# Food Chain Transport

This chapter discusses the modeling of radionuclide migration in the
food chain. The primary focus is uptake of radionuclides by plants and
animals from contaminated environmental media. These contaminated media
may include air, water, and soil. For uptake of radionuclides by
terrestrial animals, contaminated media may also include plants. The
primary reason for considering food chain transport is to estimate
radionuclide concentrations in foodstuffs, both plant and animal
products, that may be consumed by humans.

The starting point for estimating food chain transport is the
radionuclide concentrations in air and water at various exposure
locations. Previous chapters have discussed transport of radionuclides
through the primary media of air and water; air transport is discussed
in [Chapter 6](../Chapter_06_ready%20for%20CDC%20031405.doc) and water
transport is discussed in [Chapter
7](../Chapter_07_Ready%20for%20CDC%20031405.doc). For this study, food
chain transport is simulated by several models that consider a variety
of processes that include:

-   For radionuclides transported by air:

    -   Deposition of radionuclides onto vegetation and soil

    -   Uptake of radionuclides by plants from soil and plant surfaces

    -   Uptake of radionuclides by animals that consume plant products

-   For radionuclides transported by water:

    -   Deposition of radionuclides into sediments

    -   Uptake of radionuclides by aquatic animals

In general, the radionuclide concentrations in soil used to model food
chain transport need not correspond to concentrations used to model
direct exposure and inhalation; however, for this study these
concentrations generally correspond. Therefore this chapter also
discusses contamination of soil, to promote a compact presentation. This
chapter qualitatively describes the models used and summarizes the
values for many of the variables used to generate point-estimates of
radiation dose and risk. All of the variable values may be found in
[Appendix E](../Appendix_E%20ready%20for%20CDC%20031405.doc) and
[Appendix
F](../Appendix_F%20tables%20-%20Ready%20for%20CDC%20031405.doc).

## Introduction

The complete mathematical formulations of the models used for this study
are documented in the GENII Version 2 Software Design Document \[GENII
SDD\] (1). These generic models are implemented in the GENII computer
code. This chapter discusses the use of these models, as implemented in
the GENII computer code, to simulate transport of radionuclides in the
food chain in the vicinity of the SRS for the 39-year period studied.
The mathematical formulations for most of models discussed in this
chapter are documented in Chapter 9 of the GENII SDD.

Transport of radionuclides in the food chain may be viewed as transport
into and out of different compartments in the food chain, as shown
schematically in Figure 4-2 and 4-3. The food chain compartments
considered in this study of the SRS are: soil, plants, terrestrial
animals, and aquatic animals. Radionuclides enter these compartments
either directly from contaminated air and water or by transfer from
another food chain compartment. For example, radionuclides may enter
edible plants by direct deposition of radionuclides from contaminated
air or by uptake from soil contaminated by airborne radionuclides. The
mathematical models use the principle of mass conservation to estimate
the concentration in a food chain compartment based on the rates of
radionuclide input (e.g., deposition, uptake) and removal (e.g.,
radioactive decay, weathering).

The generic models implemented in the GENII computer code allow
radionuclides to enter soil, plants, and animals from contaminated
water; for example, animals may take up radionuclides from contaminated
drinking water. However, agricultural practices in the vicinity of the
SRS did not include use of the Savannah River or Lower Three Runs Creek
for irrigation of crops or watering of farm animals. Other water sources
used for agricultural purposes were considered to have low levels of
contamination; these other water sources, may have been slightly
contaminated by deposition of radionuclides from air, but did not
receive liquid releases of radionuclides from the SRS. Therefore, the
only instance of contaminated water transferring radionuclides to a food
chain compartment that has been modeled is the contamination of
shoreline sediments.

The remainder of this chapter is organized according to the four food
chain compartments considered in this study of releases from the SRS:
soil, plants, terrestrial animals, and aquatic animals; in addition, two
special models for tritium and carbon-14 migration in the food chain are
discussed in a separate section.

## Soil Concentration

Soil concentrations are calculated differently, depending on the major
physical processes involved in a particular situation. As with other
concentrations calculated, soil concentrations are based on conservation
of mass. However, depending upon the nature of the soil, different
radionuclide addition and removal processes may be applicable. Some of
the physical processes that might apply to terrestrial soils include:

-   deposition from air to surface soil,

-   deposition from water to surface soil during irrigation,

-   depletion by radioactive decay,

-   leaching from surface soil,

-   loss from surface soil during harvest of contaminated plants, and

-   removal by resuspension of contamination and subsequent transport by
    wind.

For aquatic sediments the main process is deposition of radionuclides
from contaminated water. Because irrigation with water contaminated by
liquid releases from SRS facilities was not practiced during the period
of time studied, deposition on farm land of radionuclides released to
water by the SRS was not modeled. However, contamination of farm land by
deposition of airborne radionuclides was modeled. Deposition of
radionuclide contamination from water to sediments on the shoreline was
modeled as a viable mechanism to contaminate soil.

### Radionuclide Deposition from Air 

Irrigation with contaminated water is not considered in this study
because: 1) water from the Savannah River and Lower Three Runs Creek,
which were contaminated by liquid releases from the SRS, was not used
for irrigation and 2) contamination of other waters that might have been
used for irrigation, such as reservoirs, ponds, and wells was considered
to be slight. For this reason, the only source of radionuclide
contamination in terrestrial soils is deposition of airborne
radionuclides.

Two types of deposition are usually modeled: 1) dry deposition and 2)
wet deposition. "*Dry deposition* refers to any physical removal process
that does not involve precipitation." "*Wet deposition* refers to
processes in which atmospheric chemicals are accumulated in rain, snow,
or fog droplets and are subsequently deposited onto Earth's surface."
(2).

