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4.4. ALGORITHMS FOR THE "OFF-SITE" SOURCE CATEGORY

As noted in Section 4.1, the contaminated soil is remote from the site of exposure for the "off-site" source category. A common example is an industrial site with soil contamination or a landfill with contaminated soil. Section 4.4.3. below discusses some considerations for specific types of off-site soil contamination, including the disposal of ash from incinerators, the disposal of sludge from paper and pulp mills, and sites of industrial contamination such as those on the NPL. The example setting in Chapter 5 is 10 hectares in size, has sparse vegetation, and has contamination levels of the example compounds that are in the same range as those found on NPL sites. Since many of the parameters in the algorithms discussed below are specific to particular off-site soil contamination sites, guidance in this section will be specific to the example setting in Chapter 5.

Several of the algorithms estimating exposure media concentrations for the off-site source category are the same or very nearly the same as in the on-site source category. Following now are bullet summaries for similarities and small refinements to these algorithms.

Sections below describe algorithms that are unique for the off-site source category.

. Surface water impacts:
Methodologies and assumptions for estimating surface water impacts for the on-site source category (described in Section 4.3.1.) are also used for this source category. In applying this algorithm for the example scenario demonstrating the off-site source category in Chapter 5, Example Scenario 3, the important assumption was made that the average concentration of contaminants in the watershed was very low compared to the concentration on the contaminated soil - hence, Cw (average concentrations on watershed soils other than the contaminated site) was set to 0.0. Setting Cw to 0.0 could also be justified by saying that the demonstration scenario only demonstrated the incremental impact of the contaminated site.

The unit erosion rate from the contaminated site, SLs, is assumed to be 9.6 t/ac-yr (English units). The unit erosion rate from other areas of the watershed, SLw, is assumed to be 2.9 t/ac-yr. Derivation of these terms is given above in Section 4.3.1. The contaminated site is assumed to be 150 meters away from the water body, and SDs is estimated therefore as 0.26. The effective drainage area, Aw, is 4000 ha. From Figure 4-5, it is seen the sediment delivery ratio associated with this drainage area is approximately 0.15, which is assumed for the example scenario in Chapter 5. The contaminated site for the example scenario is 10 ha.

. Vapor-phase air concentrations:
The volatilization of contaminants from soil can be estimated similarly to the way described in Section 4.3.2 for the on-site source category. Section 4.4.2 describes a dispersion model which transports contaminants through the air to the exposure site. The far-field dispersion model described in Section 4.4.2 differs from the near-field dispersion model presented in Section 4.3.2.

. Particulate-phase air concentrations:
The same model for particulate flux due to wind erosion is used for the off-site source category as described in Section 4.3.3. However, two parameters might be different than described above (Equation (4-18)) if the model is applied to off-site soil contamination when the soil is bare. One is the vegetative cover, V, which might be more appropriately assigned a 0.0 implying no ground cover for an active landfill or an industrial site. The other is the threshold wind speed, Ut.

The different assumption would be in roughness height, assumed 4 cm for a residence or farm setting, but perhaps more appropriately assumed to be 1.0 cm for bare soil. This value is appropriate for a tilled field (EPA, 1985b). With this change, a Ut is calculated as 8.25 m/sec, and F(t) is calculated as 0.5. Note that V and Ut might be the same as a residential or farm setting if the off-site soil contamination had a grass cover. Once a flux has been calculated, the far-field dispersion model described in Section 4.4.2. below is used for estimating air-borne particulate-phase contaminant concentrations at the exposure site.

. Biota concentrations:
The basic strategy for estimating biota concentrations - as a linear function of environmental media concentrations (bottom sediment concentrations, soil, air) and based on bioaccumulation or biotransfer factors (along with diet fractions, etc.) - remains the same. Section 4.4.1. describes how exposure site soil concentrations are estimated from concentrations at the off-site source. Exposure site soil then becomes a "source" for plant, beef, and milk contaminant concentrations. Similarly, air-borne particle and vapor-phase contaminants originating from the off-site become sources for pasture grass and feed concentrations, which are above ground vegetations.

As described below, exposure site soil concentrations are a function both of the amount of soil estimated to erode from the off-site contamination, and of a mixing depth which is different for "tilled" vs. "untilled" situations. The soil concentration used for cattle ingestion of soil is assumed to be untilled. The soil concentration used to estimate concentrations in underground vegetables is assumed to be tilled. The algorithm to estimate fish tissue concentrations as a function of bottom sediment concentrations remains the same for this source category.

