United Nations Scientific Committee on the Effects of Atomic Radiation

ATTACHMENT F-2

TERRESTRIAL ECOSYSTEM APPLICATION OF
ASSESSMENT METHODS AND LIFE HISTORYUNSCEAR 2013 Report, Annex A, Levels and effects of radiation exposure dueto the nuclear accident after the 2011 great east-Japan earthquake and tsunami,
Appendix F (Assessment of doses and effects for non-human biota)

Notes
The designations employed and the presentation of material in this publication do not imply theexpression of any opinion whatsoever on the part of the Secretariat of the United Nations
concerning the legal status of any country, territory, city or area, or of its authorities, orconcerning the delimitation of its frontiers or boundaries.
©

United Nations, August 2014. All rights reserved, worldwide.

This publication has not been formally edited.

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ATTACHMENT F-2: TERRESTRIAL ECOSYSTEM APPLICATION OF ASSESSMENT METHODS AND LIFE HISTORY

Contents
NOTES ......................................................................................................................................... 1
I. INTRODUCTION .................................................................................................................. 2
II. METHODOLOGY FOR THE DETAILED ASSESSMENT AT SITES WHERE
DIRECT MEASUREMENTS IN ANIMALS WERE AVAILABLE ................................... 3
III. EQUILIBRIUM ASSESSMENTS ....................................................................................... 10
IV. SETTING UP THE DYNAMIC MODEL ........................................................................... 12
V. RESULTS............................................................................................................................. 13
REFERENCES ........................................................................................................................... 19

I.

INTRODUCTION

1.
The assessment for the terrestrial system covers three of the main steps outlined in thegeneric methodology (see attachment F-1; Introduction):a. Focus was initially placed upon the locations where direct measurements interrestrial animals were available. Combining a spatial analysis of deposition
density data with cluster analyses of activity concentrations in biota resulted in
the most reliable estimation of dose rates for the selected animals at the time ofsampling;b. An equilibrium assessment for locations where no direct measurements ofactivity concentrations in biota were available was performed. Focus was placedon an area of maximum deposition density to provide an upper percentile (andthus conservative) estimate on exposures of plants and animals in the accidentaffected region at the time of the deposition density measurement. Further
calculations were made to account for the decay of short-lived radionuclides toprovide an estimate for exposures at the time of maximum deposition density.
c. A dynamic assessment was conducted to further explore the time evolution ofconcentration levels in plants and animals. Predictions of 137Cs activity
concentrations in vegetation and deer/herbivorous mammal were made, and thelatter values were compared against empirical datasets for Sika deer. The model
was applied to the area of maximum deposition density (as in b above) to
provide further refinement through insight into the temporal evolution ofconcentration levels at sites of relatively high deposition density.

ATTACHMENT F-2: TERRESTRIAL ECOSYSTEM APPLICATION OF ASSESSMENT METHODS AND LIFE HISTORYII.

3METHODOLOGY FOR THE DETAILED ASSESSMENT AT SITES
WHERE DIRECT MEASUREMENTS IN ANIMALS WEREAVAILABLE2.
The initial stage of the assessment required interpolation of deposition density data
between measurement points. This was required subsequently to predict deposition levels at
locations where biota had been sampled and, where appropriate, to average deposition density
levels over the home range of the animal, thus facilitating the derivation of concomitant
average external dose rates.
(a)Empirical Bayesian Kriging (EBK)

3.
Empirical Bayesian Kriging was selected as the method to calculate the 137Cs and 134Cs
deposition density rasters for the Fukushima area. This method estimates values based onsemivariogram models that are derived from the input data. Empirical Bayesian Kriging uses
Bayes’ rule to estimate the optimum weight for the ‘local’ semivariograms [Krivoruchko,
2011; Gribov & Krivoruchko, 2012; Pilz et al., 2012].
4.
Error estimates were modelled by running the EBK prediction on a randomly split
dataset (95% of the data) and then comparing the modelled value with the measured value
using the remaining 5%. This was repeated 20 times. The resulting error (absolute percentage
difference) is shown as a histogram (figure I).
Figure I. Histogram showing error (absolute % difference) between EBK predicted andmeasured 137Cs deposition density data

5.
The coordinate system for the spatial analysis was WGS84 UTM zone 54n. The
deposition raster for 137Cs is presented in figure II and for 134Cs in figure III.