Dry deposition may involve several processes including gravitational
settling, impaction onto plant and soil surfaces, and absorption by soil
and plant surfaces. For small particles and gases, gravitational
settling is negligible. The model implemented by the GENII code uses an
electrical analogy to calculate a net deposition velocity from three
separate "resistances" (s/m): aerodynamic resistance, surface
resistance, and transfer resistance and the gravitational settling
velocity. The aerodynamic and surface resistances depend upon the wind
speed, as detailed in the GENII Software Design Document (1). For this
study the default values for transfer resistance were used: 10 s/m for
gas (iodine) and 100 s/m for particles. To calculate the gravitational
settling velocity for particles two key variables were the particle
density and diameter; for the point estimates the values for these
variables were chosen to be 2.0 g/cm^3^ and 0.001 mm, respectively. The
deposition velocity is calculated by combining the separate resistances
and the gravitational settling velocity, if appropriate (not appropriate
for gases and very fine particles). The dry deposition rate is then
calculated from the deposition velocity and air concentration of each
radionuclide for any given year:

R~ai~ = V~d~C~ai~ (8-1)

Where R~ai~ is the rate of deposition from air, a, of radionuclide i
(Bq/m^2^/s)

V~d~ is the deposition velocity (m/s)

C~ai~ is the air concentration of radionuclide i in that year.

Clearly the concentration in air varies with the location of the
exposure location at which the radionuclide deposition is to be
calculated; however, for simplicity an index denoting this variability
has not been included in the equation. The air concentrations of the
radionuclides studied were obtained by the approach discussed in
[Chapter 5](../Chapter_05_Ready%20for%20CDC%20031405.doc).

Wet deposition for gases considers that the gas is soluble in water, but
partitions the gas between air and water according to Henry's Law.
Henry's Law describes the ratio of the concentration of a chemical in
air to the concentration in water at equilibrium. Wet deposition for
particles uses the concept of a washout coefficient that describes the
fraction of the air concentration of particles removed per unit distance
of travel of precipitation. Default values build into the GENII code
were used for the Henry's Law constants. The washout coefficient is
computed based on the precipitation rate during a precipitation event. A
key variable for wet deposition is the average daily rain rate, which
was taken to be 11.4 mm/d, based on data from 1970-99 (3).

### Radionuclide Concentration in Soil -- Direct Exposure

To estimate the concentration of radionuclides in soil for determining
dose resulting from direct exposure from the ground plane, most removal
processes are not considered; the only removal process considered is
radioactive decay. Thus a key variable in determining these
concentrations is the decay constant for each radionuclide. These are
well established and are incorporated into one of the many GENII
databases.

For direct exposure calculations, the average concentration in the soil
is used, considering the total air deposition rate, at a particular
exposure location, and radioactive decay. Every year was considered
separately and for this study the radionuclide concentration at the
beginning of each year was assumed to be zero. Some long-lived
radionuclides may have persisted in the soil from year to year, but the
results of this study suggest that dose pathways for these radionuclides
were not likely to be significant. The model used to estimate soil
concentration assumes that radionuclides deposited from the air onto the
soil surface is mixed uniformly to a certain depth. For this reason, the
concentration of radionuclides in the soil depends on the depth to which
mixing is assumed to occur and the bulk density of the soil. For this
study, the mixing depth was set to 15 cm and the bulk soil density set
to 1.6 g/cm^3^; these values are frequently used as nominal values in
studies of this type.

### Radionuclide Concentration in Soil -- Agricultural Exposure

To estimate the concentration of radionuclides in soil for determining
dose resulting from agricultural pathways, such as uptake by plants,
removal processes in addition to radioactive decay must be considered.
Two additional removal processes are 1) loss of activity by harvest and
2) leaching of radionuclides from the soil. Loss of activity by harvest
models the amount of radionuclides taken up by vegetation while growing
and assumes it is removed at harvest. For this study it was
conservatively assumed that no activity was removed by harvesting. Fresh
water from precipitation and irrigation can dissolve radionuclides in
the root zone of the soil and transport them to a lower depth, where the
activity will be unavailable for uptake by roots or resuspension into
the air. The model used to estimate removal of radionuclides from the
upper layer of soil assumes equilibrium between the pore water and soil.
The partitioning coefficient, K~D~, (i.e., distribution coefficient)
describes the ratio of the concentration in soil to the concentration in
water. This coefficient depends on a number of factors including the
chemical natures of the soils and the radionuclide ions. Appropriate
values for the radionuclides modeled were carefully studied and
selected, as shown in Table 8-1 and described in [Appendix
C](../Appendix_C%20-Ready%20for%20CDC_031405.doc). Additional important
variables in the model include the annual precipitation rate, the
surface soil moisture content fraction, and the bulk soil density. The
surface soil moisture content fraction was estimated to be 0.22 using
characteristics for sandy loam (porosity of 0.4), a saturated hydraulic
conductivity of 540 m/y, and an infiltration rate of 0.373 m/y. As with
the soil concentration used for direct exposure, the soil depth was set
to 15 cm and the bulk soil density set to 1.6 g/cm^3^. Since the K~D~
changes for each chemical species, these data yield a removal rate
constant for the activity of each radionuclide in the surface soil
layer. This removal rate constant is used in addition to and in a manner
similar to the radioactive decay constant to estimate the loss of
radioactivity in the soil layer by leaching; this provides an estimate
of the annual average radionuclide concentration in the surface soil
layer.

