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4.3.4.2. Vegetation concentrations
Vegetation concentrations are required for the estimation of exposure to homegrown
fruits and vegetables, and also for the beef and dairy food chain algorithms. Three
principal assumptions are made to estimate vegetative concentrations:
. Outer surfaces of bulky below ground vegetation are impacted by soils which
contain dioxinlike compounds. Inner portions are largely unimpacted.
. Translocation of dioxinlike compounds from roots to above ground portions of
plants are negligible compared to other mechanisms which impact above ground portions of
plants. As such, translocation into above ground portions will be assumed to be zero.
. Similar to the assumption concerning transport of contaminants from outer to
inner portions of below ground vegetation, it will be assumed that outer and not inner
portions of above ground bulky vegetation are impacted.
Concentration of contaminants in below ground vegetation is only required for
vegetables (carrots, potatoes, e.g.) grown underground. The basis for the below ground
algorithm is the experiments of Briggs, et al. (1982) on uptake of contaminants into
barley roots from growth solution, and their elaboration of a Root Concentration Factor.
The below ground concentration is given by:
Two processes, airborne vapor phase absorption and airborne particle deposition, are
assumed to contribute to above ground vegetation concentrations:
The basis for a vaporphase bioconcentration factor for various airborne contaminants,
including 1,2,3,4TCDD, from the atmosphere to vegetation was developed by Bacci, et al.
(1990, 1992), with amendments suggested by McCrady and Maggard (1993), and McCrady (1994).
Bacci and coworkers conducted laboratory growth chamber experiments on the vaporphase
transfer of 14 organic compounds from air to azalea leaves, and developed a generalized
model to predict the vaporphase bioconcentration factor based on a contaminant Henry's
Constant, H, and octanol water partition coefficient, Kow.
A similar experiment by McCrady
and Maggard (1993) conducted for 2,3,7,8TCDD vapor transfer to grass leaves suggested
that the Bacci empirical algorithm to estimate the transfer factor would greatly
overestimate it. Further details on these experiments are in the section below on this
critical bioconcentration parameter, termed B_{vpa} in this assessment. The
algorithm estimating plant concentrations as a function of vaporphase air concentrations
is:
Several exposure efforts for 2,3,7,8TCDD (Fries and Paustenbach, 1990; Stevens and
Gerbec, 1988; Connett and Webster, 1987; Travis and HattemerFrey, 1991), have modeled the
accumulation of residues in vegetative matter (grass, feed, vegetables) resulting from
deposition of contaminated particulates. Key components of their approach, as well as the
one for this assessment, include:
. Vegetative concentrations result from particulate deposition onto plant
surfaces.
. Vegetative dry matter yield is the reservoir for depositing contaminants; this
reservoir varies according to crop.
. Not all particulate deposition reaches the plant, some goes directly to the
ground surface; the "interception fraction", less than 1.0, reduces the total
deposition rate. This fraction can be related to the percent ground that is covered by the
vegetation.
. Weathering processes, such as wind or rainfall, remove residues that
have deposited onto plant surfaces via particle deposition, and this process is reasonably
modeled as a firstorder exponential loss with an associated weathering dissipation rate.
All the above references have justified a dissipation rate derived from a halflife of 14
days (based principally on field measurements described in Baes, et al. (1984)); this is
the value used for all dioxinlike compounds in this assessment as well. As well, a
portion of particles depositing as wet deposition are not retained on the vegetation after
the rainfall. A retention factor reduces total wet deposition considering this.
. Vegetative concentrations may not reach steady state because of harvesting or
grazing, but a steady state algorithm is used.
The steady state solution for plant concentrations attributed to wet plus dry particle
deposition is:
The unit contaminant wet plus dry deposition rate, F, is given as:

Following is brief guidance on assignment of values to the terms in Equations (423) to
(427). 
. C_{s} and Kd_{s}:

This is the soil concentration and
soil/water partition coefficient, respectively. The soil concentration is specified for
the onsite source category. For the two source categories where soil contamination is
distinct from the site of exposure, the soil concentration at the site of exposure is
estimated. As discussed in Section 4.4.1 below, two soil concentrations including one for
a notill and one for a tilled situation, are estimated. For estimating below ground
vegetable concentration, the tilled concentration is required. The soil partition
coefficient is a function of the contaminant organic carbon partition coefficient, Koc,
and the soil organic carbon fraction, OC_{sl}, as discussed above in Section
4.3.1. Division of C_{s} by Kd_{s} results in the equilibrium soluble
phase concentration of the contaminant, in mg/L.

