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5.5. SOURCE TERMS

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.

table Table 5-1 Environmental fate parameters for the three example compounds demonstrated for the soil contamination source categories and the effluent discharge source category.
This section does list TEQ emissions in grams per second, and the exposure site soil concentrations that result from stack emission depositions. The source term for the effluent discharge example scenario is the rate of discharge of dioxin-like compounds. This is briefly discussed in this section, with reference to a more detailed discussion in Chapter 7, Section 7.2.3.6.

Key source terms are summarized in Table 5-3. Following now are discussions on these terms for all scenarios.

Scenarios 1 and 2.
The residence in Scenario 1 is 4,000 m2 and in Scenario 2 is 40,000 m2 in size.

expand table Table V3 5-1
table Table 5-2 Key source terms and fate parameters for 2,3,7,8-TCDD and for individual dioxin and furan congeners with non-zero TEFs for the demonstration of the stack emission source category. table Table 5-3 Summary of key source terms for the six exposure scenarios and the example compounds.
expand table Table V3 5-2 expand table Table V3 5-3

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.

Scenario 3

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.

Scenarios 4 and 5

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.

Scenario 6

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.

5.6. RESULTS

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 Cj*TEFj, where Cj are exposure media concentrations for the individual congeners and TEFj are the TEF for the individual congeners.

table Table 5-4 Exposure media concentrations estimated for all scenarios and pathways.
It is important to understand that all observations made below are not generalizable comments. Different results would arise from different source strength characteristics, proximity considerations, model parameter values, different models altogether, and so on. Chapters 6 and 7 on User Considerations and Uncertainty describes many areas of this assessment which should be considered when evaluating the methodology or viewing the results.
expand table Table V3 5-4

5.6.1. Observations Concerning Exposure Media Concentrations

. 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.

table Table 5-5 Lifetime average daily dose (LADD) estimates for all scenarios and exposure pathways (all results in mg/kg-day).
The 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.
expand table Table V3 5-5

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, Sz. 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 basis, beef fat concentrations should be multiplied by approximately 0.18-0.22 (beef is roughly 18-22% fat) and milk fat by approximately 0.02-0.04 (2-4% fat). Milk fat concentrations were lower than beef fat concentrations in all cases, but within a factor of two. This was due to assumptions concerning apportioning of total dry matter intake between contaminated soil, contaminated pasture grass, and home-grown contaminated feeds.

Beef cattle were assumed to take in twice as much soil as lactating cattle, 4% of their dry matter intake versus 2%, and much more leafy vegetations than lactating cattle, 48% pasture grass versus 8% pasture grass. Another observation that can be made is similar to the observation concerning the key bioaccumulation parameters for fish, the BSAF and the BSSAF. In those cases, it was noted that the factors for the PCB example compound was much higher than those of either the dioxin or the furan - hence higher fish concentrations result. In this case, however, the PCB example compound had the lowest beef/milk biotransfer factor, BCF. The BCFs for the dioxin and furan example compound were both 4.3 and 3.1 respectively, while the PCB BCF was 2.3.

This resulted in uniformly lower PCB beef and milk concentrations. Another noteworthy trend concerns the comparison of off-site soil impacts (4 hectares at 1 ppb) to on-site soil impacts (a basin-wide 1 ppt), with regard to fish and beef/milk. Specifically, there is only an order of magnitude difference between fish concentrations in the on-site scenarios (1 & 2) versus the off-site contamination scenario (3). Said another way, a basin-wide concentration of 1 ppt has nearly the same impact to fish as a small land area of 1 ppb. However, the same is not true for beef/milk. The 1 ppt basin-wide soil concentration resulted in beef/milk concentration 2 orders of magnitude lower than the 1 ppb small area of contamination impact to beef/milk.

An examination of model performance explains this pattern. The pertinent trend is discussed further in Chapter 7, Sections 7.2.3.1, 7.2.3.2, and 7.2.4.1, but briefly has to do with dilution of soil concentrations associated with transport modeling. The ratio of surface water sediment concentration to on-site soil concentration is termed the "sediment dilution ratio", and the ratio of exposure site soil concentration to contaminated site soil concentration is called the "soil dilution ratio". The sediment dilution ratios for example Scenarios 1 and 2 demonstrating basin-wide low concentrations was 2.8, meaning that surface water sediments were in fact enriched compared to basin-wide soil concentrations; the 1 ppt basin-wide soil concentrations translated to bottom sediment concentrations of 2.8 ppt. For Scenario #3, demonstrating the impact of a smaller area of high soil concentration, the sediment dilution ratio was 0.016, meaning that the 1 ppb soil concentration resulted in a bottom sediment concentration of 16 ppt.

Note the one order of magnitude difference in bottom sediments between Scenarios 1 & 2 (2.8 ppt) and 3 (16 ppt); this explains the one order of magnitude difference in fish concentration results. However, the soil dilution ratio of Scenario 3 was 0.279, meaning that the 1 ppb contaminated site concentration translated to 0.279 ppb concentrations to which cattle are exposed. This compares to the 0.001 concentration to which cattle were exposed in Scenarios 1 and 2. Note that the orders of magnitude difference here explains the differences in estimated beef and milk concentrations between Scenarios 1 & 2 and 3. For the stack emission high end scenario, TEQ concentrations continue to be higher than 2,3,7,8-TCDD concentrations by about an order of magnitude.