Table 8‑1 Soil Distribution Coefficients for Surface Soil Leaching
Calculations

  --------------- ---------------- ------------------------ --------------
  Nuclide         Kd (mL/g)        Nuclide                  Kd (mL/g)

  ^3^H            0                ^99^Tc                   2.49\*

  ^32^P           173\*            ^103,106^Ru              55

  ^35^S           7.5              ^129,\ 131^I             1.55\*

  ^60^Co          60               ^134,\ 137^Cs            59

  ^65^Zn          200              ^141,144^Ce              490

  ^89,90^Sr       3041\*           ^231,234^Th^†^           3000

  ^90^Y^†^        510              U (all isotopes)         1000

  ^95^Zr          600              Pu (all isotopes)        4100

  ^95^Nb          160              ^241^Am                  2000
  --------------- ---------------- ------------------------ --------------

> \*This table lists the number of significant digits that were
> presented in the primary sources.
>
> ^†^These isotopes are daughters of the primary isotopes considered in
> the report

### Radionuclide Concentration in Shoreline Sediments -- Direct Exposure 

The concentration of radionuclides in shoreline sediments depends on the
deposition of radionuclides from the adjacent contaminated body of
water. The removal of radionuclides is considered to be limited to
radioactive decay. As with other soil concentrations, the average
concentration over a year is used to estimate doses. The mathematical
model describing the concentration in sediments is very similar to the
model for soil concentrations used for direct exposure. The key variable
for this model, in addition to the decay constant for each radionuclide,
is the deposition rate for sediments. The deposition rate is stated in
terms of the transfer rate constant; for this study the transfer rate
constant was set equal to the default value of 25,400 L/m^2^/y used in
the GENII computer code (The original estimate had one significant
figure, but three significant figures were retained by the GENII code to
reflect the conversion from English to metric units and to allow trace
back to the historical value). This value of the transfer rate constant
was derived for several radionuclides by using measured values of
concentrations in shoreline sediments and in the water column of the
Columbia River. Two additional important variables determining the
sediment concentration are the sediment density and the sediment depth;
for this study the shoreline sediment density was set equal to the GENII
code default of 240 kg/m^2^ and the depth, as with other soil layers,
was set equal to 15 cm.

## Radionuclide Concentration in Terrestrial Vegetation 

In general, contaminants may enter terrestrial vegetation by deposition
on plant surfaces and by uptake through plant roots. Because the rate of
contaminant entry into the plant is generally not affected by the
concentration in the plant, concentration induced by direct deposition
and by root uptake may be calculated separately. The total concentration
of a radionuclide in a particular plant may then be calculated by
summing the concentrations induced by direct deposition and by root
uptake; since the mass of the plant is the same for both types of
uptake, adding concentrations is equivalent to summing the radionuclide
mass from each entry mechanism.

### Plant Concentration from Direct Deposition

Deposition of radionuclides on the surfaces of plants (usually the
leaves) can generally result from:

1\) dry deposition from contaminated air,

2\) wet deposition from contaminated air,

3\) deposition of radionuclides resuspended from contaminated soil, and

4\) deposition from irrigation with contaminated water.

As explained elsewhere in this Section, irrigation with contaminated
water was not considered a viable pathway. Dry and wet deposition from
contaminated air is discussed in Section 8.2.1. Deposition of
radionuclides resuspended from contaminated soil is discussed in Section
8.3.1.1. Once these three deposition rates \[1), 2), 3)\] have been
determined, the radionuclide concentration in the vegetation is computed
using a mass balance relationship; processes removing radionuclides from
the plant surfaces include radioactive decay and weathering. The
calculation of concentration in vegetation from direct deposition is
discussed in Section 8.3.1.2.

#### Deposition of Resuspended Activity

Resuspension of radionuclides deposited on soil and plant surfaces
causes an incremental addition to the concentration in the air above the
affected surfaces. Radionuclides are resuspended by turbulence in the
air and by mechanical agitation of the surface, e.g., by automobile
traffic, pedestrian traffic, and agricultural operations such as
plowing, cultivating, and harvesting. For most studies of this type, the
concentration of radionuclides in the air is considered to be in
equilibrium with the surface concentration of radionuclides; i.e., the
forces tending to resuspend activity are balanced by forces tending to
cause the activity to settle on surfaces. For equilibrium conditions the
relationship between air and soil concentration is given by:

C~ar~ = RF·Ψ~S~ (8-2)

Where C~ar~ is the concentration induced in the air by resuspension
(Bq/m^3^)

RF is the resuspension factor, a constant, (m^-1^)

Ψ~s~ is the areal concentration of radionuclides in the soil (Bq/m^2^)

The areal concentration may be obtained from the mass-based
concentration, C~s~, by multiplying by the bulk soil density, ρ~s~, and
soil depth, d~s~:

Ψ~s~ = C~s~·ρ~s~·d~s~ (8-3)

The mass-based soil concentration, C~s~ (Bq/kg), is the average annual
concentration obtained as described in Section 8.2.2. As with other
cases involving the soil concentration, the soil depth was set to 15 cm
and the bulk soil density set to 1.6 g/cm^3^, so ρ~s~·d~s~ has a value
of 24 g/cm^2^ or 240 kg/m^2^.

Two separate values of resuspension factor were used in this study, one
for farms and one for other environments. These values, representative
of different land uses, were chosen from a compilation of measured
resuspension factors (4). Because agricultural operations with heavy
equipment, such as tractors, greatly disturbs the soil surface a
resuspension factor value of 10^-5^ m^-1^ was chosen for agricultural
exposure locations; this value is 100 times larger than the value of
10^-7^ m^-1^ chosen for urban, suburban, and non-agricultural rural
exposure locations. Farming locations using the higher value of
resuspension factor were taken to be: Girard (Rural Family #1), New
Ellenton (Migrant Family), and Williston (Rural Family #2). All other
exposure locations used the smaller value of resuspension factor.

Once the incremental addition to the concentration in the air above the
affected surfaces is computed by equation (8-2), the deposition rate of
this activity on plant surfaces is obtained from the product of the air
concentration and a deposition velocity, similar to equation (8-1). The
deposition velocity used for all radionuclides and all plants was chosen
to be 0.001 m/s, which is a representative value.