The soil ingestion and dermal exposure pathways are still a function of exposure site soil concentrations; i.e., no assumption of direct contact with the off-site contamination is made. Also, both of these direct soil exposure pathways used the untilled soil concentrations.

Section 4.4.1. discusses how exposure site soil concentrations are calculated from off-site concentrations. Section 4.4.2. describes adjustments to the volatilization flux algorithm and the far-field dispersion model which transports vapor and particulate-phase residues to the nearby exposure site. Section 4.4.3. discusses considerations for specific types of off-site soil contamination.

4.4.1. Exposure Site Soil Concentrations

The key assumptions for the solution strategy estimating exposure site soil concentrations resulting from an off-site soil contamination source are:

1) the exposure site soil becomes contaminated due to erosion of contaminated soil from the source to the exposure site,

2) the soil eroding off the site of contamination is "enriched" in comparison to the soil at the site - eroded soil has higher contaminant concentrations than in-situ soil,

3) the amount of soil at the exposure site does not increase, which means that soil delivered to the site via erosion is matched by an equal amount which leaves the site, and

4) not only does soil erode off the contaminated site en route to the exposure site, but soil between the contaminated site and the exposure site also erodes to the exposure site.

The third and fourth assumption translate to:

Equation V3 4-33

The mass balance equation for exposure site soil concentrations can now be qualitatively stated as (with "_C" used as shorthand for change in exposure site soil concentration over time):

Diagram V3 4-3

This can be expressed mathematically as:

Equation V3 4-34

Assuming that the contaminant concentration at the exposure site, C, is initially 0, Equation (4-34) can be solved to yield:

Equation V3 4-35

which computes C as a function of time, t (in years since k is in years). This can be solved for various increments of time starting from a time when the exposure or contaminated site initially became contaminated, or it can be simplistically assumed that the contamination has existed at the contaminated site for a reasonably large amount of time such that the exponential term approaches zero. This can be alternately stated that the assumption is made that the system has reached steady state over a long period of time. Either way, the exponential term drops out, and C is estimated as:

Equation V3 4-36

Equation (4-36) was used to estimate exposure site concentrations resulting from off-site contamination for the example scenario in Chapter 5.

Guidance for estimation of these terms including justification for their values as selected in the example settings are:

. E:

A discussion of the enrichment ratio was given in Section 4.3.1. Its use in that application was to estimate the enrichment of soil delivered to a surface water body and the resulting impacts to suspended and bottom sediments. It's value was placed in the 1 to 5 range, and the value of 3 was chosen for all contaminants. The same value will be used for estimating concentrations in soil that result from erosion of contaminated soils to nearby sites of exposure.

. k:

For the on-site source category, and for contaminated soil at the off-site contaminated location, the assumption is made that residues do not degrade or dissipate to the point of reducing the concentration of the "initial" soil levels. This was partly based on information indicating generally low rates of biological or chemical degradation for the dioxin-like compounds of this assessment, coupled with the assumption that on-site and off-site contamination was sufficiently deep implying a reservoir of contaminant that would remain available during a period of exposure.

These assumptions are less likely to be valid for residues which have migrated over the surface to deposit on the exposure site. The deposition is likely to result in only a thin layer of contaminated soil. Though very small, surface-related dissipation mechanisms such as photolysis, volatilization, or degradation, might reduce surface soil contaminant concentrations. For these reasons, a "dissipation" rate constant is assumed to apply to delivered contaminant, where the precise mechanisms of dissipation are not specified, but could include transport (volatilization, erosion) and degradation (photolysis, biodegradation) mechanisms.

The studies on 2,3,7,8-TCDD described in Young (1983) imply a dissipation half-life of 10 years. Fries and Paustenbach (1990) suggested the use of a half-life of at least 10 years, and used a 15 year half-life in their example scenarios on the impact of air-borne deposition of 2,3,7,8-TCDD originating from stack emissions. This assessment uses a dissipation half-life of 10 years for all of the three example compounds in Chapter 5. This half-life translates to a first-order dissipation rate constant of 0.0693 yr-1.

