5. DEMONSTRATION OF METHODOLOGY

This document has provided methodologies and background information for conducting site-specific exposure assessments to the dioxin-like compounds. Volume II contains key information pertinent to the methodologies described in this Volume. Chapter 2 of Volume II described physical and chemical properties of these compounds, Chapter 3 described sources of dioxin-like compound release, and Chapter 4 described their occurrence in environmental and exposure media. This Volume lays out the methodologies demonstrated in this chapter. Chapter 2 summarized an overall exposure assessment framework, Chapter 3 described mechanisms of formation of dioxin-like compounds in stack emissions and the fate and transport modeling of releases from the stack to a site of exposure, and Chapter 4 provided methodologies to estimate exposure media concentrations for four sources of contamination, which were termed source categories.

The purpose of this chapter is to put all this information together and demonstrate the methodologies that have been developed. For this demonstration, exposure scenarios are developed which are associated with the four source categories. These categories were defined in Chapter 4, and are:

• On-site soil: The source of contamination is soil and both the source and exposure site are on the same tract of land.

• Off-site soil: The source of contamination is soil and this source is located distant and upgradient/upwind from the site of exposure.

• Stack emissions: Exposed individuals reside downwind of the site where stack emissions occur and are exposed to resulting air-borne contaminants, and soil and vegetation on their property is impacted by deposition of contaminated particulates.

• Effluent discharge: A discharge of dioxin-like compounds in effluents impacts surface water and fish. Exposure occurs through consumption of the impacted fish and water.

The demonstration in this chapter is structured around what are termed exposure scenarios. As defined in Chapter 2, an exposure scenario includes a description of the physical setting of the source of contamination and the site of exposure, behavior of exposed individuals, and exposure pathways. Chapter 2 also described the objective of exposure assessors to determine "central" and "high end" exposure scenarios. This objective was an important one for this demonstration, and the strategy to design such scenarios is detailed in Section 5.2 below. For the two soil source categories and the effluent discharge source category, three dioxin-like compounds are demonstrated for each of the exposure scenarios, including 2,3,7,8-TCDD, 2,3,4,7,8-PCDF, and 2,3,3',4,4',5,5'-HPCB. For the stack emission source, a different approach is taken with regard to compounds demonstrated. One compound is 2,3,7,8-TCDD, as in the other source categories. An addition demonstration estimates TEQ exposures given emission rates of dioxin-like compounds with non-zero Toxicity Equivalency Factors (TEFs) from a stack. Individual congeners emitted by the stack are transported to a site of exposure using the dispersion/deposition model, COMPDEP. Further modeling then takes the key quantities for each congener, the air concentrations (vapor and particulate phases), in m g/m³, and the total deposition (dry and wet deposition summed), in m g/m²-yr, and determines congener specific exposure media concentrations. The toxic equivalent concentration for each congener is estimated by multiplying the individual congener concentration estimated by the individual congener's TEF. Finally, the individual TEQ concentrations are summed to arrive at an exposure media concentration equalling total TEQ for that media.

Section 5.2 describes the strategy for development of the demonstration exposure scenarios. Section 5.3 gives a complete summary of the demonstration scenarios. Section 5.4 provides some detail on the example compounds demonstrated. Section 5.5 describes the source strength terms for the scenarios. Section 5.6 summarizes the results for all scenarios, which are exposure media concentrations for all exposure pathways, and exposure estimates which are Lifetime Average Daily Doses (LADDs) for all pathways and for all example compounds.

5.2. STRATEGY FOR DEVISING EXPOSURE SCENARIOS FORDEMONSTRATION PURPOSES

Chapter 2 of this document contained Figure 2-1, a roadmap for assessing exposure to dioxin-like compounds. These procedures assess individual exposures to known sources of contamination. Central and high end exposure patterns, and exposure parameters consistent with these definitions were proposed in that chapter. The demonstration in this chapter attempts to merge procedures for estimating individual exposures to known sources of contamination and current thoughts on devising central and high end exposure scenarios.

An exposure assessor's first task in determining patterns of exposure is to fully characterize the exposed population in relation to the source of contamination. If the extent of contamination can be characterized, then the exposed population would be limited to those within the geographically bounded area. An example of this situation might be an area impacted by stack emissions. Chapter 3 demonstrated the use of COMPDEP atmospheric dispersion model to predict ambient air concentrations and depositions rates for all points surrounding the stack. Results listed in Tables 3-12 through 3-17 were only for the prevailing wind direction. As can be seen on these tables, the points of maximum impact were within 1 km of the stack. By overlaying the concentration isopleths onto a population density map, the exposed population can be identified. If the extent of contamination is not as clearly defined, such as extent of impact of nonpoint source pollution (impacts from use of agricultural pesticides, e.g.) or the compound is found ubiquitously without a clearly defined source, then the emphasis shifts from geographical bounding to understanding ambient concentrations, exposure pathways and patterns of behavior in general populations.

After identifying the exposed population, the next task is to develop an understanding of the continuum of exposures. The exposures faced by the 10 percent of the population most exposed has been defined as high end exposures. Those faced by the middle of the continuum are called central exposures. Another important estimate of exposure level is a bounding exposure, which is defined as a level above that of the most exposed individual in a population. Arriving at such an understanding can be more of an art than a science. One consideration is the proximity of individuals within an exposed population to the source of contamination. For the incinerator example discussed above, one might begin an analysis by assuming that bounding or high end exposures occur within a kilometer from the stack, in the prevailing wind direction. Another important consideration is the relative contribution of different exposure pathways to an individual's total exposure. While individuals residing at this distance from the incinerator might experience the highest inhalation exposures, they may not experience other exposure pathways associated with contaminated soil on their property - such as consumption of home grown vegetables, dermal contact, or soil ingestion. Families with home gardens and individuals who regularly work in those gardens may reside over a kilometer from the incinerator and possibly be more exposed because of their behavior patterns. Screening tools, such as the spreadsheets developed for this assessment, can be used in an iterative mode to evaluate the interplay of such complex factors. When applied to a real world situation, information should be sought as to the makeup and behavior patterns of an exposed population.

Following are bullet summaries of key features of the structure and intent of the demonstrations.

• Exposed populations: Exposed individuals are assumed to reside in a rural setting. Exposures occur in the home environment, in contrast to the work environment or other environments away from home (parks, etc.). The presumption is made that the sources of contamination of this assessment can occur in rural settings in the United States. Sources demonstrated include basin-wide soils with concentrations characteristic of background levels, much smaller areas of soils with concentrations that have been found in industrial sites, stack emissions, and effluent discharges where characteristics of the effluent stream including contaminant discharges were developed from recent data from pulp and paper mills. (see Section 5.5. below for more details on source strength terms). It is further assumed that the behavior patterns associated with the exposure pathways can exist in rural settings. Several of these behaviors characterized as high end relate to individuals on farms as compared to behaviors characterized as central for individuals not on farms. The exposed population for this demonstration, therefore, consists of rural individuals in farming and non-farming residences. For each of the four source categories demonstrated, the exposed populations can be further defined:

-- On-site soils: The on-site source category is demonstrated with soil concentrations that have been found and characterized in the literature as "background" and "rural", or not associated with an identified source. For this source, the exposed population includes all individuals within a rural area for which the background concentration can be considered representative.

