PAGE 8 OF 9 
4.4.3.3. Sites studied in the National Dioxin Study
The National Dioxin Study (EPA, 1987a) focused on sites of known or suspected
contamination of soil by 2,3,7,8TCDD. There were 7 "Tiers" of investigation,
with roughly decreasing expectations of finding 2,3,7,8TCDD. Tiers 1 and 2 included
2,4,5TCP production and associated disposal sites (Tier 1) and sites where 2,4,5TCP was
used as a precursor in the manufacture of pesticidal products and associated disposal
sites (Tier 2). These tiers had the highest expectation for finding 2,3,7,8TCDD. There
were originally thought to be 450 sites that would fall in Tiers 1 and 2, but after
investigation, only 100 sites were included for study. Some were downgraded into Tier 3.
Of the 100 sites studied, 20 were on or were proposed for inclusion in the Superfund
National Priorities List. Tiers 3 and 5 were associated with 2,4,5TCP formulation (Tier
3) and use (Tier 5). Tier 6 were organic chemical or pesticide manufacturing facilities
were 2,3,7,8TCDD was suspected of being present. Tier 4 included combustion sources and
are not discussed further in this section. Tier 7, basically an examination of background
areas, are also not discussed here.
Issues that are identified as important in fate and transport modeling for this
subcategory of offsite sources include concentrations, the possibility of ground water
contamination, and sitespecific characterization.
These are discussed in turn.
. Concentrations:
Only 11 of the 100 Tier 1 and Tier 2 sites were eventually
classified as requiring "no further action" because 2,3,7,8TCDD soil
concentrations were very low, < 1 ppb, or not detected (with detection limits generally
at 1.00 ppb). Where it was detected, a general trend was to find very high concentrations
where 2,4,5TCP production wastes were stored or disposed of, with much lower
concentrations at soils near these particular areas. At hot spots, concentrations were as
high as 2,000 parts per million, but generally soil concentrations were in the parts per
billion.
It was this parts per billion generalization that led to the assignment of a 1
ppb soil concentration for the demonstration of the offsite source category in Chapter 5.
There were findings in the low ppb range for Tiers 3, 5, and 6, but at much lower
frequency and no findings higher than the tens of ppb range. For exposure assessments, the
characterization of soil concentrations in a site containing hot spots has to be carefully
considered.
For site evaluations and proposed options for remediation, an areally weighted
average might be considered, although this could dilute loss estimates depending on the
area chosen  choosing a large area corresponding to property lines might, for example,
lead to an "average" concentration orders of magnitude lower than concentrations
found in hot spots. One approach which should be considered is a "hot spot"
impact compared to an areally averaged impact. It should also be remembered that removal
of highly contaminated soils is a common practice and another option for evaluation would
be a concentration assuming hot spots are removed.
. Potential for Ground Water Contamination:
PCBs have been found in ground
water in sites associated with dielectric fluids of transformers. Oils can migrate through
soils as a separate immiscible phase and reach ground water, which has been the common
explanation for PCB impacts to ground water. Ground water contamination by 2,3,7,8TCDD
has very rarely been found in ground water, although it has been released to the
environment in an oil matrix. The Times Beach area of Missouri is the principal example of
this release, where waste oils containing 2,3,7,8TCDD were used for dust control.
Ground
water sampling did occur in many of the Tier 1 and 2 National Dioxin Study sites, but the
results were mostly nondetects. One occurrence at 0.18 ppt was noted for the Hyde Park
site of Hooker Chemical in Niagara, NY, and a high of 1.8 ppb was found in an onsite
monitoring well at National Industrial Environmental Services in Furley, KS. There were,
however, numerous high occurrences in subsoil samples in hot spot areas, in bottom
sediments of evaporation lagoons, and so on, in the hundreds of ppb range.
There have been some limited experimentation showing different patterns of 2,3,7,8TCDD
migration in soils in the presence of solvents or in an oily matrix. Palusky, et al (1986)
studied the mobility of 2,3,7,8TCDD in soils associated with each of 6 solvents.
Migration was found to be higher with aromatic solvents and chloroform in comparison to
saturated hydrocarbons and methanol.
They speculated that the extent of migration related
to the solubility of 2,3,7,8TCDD in the solvent. Puri, et al. (1989) studied the
migration potential of 2,3,7,8TCDD in soil, water, and waste oil mixtures. Over time,
they observed a reversible sorption pattern of TCDD, and concluded that a carrier medium
with a significant amount of waste oil would play a dominant role in the movement of TCDD
through soils.
. Sitespecific Characterization:
In the case of landfills or sludge land
application sites, the assignment of a soil concentration and an area can be made with
some reasonableness. Such is not the case with the industrial contamination sites such as
those studied in the National Dioxin Study, as briefly discussed above in the
concentration bullet. Most of the sites studied in the National Dioxin Study were in the
order of tens of hectares to below ten hectares.
On the other hand, the Dow Chemical site
in Midland, Michigan is described as a site 607 ha in size (Nestrick, et al, 1986). That
area corresponds to the size of the property, and the many soil sampling sites within that
area were described as "background". Several of the pesticide formulator sites
studied in Tier 3 were 2 hectares or less in size. Many of the them were extensively or
partially paved with buildings, which complicate fate and transport modeling. Some of the
Tier 5 sites of 2,4,5TCP use were agricultural fields, which are less complicated to
describe.
However, two sites were described as 2500 acres in size, which again is very
large and makes assignment of an average soil concentration nontrivial. Other use sites
were described as railyards and railroad rights of way. While estimates of loss into air
could be made in complicated sites such as these, use of soil erosion modeling becomes
very complicated if not undoable with paved areas, buildings, drainage ditches, roads, and
the like.

4.5. ALGORITHMS FOR THE STACK EMISSION SOURCE CATEGORY 
Contaminants emitted from incinerator stacks are transported in air and deposit on the
exposure site, water bodies that may be used for drinking or fishing purposes, and on
surrounding land. Chapter 3 describes the application of the COMPDEP (reference) model to
obtain vaporphase air concentrations and deposition rates of particles at a specified
distance from an example stack emission source. These quantities are assumed to be given
for purposes of discussion in this section; further discussion of the air transport
modeling is given in Chapter 3.
Estimating soil concentrations based on particulate depositions follows a similar
approach as estimating exposure site soil concentrations resulting from erosion of
contaminated soil from offsite areas of contamination. Section 4.5.1. describes how soil
concentrations are estimated given total (wet plus dry) deposition rates. Surface water
impacts are assumed to result from direct deposition onto surface water bodies as well as
erosion from the impacted effective drainage area. This solution is an extension of the
solution given in Section 4.3.1. for the onsite source category, and is given in Section
4.5.2. Following now are bullet summaries for similarities and small refinements to
algorithms previously discussed:
. Air impacts:
The atmospheric transport modeling described in Chapter 3
was comprised of two computer simulations: one which considered that emissions were in a
vapor form and were transported as such, and one which considered that emissions were in
particle form and likewise were transported as such. The result of the vaporphase runs
was a unitized ambient air concentration at various distances up to 50 km in all
directions from the stack. Only the results in the predominant wind direction were used in
this demonstration.
The result of the particlephase runs were an ambient reservoir of
airborne contaminants sorbed to particulates (used only for inhalation exposures), and
wet and dry deposition unit rates also at various distances up to 50 km. By
"unitized", what is meant is that emissions for the vapor or particle runs can
be thought of as "1" mass/time (g/sec) emissions. Results for all distances are
linear with respect to this emission rate; that is, if the rate of vapor contaminant
determined to be emitted is "5", than ambient air concentrations at any location
are 5 times what they are when "1" is assumed to be emitted. The same holds true
for emissions in the particle phase. Chapter 3 developed a framework for assigning a vapor
and a particle fraction for specific dioxin congeners.
For example, 2,3,7,8TCDD was
assumed to have a vapor fraction of 0.55 (55% was in vapor form) and a particle fraction
of 0.45. The final model results for air concentrations, and dry and wet deposition rates
for all congeners, starting from these unit model runs and then incorporating
congenerspecific emission rates and vapor/particle splits, are given in Tables 312 to
317. The vaporphase air concentrations were used to model vapor phase transfers in the
vegetative bioconcentration algorithms. They were also used, summed with the simulated
reservoir of particlebound contaminants, to estimate the total reservoir of contaminant
available for inhalation exposures.
. Vegetative impacts:
The rates of wet and dry deposition modeled by
COMPDEL were used to determine vegetative impacts. The model for particle deposition
impacts to vegetations is described in Section 4.3.4.2 above. Of course, this above
section solves for dry deposition as a reservoir times a dry deposition velocity (for dry
deposition), and as a reservoir times rainfall and a washout factor (for wet deposition);
such a solution is not required for the stack emission source category since the
deposition totals are estimated by the COMPDEP model. Other parameters for the vegetative
model  the B_{vpa} (airtoleaf vapor transfer factor), the R_{w} (fraction of wet deposition retained on vegetation surfaces), crop yields and
interceptions, and the vegetative washout factor, k_{w}, are used for the stack
emission source category.
. Biota concentrations:
The algorithm estimating concentration in fish
tissue based on bottom sediment concentrations is the same as in previous source
categories. Modeled rates of contaminant deposition on particles onto the exposure site
are used to estimate a "tilled" and an "untilled" soil concentration,
as described below in Section 4.5.1. Underground vegetable concentrations are a function
of tilled soil concentrations. The soil concentration used for cattle soil ingestion is
untilled. Beef and milk concentrations are again a function of vegetative and soil
concentrations, diet fractions, and bioconcentration and bioavailability factors as
described in Section 4.3.4.3.