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ATTACHMENT F-2: TERRESTRIAL ECOSYSTEM APPLICATION OF ASSESSMENT METHODS AND LIFE HISTORY

Figure II. Deposition density of 137Cs (kBq/m2)

ATTACHMENT F-2: TERRESTRIAL ECOSYSTEM APPLICATION OF ASSESSMENT METHODS AND LIFE HISTORY

5

Figure III. Deposition density of 134Cs (kBq/m2)

6.
With regard to data for biota, measured activity concentrations for 131I, 134Cs and 137Cs
in mammals and birds were available. These data sets contained additional information, such as
weight, length, age and sex of measured biota. Table 1 provides an overview of the biota data.
Table 1. List of animal species and concomitant information from datasets provided by
Fukushima Prefecture [2013]
Data
Organism

Latin name

Nr. of data
Weight (kg)Length (cm)

Wild boar

Sus scrofa

153

10–150 (55)*

30–160 (101)*

Asian black bear

Ursus thibetanus

19

50–140 (72)*

100–160 (120)*

Sika deer

Cervus Nippon

10

60–120 (72)*

100–160 (139)*

Common tealAnas crecca

1

1

20

Copper pheasantSymaticus
sommerringii

11

1

90

Grey duckAnas poecilorhyncha

23

1

30–60

Japanese pheasantPhasianus versiclor

35

MallardAnas platyrhynchos

8

1

50

*Average values.

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7.
The first step involved placing all the deposition density and activity concentration dataon maps. The spatial distribution of data on radionuclide activity concentration in animals andbirds was examined using a Geographical Information System (GIS) and this resulted in thefollowing maps for 137Cs deposition density data (figures IV and V).
Figure IV. 137Cs deposition density with sampling locations of mammals and birds
superimposed

Figure V. 137Cs deposition density data with distribution of mammal species superimposed

ATTACHMENT F-2: TERRESTRIAL ECOSYSTEM APPLICATION OF ASSESSMENT METHODS AND LIFE HISTORY(b)

7

Preparation of data for detailed assessment

8.
The plan was to perform a more detailed analysis, preferably using actual radionuclide
concentrations in the biota. Initial categorization of data showed that, in fact, there were only
three species of mammal and five species of bird (see table 1). Most of the mammal data were
for wild boar so there seemed some justification (in view of the limtied number of species
covered and weighting of data towards certain species) to treat each species separately. This
involved the use of the ‘new organism’ function in the ERICA Tool [Brown et al., 2008] andcreation of geometries which correspond to the species under discussion. This, in turn,involved collection of life history data and thereafter extraction of relevant data for theassessment.
(c)Life history data and derivation of relevant parameters

9.
To derive dose conversion coefficients (DCCs), information on mass and dimensions of
the organism is needed. Furthermore, information on habitat is needed and, where applicable,
the fractional occupancy of various organisms in their habitats. This information is important toweight external dose rates in order to account for the behaviour of the organism.
10.
Life history data sheets containing basic ecological information about all organisms
involved in the assessment were collated. In addition to the list of organisms considered,table 1 shows a summary of the collected information used to derive DCCs. The assumed
tissue density for birds was 0.85 g/cm3 [Hamershock et al., 1993] and that of the mammal
species was 1.0 g/cm3. For birds, the assumed mass and dimensions were taken from ICRP andcorrespond to ICRP’s “RAP duck”. The dimensions of mammals were derived from their massand density. The approach consisted of three steps. First, the length of the animal was found by
examining the available data and also consulting literature. Second, a representative picture of
the animal was selected on which width could be measured. Third, the relationship between
mass, volume and density was used to estimate the third dimension. Table 2 summarizes all the
relevant parameters which were used in deriving DCCs.
For wild boar, there were distinct clusters whereas for other mammals and birds, thedata were rather “spread out”. For the wild boar, life history data were examined to define ahome range. This analysis showed that a wild boar’s home range changes because of manyfactors (e.g. food availibility and hunting season). A home range was assumed for further
calculations to be represented by a circular area with a radius of 3 km. Then, a circle wasoverlayed (with radius 3 km, corresponding to the home range) around the midpoint of thecluster (the exact position of which was slightly arbitrarily placed by judgement) and then all
depsoition density data within the circle were selected and used to estimate external dose rates.
11.