-- Off-site soil: Demonstration of this source category entails a finite area of soil contamination, in contrast to the demonstration of the on-site source category, where soils containing low levels of dioxin-like compounds exist throughout a large region. The site of contamination is a 10-acre site having elevated soil concentrations that have been found in the United States in industrial sites. A working hypothesis is made that the population most exposed are those residing very near the site. Their soil is assumed to become contaminated over time due to the process of erosion; these processes normally do not carry contaminants long distances across land, particularly land developed with residences or where erosion is interrupted with ditches or surface water bodies. People from the surrounding community can be impacted by visiting or trespassing on the contaminated land, volatilized residues may reach their home environments, they may obtain water and fish from impacted water bodies, and so on. It seems reasonable to assume that those residing near these sites comprise the principally exposed individuals, or equivalently, the individuals experiencing the high end or bounding exposures associated with these areas of soil contamination.

-- Stack emissions: As indicated earlier, the populations exposed to stack emissions can be identified by overlaying results of an atmospheric dispersion modeling exercise over a map containing population density information. Such an exercise was not done for this demonstration. Instead, simulated ambient air concentrations and deposition rates were taken from tables in Chapter 3 for two locations, one for use in a central scenario and another for a high end scenario.

-- Effluent discharges: This source category is unique from the others in that soils or air are not impacted by the source. Only the surface water body into which the effluent is discharged is impacted. The only exposure pathways considered for this source category are drinking water and fish ingestion. The exposure parameters used to demonstrate this source category were those developed for the central scenario. Those that could be recreationally fishing in the impacted water body or using it as a source of drinking water could be characterized as central, high end, or bounding. There is no particular rationale for selecting central exposure behavior in demonstrating this source category.

• Proximity to sources of contamination: As noted above, the on-site soil contamination source category was demonstrated using soil concentrations typical of background levels that have been found in rural settings. In this case, proximity to the source of contamination was not an issue. Proximity to a stack emitting dioxin-like compounds was identified as an important determinant for identifying the continuum of exposures. Assuming there is a uniform distribution of exposure-related behaviors among exposed populations, i.e., their behavior patterns are not a function of where they live in relation to the stack, the most exposed individuals will be those exhibiting high end exposure behavior nearest the stack. This was the assumption made for purposes of this demonstration. A set of high end exposure behaviors and pathways were demonstrated for individuals residing 500 meters east of the stack, and a set of central exposure pathways were demonstrated for individuals residing 5000 meters east of the stack. The highest ambient air concentrations, and dry and wet deposition rates were simulated to occur at 200 to 1000 meters downwind, justifying 500 meters as an appropriate point for assuming high end impacts. Tables 3-12 through 3-17 listing concentrations and depositions rates as a function show that air concentrations and dry depositions rates at 5000 meters are only about half of what they are at 500 meters, although wet deposition rates are about 20 times higher at 500 meters as compared to 5000 meters. Without rigorous justification, the model output (concentrations and deposition rates) at 500 and 5000 meters was felt to appropriately characterize high end and central exposures. The above bullet justifies a definition of principally exposed individuals as those nearest the site of high soil contamination in the demonstration of the off-site source category, while recognizing that lesser exposures can occur for other individuals in the community. These lesser exposures will not be demonstrated. Instead, the off-site soil source categories will only be demonstrated with a single, high end scenario. Individuals exposed will be assumed to reside 150 meters downgradient from the site of soil contamination. The above bullet also discussed how a surface water body impacted by effluent discharges could be used (for drinking and recreationally fishing) by individuals exhibiting central, high end, or bounding exposure behavior patterns. Intuitively, proximity should be an issue because impacts to fish and water are likely to be higher nearer to the point of discharge. However, the simplistic model estimating impacts from effluent discharges uses a simple dilution approach to obtain water and suspended sediment concentrations. The suspended sediment concentrations are used to estimate fish impacts. For this approach, therefore, proximity cannot be rigorously evaluated. Exposure parameters for water and fish ingestion corresponding to central behavior patterns were used in the demonstration of the effluent discharge source category.

• Central and high end exposure patterns: Chapter 2 described the exposure pathways that are considered in this methodology, and justified assignment of key exposure parameters (contact rates and contact fractions, exposure durations, and so on) as central or high end estimates. That chapter notes that the exposure pathways identified were those that were consistent with the sources of contamination, and consistent with literature which identified predominant media where these compounds were found. The bullet above discussing exposed population indicated that several of the behavior assumptions were specific to individuals on farm, and that these behavior patterns were evaluated as high end. High end behaviors assumed to be different for individuals on farms versus central behaviors for individuals not on farms include: residing on larger tracts of land (10 acres assumed for farmers; 1 acre assumed for non-farmers), ingestion of home produced and impacted beef and milk, tendencies to reside in a single location longer (20 years versus 9 years), tendencies to be present in the home environment longer (90% of the time versus 75% of the time), and patterns of soil dermal contact designed to be plausible for farmers working with soil versus those incidentally contacting soil. Other patterns of behavior modeled as central and high end are not specifically associated with farming and not farming, but are assumed to be plausible for individuals in rural settings. These include home gardening for fruit and vegetables, inhalation exposures, children that ingest soil, and the use of impacted surface water bodies for drinking and fish to be ingested.

• Plausibility of source strength terms: The objective to determine plausible levels of source strength contamination was an important one for this demonstration. The source terms are soil concentrations, effluent discharge rates, and stack emission rates. Section 5.5 describes the source terms in detail.

• Appropriate estimation of exposure media concentrations: The realism of estimated exposure media concentrations is dependent on the appropriateness of the models used for such estimations and the assignment of parameter values for those models. One way to arrive at a judgement as to the realism of estimated concentrations is to compare predictions with observations. To the extent possible (i.e., given the availability of appropriate data), model predictions of exposure media concentrations are compared with occurrence data in Chapter 7 on Uncertainty. As is shown, predictions fell within the realm of observed data. Chapter 4 describes the justification of all model parameter values. Many of the parameters are specific to the contaminants. Some contaminant properties were estimated as empirical functions of contaminant-specific parameters, such as the octanol water partition coefficient, Kow, and others were measured values. For non-contaminant parameters such as soil and sediment properties, patterns of cattle ingestion of soil (and other bioaccumulation/biotransfer parameters), and many others, selected values were carefully described and crafted to be plausible.

As noted above, all exposures occur in a rural setting. Exposure pathways were those which could be associated with places of residence in contrast to the work place or other places of exposure. The example scenarios are structured so that all the behaviors associated with high end exposures are included in the "high end" scenarios and all the central behaviors are in the scenarios characterized as "central". To summarize, the components which distinguished the high end exposure scenarios in contrast to the central scenarios include:

• Individuals in the central scenarios lived in their homes and were exposed to the source of contamination for only 9 years, in contrast to individuals in the high end scenarios, who were exposed for 20 years (except for the exposure pathway of soil ingestion, where the individuals are assumed to be children ages 2-6, and in both the central and high end scenarios, the exposure duration is 5 years).

• Individuals in the central scenarios lived on properties 1 acre in size, whereas individuals in the high end scenarios lived on properties 10 acres in size.

• Individuals in the high end scenario associated with the stack emission source category lived 500 meters from the incinerator, whereas individuals in central scenario lived 5000 meters from the incinerator.