. M:

The delivered contaminant mixes to a shallow depth at the exposure site. The mixing depth depends on activities which disturb the surface, such as construction, plowing, vehicle traffic, movement of cattle or other animals, burrowing action of animals, other biological activity, normal leaching, and raindrop splash. Mixing depths for fallout plutonium have been found to be 20 cm on cultivated land and 5 cm on uncultivated forest and rangeland (Foster and Hakonson, 1987). Fries and Paustenbach (1990) suggested a depth of 15 cm for agricultural tillage, but assumed values of 1 and 2 cm for various sensitivity tests.

However, they did not need to make a distinction between tilled and untilled situation because vegetation (pasture grass and forage for estimating beef and milk fat concentration; above ground fruits and vegetables for human consumption) was assumed to be impacted only by particulate deposition and not root uptake. In another assessment on indirect impacts from incinerator emissions, EPA (1990a) estimated vegetation concentrations as a function of particulate depositions, root uptake, and air-to-leaf transfer from the vapor phase.

Different mixing depths for untilled and tilled concentration estimation was required. For root uptake estimation for vegetable and other crops, the estimated soil concentrations assuming a tillage mixing depth of 20 cm. For soil concentrations in untilled situations, they assumed a mixing depth of 1 cm. The methodology of this assessment uses 5 cm for the untilled and 20 cm for the tilled conditions for the off-site soil source category. Soil concentrations for calculation of concentrations of underground vegetables will be a function of a 20-cm depth.

This assumption is made because tilling gardens is assumed to distribute surface residues to the 20-cm depth. Soil concentrations for dermal contact, soil ingestion, and pasture grass and soil intake for cattle grazing will assume a depth of 5 cm. These activities are assumed to occur on soil which has not been tilled. As will be described in Section 4.5, tilled and untilled depths of mixing are also required for the stack emission source category.

For that source category, the tilled mixing depth is also assumed to be 20 cm, but the untilled mixing depth will instead be assumed to be 1 cm. The difference is made because of the assumed differences in the processes of erosion and air deposition. Erosion is a turbulent process, mixing soils from the contaminated site with soils between the contaminated and exposure site, while air deposition of particles happens uniformly over a landscape in a less turbulent manner.

Given the area of the exposure site, the mass of soil into which the eroded contaminant is mixed can be calculated as:

Equation V3 4-37

. D1 and D2:

The first step in deriving both these amounts of soil is to use the Universal Soil Loss Equation (USLE). This approach was described above. Justification was given for an assumption of unit soil loss from the contaminated site of 9.6 t/ac-yr (see Section 4.3.1). D1 equals this unit loss times the area of contamination times a sediment delivery ratio. The example scenario in Chapter 5 assumed that the exposure site was 150 meters from the contaminated site, and using Equation (4-10), the sediment delivery ratio is 0.26. The unit loss assumed for the area between the contaminated site and the exposure site is 0.96 t/ac-yr. Since this area is adjacent to the exposure site, there is no sediment delivery, and D2 equals this unit loss times the area between the contaminated and exposure sites.

D2 and D2 can now be expressed as:

Equation V3 4-38 a and b

An adjustment is made to the sediment delivery ratio, SD1, considering the size discrepancies between the contaminated site and the exposure site. For example, if the contaminated site is larger than the exposure site, then the amount of eroded soil delivered 150 meters downgradient would not all mix with soil at the exposure site. On the other hand, if the contaminated site were smaller than the exposure site, than the full amount of eroded soil delivered 150 meters downgradient would be contained within the exposure site. A simple correction factor, equaling the ratio of a side length of the exposure site (assumed square-shaped) and a side length of the contaminated site size (also assumed square shaped), is used to adjust the sediment delivery ratio:

Equation V3 4-39

Similar considerations are pertinent to the land area between the contaminated and exposure site. Remember that the algorithm assumed that some "clean" (D2) and some "contaminated" soil (D1) erodes onto the exposure site, and that a similar amount of soil entering the exposure site (R, which equals D1 + D2) leaves the exposure site so as to maintain a mass balance. The amount of clean soil eroding from upgradient sources mixing with exposure site soil can be larger than the amount of contaminated soil if the exposure site is larger than the contaminated site.

If the exposure site is smaller than the contaminated, and similar to the solution for SD1a above, then only the small corridor defined by the size of the exposure site contributes clean soil. Either way (i.e., the exposure site is larger or smaller than the contaminated site), the size of the land area contributing clean soil is defined by the size of the exposure site. ABLE can be estimated as the product of the distance between the exposure and contaminated site, and the side length of the exposure site:

Equation V3 4-40