4.5.1. SteadyState Soil Concentrations
Chapter 3 describes the use of the COMPDEP Model to estimate the particulate phase
deposition rates at the exposure site. This total deposition rate, F, includes both dry
and wet deposition, and is used to estimate the steady state soil concentrations. The
deposition of contaminated particulates from the air is assumed to be somewhat analogous
to the process of eroding contaminated soil from an offsite source depositing on an
exposure site. Specifically, the following assumptions are also made:
1) only a thin layer
of soil becomes contaminated,
2) this layer is either "untilled" or
"tilled", depending on surface activities, and
3) surface residues are assumed
to dissipate with a halflife of 10 years corresponding to a first order decay rate of
0.0693 yr^{1}.
Considerations of upgradient erosion and exposure site soil
removal are not made. Depositions occur over the exposure site and surrounding land area
on an ongoing basis. It might be said that upgradient soil concentrations are similar to
exposure site concentrations at all times. Like the soil source categories, a tilled
mixing depth of 20 cm is assumed.
However, an untilled mixing depth of 1 cm is assumed for
this source category, in contrast to the 5 cm assumed for the offsite soil source
category. It is felt that the process of erosion assumed to transport contaminated soil in
the offsite soil source category to a site of exposure is a more turbulent process. It
assumes that contaminated soils mix with "clean" soils that are between the site
of contamination and the site of exposure.
In contrast, ongoing airborne deposition of
particles is felt to be a less turbulent process impacting all watershed soils
simultaneously; hence the assumption of a 1cm mixing depth. The qualitative mass balance
statement (similar to the one made above in Section 4.4.1, with _C equalling change in
exposure site soil concentrations over time) can now be made as:
This is mathematically stated as:
The solution to this equation is:
which computes C as function of time, t. Similar to the assumption made above in
Section 4.4.1., the steady state solution for C is simply F/kM. The deposition rates
supplied by the COMPDEP model are in units of g/m^{2}yr, so a conversion to mg/yr
requires a multiplication by the land area of the exposure site and a multiplication of
1000 mg/g. Procedures to estimate M are given above in Section 4.4.1.