• High end individuals obtained a portion of their beef and milk using home produced beef and milk - such individuals are obviously farmers. Beef and milk ingestion pathways were not assessed for non-farming rural individuals, representing the central scenarios.

• Ninety percent of the inhaled air and ingested water by the high end individuals were assumed to be contaminated, whereas only 75% of these exposures were with impacted media for the central individuals. This is based on time at home versus time away from home assumptions for central versus high end individuals. Also, individuals in high end scenarios were assumed to consume 2.0 L/day of water as compared to 1.4 L/day consumed by individuals in central scenarios.

• Although their total intake of fruit and vegetables was assumed to be the same, a larger proportion of the intake of those food products in the high end household was home grown and impacted as compared to the central household.

• The rates of ingestion of soil by children and of recreationally caught and impacted fish were higher for the high end individuals than the central individuals.

These are the distinguishing features for the central and high end exposure scenarios. For the sake of convenience mainly, all the scenarios defined below as high end are called "farms", and all central scenarios are called "residences". The assertion is not being made that all behaviors are likely to occur simultaneously (or in some cases, simply to occur) on a farm or a non-farm residence, although several of the high behavior patterns are specific to farms. In an exhaustive site-specific analysis, one might begin by evaluating all possible pathways, further evaluating pathways of most exposure, and then determining what pathways occur simultaneously for identified individuals in the exposed population. Only then can be the assessor begin to define a continuum of exposures.

The following bullets describe six exposure scenarios that are demonstrated. The numbering scheme and titles will be referenced for the remainder of this chapter:

Surface soils on a 4,000 m² (1-acre roughly) rural residence (Scenario 1) and on a 40,000 m² (10-acre roughly) small rural farm (Scenario 2) are contaminated with the three example contaminants. The concentrations of the contaminants are uniformly set at 1 part per trillion, which was evaluated as reasonable background levels (see Section 5.5 below). Bottom sediment in a nearby stream becomes contaminated. Water and fish in that stream are subsequently impacted. Fish are recreationally caught and eaten, and the water is extracted for drinking purposes, perhaps at a downstream water system intake. However, water concentration predictions are only those which are estimated to occur in the drainage area impacting the generally smaller size stream. For background soil concentrations, river system impact should be similar to local stream impact justifying the drinking water pathway for these scenarios.

A 40,000 m² rural farm is located 150 m (500 ft roughly) from a 40,000 m² area of bare soil contamination; an area that might be typical of contaminated industrial property. The surface soil at this property is contaminated with the three example compounds to the same concentration of 1 part per billion. This is evaluated as reasonable for industrial sites of contamination of dioxin-like compounds, and three orders of magnitude higher than concentrations for Scenarios 1 and 2. As in the above and all scenarios, bottom sediment in a nearby stream is impacted, which impacts the drinking water supply and fish which are recreationally caught and consumed by members of this farming household. A similarly sized stream is impacted for this source category as in the on-site source category. It is less likely that water concentrations for this stream would

be similar to concentrations at a point where water is withdrawn for drinking purposes. Nonetheless, to be consistent with the demonstration of the on-site source category and the stack emission source category, drainage area sizes and stream sizes were the same. It can be said that the stream size is plausible for recreational fishing, so impacts to fish are appropriately estimated and compared among the source categories.

A 4,000 m² rural residence (Scenario 4) is located 5000 meters from an incinerator, and a 40,000 m² (Scenario 5) rural farm is located 500 meters downwind from an incinerator. Emission data of the suite of dioxins and furans with non-zero TEFs is available. This allows for estimation of impacts from 2,3,7,8-TCDD alone, and estimation of TEQ impacts. The modeling of the transport of these contaminants from the stack to the site of exposure and other points in the watershed used the COMPDEP model. Details on the stack emission source and the COMPDEP model application are found in Chapter 3. A nearby impacted stream feeds into a drinking water system and supports fish for recreational fishing.

As has been discussed, this source category is different from others in that the air, soil, and vegetation at a site are not impacted. Rather, only surface water impacts are considered. Therefore, central and high end behaviors associated with places of residence are less pertinent for this source category. Exposure parameters associated with central behaviors for the water and fish ingestion pathways were chosen to demonstrate this source category. The source strength was developed from data on pulp and paper mill discharges of 2,3,7,8-TCDD; more detail on this source strength term development is provided in Section 5.5 below. The discharges of the other two example compounds are assumed to be the same for purposes of demonstration. Obviously, however, there is less of a tie to real data for the discharge rate for these other two example compounds. Also noteworthy for this source category as compared to the others is the size of the surface water body into which discharges occur. The other source categories all were demonstrated on water bodies with annual flow rates of 1.5 * 10¹⁰ L/yr. The river size into which the example effluent was discharged was developed from data from the 104 pulp and paper mill study (as discussed in Section 5.5 below). This river size was 4 * 10¹² L/yr, two orders of magnitude larger than the other streams. In this demonstration, therefore, use of impacted water in a drinking water system would appear to be more plausible.

Three compounds were demonstrated for the two soil source categories, on- and off-site soil contamination, and for the effluent discharge source category. For purposes of illustration, one compound was arbitrarily selected from each of the major classes of dioxin-like compounds. They are: 2,3,7,8-tetrachlorodibenzo-p-dioxin, 2,3,4,7,8-pentachlorodibenzofuran, and 2,3,3',4,4',5,5'-heptachloro-PCB. For the remainder of this chapter, these compounds will be abbreviated as 2,3,7,8-TCDD, 2,3,4,7,8-PCDF, and 2,3,3',4,4',5,5'-HPCB.

These compounds demonstrate a range of expected results because of the variability of their key fate and transport parameters. The log octanol water partition coefficients (log Kow) for 2,3,7,8-TCDD, 2,3,4,7,8-PCDF, and 2,3,3',4,4',5,5'-HPCB were 6.64, 6.92, and 7.71, respectively. Whereas the span of reported log Kow ranged from less than 6.00 to greater than 8.00, only a few reported values were at these extremes. Increasing log Kow translates to the following trends: tighter sorption to soils and sediments and less releases into air and water, less accumulation in plants and in cattle products (beef, milk), and more accumulation in fish. The Henry's Constants for the three compounds span the range of reported values, with the value of the PCB compound the highest of all reported at 3.0 * 10^-3. There were few values less than the 4.99 * 10^-6 reported for 2,3,4,7,8-PCDF. Higher Henry's Constants translate to greater amounts of volatilization flux. A summary of the chemical specific parameters for these three compounds is given in Table 5-1.

For the stack emission demonstration, Scenarios 4 and 5, a different approach was taken. Like the above source category demonstrations, exposures to 2,3,7,8-TCDD alone are determined. Given that the stack emission data included emission rates for all dioxins and furans with non-zero toxicity equivalency factors (abbreviated TEFs), and the atmospheric transport modeling led to estimates of ambient air concentrations, and wet and dry deposition rates at various distances for these compounds, an opportunity presented itself for demonstrating an approach to estimating TEQ impacts. This approach takes the individual deposition rates and concentrations for the dioxins and furans with non-zero TEFs and models the exposure media concentrations individually with unique fate and bioaccumulation parameters, and then determines a final TEQ exposure media concentration using TEFs. Results for this approach are hereafter termed "TEQ" results. The deposition rates, air concentrations, TEFs, and chemical specific parameters for 2,3,7,8-TCDD and the individual congeners are provided in Table 5-2.