. RCF:

Briggs, et al. (1982) conducted experiments measuring the uptake of
several compounds into barley roots from growth solution. He developed the following
relationship for lipophilic compounds tested (lipophilic compounds were identified as
those tested that had log Kow 2.0 and higher (n=7, r=0.981):
Since his experiments were conducted in growth solution, the RCF is most appropriately
applied to soil water in field settings. This is why the C_{s} was divided by Kd_{s} in Equation (423).

. VG_{bg}:

This correction factor and the one used to correct for
airtoleaf transfer of contaminants, VG_{ag}, are based on a similar hypothesis.
That hypothesis for VG_{bg} is that the uptake of lipophilic compounds into the
roots of this experiments is due to sorption onto root solids. High root concentrations
were not due to translocation to within portions of the root hairs. Direct use of the RCF
for estimating concentrations in bulky below ground vegetation would greatly overestimate
concentrations since an assumption (stated above) is that there is insignificant
translocation to inner parts of below ground bulky vegetation for the dioxinlike
compounds. Concentrations in outer portions of edible below ground vegetation would mirror
concentrations found in barley roots, by this hypothesis.
VG_{bg} can be estimated by assuming that the outer portion, or skin, of below
ground vegetables would contain concentrations that can be predicted directly using the
RCF, but that the inner portions would effectively be free of residue. The correction
factor can be estimated as the ratio of the mass of the outer portion to mass of the
entire vegetable:
Simplifying assumptions are now made to demonstrate this ratio for a carrot and a
potato. First, it will be assumed that the density of the skin and of the vegetable as a
whole are the same, so the above can become a skin to whole vegetable volume ratio. The
thickness of the skin will be assumed to be same as the thickness of the barley root for
which the RCF was developed. Without the barley root thickness in Briggs, et al. (1982),
what will instead be assumed is that the skin thickness is equal to 0.03 cm. This was the
thickness of a leaf from broadleaved trees assumed by Riederer (1990) in modeling the
atmospheric transfer of contaminants to trees.
The shape of a carrot can be assumed to be
a cone. The volume of a cone is given as (p /3)r^{2}l,
where r is a radius of the base and l is length. Assuming a carrot base radius of 1 cm and
a length of 15 cm, the volume is 16 cm^{3}. The curved surface area of a cone is
given as: p r(r^{2} + l^{2})^{1/2},
which equals 47 cm^{2}, given the r and l assumptions. The volume of the cone
surface area is 47 cm^{2} * 0.03 cm, or 1.41 cm^{3}. The skin to whole
plant ratio for this carrot is 0.09 (1.41/16). A similar exercise is done for a potato,
assuming a spherical shape with a radius of 3 cm. The volume is given as 4/3p r^{3}, or 113 cm^{3}. The surface area of a sphere
is 4p r^{2}, or 113 cm^{2}, and the volume of
this surface area is 3.39 cm^{3}. The skin to whole plant ratio for the potato is
0.03.
This exercise indicates upper bounds for such an empirical parameter. For exposure
assessments, other factors which reduce vegetative concentrations should also be
considered and will be considered in this empirical correction factor in this assessment.
Additional reductions in concentration result from peeling, cooking, or cleaning, for
example. Wipf, et al. (1982) found that 67% of unwashed carrot residues of 2,3,7,8TCDD
came out in wash water, and 29% was in the peels.
A peeled, washed carrot correction
factor might instead be, 0.09*0.04, or 0.004 (0.09 from above; 0.04 = 100%  67%  29%). A
96% reduction in the estimated VG_{bg} for the potato (the potato is cleaned and
the skin is not eaten; additional reductions possibly when cooking the potato) would equal
0.001. In a sitespecific application, the type of vegetation, preparation, and so on,
should be considered. The VG_{bg} for underground vegetables for this assessment
is assumed to be 0.01. This is less than the estimates of 0.09 and 0.03 for the carrot and
potato above, but greater than it might be if based on this discussion on cleaning,
washing, peeling, and so on.

. C_{va}, C_{pa}:

The vaporphase concentration of
contaminant in air, C_{va}, used in this algorithm is estimated using procedures
described in Section 4.3.2 above. The particlephase concentration of contaminant in air,
C_{pa}, is estimated using procedures described in Section 4.3.3.

. B_{vpa}:

Bacci, et al. (1990, 1992) conducted laboratory
experiments on the airtoleaf transfer of vaporphase concentrations of 14 organic
contaminants to azalea leaves. With their results, they developed an empirical
relationship for a vaporphase bioconcentration factor from air to azalea leaves, termed
in this assessment the B_{vpa}, but which was termed BCF by Bacci and coworkers.
They related the B_{vpa} to the chemical octanolwater and airwater partition
coefficients, Kow and Kaw. The airwater partition coefficient, Kaw, is a dimensionless
form of Henry's Law constant, H, derived by dividing H by the product of the ideal gas
constant, R, and the temperature, T. The most general form of the airtoleaf transfer
factor is on a unitless volumetric basis: [ng contaminant/L or leaf]/[ng contaminant/L of
air], and is given as:
Bacci, et al. (1990) showed that the volumetric transfer factor can be transformed to a
massbased transfer factor by assuming that 70% of the wet leaf is water, the leaf density
is 890 g/L, and the air density is 1.19 g/L:
Bacci's experiments were conducted under conditions which would not account for
photodegradation of his test chemicals from the leaf surfaces. A recent study by McCrady
and Maggard (1993) which investigated the uptake and photodegradation of 2,3,7,8TCDD
sorbed to grass foliage suggests a significant difference in experimental B_{vol} for grass plants. The authors note that the log B_{vol} for 2,3,7,8TCDD and
azalea plants, using Bacci's empirical relationship, was estimated as 8.5. The
experimental log B_{vol} for 2,3,7,8TCDD and grass plants reported by McCrady was
6.9 when photodegradation was accounted for, and 7.5 in the absence of photodegradation.
Since the photodegradation experiments by McCrady best represent outdoor conditions, their
work suggests that the airtoleaf transfer factor estimated by Bacci's algorithm may be
40 times too high for vaporphase transfer of 2,3,7,8TCDD onto grass leaves.
While McCrady's experiments included consideration of photodegradation of 2,3,7,8TCDD,
it is uncertain as to how their results can be generalized to other dioxinlike compounds
and vegetations other than grass. There is very little information in the literature on
the photodegradation of dioxins and furans on plant surfaces. McCrady and Maggard (1993)
cite Crosby and Wong (1977) as the only other work measuring photodegradation of
2,3,7,8TCDD from leaf surfaces. In that work, 2,3,7,8TCDD was applied as a 15 ppm
concentration in Agent Orange, and McCrady speculated that the rapid photodegradation
measured in those experiments occurred because the herbicide formulation contained
carriers and organic solvents that may have promoted photodegradation.
Some experiments
conducted in organic solvents (Crosby, et al., 1971; Buser, 1976) and in water (Friessen,
et al., 1990) noted reductive dechlorination resulting in dioxin compounds of lower
chlorination. Other experiments did not find such reductive dechlorination (Dulin, et al.,
1986; Friessen, et al., 1990 who found reductive dechlorination in one experiment, but not
in another). An important issue to consider, at least, for the process of photodegradation
of dioxins and furans on leaf surfaces is the possible formation of lower chlorinated
