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Part I Volume III
IIISITE-SPECIFIC ASSESSMENT PROCEDURES
III.1. EXPOSURE EQUATION

Volume III describes procedures for conducting site specific exposure assessments to estimate potential dose. A potential dose is defined as a daily amount of contaminant inhaled, ingested, or otherwise coming in contact with outer surfaces of the body, averaged over an individual's body weight and lifetime. The general equation used to estimate potential dose normalized over body weight and lifetime is as follows:

Diagram V1 III-1

This procedure is used to estimate dose in the form needed to assess cancer risks. Each of the terms in this exposure equation is discussed briefly below:

Exposure media concentrations:
These include the average concentrations in the media to which individuals are exposed. Media considered in this assessment include soil, air, water, vegetables/fruits, fish, beef, and milk.

Contact rate:
These include the ingestion rates, inhalation rates, and soil contact rates for the exposure pathways.

Contact fraction:
This term describes the distribution of total contact between contaminated and uncontaminated media. For example, a contact fraction of 0.8 for inhalation means that 80% of the air inhaled over the exposure period contains dioxin-like compounds in vapor form or sorbed to air-borne particulates.

Exposure duration:
This is the overall time period of exposure, mostly pertinent to adult exposures. Another exposure duration considered in this methodology is one associated with a childhood pattern of soil ingestion. The exposure duration in this case is 5 years.

Body weight:
For all the pathways, the human adult body weight of 70 kg is assumed. This value represents the United States population average. The body weight for child soil ingestion is 17 kg (EPA, 1989).

Lifetime:
Following convention, and because cancer risk slope factors are derived based on a 70-year human lifetime, the average adult lifetime assumed throughout this document is 70 years.

III.2.PROCEDURE FOR ESTIMATING EXPOSURE

Before making exposure estimates, the assessor needs to gain a more complete understanding of the exposure setting and the contamination source. The approach used for this assessment is termed the exposure scenario approach.

A "road map" of that procedure including identification of chapters in Volumes II and III where key information can be found, is shown in Figure III-1.
Brief descriptions of 7 steps in this approach are:

Step 1. Identify Source:
Three principal sources are addressed in this document: contaminated soils, stack emissions, and effluent discharges.

Step 2. Estimate Release Rates:
Estimating the release of contaminants from the initial source is the first step towards estimating the concentration in the exposure media. Releases from soil contamination include volatilization, and wind and soil erosion. Stack emissions and effluent discharges are point source releases into the environment.

Step 3. Estimate Exposure Point Concentrations:
Contaminants released from soils, emitted from stacks, or discharged into surface waters move through the environment to points where human exposure may occur, and/or to impact environmental media to which humans are exposed. Various fate, transport, and transfer models are used to predict exposure media concentrations given source releases.

Step 4. Characterize Exposed Individuals and Exposure Patterns:
Exposed individuals in the scenarios of this assessment are individuals who are exposed in their home environments. They are residents who breathe air at their residence, fish recreationally, have a home garden, farm, and are children ages 2-6 for the soil ingestion pathway.

table Figure III-1. Roadmap for assessing exposure and risk to dioxin-like compounds 50.
Exposures which are occur at the workplace or other locations are not discussed in this assessment, although the procedures could be adapted for other exposure sites.

Each of these pathways are evaluated separately.

Since it is unlikely that single individuals would experience all of these pathways, the exposures across pathways are not added.

Each pathway has a set of exposure parameters including contact rates, contact fractions, body weights, exposure durations, and a lifetime..
expand table Figure VX X-X

Step 5. Put It Together in Terms of Exposure Scenarios: A common framework for assessing exposure is with the use of "settings" and "scenarios." Settings are the physical aspects of an exposure area and the scenario characterizes the behavior of the population in the setting and determines the severity of the exposure.

A wide range of exposures are possible depending on behavior pattern assumptions. An exposure scenario framework offers the opportunity to vary any number of assumptions and parameters to demonstrate the impact of changes to exposure and risk estimates.

Step 6. Estimate Exposure:
The end result of having followed the above 5 steps are estimates of individual exposures to a characterized source of contamination.

Step 7. Assess Uncertainty:
Uncertainties should be considered when applying procedures in this document to a particular site. Pertinent issues explored in this assessment include:

1) model predictions of exposure media concentrations compared to field measurements,
2) similarities and differences for alternate models for estimating exposure media concentrations,
3) sensitivity of model results to a range of values for methodology parameters,
4) mass balance checks, and
5) qualitative and quantitative discussions on the uncertainties with the model parameters and exposure estimates generated for the demonstration scenarios.

