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Exposure Scenarios

1 & 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-acres) small rural farm

(Scenario 2) contained residues of the three example contaminants. The concentrations of the contaminants are uniformly set at 1 part per trillion, which was evaluated as reasonable background levels.

3: Off-site Soil Contamination, Farm

A 40,000 m2 rural farm is located 150 m (500 ft) 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.

4 &5: Stack Emissions, Residence and Farm

A 4,000 m2 rural residence

(Scenario 4) is located 5000 meters downwind from a stack emission source, and a 40,000 m2 rural farm

(Scenario 5) is located 500 meters from the same stack emission source. The emissions of dioxin-like compounds were evaluated as within the range observed for various stack emission sources which have sophisticated air pollution control devices (e.g., scrubbers combined with fabric filters).

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, and exposures to ingestion of drinking water and fish, are considered.

The source strength was developed from data on pulp and paper mill discharges of 2,3,7,8-TCDD. Discharge rates were based on data from EPA's 104-mill study (EPA, 1990c), and then reduced considering recent improvements in the bleaching process which have reduced discharges.

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 (abbreviated 2,3,7,8-TCDD),
2,3,4,7,8-pentachlorodibenzofuran (2,3,4,7,8-PCDF), and
2,3,3',4,4',5,5'-heptachloro-PCB (HPCB).

For the stack emission demonstration, Scenarios 4 and 5, a different approach was taken. Exposures to 2,3,7,8-TCDD alone are determined, as in the other demonstrations. Emission rates for all dioxins and furans with non-zero toxicity equivalency factors (abbreviated TEFs) were available for the demonstration of the stack emission source category.

Use of the full suite of emissions allowed for the opportunity to demonstrate an appropriate methodology for estimating TEQ exposures. The framework takes the individual deposition rates and concentrations for the individual congeners and models the exposure media concentrations individually with unique fate and bioaccumulation parameters, and then determines a final TEQ exposure media concentration using TEFs.

III.4.1.Results from the Demonstration of the Stack Emission Source Category

For brevity, only the results from the stack emission source category will be summarized. Table III-2 gives the exposure media concentrations estimating for 2,3,7,8-TCDD and for TEQs for Example Scenario #5, the high end scenario for the stack emission source category. Table III-3 gives the estimated Lifetime Average Daily Doses, LADDs, for the exposure pathways modeled in this assessment.

Much of the differences between exposure pathways and scenarios is due to differences in exposure media estimation. Therefore, the discussion below on trends for LADD follows directly from how the methodologies estimate exposure media concentrations.

It is important to understand that exposure estimates generated for the demonstration scenarios are specific to the site conditions assumed for the examples and are not generalizable to other sites. Following are some key observations:

table Table III-2. Exposure media concentrations estimated for the demonstration of the stack emission source category1.. table Table III-3. Lifetime Average Daily Doses, LADD, for the high end stack emission demonstrations scenario (LADD in units of ng/kg-day).
expand table Table VX X-X expand table Table VX X-X

1) The highest exposures were associated with the off-site soil contamination scenario, Scenario #3. This scenario had the highest exposure media concentrations for all exposure media. The source of contamination was a 40,000 m2 land area with soil concentrations initialized at 1 ppb for the three example compounds. The lowest LADDs were estimated for the demonstration of the stack emission source category. Although the intensity of the source strength between a stack emission source and a soil source cannot be directly related, it is noted that the releases of 2,3,7,8-TCDD and TEQs used to demonstrate the stack emission source were comparable to other stack emission sources with sophisticated air pollution control devices. Exposures to 2,3,7,8-TCDD were about 5% of exposures to TEQs.

This mirrors the comparison of the 2,3,7,8-TCDD release rate and total TEQ release rate from the stack. Only a fish and a water ingestion pathway were considered for the effluent discharge source category.

The exposures estimated for these two pathways were similar in magnitude to the fish and water ingestion exposures estimated for demonstration of the on-site soil source category, demonstrations #1 and #2. For those demonstrations, watershed soils were initialized at 1 ppt, a concentration that researchers have found for 2,3,7,8-TCDD in background settings.

2) Differences between analogous "central" and "high end" exposures for the on-site soil source demonstration scenarios were near or less than an order of magnitude. "Analogous" exposures are those estimated for both scenarios.

They include inhalation, soil ingestion and dermal contact, water, vegetable/fruit, and fish ingestion exposures. Only beef and milk are not analogous since they were only estimated for the high end scenario.

Analogous exposures were within an order of magnitude of each other because the exposure parameters used to distinguish typical and high end exposures, the contact rates, contact fractions, and exposure durations, themselves did not differ significantly, and these were the only distinguishing features for the central and high end demonstrations of the on-site soil source category. In the stack emission scenario, placing exposed individuals either 500 or 5000 meters away from the incinerator did significantly impact the results.

In this case, the difference was closer to 2 orders of magnitude for all analogous exposures except water and fish exposures, which were not a function of distance from the stack. The order of magnitude difference in distance added about an order of magnitude difference in exposure media concentrations and hence LADD estimates.

3) It is inappropriate to compare and rank exposure pathways across all scenarios because the source terms are different. However, relationships between different pathways within each scenario can be discussed. Table III-4 was constructed by summing the LADDs for all pathways, and then determining the percent contribution by each pathway.

Before the summation, LADDs were corrected to account for absorption - all ingestion LADDs assumed 50% absorption and inhalation LADDs assumed 75% (data on bioavailability from animal feeding studies, suggests that the absorption of 2,3,7,8-TCDD is around 50%; 75% for inhalation reflects a general assumption of greater absorption for this pathways; both simple assumptions made only for the purpose of this comparative exercise).