#### Radionuclide Concentration in Plants from Direct Deposition

Radionuclide concentration in plants from direct deposition is
calculated from a mass balance that considers radionuclide deposition
from the air and removal by radioactive decay and weathering of the
radionuclides from the plant surfaces. The net uptake of radionuclides
is computed by integrating the uptake rate over the growing period for
the particular crop. Radionuclide concentration is given by:

C~dci~ = \[R~aid~·r~dc~ + R~aiw~·r~dw~ +
C~ci~·RF~c~·V~di~·r~ac~\]·\[Tvc/Bc\]·\[(1-exp{-λ~ei~T~gc~/365d/y})/λ~ei~\]
(8-4)

Where,

C~dci~ = concentration from deposition, d, of radionuclide i in crop
type c

R~aid~ = the rate of dry (d) deposition of radionuclide i from air (a)

R~aiw~= the rate of wet (w) deposition of radionuclide i from air (a)

r~dc~ = the dry (d) deposition fraction for crop type c

r~dw~ = the wet (w) deposition fraction for crop type c

C~ci~ = the concentration of radionuclide i in the soil for crop type c

RF~c~ = resuspension factor for the soil for crop type c

V~di~ = deposition velocity for radionuclide i

r~ac~ = the deposition fraction for resuspension for crop type c

Tvc= the translocation factor for crop type c

Bc = the standing biomass for crop type c

> λ~ei~ = λ~wi~ + λ~i~ = effective loss rate contant (y-1), which is the
> sum of the weathering rate constant, λ~wi~, and the radioactive decay
> constant, λ~i~
>
> T~gc~ = the growing period for crop type c

Growing periods for different types of crops are shown in Table 8-2. The
dry and wet deposition rates are discussed in Section 8.2.1; deposition
rates from resuspended activity are discussed in Section 8.3.1.1. Since
plant surfaces do not generally cover the entire area subject to
radionuclide deposition, a deposition fraction is applied to account for
the fraction of the surface covered by vegetation; a standard value of
0.25 is used in this study for both dry and wet deposition calculations.
Variables related to radionuclide removal from plants include the
radioactive decay constant and the weathering constant. Radioactive
decay constants are included in a data library in the GENII code. The
weathering constant, which describes removal of radionuclides by
precipitation, wind, and similar processes, is expressed as a weathering
rate half-life; for this study a standard value of 14 days was selected
for the weathering rate half-life.

Table 8‑2 Values used for the point-estimate case for the variables:
growing period (days), translocation factor, and standing biomass
(kg/m2).

  -------------------------- -- -------------- ---------------- ------------
  Crop Types                    Growing Period Translocation    Standing
                                (days)         Factor           Biomass
                                                                (kg/m^2^)

  Leafy vegetables              70             1                0.7

  Root vegetables               70             0.1              0.7

  Fruit and Grain               145            0.1              0.7

  Poultry, Milk, and Egg        145            0.1              2.4
  Animal Feed                                                   

  Beef and Milk Animal          30             1                1.8
  Forage                                                        
  -------------------------- -- -------------- ---------------- ------------

Although the radionuclides are absorbed through the plant surfaces, once
inside the plant radionuclides migrate to different parts of the plant
at different rates. Since the interest is in the edible portion of the
crop, the fraction of absorbed radionuclides that migrate to that edible
part, termed the "translocation factor", is an important quantity; the
translocation factor has been measured for a variety of crops. Since the
absorbed radionuclides spread throughout the plant, the concentration in
the plant depends on the total biomass present; this is accounted for by
the "standing biomass", which represents the total biomass per unit area
of cultivated ground. Values of translocation factor and standing
biomass used in this study for various crops are shown in Table 8-2.

### Plant Concentration from Root Uptake

The plant concentration at harvest from root uptake is based on a
radionuclide partitioning model that assumes equilibrium between the
radionuclide concentration in the plant and the radionuclide
concentration in the soil. The concentration of radionuclides in the
plant is given by the following relation:

C~rci~ = \[C~ci~·RP~sc~·Bv~ci~·fc\]/P~3~ (8-5)

Where C~rci~ = concentration of radionuclide i in crop type c from root
uptake for a one-year period (Bq/kg wet weight)

> C~ci~ = average concentration of radionuclide i in farmland soil for
> crop type c for the current one year period (Bq/m^2^)

RP~sc~ = fraction of plant type c having roots in surface soil zone
(dimensionless)

Bv~ci~ = concentration ratio for root uptake of radionuclide i in crop
type c (Bq/kg dry plant per Bq/kg dry soil)

f~c~ = dry-to-wet ratio for plant type c (kg dry plant/kg wet plant)

P~3~ = areal soil density of farmland soil (kg/m^2^)

The fraction of plant roots in the surface soil, RP~sc~, was
conservatively determined to be 1.0 for all plants, which indicates that
the entire root system is in the soil zone where contamination is
modeled to occur. Many crops have roots mainly in the surface soil zone.
Assuming the entire root is in contaminated soil, when part may be in
uncontaminated soil, will overestimate radionuclide uptake by the plant.
Because this fraction was set to a value of 1.0, the fraction of the
root below the surface soil (15 cm depth) was set to 0 in all cases. The
areal density of farmland was set to 240 kg/m^2^, consistent with values
previously used for bulk soil density and soil depth. Values used for
the concentration ratio for root uptake of radionuclide i in crop type c
(Bq/kg dry plant per Bq/kg dry soil) are shown in Table 8-3. The source
for these values is provided in [Appendix
E](../Appendix_E%20ready%20for%20CDC%20031405.doc). The dry to wet ratio
for various plant types is shown in Table 8-4.