III.3.ESTIMATING EXPOSURE MEDIA CONCENTRATIONS

Literally hundreds of fate and transport models have been published which differ widely in their technical sophistication, level of spatial or temporal resolution, need for site specific parameterization, and so on. This makes selection of the most appropriate one for any particular situation very difficult.

For this assessment, relatively simple, screening level models are used to model fate, transport, and transfer of dioxin-like compounds from the source to the exposure media. Simple assumptions are often made in order to arrive at the desired result, which is long-term average exposure media concentrations. Perhaps the most critical of the assumptions made is that the source strength remains constant throughout the period of exposure.


It is important to understand that EPA is not endorsing the algorithms of this assessment as the best ones for use in all dioxin assessments. They are suggested as reasonable starting points for site-specific or general assessments. All assumptions for the models and selection of parameter values are carefully described.


If these assumptions do not apply to a particular situation, or where assessors require more spatial or temporal resolution, more complex models should be selected. Finally, it cannot be overemphasized that measured concentrations are generally more reliable than modeled ones. Assessors should use measured concentrations if available and if such measurements can be considered spatially and temporally representative for the exposed populations.

III.3.1.Overview of Fate, Transport, and Transfer Algorithms of the Methodology

Figures III.2 through III.5 provide an overview of algorithms used to evaluate the fate, transport, and transfer of dioxin-like compounds from contaminated soil, stack emissions, and effluent discharge (called "source categories" in this document).

Algorithms are presented which link each of these sources to estimated concentrations in a number of media which may be contaminated as a result, and are therefore potential "exposure media":

1) surface soils,
2) surface-water associated media: suspended and bottom sediment and dissolved phase concentrations,
3) air including the vapor phase and in particulate form, and
4) biota including beef, milk, fruit and vegetables, and fish. The remainder of this section describes how each potential exposure medium can be affected by each source, and the algorithms used to make this link.

Surface soils:
Exposure to contaminated soil may be a result of direct contact with soil on the site of the "source" contamination, or indirectly after the contaminated soil has been transported off-site. These cases are known as the "on-site" scenario and the "off-site" scenario, respectively. In either case, soil concentrations are specified for the contaminated source. For the on-site scenario, the soil at the residence or farm (where exposures occur) is contaminated. In the off-site scenario, soil contamination is assumed to be adjacent to an accessible area known as the "exposure site".

Examples here would include a landfill or a Superfund site. Residues which reach the exposure site mix with soil already there; the mixing is assumed to take place to either a "tilled" depth or a "non-tilled depth". The tilled depth is assumed to be 20 cm (approximately 8 inches), typical of soil mixing for growing below-ground vegetables.

The concentrations derived from using a 20 cm mixing depth are also used to estimate concentrations for dermal contact for individuals in farming families (i.e., dermal contact is assumed to occur as a result of farming activities).

table Figure III-2 Diagram of the fate, transport, and transfer relationships for the on-site source category 53m. table Figure III-3  Diagram of the fate, transport, and transfer relationships for the off-site source category.
expand table Figure VX X-X expand table Figure VX X-X
table Figure III-4  Diagram of the fate, transport, and transfer relationships for the stack emission source category. table Figure III-5  Diagram of the fate, transport, and transfer relationships for the effluent discharge source category.
expand table Figure VX X-X expand table Figure VX X-X

The non-tilled mixing depth is assumed to be 5 cm (approximately 2 inches) when erosion transports residues to a site of exposure where deep tilling or plowing does not routinely occur. The concentrations derived using this mixing depth are used for dermal contact exposures in residential settings, for childhood soil ingestion in residential and farm settings, and for cattle soil ingestion (used in estimation of beef and milk concentrations).

Exposure site soils can also be impacted from stack emissions due to air transport of either vapor or particulate residues from the stack to the exposure site. Deposition modeling for particles allows for estimation of tilled and non-tilled soil concentrations. When stack emissions are the source, however, the nontilled depth of mixing is assumed to 1 cm (about 0.4 inch) instead of 5 cm, on the assumption that particle deposition is a less turbulent process than soil erosion.

A key assumption for evaluating the exposure site as a result of both off-site erosion and stack emissions is that contaminants impact a thin layer of soil and do, in fact, dissipate. For the on-site soil scenario, on the other hand, the contamination is assumed to extend into the soil and surface concentrations are not dissipated over time. Dissipation processes could include volatilization, photolysis, or other processes. A soil dissipation half-life of ten years is assumed for all dioxin-like compounds.

