their own experimental data and field obser- ence in their own countries. Aggregation of these data,
vations. Further details of the rationales adopted by the in most cases, was considered appropriate to give gen-
experts can be found in Reference 3.
eric values for European conditions. In general, the
range factors observed across the range of animals and
feeds were less than 20, varying from a factor of 4 for
the consumption of pasture grass by dairy cows and
RESULTS OF EXPERT JUDGEMENT EXERCISE
This section presents a selection of the uncertainty sheep to 20 for consumption of cereals by pigs. The
distributions obtained from the expert panels. Following range factor on the consumption of soil by animals
the expert elicitation, the 5th, 50th and 95th percentiles varied between 40 and 200, depending on the animal.
of the distributions of each elicited variable were
For the transfer to meat, milk and eggs, the largest
obtained from each expert. For each elicitation variable, range factors were generally observed for transfer to
these distributions were then combined into single dis- lamb, pork and chicken. The experts were least uncer-
tributions, with each expert’s distributions being given tain for caesium with range factors from 20 to 80. The
equal weight(3). The distributions discussed in this paper range factors for strontium were larger, particularly for
are these aggregations of the individual elicited distri- meat, where they varied from 200 for dairy cows to
butions for each elicited parameter. Most of the results 2000 for poultry. The transfer of iodine to eggs and
are discussed in terms of a range factor, which is the sheep milk is also very uncertain with range factors of
ratio between the 95th and 5th percentiles of the aggre- about 1100 and 600, respectively, being observed. For
gated distributions.
the biological half-lives in meat, for the animal species
For soil migration parameters, the aggregated range considered, the aggregated range factors were smallest
factors vary from about 20 to 80, increasing with for caesium, ranging from 10 to 30, and highest for
migration below greater depths, and are smaller for cae- strontium, ranging from 500 to 800. The range for iod-
sium than for strontium, typically by about a factor of ine was 200–500.
2 for generic soils. For fixation in soil, the range factors
A comparison of the aggregated distributions
for caesium and strontium are similar, ranging from 3 obtained from expert elicitation has been made with the
to 60, being largest for organic soils, but are typically default values currently used in COSYMA in the
less than 10 for times greater than 1 year following FARMLAND model(5). Also, where possible, uncer-
deposition. The range factors observed for root uptake tainty distributions used in past uncertainty studies
concentrations for caesium and strontium for all crops carried out with FARMLAND and ECOSYS have
were broadly similar and typically in the range of 50– been made(4)
200. The exceptions were those for caesium uptake by
.
In general, the default FARMLAND values lie fairly
root vegetables, which were between 400 and 4000, close to the 50th percentile of the expert aggregated dis-
depending on the time after deposition. The overall tribution, typically between the 35th and 65th percen-
trend was that the range factors for root uptake of stron- tiles. As an example, Table 3 shows the comparison for
tium were smaller than those for caesium for all crops, the root uptake concentration factors for cereals, where
typically by a factor of 2. The aggregation of data for it can be seen that there is good agreement between the
interception factors results in range factors of about 50th percentile values for a generic soil and the model
10–40 for all crops. In all cases, the aggregated 95th default. Notable exceptions are the transfer of strontium
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