A three-step process, shown in Figure 8.1, was used to select transfer
factor values for this report (i.e., the concentration ratios shown in
Table 8-3 and Table 8-5). The first step was to consider values based on
local data. Where local data were not available, generic values were
used. Among generic values, those that were applicable to conditions at
the SRS were preferred; e.g., choosing an uptake factor of 1.7 for root
vegetables, which was the average for sand and pH of 5, corresponding to
conditions at the SRS. The first reference source for generic values was
*Handbook of Parameter Values for the Prediction of Radionuclide
Transfer in Temperate Environments* (5). Where (5) did not contain a
generic value, the GENII default was used.

Table 8‑3 Values of ratio of concentration in plants to concentration in
soil (kg/kg) for various plant types and elements.

  ------------------------------------------------------------------------
               PLANT TYPE                                    
               (c)                                           
  ------------ ------------ --------------- ---------------- -------------
  Element (i)  Cereal and   Leafy           Other Vegetables Fruit
               Grain        Vegetables,     and Root         
                            Forage, and Hay Vegetables       

  P            3.5          3.5             3.5              3.5

  S            1.5          1.5             1.5              1.5

  Co           3.7E-03      0.20            0.11             7 E-03

  Zn           1.6          3.3             12               0.9

  Sr           0.15         1.7             1.7              0.2

  Y            0.01         0.01            0.01             0.01

  Nb           2.5 E-2      2.5 E-2         1.7E-02          2.5E-02

  Zr           1E-03        1E-03           1E-03            1E-03

  Tc           0.73         940             28               1.5

  Ru           5E-03        0.2             0.04             0.04

  I            2E-02        3.4E-03         0.02             0.02

  Cs           0.02         5.31            0.9              0.22

  Ce           3E-02        3E-02           3E-02            3E-02

  Th           3.4E-05      1.1E-02         6.9E-03          2.5E-04

  U            1.3E-03      2.3E-02         0.011            4 E-03

  Pu           6E-05        2.2E-03         8.2E-04          4.5E-05

  Am           2.2E-05      0.067           8.4E-04          2.5E-04
  ------------------------------------------------------------------------

Table 8‑4 Variable values for the dry-to-wet ratio for plant type c (kg
dry plant/kg wet plant).

+--------------------------------------+---+----------------------------+
| Crop Types                           |   | Ratio of Dry to Wet Weight |
|                                      |   |                            |
| \(c\)                                |   |                            |
+--------------------------------------+---+----------------------------+
| Leafy vegetables                     |   | 0.1                        |
+--------------------------------------+---+----------------------------+
| Root vegetables and Fruit            |   | 0.15                       |
+--------------------------------------+---+----------------------------+
| Grain                                |   | 0.91                       |
+--------------------------------------+---+----------------------------+
| Poultry and Egg Animal Feed          |   | 0.91                       |
+--------------------------------------+---+----------------------------+
| Beef and Milk Animal Forage          |   | 0.22                       |
+--------------------------------------+---+----------------------------+

Figure 8‑1 Process for Selecting Transfer Factor Values

## Radionuclide Concentration in Terrestrial Animals

The concentration of radionuclides in food products from terrestrial
animals (e.g., eggs, milk, and beef) depends on the amount of
radionuclides ingested by those animals. In general, animals may ingest
contaminated water, soil, and feed (food). For this study drinking water
for either animals or humans was not considered to be contaminated in
the vicinity of the SRS. This is because water contaminated with liquid
releases from the SRS was not used for such purposes, while the water
that was used was likely to be uncontaminated (well water) or only
slightly contaminated (reservoirs and ponds). Consequently, uptake of
radionuclides by terrestrial animals from drinking water was not modeled
in this study. Because ingestion of soil was not considered to be a
significant pathway for animal product contamination, it was not modeled
in this study.

For ingestion of feed as the viable pathway for radionuclide ingestion,
the concentration of radionuclides in animal products is given by
equation (9.9) in the GENII SDD and which is adapted here:

$C_{\text{hai}} = F_{\text{ai}} \cdot \sum_{f = 1}^{N_{\text{af}}}{C_{\text{cfi}}d_{\text{af}}}U_{\text{af}}$
(8-5)

Where,

> C~hai~ = concentration of radionuclide i in animal product a at
> harvest of the animal product for a one-year period (Bq/kg)
>
> F~ai~ = transfer coefficient that relates daily intake rate by an
> animal to the concentration in an edible animal product a (e.g., Bq/(L
> milk) per Bq/d for milk, and Bq/(kg meat) per Bq/d for beef)
>
> C~cfi~ = concentration of radionuclide i in animal feed type f at the
> time of consumption for a one-year period (Bq/kg wet weight)

N~af~ = number of feed types, f, fed to animal type a

> d~af~ = fraction of animal type a feed type f intake that is
> contaminated (dimensionless)
>
> U~af~ = daily feed intake rate for animal type a of feed type f
> (kg/d).

The calculation of the concentration in animal feed grown in the
vicinity of the SRS has been discussed in Section 8.3. There were 5
types of animal feed considered for this study: poultry, milk, and egg
animal feed and beef and milk animal forage. Although beef animal feed,
in general, is a possibility, site specific information indicated that
cattle farmers near the SRS used forage exclusively. Table 8-5 shows
values for each feed type for d~af~ (the fraction of feed type f intake
that is contaminated) and U~af~ (daily feed intake rate for feed type f
(kg/d)). Table 8-6 shows the values of the transfer factors, F~ai~, for
the various radionuclides and the four animal types considered.