Surface Water:
The principal assumption driving the solutions for the soil and stack emission source categories is that the suspended and bottom sediments of water bodies originate as watershed soils, which are subsequently eroded. For the stack emission source category, a portion of the sediments also originates from directly-depositing particulates. The process of erosion transports soils within the watershed to the water body.


Unit rates of erosion along with watershed size determine the total potential amount of soil which could be delivered to the water body. Sediment delivery ratios reduce that potential amount. A mass balance assures that soil eroding on an annual basis becomes either suspended or bottom sediment within an annualized volume of surface water. "Enrichment" of eroded soil is assumed, which means that eroded soil from a contaminated source is assumed to be higher in concentration of dioxin-like compounds than in situ, off-site soils.


Once in the water body, a standard partitioning model based on the organic carbon partition coefficient, Koc, determines the concentration of contaminant in the water in truly dissolved form and the concentration on suspended sediments. The organic carbon normalized concentrations of suspended and bottom sediment are assumed to be equal. Watershed soil concentrations are model input parameters for determining the effect on surface water from contaminated soils.

For stack emissions, a total (dry + wet) deposition rate of contaminant which represents average depositions onto the watershed is specified as an input parameter, as well as a mixing depth representing the watershed. In this way, average watershed soil concentrations are calculated for the stack emission source category.

For effluent discharges as sources, watershed soils are not considered. An amount of contaminant is discharged into an annual flow volume to obtain a simple dilution concentration. This total concentration is partitioned into a truly dissolved phase and a phase sorbed to suspended sediments using the organic carbon partition coefficient, the Koc. Bottom sediments are not considered for effluent discharges.

Soil to Air:
From contaminated soils, residues become airborne via the processes of volatilization and wind erosion. For on-site soil contamination, these vapor and particle phase fluxes are translated to ambient air concentrations using a near-field dispersion model. For the off-site scenario, the same approach is used to estimate ambient air exposure site concentrations, except that a far-field dispersion model is used. These airborne reservoirs are the basis for inhalation exposures, and are also used to estimate plant concentrations for vegetable ingestion and in grass and feed for estimating beef and milk concentrations.

Stack Emissions, Atmospheric Transport Modeling:
Air dispersion/deposition models consider the basic physical processes of advection, turbulent diffusion, and removal via wet and dry deposition to estimate the atmospheric transport, resulting ambient air concentration, and settling of particles. Volume III uses the COMPDEP model for air dispersion and deposition modeling. Besides discussions in Volume III, further discussions on the COMPDEP model can be found in EPA (1990d).

COMPDEP
contains modifications of the Industrial Source Complex model (Short-Term version), and COMPLEX I to incorporate algorithms to estimate dispersion, and resulting ambient air concentrations, and wet and dry deposition flux. COMPLEX I is a second level screening model applicable to stationary combustion sources located in complex and rolling topography (EPA, 1986).

The model was developed specifically to evaluate the effects of complex terrain that exceeds the stack height of the source as developed by Turner (1986).

To account for pollutant deposition, the concentration algorithms in COMPLEX 1 were replaced with those from the Multiple Point Source Algorithm with Terrain Adjustments Including Deposition and Sedimentation (MPTER-DS) model (Rao and Sutterfield, 1982). The MPTER-DS algorithms incorporate the gradient transfer theory described by Rao (1981), and are extensions of the traditional Gaussian plume algorithms.

The dispersion algorithms contained in the Industrial Source Complex, Short-term version (ISCST), have been incorporated in COMPDEP to analyze ground-level receptors located below the height of the emission plume. COMPDEP uses the generalized Briggs (1975, 1979) equation to estimate plume-rise and downwind dispersion as a function of wind speed and atmospheric stability.

A wind-profile exponent law is used to adjust the observed mean wind speed from the measurement height to the emission height for the plume rise and pollutant concentration calculations. The Pasquill-Gifford curves are used to calculate lateral and vertical plume spread (EPA, 1986).

These curves are based on Pasquill's definitions of atmospheric stability classes, e.g., extremely unstable, moderately unstable, slightly unstable, neutral, slightly stable, and moderately stable, that correspond to various intensities of solar radiation and wind speeds (Seinfeld, 1986).

The incorporation of these two basic models into COMPDEP permits analysis of a source located in all types of terrain. Further details on the use of the COMPDEP model are: