The methodology in this document has been earlier described as screening level in terms of theoretical sophistication, but site specific in its application. Chapter 2 described concepts of exposure and assigned values to exposure parameters which define, for purposes of demonstration, a central and a high end exposure pattern. Chapters 3 and 4 described algorithms for the fate, transport, and transfer of dioxin-like compounds, and also assigned parameter values for purposes of demonstration. The methodology was demonstrated in Chapter 5, using exposure and fate and transport parameters which had been laid out in earlier chapters. Those who wish to use the methodology for further analysis of incremental exposures to sources of dioxin-like compounds are now in a position to use the same algorithms, perhaps many of the same parameter values. The purpose of this chapter is to provide guidance on some key issues for potential users.

Section 6.2 discusses the use of the parameter values selected for the demonstration scenarios in Chapter 5 for other applications. Section 6.3 is a sensitivity analysis exercise on the parameters required for algorithms estimating exposure media concentrations. Section 6.4 addresses the issue of mass balance with regard to the source strength terms of the four source categories.

Table 6-1 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 of Chapter 2. 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 in Section 6.3. below. The parameter values labeled "selected" were the ones used in the demonstration of the methodologies in Chapter 5. Section 6.3. below justifies the high and low values of parameters selected for sensitivity analysis. Other users of this methodology may wish to view these high and low values as reasonable high and low possible values for their applications; note however that 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

Table 6-1. Parameters used to estimate exposure media concentrations for this assessment.

Parameter Name Definition Low Selected¹ High Rating²

1. Contaminated and Exposure Site Characteristics

A. Site of Exposure

AES area of exposure site, m² 4,000 40,000 400,000 SS

E_slp soil porosity, unitless 0.35 0.50 0.60 SS

P_soil particle bulk density, g/cm³ 2.55 2.65 2.75 FOD

B_soil soil bulk density, g/cm³ 1.20 1.50 2.00 SS

OC_sl soil organic carbon fraction 0.005 0.01 0.05 SS

d_t tillage mixing depth, m 0.10 0.20 0.30 SOD

d_not no-till mixing depth, soil source categories, m 0.01 0.05 0.10 SOD

B. Site of Contamination, Off-site Soil Source Category

ASC area of off-site contamination, m² 4,000 40,000 400,000 SS

E_slp soil porosity, unitless 0.35 0.50 0.60 SS

P_soil particle bulk den, g/cm³ 2.55 2.65 2.75 FOD

OC_sl soil organic carbon fraction 0.005 0.01 0.05 SS

2. Soil and Sediment Delivery Parameters

SL_s contaminated site soil loss, kg/ha-yr 2100 21520 42000 SS

SL_ec soil loss between exp. and cont. site, kg/ha-yr 0 2152 21000 SS

(cont'd on next page)

Table 6-1. (cont'd)

Parameter Name Definition Low Selected¹ High Rating²

2. Soil and Sediment Delivery Parameters (cont'd)

SL_w watershed soil loss, kg/ha-yr 2100 6455 21500 SS

ER enrichment ratio, unitless 1 3 5 SOD

C_w watershed contaminant conc, mg/kg 0 0 1x10^-6 SS

OC_ssed suspended sediment organic carbon fraction 0.02 0.05 0.10 SS

OC_sed bottom sediment organic carbon fraction 0.01 0.03 0.05 SS

A_w watershed drainage area, ha 400 4,000 400,000 SS

SD_w watershed sediment delivery ratio, unitless 0.25 0.15 0.04 SS

TSS total suspended sediment, mg/L 2 10 70 SS

DL_e distance to exposure site, m 50 150 1000 SS

DL_w distance to water body, m 50 150 1000 SS

V_wat: volume of water body, L/yr 1.5x10⁹ 1.5x10¹⁰ 1.5x10¹² SS

3. Volatilization and Dust Suspension Parameters

ED exposure duration, yrs 1 20 70 SS

V fraction of vegetative cover, onsite, unitless 0.0 0.5 0.9 SS

V fraction of vegetative cover, offsite, unitless 0.0 0.0 0.9 SS

U_m average windspeed, m/sec 2.8 4.0 6.3 SS

U_t threshold wind speed, onsite, m/sec 2.5 6.5 11.3 SS

(cont'd on next page)

Table 6-1. (cont'd)

Parameter Name Definition Low Selected¹ High Rating²

3. Volatilization and Dust Suspension Parameters (cont'd)

F(x) model-specific parameter, onsite 0.87 1.05 0.05 SS

U_t threshold wind speed, offsite, m/sec 2.5 8.25 11.3 SS

F(x) model-specific parameter, offsite 0.87 0.50 0.05 SS

FREQ frequency wind blows to site, unitless 0.05 0.15 0.50 SS

4. Bioconcentration and Biotransfer Parameters

- FISH:

f_lipid fish lipid fraction 0.03 0.07 0.20 SS

- VEGETATION

FDW dry to fresh weight conversion 0.05 0.15 0.30 SS

V_p particle deposition vel, m/yr 1.5x10⁵ 3.2x10⁵ 7.0x10⁵ SOD

R annual rainfall, m/yr 0.3 1.0 2.0 SS

W_p washout factor, unitless 2x10³ 5x10⁴ 1x10⁶ SOD

R_w retention of wet deposition, unitless 0.00 0.30 1.00 SOD

Y_gr yield of grass, kg/m² dry 0.05 0.15 0.35 SS

INT_gr grass intercept fraction, unitless 0.13 0.35 0.64 SS

Y_feed cattle feed yield, kg/m² dry 0.25 0.63 1.30 SS

(cont'd on next page)