The dermal contact LADD was the only one where absorption was already considered in its estimation: absorbed dose was estimated as 3% of dose contacting the body. Also, this exercise assumes all pathways occur simultaneously. Table III-4 was generated only for the 2,3,7,8-TCDD example compound, and the rows are listed generally from the highest to lowest percentage contribution.

table Table III-4. Percent contribution of the different exposure pathways within each exposure scenario.*

The following observations are made:
In high end scenarios which assumed exposure to home grown beef, milk, and fish, Scenarios 2, 3, and 5, exposures to these three foods dominated the results.

In Scenarios where beef and milk were not considered, but fish was considered, Scenarios 1, 4, and 6, fish exposures dominated.

The general dominance of beef, fish, and milk exposures underscores the importance of food chain exposures.

Milk exposures were lower than beef exposures because of less milk fat
expand table Table VX X-X

Fish
was the principal impacted media for the effluent discharge source category, with fish ingestion 19 times higher than water ingestion, the only two pathways considered for the effluent discharge category. However, fish is much less important than beef or milk for the high end stack emission scenario which had a beef and a milk pathway, and when a small site of contamination is near a farm raising a portion of the farming families beef and milk ingestion.

Soil ingestion
exposures were also noteworthy, particularly in scenarios that did not consider beef and milk, the central on-site scenario, #1, and the central stack emission scenario, #4. Soil ingestion was also the second highest pathway in the scenario evaluating the impact of nearby soil contamination, #3, ranking higher than milk or fish ingestion. Dermal exposures were non-trivial, but ranked behind the four ingestion pathways previously discussed: beef, milk, fish, and soil.

Inhalation
was the highest impact for the stack emission scenario when farm animal products were not considered, in Scenario #4. Fruit and vegetable exposures were noteworthy only in this same scenario. These trends imply that, where farm animal products are not being produced near a stack emission source, fish and vegetative food products still may dominate the overall exposure, but inhalation exposure can become critical.

Water ingestion
exposures were very low in comparison to the other exposures in these scenarios.

These demonstration scenarios represent only one approach to scenario development; other approaches might consider the quality of exposure media not associated with the home environment.

For example, if the bulk of an individual's ingestion of produce comes from local farms, and local farms may be impacted by an stack emission source, then perhaps 90-100% of an individual's fruit and vegetable ingestion, rather than the 20-40% assumed in this assessment, should be considered impacted.

III.5.USER CONSIDERATIONS

This section discusses three issues pertinent to use of the methodologies. The first subsection below discusses the use of the parameter values selected for the demonstration scenarios for other applications. The next subsection is a sensitivity analysis exercise on the parameters required for algorithms estimating exposure media concentrations. The last subsection addresses the issue of mass balance with regard to the source strength terms of the four source categories.

III.5.1.Categorization of Methodology Parameters.

Table 6.1 in Chapter 6 of Volume III lists all the parameters, including names, definitions, and units, that are required for the methodologies of this assessment except the exposure parameters. Exposure parameters are given in Table 2.1 in Chapter 2 of Volume III. Table 6.1 also gives four additional pieces of information for each parameter listed.

Three are numerical values which were used in the sensitivity analysis exercises that are described below. One of those parameters is labeled "selected", which were the ones used in the demonstration exposure scenarios.

High and low values of parameters selected for sensitivity analysis were carefully developed and might be considered a reasonable range of values for other uses of the methodology (with obvious exceptions such as areas of contamination, distances from contaminated to exposure site, and so on).

The chemical specific parameters are those only for 2,3,7,8-TCDD. The fourth piece of information is a qualitative judgement on the part of the authors of this document as to the appropriateness of using the "selected" parameter values for other assessments. This judgement is categorized in three ways:

1) First Order Defaults:
As defaults, these parameters are independent of site specific characteristics. As first order defaults, it is felt that the values selected for the demonstration scenarios carry a sufficient weight of evidence from current literature such that these values are recommended for other assessments.

Several of the chemical specific parameters, such as the Henry's Constant, H, and the organic carbon partition coefficient, Koc, fall into this category. The qualifier above, "current literature", indicates that new information could lead to changes in these values.

2) Second Order Defaults:
Like the above category, these parameters are judged to be independent of site specific characteristics. However, unlike the above category, the current scientific weight of evidence is judged insufficient to describe values selected for demonstration purposes as first order defaults.

Parameters of principal note in this category are the bioconcentration parameters specific to the chemicals, such as the Biota Sediment Accumulation Factor, or BSAF. This parameter translates a bottom sediment concentration to a fish tissue concentration. Users should carefully review the justification for the SOD values selected for the demonstration scenarios before using the same values.

3) Site Specific:
These parameters should or can be assigned values based on site-specific information. The information provided on their assignment for the demonstration scenarios, and for selection of high and low values for sensitivity analysis testing, is useful for determining alternate values for a specific site.

A key class of SS parameters which are the source strength terms - the soil concentrations, effluent discharge rates, and stack emission rates. If users are unable to obtain site-specific information, or their use of the methodologies is for general purposes, they should review the justification for selection of values for methodology demonstration, as well as information provided giving ranges of likely values for model parameters.

The exposure parameters can be categorized as have the contaminant fate and transport/transfer parameters. Assignment of these values are critical as LADD estimates are linearly related to parameter assignments - doubling exposure duration assumptions double LADDs, and so on. Some of the exposure parameters are appropriately described as first order defaults.

These include:
lifetime, body weights, water ingestion rates, inhalation rates, and an exposure duration for a childhood pattern of soil ingestion. All of the other exposure parameters are better described as either second order defaults or site-specific.

All exposure parameters were developed based on information and recommendations in EPA's Exposure Factors Handbook (EPA, 1989) and Dermal Exposure Assessment: Principals and Applications (EPA, 1992c). Attaining site-specific information is recommended for exposure parameters.