(d)

Sample points clustering — option 1

Caesium-134 measurements of wild boar were collected in the Fukushima area. No
information about home ranges of the sampled animals was available; therefore, the clusters ofsample points were identified by their spatial location only. The method used assumed thatpoints within one cluster were within a certain distance of one other. This distance was set to
the nearest observed mean distance for all points in the dataset. Clusters were grown fromrandom seed points in an iterative process. One point could belong to more than one cluster.
12.

105–155

25–110Sika deer (10)Length (cm)

50

30–60

90

20

100–160 (139)*

100–160 (120)*

30–160 (101)*

Data*Average; # “ICRP duck” values.
** For birds, a more conservative occupancy factor would have resulted if ‘in air’ had been changed to either ‘on water’ or ‘on soil’.

Mallard (8)11Grey duck (23)Japanese pheasant (35)11

60–120 (72)*

50–140 (72)*

10–150 (55)*Weight (kg)

Copper pheasant (11)25–80

120–195

Male: 90–150
Female: 65–90

Asian black bear (19)

0.7–1.4

110–150

50–90

Wild boar (153)

Common teal (1)Length (cm)Weight (kg)Organism(no. of samples)

Literature

Table 2. Dimensions and selected life history data for samples mammals and birds

1.26#

1.26#

1.26#

1.26#

1.26#

70

120

55

Weight
(kg)

30#

30#

30#

30#

30#

103

126

80

10#

10#

10#

10#

10#

50

45

40

8#

8#

8#

8#

8#

26

40

33

Dimensions (cm)Assumed

1–3~2.6

Not large

~2.6

~4

~2

~1

~26

Home range(km2)

0.75 on water
0.25 in air

0.75 on ground0.25 in air

0.75 on water
0.25 in air

0.75 on ground0.25 in air

0.75 on water
0.25 in air

1 on ground0.85 on ground0.15 on trees

1 on groundOccupancy
factor**8
ATTACHMENT F-2: TERRESTRIAL ECOSYSTEM APPLICATION OF ASSESSMENT METHODS AND LIFE HISTORY

13.
The “cluster numbers per point” stabilize after a certain number of iteration steps. The
number of iteration steps depends on the number of sampling points in the dataset and theestimated variance [Clarke and Evans, 1954]. For each individual point, a cluster score was
kept. The scores were then ranked. The highest scoring cluster number was assigned to thepoint. The smallest cluster size was two points.
(e)

Sample points clustering — option 2

14.
For the spatial clustering of the wild boar data, a method was used based upon the“Partitioning around Medoids (PAM)” algorithm [Kaufman and Rousseeuw, 2005]. The
algorithm was applied to wild-boar groups with an average home range of 3 km. This
information was then used to extract concomitant deposition density data for the derivation of
external absorbed dose rates.
Figure VI. Results from cluster analyses: clusters denoted by colours and specific numbers

15.
In order to extend the information on 137Cs to all other radionuclides, use was made ofradionuclide ratios. In essence, the data set for each organism group was used to examine
location-specific and generic ratios. The location-specific approach used ratios from theparticular area where an animal was sampled, whereas the generic ratios simply related to all
available ratio data for that particular animal. The ratios were thus applied to the 137Cs
deposition density data to provide information for the poorly characterized radioisotopes(110mAg, 129mTe, 131I, 89Sr, 90Sr and 239Pu).