This section describes the source terms for the example scenarios. Source terms for the soil contamination sources, the on- and off-site soil sources, include the areas of contamination and soil concentrations. This section also summarizes the exposure site soil concentrations that result from erosion of contaminated soil from the nearby soil contamination site in the example scenario demonstrating the off-site soil source category, Scenario 3. The source terms for the stack emission scenarios, 4 and 5, are the emission rates of contaminants from the stacks. Discussions of these rates are provided in Chapter 3. As noted in that Chapter, emission rates were determined from actual test data. This

Table 5-1. Environmental fate parameters for the three example compounds demonstrated for the soil contamination source categories and the effluent discharge source category.

2,3,7,8- 2,3,4,7,8- 2,3,3',4,4,'

Description TCDD TCDF 5,5- HPCB

Kow: log octanol water part coef^* 6.64 6.92 7.71

Koc: organic carbon part coef., L/kg 2.7*10⁶ 5.1*10⁶ 3.2*10⁷

H: Henry's Constant, atm-m³/mole 1.7*10^-5 5.0*10^-6 1.0*10^-3

Da: molecular diffusivity in air, cm²/s 0.05 0.05 0.05

k: dissipation rate for eroding

contaminants, yr^-1 0.0693 0.0693 0.0693

kw: first-order plant wash-off rate, yr^-1 18.02 18.02 18.02

B_vpa: air-to-leaf transfer factor, unitless 1.0*10⁵ 5.3*10⁵ 2.3*10⁴

BCF: beef/milk biotransfer factor, unitless 4.3 3.1 2.3

BSAF/BSSAF: bottom sediment (BSAF) or

suspended sediment (BSSAF) biota

sediment accumulation factor, unitless 0.09 0.09 2.0

RCF: root concentration factor, unitless 3.9*10³ 6.4*10³ 2.6*10⁴

Compound TEF deposition air concen. H log RCF B_vpa Koc BSAF BCF

m g/m²-yr m g/m³ Kow

2378-TCDD 1.0 1.1*10^-6 1.4*10^-11 1.6*10^-5 6.64 3.9*10³ 1.0*10⁵ 2.7*10⁶ 0.09 4.32

12378-PeCDD 0.5 3.9*10^-6 2.9*10^-11 2.6*10^-6 6.64 3.9*10³ 6.3*10⁵ 2.7*10⁶ 0.09 4.16

123478-HxCDD 0.1 6.3*10^-6 3.7*10^-11 1.2*10^-5 7.79 3.0*10⁴ 2.3*10⁶ 3.8*10⁷ 0.04 2.02

123789-HxCDD 0.1 9.6*10^-6 5.4*10^-11 1.2*10^-5 7.79 1.3*10⁴ 6.9*10⁵ 1.2*10⁷ 0.04 2.24

123678-HxCDD 0.1 8.6*10^-6 4.9*10^-11 1.2*10^-5 7.30 1.3*10⁴ 6.9*10⁵ 1.2*10⁷ 0.04 1.74

1234678-HpCDD 0.01 8.6*10^-5 4.9*10^-10 7.5*10^-6 8.20 6.2*10⁴ 1.0*10⁷ 9.8*10⁷ 0.005 0.36

OctaCDD 0.001 1.8*10^-4 1.0*10^-9 7.0*10^-9 7.59 2.1*10⁴ 2.4*10⁹ 2.4*10⁷ 0.0001 0.52

2378-TCDF 0.1 4.7*10^-5 9.0*10^-10 8.6*10^-6 6.53 3.2*10³ 1.5*10⁵ 2.1*10⁶ 0.09 0.94

23478-PeCDF 0.5 1.2*10^-5 9.4*10^-11 6.2*10^-6 6.92 6.4*10³ 5.3*10⁵ 5.1*10⁶ 0.09 3.10

12378-PeCDF 0.05 6.1*10^-6 5.8*10^-11 6.2*10^-6 6.79 5.1*10³ 3.8*10⁵ 3.8*10⁶ 0.09 0.73

123478-HxCDF 0.1 2.3*10^-5 1.4*10^-10 1.4*10^-5 7.30 1.3*10⁴ 5.9*10⁵ 1.2*10⁷ 0.04 2.34

123678-HxCDF 0.1 2.2*10^-5 1.3*10^-10 6.1*10^-6 7.30 1.3*10⁴ 1.4*10⁶ 1.2*10⁷ 0.04 2.00

123789-HxCDF 0.1 1.4*10^-5 8.5*10^-11 1.0*10^-5 7.30 1.3*10⁴ 8.3*10⁵ 1.2*10⁷ 0.04 2.00

234678-HxCDF 0.1 8.4*10^-6 5.0*10^-11 1.0*10^-5 7.30 1.3*10⁴ 8.3*10⁵ 1.2*10⁷ 0.04 1.78

1234678-HpCDF 0.01 3.0*10^-5 1.7*10^-10 5.3*10^-5 7.90 3.7*10⁴ 6.8*10⁵ 4.9*10⁷ 0.005 0.41

1234789-HpCDF 0.01 1.3*10^-5 7.5*10^-11 5.3*10^-5 7.90 3.7*10⁴ 6.8*10⁵ 4.9*10⁷ 0.005 0.99

OctaCDF 0.001 6.0*10^-5 3.3*10^-10 1.9*10^-6 8.80 1.8*10⁵ 1.7*10⁸ 3.9*10⁸ 0.0001 0.20

Table 5-3. Summary of key source terms for the six exposure scenarios and the three example compounds.

2,3,7,8- 2,3,4,7,8- 2,3,3',4,4,

I. Soil Concentrations, m g/kg (ppb) TCDD PCDF 5,5'-HPCB

Source Category 1: On-Site Soil

Scenario 1. Central 0.001 0.001 0.001

Scenario 2. High End 0.001 0.001 0.001

Source Category 2: Off-Site Soil

Off-site soil concentration 1.000 1.000 1.000

Scenario 3. High End No-till 0.279 0.279 0.279

Tilled 0.077 0.077 0.077

2,3,7,8-TCDD TEQ

Source Category 3: Stack Emissions

Scenario 4. Central No-till 1*10^-7 3*10^-6

Tilled 5*10^-9 2*10^-7

Scenario 5. High End No-till 1*10^-6 2*10^-5

Tilled 5*10^-8 1*10^-6

Watershed soils 1*10^-7 2*10^-6

II. Emission Rates for Stack

Emissions (g/sec) and Effluent

Discharges (mg/hr)