Table 8‑5 Values used for the point-estimate case for the variables:
fraction of feed that is contaminated and daily feed intake rate (kg/d)

+------------------------+------+--------------+--------------------+---+
| Feed Types             |      | Fraction     | Feed Intake Rate   |   |
|                        |      | Contaminated |                    |   |
|                        |      |              | (kg/d)             |   |
+------------------------+------+--------------+--------------------+---+
| Poultry Animal Feed    |      | 0.5          | 0.12               |   |
+------------------------+------+--------------+--------------------+---+
| Milk Animal Feed       |      | 1.0          | 13                 |   |
+------------------------+------+--------------+--------------------+---+
| Egg Animal Feed        |      | 0.5          | 0.12               |   |
+------------------------+------+--------------+--------------------+---+
| Beef Animal Forage     |      | 1.0          | 36                 |   |
+------------------------+------+--------------+--------------------+---+
| Milk Animal Forage     |      | 1.0          | 29                 |   |
+------------------------+------+--------------+--------------------+---+

Table 8‑6 Values of transfer factors used for the point estimate case;
i.e., the ratio of concentration in animal products to daily rate of
intake of radionuclides

+---------+--------------+--------------+--------------+--------------+
| Element | ANIMAL       |              |              |              |
| (i)     | PRODUCT (a)  |              |              |              |
+=========+==============+==============+==============+==============+
|         | Animal Milk  | Beef Animal  | Poultry      | Egg          |
|         |              | Meat (kg/kg) |              |              |
|         | (kg/L)       |              | (kg/kg)      | (kg/kg)      |
+---------+--------------+--------------+--------------+--------------+
| P       | 1.6E-02      | 5E-02        | 0.19         | 1.0          |
+---------+--------------+--------------+--------------+--------------+
| S       | 1.6E-02      | 2E-01        | 2.3          | 7.0          |
+---------+--------------+--------------+--------------+--------------+
| Co      | 3E-04        | 1E-02        | 2            | 0.1          |
+---------+--------------+--------------+--------------+--------------+
| Zn      | 0.01         | 0.1          | 7            | 3            |
+---------+--------------+--------------+--------------+--------------+
| Sr      | 2.8E-03      | 8E-03        | 0.08         | 0.2          |
+---------+--------------+--------------+--------------+--------------+
| Y       | 2E-05        | 1E-03        | .01          | .002         |
+---------+--------------+--------------+--------------+--------------+
| Nb      | 4.1E-07      | 3E-07        | .0004        | .001         |
+---------+--------------+--------------+--------------+--------------+
| Zr      | 5.5E-07      | 1E-06        | 6E-05        | .0002        |
+---------+--------------+--------------+--------------+--------------+
| Tc      | 2.3E-05      | 1E-04        | 0.03         | 3            |
+---------+--------------+--------------+--------------+--------------+
| Ru      | 3.3 E-06     | 0.05         | 8            | .005         |
+---------+--------------+--------------+--------------+--------------+
| I       | 0.01         | 0.04         | 0.01         | 3            |
+---------+--------------+--------------+--------------+--------------+
| Cs      | 7.9E-03      | 0.05         | 10           | 0.4          |
+---------+--------------+--------------+--------------+--------------+
| Ce      | 3E-05        | 2E-05        | .003         | 6.5E-05      |
+---------+--------------+--------------+--------------+--------------+
| Th      | 5E-06        | 4E-05        | .006         | .004         |
+---------+--------------+--------------+--------------+--------------+
| U       | 4E-04        | 3E-04        | 1            | 1            |
+---------+--------------+--------------+--------------+--------------+
| Pu      | 1.1E-06      | 1E-05        | .003         | .0005        |
+---------+--------------+--------------+--------------+--------------+
| Am      | 1.5E-06      | 4E -05       | .006         | .004         |
+---------+--------------+--------------+--------------+--------------+

## Radionuclide Concentration in Aquatic Animals

Uptake of radionuclides by aquatic animals can be complex because these
animals eat other organisms that are contaminated and may take up
radionuclides directly from the water they inhabit. For fish, which is
an important human food, the uptake of radionuclides is generally
simplified. The simplified model states that the radionuclide
concentration in a given type of aquatic food is equal to the product of
the radionuclide concentration in water and a bioaccumulation factor,
which is specific for the organism and radionuclide. This simplified
approach, which is implemented in the GENII computer code, is stated by
the equation:

C~hqi~ = C~wi~·B~qi~ (8-6)

Where,

> C~hqi~ = concentration of radionuclide i in aquatic food type q at the
> time of harvest (Bq/kg);
>
> C~wi~ = average concentration of radionuclide i in water over the
> year;
>
> B~qi~ = bioaccumulation[^1] factor for radionuclide i in aquatic food
> type q (Bq/kg in wet fish per Bq/L of water).

The concentration in water at the two water exposure locations used in
the study is obtained as described in [Chapter
7](../Chapter_07_Ready%20for%20CDC%20031405.doc). The bioaccumulation
factors for freshwater fish that have been used in this study are
presented in Table 8-7; [Appendix
F](../Appendix_F%20tables%20-%20Ready%20for%20CDC%20031405.doc) contains
a description of how these were obtained. Some are generic values, but
several (marked with "‡") are specific to the SRS site.

Table 8‑7 Bioaccumulation Factors Selected for Freshwater Fish
Consumption (L/kg)

  ----------------- ----------------- ----------------- -----------------
  Element\*         Factor            Element\*         Factor

  P                 50,000†           Ru                10†

  S                 800†              I                 40†

  Co                300†              Cs                4,700‡

  Zn                1,000†            Ce                30†

  Sr                450‡              Th                100†

  Y                 30†               U                 10†

  Nb                300†              Pu                4,700‡

  Zr                300†              Am                2,400‡

  Tc                20†                                 
  ----------------- ----------------- ----------------- -----------------

> \*The same factor is used for each isotope of an element.
>
> ^†^International Atomic Energy Agency (IAEA) default value based on
> several references (5).
>
> ^‡^Selected value calculated from data in (6), or from a combination
> of data from (6) and (5).