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ATTACHMENT F-2: TERRESTRIAL ECOSYSTEM APPLICATION OF ASSESSMENT METHODS AND LIFE HISTORY

III. EQUILIBRIUM ASSESSMENTS(a)Assessment for sites of relatively high deposition density — data selection andpreparation

16.
An assessment was conducted in order to indicate the maximum environmental dose
rates that could have been encountered in areas where direct measurements on animals andplants were unavailable. The approach adopted was to identify the maximum reported
deposition density levels (attachments C-1 to C-7) applying concentration ratios (CRs) toderive whole-body activity concentrations in selected plants and animals. By applying CRs, it
was recognized that poor estimates of internal activity concentrations in biota might result,reflecting a lack of suitability in applying these ratios to a system clearly not under equilibrium.
Whether the application of CRs in the intermediate phase (i.e. first months) after deposition
leads to over-estimation or under-estimation of internal activity concentrations was not
immediately evident. On the one hand, a consideration that the rate constants governingtransfer of radionuclides from soil through food chains corresponded to half-times in the orderof days and weeks led to the inference that substantial time is required for equilibrium to beestablished. As a consequence, activity concentrations in any given animal needed time toattain levels that would reflect the longer-term, equilibrated partitioning between activity
concentrations in animals and soil that CRs are intended to characterize. Thus, applying CRs tosoil activity concentrations in the intermediate phase after deposition might have a tendency toover-estimate activity concentrations in animals because the time necessary to allow
accumulation in the animals’ bodies had not elapsed. On the other hand, radionuclides in theintermediate phase after deposition would be expected to be relatively available for transfer toherbivores because they are present on the surface of vegetation. From such a perspective, theapplication of terrestrial CRs, based as they are on conditions where root uptake dominates theinitial link in radionuclide transfer from soil to the food chain, might be expected to underestimate activity concentrations in animals. In the absence of direct measurements, these
speculations are best resolved through the application of dynamic models.
17.
The following approach was adopted for the elevated deposition density, equilibriumbased assessment: The sampling location from requested dataset (attachment C-2) that gave the highest
deposition density values was selected;
 The maximum and arithmetic mean deposition (Bq/m2) for the selected location and for
all radionuclides were derived (table 3);
 Deposition values were converted to activity concentrations in soil using a genericdensity of 1300 kg/m3 [Yasunari et al., 2011] and specified soil depth of 5 cm (table 3);
 These activity concentration data for soil were used as input data to the ERICA Tool
using CRs given by ICRP [2009].

ATTACHMENT F-2 TERRESTRIAL ECOSYSTEM APPLICATION OF ASSESSMENT METHODS AND LIFE HISTORY

11

Table 3. Radionuclide deposition data for Okuma Town
134

137

Cs

Cs

89

Sr

90

Sr

239,240

Pu

129m

Te

110m

131

Ag

I

Maximum Deposition(Bq/m2)

1.40E+07

1.55E+07

6.26E+03

1.46E+03

3.06E+00

2.66E+06

1.36E+04

3.18E+04

Maximum
Concentration (Bq/kg)

2.16E+05

2.38E+05

9.63E+01

2.25E+01

4.71E-02

4.10E+04

2.10E+02

4.89E+02

Arithmetic mean
deposition (Bq/m2)

2.49E+06

2.77E+06

3.10E+03

7.25E+02

2.16E+00

5.16E+05

4.89E+03

3.18E+04

Arithmetic mean
concentration (Bq/kg)

3.83E+04

4.27E+04

4.77E+01

1.12E+01

3.32E-02

7.94E+03

7.53E+01

4.89E+02

Number of samples

14

14

3

3

3

13

7

1

(b)