Source Category 3: Stack Emissions 2,3,7,8-TCDD TEQ

Scenarios 4 & 5: 9.2*10^-11 1.5*10^-9

Source Category 4: Effluent Discharges

Scenario 6: 0.0315 0.0315 0.0315

III. Land Areas, m²

Source Category 1: On-site Soil Source Category 2: Off-site Soils

Scenario 1. Central 4,000 Contamination Site: 40,000

Scenario 2. High End 40,000 Scenario 3. High End 40,000

Source Category 3: Stack Emissions

Scenario 4. Central 4,000

Scenario 5. High End 40,000

Chapter 4 in Volume II discussed soil concentrations of the dioxin-like compounds found in the literature. As noted in that chapter, concentrations of the coplanar PCBs were not found in the literature; soil concentrations assigned for 2,3,3',4,4',5,5'-HPCB will be the same as the other two compounds. Scenarios 1 and 2 were designed to demonstrate exposures to low concentrations which might be considered "background" soil concentrations. Soil concentrations of 2,3,7,8-TCDD and 2,3,4,7,8-PCDF described as "background" or "rural" by researchers were found in the non-detect to low ng/kg (ppt) in Illinois, Ohio, and Minnesota in the United States (EPA, 1985; Reed, et al., 1990), and in Sweden (Broman, et al. 1990) and England (Creaser, et al., 1989; Stenhouse and Badsha, 1990). Tier 7 of EPA's National Dioxin Study (EPA, 1987) consisted of "background" sites, or sites that did not have previously known sources of 2,3,7,8-TCDD contamination. The purpose of this tier was to provide a basis for comparison for the other 6 tiers of study, which did include sites of known or suspected 2,3,7,8-TCDD contamination. The results were that 17 of 221 urban sites and only 1 of 138 rural sites had detectable levels of 2,3,7,8-TCDD, with a range of positives of 0.2 to 11.2 ng/kg (ppt). While the value of 1 ng/kg selected for these scenarios may not be a true "background" concentration, the intent in designing Scenarios 1 and 2 was to select a concentration that might be typical of areas where no known identifiable source impacts the soil.

This scenario was designed to be plausible for properties located near inactive industrial sites with contaminated soil. The selection of 1 m g/kg (ppb) for the three compounds was based on 2,3,7,8-TCDD findings associated with the Dow Chemical site in Midland, MI (EPA, 1985; Nestrick, et al. 1986) as well as the 100 industrial sites evaluated in the National Dioxin Study (which included the Dow Chemical site; EPA, 1987). In that study, most of the sites studied had soil concentrations in the parts per billion range. The farm size was 40,000 m², as in all high end scenarios. Table 5-3 shows these concentrations for the example compounds at the site of contamination, and also for the tilled and untilled condition at the sites of exposure. Exposure site soil is assumed to become contaminated over time due to erosion of soil from the contaminated site. The "tilled" condition distributes the eroded contaminants to a depth of 20 cm and impacts the estimated concentrations on underground vegetables grown at home. The"untilled" condition distributes the eroded contaminants only to a depth of 5 cm, and results in a soil concentration for which soil exposure pathways, ingestion and dermal contact, are estimated. Note that there are no differences in concentrations at the exposure site among the three example contaminants. Three key factors influence the concentrations estimated to occur at sites near a site of soil contamination. First are the source strength terms for the contaminated site - area of contamination and concentration. These were the same for each of the example compounds. Second are the components of the erosion algorithm - quantities of erosion, enrichment ratio, and distance from the site of contamination. Again these were the same for all example compounds. Finally, there are the key parameters determining exposure site concentration - the depth of mixing layers and the contaminant dissipation rate. For all three compounds, a dissipation rate corresponding to a 10-year half-life was assumed, and 5 and 20 cm mixing depths were used in all cases.

Chapter 3 described the application of the COMPDEP model to estimate air-borne concentrations and deposition rates of the contaminants in the vicinity of the hypothetical incinerator, given contaminant emission rates in units of g/sec. Table 5-3 shows the emission rates of 2,3,7,8-TCDD and TEQs. As discussed in Chapter 3, the emission factors (mass compound emitted per mass feed material combusted) were typical of incinerators with a high level of air pollution control, e.g., scrubbers with fabric filters. The TEQ emission factor for the hypothetical incinerator, 4.5 ng TEQ/kg material combusted, was within a range of 0.3 ng TEQ/kg municipal solid waste incinerated, to 200 ng TEQ/kg hospital waste incinerated. This range was developed from representative test data for source-specific incinerators with a similar high level of pollution control technology. Two hundred metric tons per day of material was assumed to be incinerated at the hypothetical incinerator in order to arrive at emissions in appropriate units of g/sec. Wet and dry particle deposition rates, in units of g/m²-yr, were determined for all dioxins and furans, at various distances from the stack and in the prevailing wind direction. The exposure sites of Scenarios 4 and 5 are located 500 and 5000 meters, respectively, from the emission source. Although the deposition rate for a site whose midpoint is 500 meters away can be precisely calculated as the average of several rates between, say 300 and 700 meters, the deposition rates at 500 meters as listed in Tables 3-15 and 3-16 were used. The same was done for the site at 5000 meters. Other deposition rates needed for the stack emission source category were those used to estimate average watershed soil concentrations and direct deposition onto the impacted water body. For both the central and high end scenarios, rates of deposition at 500 meters were used for these purposes. This might translate to an assumption that the stack was located near the impacted water body. The soil concentrations at the sites of exposure and within the watershed resulting from these depositions are listed in Table 5-3. It is noted that the soil mixing depth for the untilled circumstance is not the same as in the off-site soil category, demonstrated in Scenario #3. The mixing depth for untilled conditions is assumed to be 1 cm, instead of 5 cm. The reasoning is that particle deposition is a less turbulent process of transport as compared to soil erosion - soil erosion was assumed to transport residues from a site of off-site contamination to a site of exposure. The tilled mixing depth was 20 cm, as in the off-site soil category. Finally, the mixing depth assumed to characterize watershed soils on the average was 10 cm. This might assume, for example, that the watershed soils include tilled (agricultural fields) and untilled (residential) soils.

All key parameters used in Scenario 6 demonstrating the effluent discharge source category were developed using data associated with the 104 pulp and paper mill study (EPA, 1990). Derivation of the physical parameters including the flow rate of the receiving water body, flow rate of the effluent stream, suspended solids concentrations of the receiving water body and the effluent stream, and so on, are described in Section 4.6 of Chapter 4. An exercise evaluating the simple dilution model for predicting impacts to suspended solids in water body and subsequently to fish tissue concentrations resulting from discharges from these mills is described in Section 7.2.3.6, Chapter 7. The bottom line conclusion from that exercise was that the simple dilution model appears to work satisfactorily for a screening model: predicted whole fish tissue concentrations for the majority of mills were half as much as measured fish tissue concentrations. For the minority of mills, those with the highest volumes of receiving water, the model did not work as well. Predicted fish tissue concentrations were around an order of magnitude lower than measured concentrations. The precise reason for this discrepancy is not known, but the most likely explanation that larger water bodies have more uses and more sources of dioxin-like input - assuming that the fish tissue concentrations result singly from the mill discharge and a few proximate mills may be inappropriate.

Parameters for Scenario 6 were derived from the mills for which the model best performed. The average discharge rate from these mills was 0.197 mg 2,3,7,8-TCDD/hr. However, this data was valid for the time of sampling, which was 1988. Since then, pulp and paper mills have reduced the discharge of dioxin-like compounds in their effluents by altering the pulp bleaching processes. Gillespie (1992) reports that data on effluent quality from all 104 mills demonstrate reductions in discharges of 2,3,7,8-TCDD of 84% overall. On this basis, the discharge rate assumed for 2,3,7,8-TCDD was 0.0315 mg/hr (16% of 0.197 mg/hr). This same rate was assumed for the other two example compounds.

It is important to note that these discharge assignments are not intended to reflect current discharges of dioxin-like compounds from pulp and paper mills, even for 2,3,7,8-TCDD, but particularly for the other two example compounds. Data from the 104-mill study did allow for development of a "composite" effluent discharger in certainly a plausible setting (receiving water body and discharge flow rates, suspended solids, etc.) for pulp and paper mills. Assigning what might be evaluated as a reasonable discharge rate of 2,3,7,8-TCDD from pulp and paper mills for current conditions allows for the example scenario to placed in some context, which was a primary objective of crafting all example scenarios. Individual sources must be evaluated on an individual basis.