4.5.2 Surface Water Impacts
The solution for stack emission impacts to surface water bodies is an extension of the
solution for soil contamination described in Section 4.3.1. Stack emissions deposit onto
soils within the effective drainage area to result in an average basinwide soil
concentration. Soil erosion then delivers contaminants to surface waters as in Section
4.3.1. Stack emissions also directly deposit onto and impact the surface water body as
well. All the assumptions laid out at the beginning of Section 4.3.1 apply here as well.
New quantities needed for this solution include: a rate of contaminant deposition onto
soils of the effective drainage area used to estimate average soil concentrations (such
concentrations are estimated using the approach given in Section 4.5.1. above), a rate of
contaminant deposition onto the water body, and a rate of particulate matter deposition
onto the water body.
Equations (41) through (48) are now displayed again with these additions.
Mass balance and equilibrium equations continue:
Substituting again as in Equation (47):
As before, the bracketed quantity in the right hand side of Equation (456) can be
termed f , so that C_{ssed} can be solved as (C_{swb} ER_{w} + DEP_{c})/f . The numerator in this
term can be expanded to describe contaminant contributions by the effective drainage area
which has received depositions, the first quantity in the numerator, and to describe
direct depositions, the second quantity:
Again as before, the right hand side of Equation (457) can be termed, r , and the concentration in suspended sediment, C_{ssed}, is
equal to r /f . Other water body
concentration terms, C_{wat} and C_{sed}, can now be solved using
Equations (454) and (455).

Guidance on these terms and assignment of values for the
demonstration scenarios in Chapter 5 is now given. 
. C_{swb} and ER_{w}:

Equation (457) shows all the terms
necessary to arrive at an estimate of the annual contaminant entry into the water body via
erosion, the C_{swb} * ER_{w} term. Section 4.5.1 describes the algorithm
to estimate soil concentrations given a deposition rate of contaminant. One deposition
rate will be chosen to represent average deposition rates over the effective drainage area
of the watershed (the effective drainage area is termed A_{w}). This rate will be
the rate given in COMPDEP modeling at 0.5 kilometers, which implies that the water body
and the effective drainage area into the water body are near the stack. Tables 315 and
316 (Chapter 3) display wet and dry deposition rates for this distance.
These rates are
added to arrive at total deposition, shown in Table 317. Second, a representative mixing
depth to characterize average watershed soil concentrations needs to be selected. Previous
algorithms used a mixing depth of 20 cm for tillage activities, specifically home
gardening, and 1 and 5 cm for nontilled soil concentrations (1 cm for the stack emission
and 5 cm for the offsite soil source category). For the sake of demonstration, it will be
assumed that a representative watershed depth will equal 10 cm, which might be interpreted
as an average of tilled and untilled lands within the effective drainage area. The values
for SL_{w} (6455 kg/hayr), A_{w} (4000 ha), ER (3), and SD_{w} (0.15) were all given and discussed in Section 4.3.1. and will not be repeated here.

. DEP_{c}:

The second quantity of Equation (457) describes the annual
input to the surface water body that comes from direct deposition. This term is RDEP_{c} * A_{wat} * 1000, where RDEP_{c} is the rate of contaminant deposition
onto the water body, A_{wat} is the area of the water body, and 1000 converts g to
mg. The rate of contaminant deposition at 0.5 km will also be used to describe direct
deposition impact to the surface water body, since for demonstration purposes, there is no
justification for saying this distance is further or nearer the point of stack emission.
The area of the water body has not been required for any other reason, and one will now be
given. First, the effective drainage area of 4000 ha is relatively small and will result
in a relatively small stream, at 1.524 x 10^{7} m^{3}/yr flow volume.
This
volume is also equal to the average cross sectional area of the stream (m^{2})
times stream velocity (m/yr). Assuming a stream velocity of 4.73 * 10^{6} m/yr (15
cm/sec; 1/2 ft/sec), which is reasonable for a small stream, the cross sectional area is
solved as 3.22 m^{2}. An average 1 meter depth and 3.22 meter width appear
reasonable. This width times the stream length would give stream surface area, A_{wat}.
Assuming a rectangular shaped watershed, dimensions of 40 ha wide by 100 ha long (to
arrive at the 4000 ha effective drainage) seem reasonable. This length of 100 ha
translates to 10000 meters, and the full surface area of the stream is 32200 m^{2}.
This will be the value assumed for A_{wat}.