Input data for an area covering a high variability of deposition – Koriyama City

18.
Owing to the limitation of available empirical data, this method (the cluster approach)
was not applicable to biota other than wild boar. Therefore, to characterize the exposure
situation for other mammals and birds, another approach was taken. Upon closer inspection of
the existing datasets, the biota dataset for Koriyama City was found to be the best, because it
provided coverage of all considered species. Hence deposition density and activity
concentration data for this location were extracted and presented giving minimum, mean andmaximum values. The wildlife dataset contained data for five different species of bird, but as
their numbers were very few they were pooled under a single category. Table 4 shows theavailable wildlife data for Koriyama City.
Table 4. Activity concentrations in soil for Koriyama City
134

137

Cs (Bq/kg)

SpeciesCs (Bq/kg)

n

Mean

Range

n

Mean

Range

Wild boar

12134

38 – 267

12

156

54 – 289

Asian black bear

2

175

113 – 2372

201

132 – 270

Sika deer

3

85

63 – 1083

106

85 – 143

Birds

6

40

22 – 718

41

18 – 8119.
A further equilibrium analysis was conducted to account for the radioactive decay of allradionuclides with physical half-lives of less than one year — i.e. 131I, 110mAg, 89Sr and 129mTe— between the time of sampling and the initial deposition event. According to Yoshida and
Takahashi [2012], the main wet deposition events after the Fukushima accident occurred on
15–16 March in the northern Fukushima prefecture. The reference date for most of theradionuclides from Okuma Town was specified as 14 June 2011 (there were several samples
with a reference date of 13 June 2011 and one sample, for which the 131I value was reported of
20 June 2011). The reference date of 14 June 2011 was used in all calculations giving anelapsed time between the main deposition event and sampling/measurement of 90 days fordecay correction.
20.
The IAEA [2010] noted that CRs based on data pertaining to stable elements (orradionuclides with relatively long physical half-lives) should be modified when applied to12

ATTACHMENT F-2: TERRESTRIAL ECOSYSTEM APPLICATION OF ASSESSMENT METHODS AND LIFE HISTORY

radionuclides with relatively short physical half-lives to account for the fact that physical decay
will substantially reduce activity concentrations actually observed in biota. Although the
adoption of factors to modify CRs might be applicable to the present case in which
determinations for 131I, 129mTe, 110mAg and 89Sr are needed, for the sake of simplicity andtraceability the original CR values were applied. Because the factors used to modify CR values
as applied to short-lived radionuclides are fractions of 1, the calculations made in theassessment can be considered conservative.

IV. SETTING UP THE DYNAMIC MODEL21.
The model described in the methodology was implemented in ECOLEGO (figure VII).
Ecolego is a simulation software tool that is used for creating dynamic models and performing
deterministic and probabilistic simulations [Avila et al., 2005].
Figure VII. Screenshot showing model configuration in ECOLEGO-6

22.
Initial focus was placed upon 137Cs and mammal (deer) using the representative
example of Sika deer.
23.
Following exchanges with MEXT, it was clear that the MEXT data pertained to total
deposition density and not just the component of deposition density associated with soil. Theycollected soil together with small weeds or other vegetation if there were any and also collectedroot zones as soil samples. The model was run using a deposition density of 1 kBq/m2.

ATTACHMENT F-2 TERRESTRIAL ECOSYSTEM APPLICATION OF ASSESSMENT METHODS AND LIFE HISTORY

13

IV.V. RESULTS(a)

Detailed assessment with direct measurements in animals

24.
Dose rates derived for all wild boar clusters were in the range of 0.8 to 1.1 µGy/h withthe dose primarily attributable to 137Cs and 134Cs. However, as mentioned earlier, owing to thelimitation of data, the cluster appoach was restricted to wild boar only. To expand the dose
assessment to include other organisms, the city of Koriyama was selected because of theavailability of in situ data. Using deposition density and activity concentration data reported forthis location as input into the ERICA Tool, estimates of dose rates for four species were made(see table 5). The derived total dose-rate values were based on the arithmetic mean and therange was generated by considering minimum and maximum values for both soil and biota. As
seen from the results (table 5), the external exposure component of the dose rates dominatesand caesium is the main contributor to the estimated dose rates, and 134Cs has the higher share(figure VIII).
Table 5. Total weighted dose rates for wildlife in Koriyama city. Based on empirical data;
deposition density and activity concentration (for caesium isotopes)