## Effect of Delay and Storage Times on Concentrations

The expressions for radionuclide concentrations in plants, terrestrial
animals, and aquatic animals presented respectively in Sections 8.2,
8.3, and 8.4 are correct for the time of harvest or collection. However,
the consumption of these foods, by either humans or animals, may be
delayed. Animal foods, such as forage or feed, may be stored after
harvest and fed to the animals at a later time; during storage
radionuclides will continue to undergo radioactive decay, which results
in lower concentrations in the foods when they are consumed. Similarly
human foods, such as vegetables, slaughtered meat, fish, and milk, may
be delayed getting to market and table, because the food must be
processed, or transported, or both. Again radioactive decay will reduce
the radionuclide concentration in the foods consumed.

The concentration of the food consumed is given by the following
expression:

C~efi~ = C~hfi~·exp{-λ~i~T~d~} (8-7)

Where,

> C~efi~ = the concentration in animal or human food at the time it is
> consumed;
>
> C~hfi~ = the concentration in animal or human food at the time it is
> harvested;
>
> λ~i~ = radioactive decay constant for radionuclide i in units
> commensurable with T~d~;
>
> T~d~ = the storage time for animal feed or the delay time for human
> food.

Radionuclide concentrations in animal and human food are derived
according to the methods described in the three sections preceding this
one. Radioactive decay coefficients are contained in a library file in
the GENII code. Storage times for animal foods are listed in Table 8-8.
These times were based on site specific information (7). Both beef and
milk animals were allowed to forage all year long, so there was never
any storage time associated with this type of animal food.

Table 8‑8 Values used for the point-estimate case for the storage time
for animal foods

A value for each animal feed type (f) used is shown.

+-----------------------------------+----------------+-----------------+
| Feed Types                        | Index          | Storage Time    |
|                                   |                |                 |
|                                   | \(f\)          | \(d\)           |
+-----------------------------------+----------------+-----------------+
| Poultry Animal Feed               | 2              | 180             |
+-----------------------------------+----------------+-----------------+
| Milk Animal Feed                  | 3              | 90              |
+-----------------------------------+----------------+-----------------+
| Egg Animal Feed                   | 4              | 180             |
+-----------------------------------+----------------+-----------------+
| Beef Animal Forage                | 5              | 0               |
+-----------------------------------+----------------+-----------------+
| Milk Animal Forage                | 6              | 0               |
+-----------------------------------+----------------+-----------------+

Delay times between food product harvest and consumption are listed in
Table 8-9 (plant products) and Table 8-10 (animal products). These delay
times were based on a combination of generic and site specific data, as
explained and referenced in [Appendix
F](../Appendix_F%20tables%20-%20Ready%20for%20CDC%20031405.doc). In
addition, the characteristics stated for the scenarios were used to
interpret this source data and obtain values appropriate for this study.
For example, longer delay times were assumed for persons living in
cities or towns than for farmers or persons eating food grown in their
own gardens or raised in their backyards. Three families consumed fish
obtained from waters contaminated by liquid releases from the SRS: the
Outdoors Person Family, Family Living Near the River, and Delivery
Person Family; the delay time for fish consumed by these receptors was
taken to be one day. Fish consumed by other families was considered to
be uncontaminated, because the waters they fished were not contaminated
by liquid releases from the SRS.

Table 8‑9 Delay Times for Consumption of Plant Products (days)

  -------------- -------------- -------------- -------------- --------------
  Location       Leafy          Root           Fruit          Grain
                 Vegetables     Vegetables                    

  Girard         1              14             1              1

  Waynesboro     NA\*           14             NA             NA

  Augusta        7              14             7              7

  Jackson        1              14             1              1

  New Ellenton   1              14             1              1

  Barnwell       7              14             7              7

  Martin         1              14             1              1

  Allendale      NA             14             NA             NA

  Williston      1              14             1              1

  Onsite         NA             14             NA             NA

  \*NA -- not                                                 
  applicable.                                                 
  -------------- -------------- -------------- -------------- --------------

Table 8‑10 Delay Times for Consumption of Terrestrial Animal Products
(days)

+-------------+-------------+-------------+-------------+-------------+
| Location    | Beef        | Poultry     | Milk        | Eggs        |
+-------------+-------------+-------------+-------------+-------------+
| Girard      | 6           | 6           | 3           | 1           |
+-------------+-------------+-------------+-------------+-------------+
| Waynesboro  | NA\*        | NA\*        | NA\*        | NA\*        |
+-------------+-------------+-------------+-------------+-------------+
| Augusta     | 6           | 6           | 3           | 7           |
+-------------+-------------+-------------+-------------+-------------+
| Jackson     | 6           | 6           | 3           | 1           |
+-------------+-------------+-------------+-------------+-------------+
| New         | 6           | 6           | 3           | 1           |
| Ellenton    |             |             |             |             |
+-------------+-------------+-------------+-------------+-------------+
| Barnwell    | 6           | 6           | 3           | 7           |
+-------------+-------------+-------------+-------------+-------------+
| Martin      | 6           | 6           | 3           | 1           |
+-------------+-------------+-------------+-------------+-------------+
| Allendale   | NA^‡^       | NA^‡^       | NA^‡^       | NA^‡^       |
+-------------+-------------+-------------+-------------+-------------+
| Williston   | 6           | 6           | 3           | 1           |
+-------------+-------------+-------------+-------------+-------------+
| Onsite      | NA^‡^       | 6           | NA^‡^       | NA^‡^       |
+-------------+-------------+-------------+-------------+-------------+
| \*NA -- not |             |             |             |             |
| applicable; |             |             |             |             |
| location    |             |             |             |             |
| for school  |             |             |             |             |
| only.       |             |             |             |             |
|             |             |             |             |             |
| ^‡^NA --    |             |             |             |             |
| not         |             |             |             |             |
| applicable; |             |             |             |             |
| location    |             |             |             |             |
| for work    |             |             |             |             |
| only.       |             |             |             |             |
+-------------+-------------+-------------+-------------+-------------+