The results of this exercise include the exposure media concentrations for all exposure pathways and scenarios, and the LADD exposure estimates. These two categories of results are summarized in Tables 5-4 and 5-5. Following now are several observations from this exercise. As a reminder for the TEQ demonstration for the stack emission demonstration scenarios, #4 and #5, individual dioxin and furan congeners with non-zero toxic equivalency factors (TEFs) were modeled with unique fate and transport parameters until estimates of exposure media concentration were made. At that point, the TEQ exposure media concentrations were estimated as: S C_j*TEF_j, where C_j are exposure media concentrations for the individual congeners and TEF_j are the TEF for the individual congeners.

Table 5-4. Exposure media concentrations estimated for all scenarios and pathways.

Exposure pathway/ Scenarios 2378-TCDD 23478-PCDF 233'44'55'-

scenario #1,2,3,6: HPCB

Scenarios #4,5: 2378-TCDD TEQ

1. Concentration of contaminants in soil

for soil ingestion and dermal contact

pathways, m g/kg (ppb)

#1 On-site soil, central 0.001 0.001 0.001

#2 On-site soil, high end 0.001 0.001 0.001

#3 Off-site soil, high end 0.279 0.279 0.279

#4 Stack emissions, central 1*10^-7 3*10^-6

#5 Stack emissions, high end 1*10^-6 2*10^-5

2. Concentration of contaminants in air

in vapor phase for vapor inhalation

pathway, m g/m³

#1 On-site soil, central 4*10^-11 2*10^-11 9*10^-11

#2 On-site soil, high end 4*10^-11 2*10^-11 9*10^-11

#3 Off-site soil, high end 4*10^-9 2*10^-9 2*10^-8

#4 Stack emissions, central 5*10^-12 8*10^-11

#5 Stack emissions, high end 1*10^-11 2*10^-10

(note: for stack emission scenarios, #4 and #5, air

concentrations are total, including vapor + particle phases)

3. Concentration of contaminants in air

in particulate phase for particulate

inhalation pathway, m g/m³

#1 On-site soil, central 6*10^-13 6*10^-13 6*10^-13

#2 On-site soil, high end 5*10^-12 5*10^-12 5*10^-12

#3 Off-site soil, high end 2*10^-10 2*10^-10 2*10^-10

4. Concentration of contaminants in water

for water ingestion pathway, mg/L

#1 On-site soil, central 3*10^-11 2*10^-11 3*10^-12

#2 On-site soil, high end 3*10^-11 2*10^-11 3*10^-12

#3 Off-site soil, high end 2*10^-10 1*10^-10 2*10^-11

#4 Stack emissions, central 4*10^-15 5*10^-14

#5 Stack emissions, high end 4*10^-15 5*10^-14

#6 Effluent discharge, central 2*10^-11 1*10^-11 3*10^-12

(continued on the following page)

Table 5-4. (continued)

Exposure pathway/ Scenarios 2378-TCDD 23478-PCDF 233'44'55'-

scenario #1,2,3,6: HPCB

Scenarios #4,5: 2378-TCDD TEQ

5. Concentration of contaminants in

fish for fish ingestion pathway,

mg/kg

#1 On-site soil, central 6*10^-7 6*10^-7 1*10^-5

#2 On-site soil, high end 6*10^-7 6*10^-7 1*10^-5

#3 Off-site soil, high end 3*10^-6 3*10^-6 8*10^-5

#4 Stack emissions, central 6*10^-11 1*10^-9

#5 Stack emissions, high end 6*10^-11 1*10^-9

#6 Effluent discharge, central 4*10^-7 5*10^-7 1*10^-5

6. Concentration of contaminants in below

ground vegetables (no below ground fruit

was assumed) for their respective

pathways, mg/kg fresh weight

#1 On-site soil, central 1*10^-9 1*10^-9 8*10^-10

#2 On-site soil, high end 1*10^-9 1*10^-9 8*10^-10

#3 Off-site soil, high end 1*10^-7 1*10^-7 6*10^-8

#4 Stack emissions, central 1*10^-14 2*10^-13

#5 Stack emissions, high end 8*10^-14 1*10^-12

7. Concentration of contaminants in above

ground fruit and vegetables for their

respective pathways, mg/kg fresh weight

#1 On-site soil, central 6*10^-12 1*10^-11 3*10^-12

#2 On-site soil, high end 1*10^-11 2*10^-11 1*10^-11

#3 Off-site soil, high end 8*10^-10 1*10^-9 7*10^-10

#4 Stack emissions, central 5*10^-13 3*10^-11

#5 Stack emissions, high end 3*10^-12 1*10^-10

8. Concentration of contaminants in

beef fat for beef ingestion

pathway, mg/kg dry weight

#2 On-site soil, high end 1*10^-7 1*10^-7 6*10^-8

#3 Off-site soil, high end 3*10^-5 2*10^-5 2*10^-5

#5 Stack emissions, high end 2*10^-9 4*10^-8

(continued on the following page)

Table 5-4. (continued)

Exposure pathway/ Scenarios 2378-TCDD 23478-PCDF 233'44'55'-

scenario #1,2,3,6: HPCB

Scenarios #4,5: 2378-TCDD TEQ

9. Concentration of contaminants in

milk fat for milk ingestion pathway,

mg/kg dry weight

#2 On-site soil, high end 6*10^-8 5*10^-8 3*10^-8

#3 Off-site soil, high end 2*10^-5 1*10^-5 9*10^-6

#5 Stack emissions, high end 2*10^-9 3*10^-8

• Soil Concentrations:

The lowest soil concentrations resulted from deposition of particles from the example stack emission source. Concentrations for the stack emission central and high end scenario were 4 and 3 orders of magnitude lower than the central and high end scenarios demonstrating the on-site source category, respectively. This implies that the example stack emission source would have little impact to nearby soils, since the on-site source category was demonstrated with soil concentrations evaluated as typical of background conditions. The order of magnitude difference in distance from the stack between the central (5000 meters away) and high end (500 meters) scenarios is matched by the same order of magnitude difference in soil concentrations. TEQ soil concentrations were over an order of magnitude higher than 2,3,7,8-TCDD concentrations. The

Table 5-5. Lifetime average daily dose, LADD, estimates for all scenarios and exposure pathways (all results in mg/kg-day)