Species

Total dose rate(µGy/h)Range (µGy/h)Contribution fromexternal exposure(%)

Contribution of
caesium to total dose
rates (%)

Wild boar

0.55

0.06–2.0

73

92

Asian black bear

0.57

0.13–1.9

65

93

Sika deer

0.50

0.08–1.9

78

91

Birds

0.7

0.03–2.8

94

95

Figure VIII. Estimated absorbed dose rates (µGy/h) for representative organisms in Koriyama.
‘Others’ includes radionuclides such as 131I, 110mAg, 89Sr and 90Sr(b)

Equilibrium assessment using maximum deposition density values

25.
Weighted absorbed dose rates for selected organism groups using the maximum (a)reported and (b) decay-corrected activity concentrations from Okuma Town (table 3) arepresented in figure IX. Because these calculations were based on data from a single

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ATTACHMENT F-2: TERRESTRIAL ECOSYSTEM APPLICATION OF ASSESSMENT METHODS AND LIFE HISTORY

measurement point, they were considered to provide only a highly uncertain indication of
upper percentile exposure rates for plants and animals in the accident-affected region.
26.
For exposure estimates based upon maximum deposition density levels at the reference
date in July 2011, the highest weighted absorbed dose rate of approximately 400 µGy/h wasestimated for deer/herbivorous mammal. A large fraction of this dose rate was due toradioisotopes of caesium (134Cs = 61 % and 137Cs = 35 % of total weighted absorbed dose rate)
and to internal exposure (approximately 88 %). Exposure estimates for vegetation at thereference date were substantially lower than those determined for mammals, falling at levels of
100 and 140 µGy/h for pine tree and grass, respectively.
27.
The maximum weighted absorbed dose rates estimated by accounting for the physicaldecay of short-lived radionuclides between the time of primary deposition and sampling wereconsiderably greater (by a factor of 2–3 times depending on organism type) than exposuresdetermined at the reference date. For the decay-corrected estimates, a highest weighted
absorbed dose rate of approximately 600 µGy/h was calculated for deer/herbivorous mammal.
The primary contribution to the exposure estimate was from internal exposure due toradioisotopes of caesium.
28.
The validity of decay-correcting short-lived radionuclides to the time of deposition andthen applying a CR to derive dose rate estimates for internal exposure is questionable becauseof the underlying assumption that instantaneous equilibration has occurred between activity
concentrations in soil and biota. Nonetheless, the observations that (a) for the model as applied,external exposure appears to be an important component of exposure for all of the organisms
considered, with the exception of deer/herbivorous mammal, and that (b) the time evolution ofradionuclide activity concentrations in soil, upon which external exposure is assessed, isdictated primarily by physical decay processes, mitigate this potentially confounding feature of
the analysis.
29.
The exposure estimates based on arithmetic mean of the deposition density levels forthe areas of highest deposition density (table 3) provide a more reliable, less uncertain,
indication of upper percentile dose rates for plants and animals in the accident-affected region
because values are based upon more samples and are, therefore, supported by better underlying
statistics. Furthermore, the data used provide a more representative indication of deposition
density over an area that might feasibly be considered as harbouring viable populations ofbiota. Weighted absorbed dose rates for selected organism groups using the arithmetic (a)reported and (b) decay-corrected activity concentrations from Okuma Town (Table 3) arepresented in figure X.
30.
The maximum weighted absorbed dose rates associated with the calculations made forthe reference date were determined for deer/herbivorous mammal and amounted to 71 µGy/h(or 1.7 mGy/day). Virtually the entire dose rate (99 %) was due to radioisotopes of caesium
and internal exposure predominated (figure X(c)). Somewhat lower weighted absorbed dose
rates were determined for vegetation, with exposures of 26 µGy/h and 17 µGy/h estimated forgrass and trees respectively.