. DEP_{p}:

The rate of particulate deposition onto the lake is
required to achieve a mass balance of all annual soil erosion + particle deposition
contributions to water body solids. The rate of particulate matter emitting from the stack
and arriving at downwind locations was not supplied in Chapter 3. Instead, a literature
value of 0.03 g/m^{2}yr developed by Goeden and Smith (1989) will be used. This
value was based on modeling emissions from a resource recovery facility. Total particulate
emissions from the stack were projected to be 4.63 g/s, and the deposition rate onto a
nearby lake was modeled to be 0.03 g/m^{2}yr. No further information was
supplied. Now, with the surface area as solved for above at 32200 m^{2}, the total
particle deposition, DEP_{p} in kg/yr, is 966 g/yr.

. f_{s} and f_{sd}:

These are the fractions of total erosion
and depositing particles remaining as suspended materials within a year. As discussed in
the solution for the "onsite source category" in Section 4.3.1, f_{s} was solved for as:
a value for total suspended solid, TSS of 10 mg/L, multiplied by a
total flow volume V_{wat} of 1.524 x 10^{10} L/yr, divided by the total
erosion into the water body, 3.87 x 10^{12} mg/yr. This resulted in an f_{s} of 0.039. Note that this implies a total suspended load of 152,400 kg/yr. It could be
assumed that the minuscule 1 kg/yr of particles directly depositing onto the stream remain
in suspension during the year, on the basis of being smaller in size than eroded soil.
This assumption will, in fact, be made, but it will be supported as follows.
In a quiescent water body, settling occurs through gravity and can be expressed in
terms of Stokes Law:
For purposes of this discussion, a reasonable assignment of particle density of is 2.5
g/cm^{3} for depositing particles or eroding soil. Therefore, making
substitutions, the right hand side of Equation (458) reduces to 918 d^{2}.
Now, assumptions for the particle sizes of eroding soil and depositing particles can be
made to arrive at a ratio of settling velocities, V_{ssoil}/V_{spart}. The
basis for assigning an enrichment ratio for delivery of contaminants via soil erosion was
that finesized particles were the ones eventually reaching the water body via erosion.
Lick (1982) states that a major fraction of the sediments (suspended and bottom) in the
Great Lakes are fine grained, silts and clays, and that data from Lake Erie indicates that
90% of the sediments are of this category.
Brady (1984) shows USDA's classification of
soils according to particle size, and gives a range of 0.0002 to 0.005 cm for silt sized
particles and less than 0.0002 for clay size particles. The following assumptions are made
to arrive at a representative diameter for particles in eroded soil: eroded soil is
comprised of a 50/50 split of these two sized particles, siltsized particles are, on the
average 0.0026 cm in diameter, and clay size particles are 0.0001 cm in diameter. With
these assumptions, the average particle size for eroding soil is 0.0014 cm.
The settling
velocity for a 0.0014 cm particle is 1.8 x 10^{3} cm/sec. In Section 3.4.3,
Chapter 3, the argument was developed that 87.5% of the total emission rate of dioxinlike
congeners would be associated with particles less than 2 m m.
The basis of this argument was a surface area to volume ratio, with smaller particle sizes
having significantly larger ratios. This does not mean that 87.5% of the 1 kg/yr
depositing particles are of this size. However, for this discussion, the size of
depositing particles will be assumed to be 2 m m (2 x 10^{4} cm), since these size particles deliver most of the dioxinlike compounds to the water
body (and the ultimate purpose of this exercise is to determine a value for the fraction
of depositing particles which remain suspended and impact suspended sediment
concentrations). The settling velocity, V_{spart}, is estimated as 3.7 x 10^{5} cm/sec.
The ratio V_{ssoil}/V_{spart} is about 50. Said another way and with
all the assumptions and simplifications made above, depositing particles will remain in
suspension 50 times longer than eroding soil in a quiescent water body.
Given this high a difference in settling velocities, it seems reasonable to assume f_{sb} equals 1.0. The fraction of soil erosion remaining in suspension, f_{s}, will be
estimated given TSS, V_{wat}, etc., as before (see Section 4.3.1), only DEP_{p} (the total amount of depositing particles, in kg/yr) will comprise a given increment of
suspended materials when solving for f_{s}.

. V_{wat}, OC_{ssed}, OC_{sed}, and Kd_{ssed}:

These have all been discussed in Section 4.3.1.
The values for these parameters in the
demonstration scenarios in Chapter 5 are:
V_{wat} = 1.524 x 10^{10} L/yr,
OC_{ssed} = 0.05, OC_{sed} = 0.03, and Kd_{ssed} = OC_{sed} * Koc,
where Koc is the organic partition coefficient of the contaminant.