Exposure scenario/ Scenarios 2378-TCDD 23478-PCDF 233'44'55'-

pathway #1,2,3,6: HPCB

Scenarios #4,5: 2378-TCDD TEQ

#1 On-site soil contamination

Central exposure scenario

a. Soil ingestion 8*10^-13 8*10^-13 8*10^-13

b. Soil dermal contact 4*10^-14 4*10^-14 4*10^-14

c. Inhalation-vapor 1*10^-15 5*10^-16 3*10^-15

d. Inhalation-particle 2*10^-17 2*10^-17 2*10^-17

e. Water ingestion 7*10^-14 4*10^-14 6*10^-15

f. Fish ingestion 1*10^-12 1*10^-12 3*10^-11

g. Fruit ingestion 2*10^-16 4*10^-16 1*10^-16

h. Vegetable ingestion 2*10^-14 2*10^-14 1*10^-14

#2 On-site soil contamination

High end exposure scenario

a. Soil ingestion 3*10^-12 3*10^-12 3*10^-12

b. Soil dermal contact 9*10^-13 9*10^-13 9*10^-13

c. Inhalation-vapor 3*10^-15 1*10^-15 7*10^-15

d. Inhalation-particle 3*10^-16 3*10^-16 3*10^-16

e. Water ingestion 3*10^-13 1*10^-13 2*10^-14

f. Fish ingestion 1*10^-11 1*10^-11 2*10^-10

g. Fruit ingestion 1*10^-15 2*10^-15 1*10^-15

h. Vegetable ingestion 7*10^-14 6*10^-14 4*10^-14

i. Beef ingestion 6*10^-12 4*10^-12 2*10^-12

j. Milk ingestion 1*10^-12 9*10^-13 6*10^-13

#3 Off-site soil contamination

High end exposure scenario

a. Soil ingestion 9*10^-10 9*10^-10 9*10^-10

b. Soil dermal contact 7*10^-11 7*10^-11 7*10^-11

c. Inhalation-vapor 3*10^-13 1*10^-13 1*10^-12

d. Inhalation-particle 2*10^-14 2*10^-14 2*10^-14

e. Water ingestion 1*10^-12 8*10^-13 1*10^-13

f. Fish ingestion 6*10^-11 6*10^-11 1*10^-9

g. Fruit ingestion 9*10^-14 2*10^-13 8*10^-14

h. Vegetable ingestion 5*10^-12 5*10^-12 3*10^-12

(continued on the following page)

Table 5-5. (continued)

Exposure scenario/ Scenarios 2378-TCDD 23478-PCDF 233'44'55'-

pathway #1,2,3,6: HPCB

Scenarios #4,5: 2378-TCDD TEQ

#3 Off-site soil contamination

High end exposure scenario

(continued)

i. Beef ingestion 1*10^-9 9*10^-10 7*10^-10

j. Milk ingestion 3*10^-10 2*10^-10 1*10^-10

#4 Stack emissions

Central exposure scenario

a. Soil ingestion 1*10^-16 3*10^-15

b. Soil dermal contact 3*10^-19 7*10^-18

c. Inhalation-total 1*10^-16 2*10^-15

d. Water ingestion 7*10^-18 1*10^-16

e. Fish ingestion 1*10^-16 2*10^-15

f. Fruit ingestion 2*10^-17 1*10^-15

g. Vegetable ingestion 3*10^-17 1*10^-15

#5 Stack emissions

High end exposure scenario

a. Soil ingestion 4*10^-15 8*10^-14

b. Soil dermal contact 5*10^-17 1*10^-15

c. Inhalation-total 1*10^-15 2*10^-14

d. Water ingestion 3*10^-17 4*10^-16

e. Fish ingestion 1*10^-15 2*10^-14

f. Fruit ingestion 3*10^-16 1*10^-14

g. Vegetable ingestion 4*10^-16 1*10^-14

h. Beef ingestion 9*10^-14 2*10^-12

i. Milk ingestion 3*10^-14 6*10^-13

#6 Effluent discharge

Central exposure scenario

a. Water ingestion 5*10^-14 3*10^-14 6*10^-15

b. Fish ingestion 9*10^-13 1*10^-12 3*10^-11

difference in 2,3,7,8-TCDD and TEQ impacts to all media mirrors the difference in stack emissions of 2,3,7,8-TCDD and stack emissions of TEQ. As seen Table 5-3, 2,3,7,8-TCDD emissions are 6% of TEQ emissions, and soil concentrations of 2,3,7,8-TCDD are 6% of TEQ soil concentrations. This trend in differences between 2,3,7,8-TCDD and TEQ impacts occurs in all exposure media estimations. The highest soil concentrations at the site of exposure resulted from erosion of contaminated soil originating at the 10-acre contaminated site of Scenario 3. Concentrations at the sites of exposure were 0.279 m g/kg (279 ppt) for the no-till algorithm which mixed delivered residues to a depth of 5 cm and 0.070 (70 ppt) for the till algorithm which had a mixing depth of 20 cm. The soil at the site of contamination 150 meters away was 1 m g/kg (1000 ppt or 1 ppb). Exposure site soil concentrations resulting from erosion were the same for all three compounds. This is because the same initial soil concentration was assumed at the site of contamination, and the erosion algorithm contains only one chemical specific parameter. This is the rate of dissipation for eroding contaminants. It was assigned a value of 0.0693 yr^-1 (10-year half life) for all three example compounds.

• Vapor and Particle-Phase Air Concentrations:

One statement to make up front about vapor-phase air concentrations is that using the descriptor "vapor-phase" does not necessarily mean that the contaminants are expected to remain in a pure vapor state while air-borne. Residues which volatilize from the soil are expected to initially be a vapor phase. However, it is possible that dioxin-like compounds released into the air this way would not remain in vapor phase, but would partly sorb to air-borne particulates. The assumption is made is this assessment that contaminants released from the soil remain in the vapor phase for further modeling. This assumption influences air-to-leaf transfers of vapors for estimating impacts to vegetations.

It also impacts the relative magnitudes of predicted concentrations in the vapor as compared to the particulate phase for the soil source categories. As seen in Table 5-4, the vapor phase concentrations of 2,3,7,8-TCDD are 1 to 2 orders of magnitude higher than the particle phase concentrations for the soil contamination source demonstrations - Scenarios 1, 2, and 3. In contrast, the reservoirs in the vapor and particle phases for 2,3,7,8-TCDD are comparable for the demonstration of the stack emission source category, Scenarios 4 and 5. In that case, partitioning of 2,3,7,8-TCDD as released and transported is assumed to be 45% in the particle phase and 55% in the vapor phase. This close partitioning results in comparable reservoirs at sites of exposure.

Concentrations of contaminants in the vapor phase range from 10^-11 to 10^-8 m g/m³. Similar and lower concentrations, in the 10^-11 m g/m³ range, resulted from the volatilization of background soil concentrations of 0.001 m g/kg of the three example compounds, Scenarios 1 and 2. When the soil concentration of these compounds were three orders of magnitude higher at a site 150 meters away, air concentrations at the exposure site were about two orders of magnitude higher.

One interesting trend of note is that the vapor-phase concentrations for the central and high end scenarios of Scenarios 1 and 2 are similar for each compound; i.e., the 2,3,7,8-TCDD concentration for Scenario 1 is the same as the 2,3,7,8-TCDD concentration of Scenario 2 (although they are different within a Scenario for different compounds; that will be discussed shortly). This is, in fact, the result of two inverse trends of the solution algorithm. First, the average volatilization flux (mass/area-time) will always be lower for the high end scenario as compared to the central scenario. This is due to the solution algorithm assumption that residues available for volatilization originate from deeper in the soil profile over time, so that the average flux is lower for longer periods of volatilization. This is seen in the volatilization flux equation - Equation (4-13), Chapter 4 - which has a time term (ED, or exposure duration) in the denominator. The high end scenarios assume 20 years exposure duration compared to 9 years for the central scenarios. This alone would have resulted in lower air concentrations in the high end as compared to the central scenario. However, the dispersion of volatilized residues is a direct function of the area over which volatilization occurs. This is expressed in terms of a side length, parameter "a" in Equation (4-16), Chapter 4, as well as a dispersion term, S_z. It is easy to show that increasing the area alone would have resulted in higher air concentrations at the larger farm site of the high end scenario, 10 acres, as compared to the smaller residence of the central scenario, 1 acre. The two trends cancel each other and vapor phase concentrations for a given compound are similar for both scenarios. However, for different compounds within the same scenario, vapor phase concentrations are different. This difference is due to chemical parameters, principally the Henry's Constant, H. 2,3,3',4,4',5,5'-HPCB had the highest value for H, and it was 2 orders of magnitude higher than the value for 2,3,7,8-TCDD and 3 orders of magnitude higher than the value for 2,3,4,7,8-PCDF. This drove the trend for air concentrations, as 2,3,3',4,4',5,5'-HPCB had the highest air concentrations, followed by 2,3,7,8-TCDD at a concentration 1 order of magnitude lower and 2,3,4,7,8-PCDF at slightly lower than 2,3,7,8-TCDD.

