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5.0. DEMONSTRATION OF METHODOLOGY
5.1. INTRODUCTION

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.

The demonstration in this chapter attempts to be consistent with the goal of quantifying central and high end exposures. However, it is not exhaustive in its analysis, nor should it be construed as a case study with generalizable results. All the scenario definitions, parameter values, and so on, were construed to be plausible and reasonable, and to demonstrate the application of a site-specific methodology, not to set any regulatory precedent.

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.

5.3. EXAMPLE EXPOSURE SCENARIOS

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:

Exposure Scenarios 1 and 2: On-site Soil Contamination, Residence and Farm

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.

Exposure Scenario 3: Off-site Soil Contamination, Farm

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.

Exposure Scenarios 4 and 5: Stack Emissions, Residence and Farm

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.

Exposure Scenario 6: Effluent discharge into a river

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.

5.4. EXAMPLE COMPOUNDS

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.