4.6. ALGORITHMS FOR THE EFFLUENT DISCHARGE SOURCE CATEGORY 
As discussed in Volume II, Chapter 3, dioxinlike compounds can be released to
waterways via various types of effluent discharges such as discharges from municipal waste
water treatment facilities and pulp and paper mills using chlorine bleaching. Also
discussed is the fact that these emissions have declined substantially in recent years,
especially from pulp and paper mills. Since the procedures for considering point source
discharges to waterways are somewhat different than those associated with the nonpoint
source procedures for soil contamination and stack emissions, they are covered separately
in this section. This source category is also different from others in that effluent
discharges into surface water bodies are assumed only to impact fish and water.
The approach used in this report is an extension of the "simple dilution"
model described in the Superfund Exposure Assessment Manual (EPA, 1988c). Other models are
available which offer more spatial and temporal resolution than the model described here.
One such model is the EXposure Analysis Modeling System, or EXAMS (Burns, et al., 1982,
and Burns and Cline, 1985). The EXAMS and a simple dilution model were both applied in an
assessment of effluent discharges from pulp and paper mills (EPA, 1990d). In this
assessment, 98 of the 104 pulp and paper mills were modeled with both models using
sitespecific information (water body flow rates from STORET for all but 6 of the mills,
effluent flow rates and contaminant discharges, etc.).
Three key quantities  one model
result and two model parameters
 led to a range of exposure conditions for humans
consuming fish impacted by discharges from these pulp and paper mills: a water column
concentration, a bioconcentration factor (BCF) applied to the water column concentration
to get fish tissue concentration, and a fish ingestion rate.
The simple dilution model was
used to estimate total water concentrations  i.e., mg TCDD total/L water. The EXAMS model
was used to estimate dissolved phase water column concentration  i.e., mg TCDD dissolved
in water column/L water. Then, with each set of water concentrations, two sets of exposure
estimates (a low and a high estimate, in one sense) were generated  one with a BCF of
5,000 and a fish ingestion rate of 6.5 g/day, and one with a BCF of 50,000 and a fish ingestion rate of 30 g/day. Note that in deriving the range of results in that
exercise, the BCF was applied to both a total and a dissolved phase water concentrations.
EPA (1993) discusses several bioconcentration/bioaccumulation empirical parameters for
2,3,7,8TCDD, and makes the clear distinction for those which are to be applied to a total
water concentration versus those applied to a concentration in the dissolved phase. The
dilution and EXAMS model study indicated that the simple dilution model generally
estimated higher water column contaminant concentrations compared to the EXAMS model,
although this trend was not consistent among all water bodies modeled. The results from
both models were comparable when the receiving water body had relatively low suspended
solids concentration.
One key limitation of the EXAMS and the simple dilution model for use with dioxinlike
compounds in aquatic systems is that they do not account for sediment transport processes.
The EXAMS model was designed to determine the fate of transport of contaminants in the
dissolved phase. Another spatially and temporally resolved model for this source category
is the Water Analysis Simulation Package, the most uptodate version termed WASP4
(Ambrose, et al., 1988). This model does include sediment processes and has been applied
in a comprehensive evaluation of 2,3,7,8TCDD bioaccumulation in Lake Ontario (EPA,
1990b). It requires extensive sitespecific parameterization, but should be considered for
more detailed sitespecific evaluations of strongly hydrophobic and bioaccumulating
contaminants such as the dioxinlike compounds.
The dilution model described below will be demonstrated in Chapter 5 with a set of data
developed using sitespecific data from the 104 pulp and paper mills of the 104mill
study. As will be discussed below, a hypothetical effluent discharge will have
characteristics developed as the average of key characteristics from the 104 mill study.
These key data include: flow rates of the receiving water bodies, suspended solids
concentration in these receiving water bodies, effluent discharge flow rates, suspended
solids in the effluent discharges, organic carbon content of solids in the effluent
stream, and discharges of 2,3,7,8TCDD.