Total air concentrations of 2,3,7,8-TCDD predicted to occur at exposure sites at 500 meters and 5,000 meters from a stack emission were in the 10^-12 to 10^-11 m g/m³ range. The air concentration estimated to result from a background soil concentration of 1 ppt (example Scenarios 1 and 2) was dominated by the vapor phase and equalled 4*10^-11 m g/m³. The TEQ vapor and particle concentrations exceeded the analogous concentrations of 2,3,7,8-TCDD by about a factor of 20. As in the soil concentration discussion above, this difference is driven by the difference in emission rates of 2,3,7,8-TCDD and TEQs. Even though air concentrations of 2,3,7,8-TCDD are the similar for Scenarios 4 and 5, the stack emission source category, and Scenarios 1 and 2, the soil contamination source category demonstrated at background soil concentrations, the soil concentrations are much different, as noted above in the discussion on soil concentrations. Chapter 6, Sections 6.3.3.9 and 6.3.3.11 discuss this dichotomy in performance between the soil contamination source categories and the stack emission source categories - the dichotomy being that while air concentrations from the stack emission demonstration and background soils appear similar, the soil concentrations are much different.

Particulate-phase concentrations at the exposure sites of Scenarios 1 and 2 were 2 to 3 orders of magnitude lower than exposure site concentrations predicted to occur from emissions at a contaminated site which is 150 meters away at the off-site location of contaminated soil, Scenario 3. This was due principally to the 3 orders of magnitude higher soil concentrations at these off-site soil contamination locations. Another trend is that the particle-phase concentrations are the same for all three compounds within Scenarios 1-3. This is because the algorithm to estimate particle-phase concentrations is independent of chemical properties. The trend discussion above concerning vapor phase concentrations resulting from volatilization and dispersion is not true for particulate phase estimation. In this case, a steady flux is estimated which is not a function of time. The same dispersion algorithm is used, however, so that the high end concentrations in Scenario 2 are higher than the central concentrations in Scenario 1.

• Drinking Water and Fish:

Concentrations of the example contaminants in water were 10^-15 to 10^-10 mg/L (ppm; or equivalently 10^-6 to 10^-1 pg/L or ppq). Concentrations in fish ranged from 10^-11 to 10^-5 mg/kg, or in parts per trillion terms, which are common units used in expressing fish concentrations in the literature, 10^-5 to 1 ppt. The concentrations resulting from the stack emissions were 4 to 5 orders of magnitude lower than the concentrations resulting from the soil and effluent source discharge source categories. The concentrations resulting from the effluent discharge were nearly identical to the concentrations resulting from basinwide background soil concentrations of 1 ppt, which were used to demonstrate the on-site soil source category. The fish concentrations resulting from the bounded area of high soil contamination, where 10 hectares within the watershed had soil concentrations of 1 ppb, were about an order of magnitude higher than the effluent discharge or on-site soil sources.

The PCB concentrations were 1-2 orders of magnitude higher than the dioxin and furan because the key bioaccumulation variables estimating fish tissue concentrations, the Biota Sediment Accumulation Factor, BSAF, and the Biota Suspended Solids Accumulation Factor, BSSAF (used only for the effluent discharge source category), is 2.0 for the example PCB while it is 0.09 for the example dioxin and furan.

Concentrations of 2,3,7,8-TCDD are about an order of magnitude lower than concentrations for TEQs. This mirrors the results for the air and soil, and reflects about an order of magnitude higher stack emissions of TEQs than 2,3,7,8-TCDD. Also, there is no difference between the central and high end scenarios for the stack emission source category. The exposure sites are located at different points with respect to the stack - the site for the central scenario is 5000 meters away from the stack, and the site for the high end scenario is 500 meters from the stack. This impacts all exposure media estimations except the fish and water estimates. Those two are a function of average watershed impact to the stack emissions, not impact to the site of exposure.

• Fruit and Vegetable Concentrations:

Concentrations in these foods ranged from 10^-14 to 10^-7 mg/kg (ppm) expressed on a fresh weight basis. Concentrations in below ground vegetables are found to exceed those in above ground vegetables when the source of contamination is soil - the on-site and off-site examples scenarios, #1 - #3. When the source of contamination is stack emissions, however, above ground concentrations exceed those of below ground. The causes for this trend follow from the trend discussions on soil and air concentrations above. First, the air concentrations for the stack emission demonstrations, 4 and 5, were comparable to the air concentrations for the background soil scenarios, 1 and 2. This in itself would lead to roughly similar above ground vegetation concentrations, and that in fact is what happened. On the other hand, the soil concentrations were 3 to 4 orders of magnitude lower for the stack emission demonstrations as compared to the background soil demonstrations. This is the reason why above ground vegetation concentrations exceeded below ground concentrations for the stack emission source category, while the reverse was true for the soil contamination source category. Trends regarding vapor phase transfers and particle depositions to vegetations are discussed more extensively in Chapter 6, Section 6.3.3.8.

As in the air and soil trends discussed above, off-site soil contamination in the range of 1 m g/kg (1 ppb; example Scenario #3) results in higher concentrations than on-site background soil concentrations of 0.001 m g/kg (1 ppt; example Scenarios #1 and #2). Another trend noted for Scenarios 1-3, where the initial soil concentrations were the same among the three compounds, is that transfers from soil to plant are driven by chemical parameters, particularly the octanol water partition coefficient, Kow. 2,3,3',4,4',5,5'-HPCB had the highest Kow, with 2,3,4,7,8-PCDF and 2,3,7,8-TCDD at lower but similar Kow. Higher Kow translates to tighter sorption to soil, and less transfer to plant, either through root uptake or air-to-leaf transfer. This trend translated to the lowest fruit/vegetable concentrations for 2,3,3',4,4',5,5'-HPCB. 2,3,7,8-TCDD and 2,3,4,7,8-PCDF had similar fruit/vegetable concentrations for Scenarios 1-3.

The results for the stack emission source category indicate once again that TEQ fruit and vegetable concentrations exceed those of 2,3,7,8-TCDD by about an order of magnitude.

• Beef and Milk Fat Concentrations:

These concentrations ranged from 10^-9 to 10^-5 mg/kg, or equivalently, 0.001 to 10 ppt. These results were in terms of fat concentrations, which assumes that all the compound bioconcentrates in the fat of beef and milk. To convert to a whole product