ATTACHMENT F-2 TERRESTRIAL ECOSYSTEM APPLICATION OF ASSESSMENT METHODS AND LIFE HISTORY

15

Figure IX. a) Weighted absorbed dose rate for selected biota based on maximum reported
deposition densities in soils at Okuma Town, blue solid bars = dose rates based on reported
activity concentrations at reference date and yellow striped bars = dose rates based on activity
concentrations, decay-corrected to 15 March 2011. Radionuclide and internal/external
contribution to total weighted absorbed dose rates for mammal at b) time of maximum
deposition (decay-corrected to 15 March 2011) and c) reference date

Figure X. a) Weighted absorbed dose rate for selected biota based on average reported
deposition density in soils at Okuma Town, blue solid bars = dose rates based on reported
activity concentrations at reference date, yellow striped bars = dose rates based on activity
concentrations decay-corrected to 15 March 2011. Radionuclide and internal/external
contribution to total weighted absorbed dose rates for deer/herbivorous mammal at b) time of
maximum deposition (decay-corrected to 15 March 2011) and c) reference date

31.
When the radioactive decay of short-lived radionuclides was accounted for, theestimated weighted absorbed dose rates at the time of the assumed main deposition event
increased substantially. Maximum exposures were then calculated to be associated with soil-

16

ATTACHMENT F-2: TERRESTRIAL ECOSYSTEM APPLICATION OF ASSESSMENT METHODS AND LIFE HISTORY

dwelling organisms with derived dose rates of 290 µGy/h and 330 µGy/h for earthworm/soil
invertebrate and rat/burrowing mammal respectively.
32.
Weighted absorbed dose rates for deer/herbivorous mammal were estimated to be
240 µGy/h with 131I and internal exposure making dominant contributions to this exposure(figure X (b)). In contrast, the external exposure predominated for all other organisms
considered. Calculated dose rates for vegetation fell in the range 110 to 130 µGy/h.
(c)Dynamic modelling predictions

33.
Model predictions for activity concentrations of 137Cs in grass, tree and deer following adeposition of 1 kBq/m2 are presented in figure XI. Because the modelled activity
concentrations in deer exhibit a linear response to deposition, predictions can be easily madefor any given deposition by applying a correction factor based on the relative magnitude of thedeposition to 1 kBq/m2.
Figure XI. Changes in 137Cs activity concentration with time followin an initial deposition of
1 kBq m-2. Light green squares and solid line =137Cs in grass (Bq/kg f.w.); dark green circles andbroken line = 137Cs in tree (Bq/kg f.w.); red triangles and dashed line = 137Cs in deer (Bq/kg f.w.)

34.
Activity concentrations in grass fell rapidly, reflecting the relatively high weathering
rates and biomass dilution associated with the start of the growing season, falling from amaximum activity concentration of approximately 1000 Bq/kg f.w. to levels slightly in excess
of 10 Bq/kg f.w. within the first 50 days or so after deposition. Initial activity concentrations intrees were approximately 60 Bq/kg f.w. but lower weathering rates led to a much slower
decline in activity concentrations with time compared to grass and falling to levels below
10 Bq/kg f.w. approximately 250 days after deposition. Modelled activity concentrations indeer increased for the initial 65 days after deposition attaining a maximum activity

ATTACHMENT F-2 TERRESTRIAL ECOSYSTEM APPLICATION OF ASSESSMENT METHODS AND LIFE HISTORY

17

concentration of ca. 70 Bq/kg f.w before declining gradually to levels around 30 Bq/kg f.w.
1 year after deposition.
35.
In the process of initial model testing, paired samples of deposition density on soil andactivity concentration for Sika deer of 137Cs were selected. The simplest way to do this was toselect the deposition density sample nearest to the location of the animal sampling to provide a
corresponding pair. The dynamic model was then run using the deposition density data andestimates made for 137Cs concentration in deer for comparison with the empirical data
pertaining to Sika deer. The starting date for the model was taken to be 15 March 2011,
assuming that the deposition occurred in one discrete event at this time. The time for estimation
was thereafter selected as the time when the deer sample was taken. Therefore, 15 November
2011 would correspond to an estimate for eight months after deposition.
36.
The comparison between modelled (predicted) and measured values is provided infigure XII.
Figure XII. Predicted versus measured activity concentrations of 137Cs in (Sika) deer; themeasured data include 2 values below detection limits (where activity concentrations wereassumed to be at the detection limit)