## Special Models for Tritium and Carbon-14 

The models describing uptake of radionuclides by vegetation and animals
presented in Section 8.3 and 8.4 are largely based on an assumption that
the radionuclides are trace elements in the plant and animal tissues;
i.e., the concentrations of radionuclides and non-radioactive isotopes
of the same element in the organism are small. Generally this is the
case and the models are appropriate. However, for two important
radionuclides, tritium and carbon-14, this assumption is not true. In
the environment, carbon-14 behaves much like its non-radioactive, more
common isotope carbon-12. Similarly tritiated water, with each water
molecule containing either one tritium atom and one normal hydrogen atom
or two tritium atoms, behaves like normal water in the environment. A
principal component of plant and animal tissue is carbon and both plants
and animals contain significant percentages of water (plant tissue is
usually 75% or higher water by weight).

The GENII code has incorporated special models for plant uptake of
tritium and carbon-14, as described in Section 9.6 of the GENII SDD (1):

The concentrations of tritium or carbon-14 in environmental media (soil,
plants, and animal products) are assumed to be related to the specific
activity (Becquerels of radionuclide per kilogram of soluble element) in
the contaminating medium (air or water). The fractional content of
hydrogen or carbon in a plant or animal product is then used to compute
the concentration of tritium or carbon-14 in the food product under
consideration. The hydrogen contents in both the water and the non-water
(dry) portion of the food product are used when calculating the tritium
concentration. The creation of organically-bound tritium \[OBT\] in
plant and animal products from intake of HTO \[tritiated water\] is also
addressed.

The model for tritium also considers tritium released in elemental form.
Further details of these special models may be found in the cited
reference.

Because of the specialized nature of these models, the default values
for various coefficients built into the GENII code were used. Until the
results of this study were evaluated to estimate the significance of
doses from tritium, it was not considered appropriate to evaluate
whether site-specific characteristics warranted changes in these values
or if any site-specific data to support such changes were available.
Nevertheless, a few variables used in these models were specifically
selected for this study.

The tritium model for uptake by plants has as a key parameter the
absolute humidity of the air; this is because the relative fraction of
tritiated water to normal water in the air is given by the ratio of the
tritium concentration to the absolute humidity. The value selected for
the point estimate case is a site-specific average over 42 years and has
a value of 0.01125 kg/m3, with a standard deviation of 0.00053 kg/m3
(8). Tritium concentrations in animals are based on their uptake of
tritium from contaminated water and food. Since the water consumed by
livestock was considered to be uncontaminated (as explained elsewhere),
the fraction of contaminated water was set to zero, which meant the
uptake from water was zero. However, as explained in Section 8.4,
livestock did consume contaminated crops; the quantities of crops
consumed and fractions contaminated specified in Section 8.4 were also
employed in the tritium model.

A key variable in determining the uptake of carbon-14 by plants from the
atmosphere is the ratio of carbon-14 to normal carbon. The concentration
of carbon-14 in the vicinity of the SRS is calculated according to the
approach described in [Chapter
5](../Chapter_05_Ready%20for%20CDC%20031405.doc); the concentration of
normal carbon in the atmosphere is based on a nominal value, which is
built-into the code.

## References

1.  Napier et al., 2002. Napier, B.A., et al. GENII Version II Software
    Design Document. PNNL for U.S. Environmental Protection Agency.
    November 2002.

2.  Hemond and Fechner-Levy, 2000. Hemond, Harold F. and Elizabeth J.
    Fechner-Levy. Chemical Fate and Transport in the Environment.
    Academic Press, 2000, New York. ISBN 0-12-340275-1.

3.  National Climatic Data Center, 2003, National Oceanographic and
    Atmospheric Administration. Normal Monthly precipitation (Inches).
    <http://www.ncdc.noaa.gov/oa/climate/online/ccd/nrmlprcp.html>.
    Downloaded June 28,2004

4.  Kennedy and Strenge, 1992. Kennedy, W.E. and D.L. Strenge. Residual
    Radioactive Contamination from Decommissioning, Technical Basis for
    Translating Contamination Levels to Annual Total Effective Dose
    Equivalent, Final Report. NUREG/CR-5512, PNL-7994. October 1992.

5.  IAEA, 1994. International Atomic Energy Agency. Handbook of
    Parameter Values for the Prediction of Radionuclide Transfer in
    Temperate Environments. Technical Reports Series No. 364. Vienna,
    Austria. 1994.

6.  Friday, 1996. Friday GP, Cummins CL, and Schwartzman AL.
    Radiological Bioconcentration Factors for Aquatic, Terrestrial, and
    Wetland Ecosystems at the Savannah River Site (U). Aiken, SC,
    Westinghouse Savannah River Company Report: WSRC-TR-96-0231; 1996.

7.  Hamby, 1991. Land and water use characteristics in the vicinity of
    the Savannah River Site. WSRC-RP-91-17. Westinghouse Savannah River
    Company. Aiken, South Carolina. March 1, 1991.

8.  Hamby, 1993. Hamby DM. A Probabilistic Estimation of Atmospheric
    Tritium Dose. Health Physics; 65(1):33-40. July 1993.

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[^1]: Some authors reserve the term "bioaccumulation" for processes in
    which uptake of contaminants is from both water and food, while the
    term "bioconcentration" is used to describe uptake that depends only
    on the water concentration, as stated in equation (8-6). (2)