congeners of nonzero toxic equivalency.
Another issue discussed by McCrady is that the theoretical time for the grass tissue to
reach a steady state in his experiments is much shorter than that indicated in the Bacci
experiments. Using Bacci's results, McCrady noted that the azalea leaves theoretically
take greater than 400 days to reach equilibrium, in comparison to less than 20 days to
reach equilibrium for the grass plants in his experiments. This difference is not entirely
due to photodegradation. McCrady (personal communication, J. McCrady, Corvallis
Environmental Research Laboratory, EPA) suggests that the 50day exposure time used in
Bacci's experiments may allow for considerable diffusion into the newly formed plant
surface wax.
The sorbed TCDD residues may be trapped and unable to volatilize. Thus, for
estimating contaminant concentrations in animal feeds such as relatively shortlived grass
plants, the equilibrium B_{vol} from the Bacci azalea model may overestimate the
contaminant concentration in grass. On the other hand, McCrady's experiments may have been
conducted in too short a time frame, with the sum of uptake and elimination phases being
less than 10 days in the various experimental designs. The volatilization and
photodegradation rates reported by McCrady may be higher than what might occur for the
longer exposure times expected in real world situations, where growth and residue trapping
may occur.
These arguments are being presented to demonstrate the uncertainty in choosing either
of the two reported B_{vol} values for estimating plant contaminant
concentrations. McCrady's results pertaining to 2,3,7,8TCDD cannot be generalized to
other dioxinlike compounds or other contaminants in terms of commonly available
contaminant parameters such as H or Kow. Therefore, a McCrady framework similar to Bacci's
for estimating congenerspecific B_{vpa} cannot be offered at this time. On the
other hand, their work strongly suggests that the Bacci model may be inappropriate for
terrestrial vegetations of this assessment, including vegetables/fruits and vegetations of
the beef/milk food chain model, and Bacci's experiments, because of their length of time,
the use of an azalea leaf of high wax content, and lack of an artificial light source
simulating photodegradation, are likely to have overestimated the air to leaf transfers.
What will be done for this assessment is to first estimate a congenerspecific B_{vol} using the Bacci algorithm of Equation (430) above. Then, it will be transformed into a
massbased B_{vpa} as in Equation (431), except that the assumptions McCrady and
Maggard used for fraction of grass plant that is wet weight, 85%, and the grass leaf
density, 770 g/L, will instead be used as more representative of vegetations of this
assessment. Most importantly, the B_{vpa} calculated this way will be empirically
reduced by a factor of 40 for all dioxinlike congeners as suggested by the difference in
McCrady's experiments as compared to Bacci's.
It should be noted that all bioconcentration or biotransfer parameters, such as the B_{vpa},
are qualified as second order defaults for purposes of general use. Section 6.2. of
Chapter 6 discusses the use of parameter values selected for the demonstration scenarios,
including a categorization of parameters. Second order defaults are defined there as
parameters which are theoretical and not site specific, but whose values are uncertain in
the published literature. The parameter values in this category should be considered
carefully by users of the methodology.