37.
In view of the large uncertainties associated with numerous parameters adopted within
the model implementation, especially those characterizing activity levels within the diet of Sikadeer, the predictions were quite reasonable. The model appeared to have a slight tendency toover-estimate activity concentrations but by no more than a factor of approximately 2. In only
one case was there a substantial under-estimation for which the modelled value was a factor of
approximately 5 below the measurement.
38.
Dose rates arising from 134Cs, 137Cs and 131I were calculated using the arithmetic meanof the deposition density data for Okuma Town (figure XIII). The deposition density levels for131
I were decay-corrected to 15 March 2011. This might be considered as a hypothetical
exercise, because the presence of Sika deer in the Okuma Town area had not been established.
Nonetheless, the dose rates might be considered as indicative of exposures for any deer-sized
herbivorous mammal present in this area.

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ATTACHMENT F-2: TERRESTRIAL ECOSYSTEM APPLICATION OF ASSESSMENT METHODS AND LIFE HISTORY

Figure XIII. Dose rates versus time for (i) grass: light green squares, solid lines, (ii) tree: darkgreen circles broken line and (iii) deer: red triangles and dashed line

39.
Highest weighted absorbed dose rates were estimated for grass, with levels of
approximately 5.6 mGy/h at the time of deposition. A large fraction of this original dose rate(>85%) was due to 131I present on the vegetation surfaces and modelled most appropriately as
an internal exposure. The dose rates for grass decreased rapidly, falling to levels below
100 µGy/h in the 15 days following deposition. The accumulated dose in the first 30 days after
deposition slightly exceeded 0.45 Gy. Weighted absorbed dose rates for trees were initially at a
substantially lower level than those modelled for grass with exposures of about 1.4 mGy/h.
However, dose rates declined less rapidly than those for grass and were estimated not to havefallen below 100 µGy/h until about 80 days of elapsed time following deposition. The main
contribution to dose, as for grass, was due to 131I on vegetation. The accumulated dose in thefirst 30 days was commensurate with the value estimated for grass, falling just below 0.45 Gy.
Maximum dose rates for deer/herbivorous mammals were estimated to have occurred within
the first week following the main deposition event attaining dose rates of 370 µGy/h. A large
fraction of this initial dose rate (64%) was associated with internal exposure due to 131I.
Although dose rates fell quite rapidly, with levels below 200 µGy/h following 40 days ofsimulation time, dose rates did not fall below 100 µGy/h until time=275 days. The accumulated
dose in the first month following the accident was estimated to be approximately 0.2 Gy.
40.
The initial, maximum dose rates estimated for vegetation resulting from the dynamic
modelling approach were quite different from values derived using an equilibrium-based
approach even when decay was accounted for in the latter. This could be quite clearly seen forgrass where the dynamic approach predicted dose rates exceeding the equilibrium-based
estimate by a factor of approximately 44. This discrepancy was even more apparent if
consideration is given to the fact that the equilibrium simulation included a broader suite ofradionuclides. For deer/herbivorous mammals, the discrepancy between the models was lesspronounced, although the dynamic model still produced estimates that fell a factor of 1.6 above
the decay-corrected equilibrium-based model when exposures were at a maximum. For allorganisms, over the first few months the absorbed dose rates estimated using the two
approaches converged and thereafter the dynamic model produced lower dose-rate estimates

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19

than the equilibrium model (presumably reflecting the decay of 131I being accounted for in theformer model).

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