. VG_{ag}:

The same discussion for this correction factor for below
ground vegetation applies here. Fruits such as apples, pears, plums, figs, peaches, and so
on, can be approximated by spheres, and upper bound estimates of correction factors would
be less than 0.05. Peeling, cooking, and cleaning further reduces residues. The VG_{ag} for unspecified above ground fruits and vegetables in this assessment is assumed to be
0.01. Like VG_{bg}, this value is assigned considering that it should be less than
estimated just based on surface volume to whole fruit volume ratios.
Two other VG_{ag} values are required for this assessment. One is for pasture
grass and the other for other vegetations consumed by cattle. Both are required to
estimate concentrations in these vegetations consumed by cattle in order to estimate beef
and milk concentrations. A VG_{ag} value of 1.0 was used to estimate pasture grass
concentrations since there appears to be a direct analogy to exposed azalea and grass
leaves. However, VG should be less than the other general category of cattle vegetations
defined in this assessment, "hay/silage/grain". Recognizing that pasture grass
is important in terms of amount consumed in the lifetime of a beef cow, and the fact that
it is a leafy vegetation, it is considered seperately, whereas other cattle vegetations
are lumped together in this second category.
As described below in Section 4.3.4.3, this
second general category of nongrass cattle vegetations include some thin leafy (hay) as
well as bulky (corn silage and other grains) vegetations to consider. A volume ratio of
outer surface to whole surface area to volume vegetation could be used to assign a value
to VG, if specific assumptions concerning proportions of each type of vegetative cattle
intake were made. An appropriate assumption for a fully protected vegetation such as grain
would be zero. Silage can be considered part protected and part leafy. Since specific
assumptions concerning hay/silage/grain intake are not being made for this exercise, a
simple assumption that VG equals 0.50 for hay/silage/grain is instead made, without
rigorous justification.
The only experimental evidence that a VG_{ag} for vapor transfers of
dioxinlike compounds is justified came in a recent study by McCrady (1994). McCrady
experimentally determined uptake rate constants, termed k_{1}, for vapor phase
2,3,7,8TCDD uptake into several vegetations including kale, grass, pepper, spruce
needles, apple, tomato, and azalea leaves. Recall that the similar experimental design of
both McCrady and Maggard (1993), and Bacci, et al. (1990; 1992), included an initial phase
where vegetations in experimental chambers were exposed to the vaporphase organic
chemicals. The uptake which occurs during this initial phase is described with the rate
constant, k_{1}.
A second "elimination" phase then occurs where organic
vapors are removed from the chambers and the chemicals allowed to volatilize or otherwise
dissipate from the vegetation. The rate constant for this phase is termed k_{2}. A
steady state bioconcentration factor, or B_{vpa} in this assessment, is then
estimated as k_{1}/k_{2}. The uptake rate constants from air to the whole
vegetations estimated in the recent experiments by McCrady (1994) demonstrate the concept
behind the VG parameter. The uptake rate for an apple divided by the uptake rate for the
grass leaf was 0.02 (where uptake rates were from air to whole vegetation on a dry weight
basis).
For the tomato and pepper, the same ratios were 0.03 and 0.08. The VG_{ag} was 0.01 for fruits and vegetables in this assessment, but note above that the simple
exercise with a conical carrot and spherical potato estimated a surface volume to whole
fruit volume ratio of 0.09 (carrot) to 0.03 (potato); a value of 0.01 for fruits and
vegetable empirically considers factors such as washing or peeling which would reduce
exposures. McCrady (1994) then went on to normalize his uptake rates on a surface area
basis instead of a mass basis; i.e., air to vegetative surface area instead of air to
vegetative mass. Then, the uptake rates were substantially more similar, with the ratio of
the apple uptake rate to the grass being 1.6 instead of 0.02; i.e., the apple uptake rate
was 1.6 times higher than that of grass, instead of 1/50 as much when estimated on an air
to dry weight mass basis. The ratios for tomato and pepper were 1.2 and 2.2, respectively.
In his article, McCrady (1994) concludes, "The results of our experiments have
demonstrated that the exposed surface area of plant tissue is an important consideration
when estimating the uptake of 2,3,7,8TCDD from airborne sources of vaporphase
2,3,7,8TCDD. The surface area to volume ratio (or surface area to fresh weight ratio) of
different plant species can be used to normalize uptake rate constants for different plant
species." McCrady does caution, however, that uptake rates are only part of the
bioconcentration factor estimation, and is unsure of the impact of surface area and volume
differences on the elimination phase constant, k_{2} (personnal communication, J.
McCrady, US EPA, ERLCorvallis, Corvallis, OR 97333). Still, his recent experiments do
appear to justify the use of a VG parameter since the B_{vpa} were developed on an
air to whole plant mass basis, and his results are consistent with the assignment of 0.01
for fruits and vegetables.

. kw:

Fries and Paustenbach (1990) note that this approach may overestimate
concentrations because crops can be harvested or pastures grazed before the plant
concentrations reach steady state, and that a kw based on a weathering halflife of 14
days may be too long given experimental results of Baes, et al. (1984) which showed a
range of 234 days, and a median value of 10 days. Stevens and Gerbec (1987) considered
harvest intervals by including the exponential term, (1e^{kt}), and assigning
values of t based on harvest intervals of different crops. This assessment uses a kw of
18.02 yr^{1}, which is equivalent to a halflife of 14 days.