Table 6-1. (cont'd)

Parameter Name Definition Low Selected¹ High Rating²

4. Bioconcentration and Biotransfer Parameters (cont'd)

INT_feed feed intercept fraction 0.20 0.62 0.93 SS

Y_veg vegetable yield, kg/m² fresh 2.7 7.8 8.6 SS

INT_veg vegetable intercept fraction, unitless 0.18 0.48 0.72 SS

VG_bg below ground veg. correction factor, unitless 0.001 0.01 0.10 SOD

VG_veg veg/fruit air-to-leaf correction factor, unitless 0.001 0.01 0.10 SOD

VG_gr grass air-to-leaf correction factor, unitless 0.50 1.00 1.00 SOD

VG_feed feed air-to-leaf correction factor, unitless 0.25 0.50 0.75 SOD

- BEEF & MILK

BCSDF beef cattle soil diet fraction, unitless 0.01 0.04 0.15 SOD,SS³

DCSDF dairy cattle soil diet fraction, unitless ---- 0.02 ---- SOD,SS³

BCFDF beef cattle feed diet fraction, unitless 0.02 0.48 0.90 SS

DCFDF dairy cattle feed diet fraction, unitless ---- 0.90 ----   SS

BCGDF beef cattle grass diet fraction, unitless 0.02 0.48 0.90 SS

DCGDF dairy cattle grass diet fraction, unitless ---- 0.08 ---- SS

BCGRA beef cattle fraction of cont. grazing land 0.25 1.00 1.00 FOD,SS⁴

DCGRA dairy cattle fraction of cont. grazing land ---- 1.00 ---- FOD,SS⁴

BCFOD beef cattle fraction of cont. feed 0.25 1.00 1.00 FOD,SS⁴

DCFOD dairy cattle fraction of cont. feed ---- 1.00 ---- FOD,SS⁴

B_s bioavailability of cont. on soil relative to vegetation 0.30 0.65 0.90 SOD

(cont'd on next page)

Table 6-1. (cont'd)

Parameter Name Definition Low Selected¹ High Rating²

5. Effluent Discharge Source Category

LD loading to surface water body, mg/hr 0.00315 0.0315 0.315 SS

Q_e effluent flow rate, L/hr 10⁵ 4.1x10⁶ 10⁷ SS

Q_u upstream receiving water flow, L/hr 10⁷ 4.7x10⁸ 10⁹ SS

OC_e effluent organic carbon content, unitless 0.15 0.36 0.50 SS

OC_u upstream organic carbon content, unitless 0.02 0.05 0.10 SS

TSS_e effluent total suspended solids, mg/L 10 70 250 SS

TSS_u upstream total suspended solids, mg/L 2 9.5 50 SS

6. Stack Emission Source Category

RDEP_e wet+dry deposition onto exposure site, m g/m²-yr 2.2*10^-6 1.2*10^-6 1.5*10^-7 SS

RDEP_wat wet+dry deposition onto watershed, m g/m²-yr 2.2*10^-6 1.2*10^-6 1.5*10^-7 SS

RDEP_sw wet+dry deposition onto water body, m g/m²-yr 2.2*10^-6 1.2*10^-6 1.5*10^-7 SS

C_va vapor phase air concentration at exp. site, m g/m³ 4.7*10^-13 7.6*10^-12 2.6*10^-12 SS

RDEP_p deposition of particles onto water body, g/m²-yr 0.003 0.03 3.00 SS

d_not no-till mixing depth, stack emission source, m 0.01 0.01 0.05 SOD

d_wmx average mixing depth of deposition over watershed, m 0.01 0.10 0.20 SOD

f_sd fraction of particles depositing onto water which 0.00 1.00 1.00 FOD

remain in suspension

(cont'd on following page)

Table 6-1. (cont'd)

Parameter Name Definition Low Selected¹ High Rating²

7. Contaminant Physical, Chemical, and Bioconcentration/Biotransfer Parameters

H Henry's Constant, atm-m³/mole^-5 1.65x10^-6 1.65x10^-5 1.65x10^-4 FOD

D_a molecular diffusivity in air, cm²/s 0.005 0.047 0.10 FOD

Koc organic carbon partition coefficient, L/kg 2.7x10⁵ 2.69x10⁶ 2.7x10⁷ FOD

f fraction of airborne reservoir sorbed, unitless 0.80 0.45 0.20 FOD

B_vpa air-to-leaf transfer factor, unitless 1.0x10⁴ 1.00x10⁵ 1.00x10⁶ SOD

BCF beef/milk bioconcentration factor, unitless 1.00 4.32 10.00 SOD

BSAF biota sediment accumulation factor, unitless 0.03 0.09 0.30 SOD

BSSAF biota suspended solids acc. factor, unitless 0.03 0.09 0.30 SOD

k dissipation rate for eroding/depositing cont., yr^-1 0.00693 0.0693 0.693 SOD

kw plant weathering rate constant, yr^-1 51 18 8.4 FOD

RCF root bioconcentration factor, unitless 1,600 3,916 106,000 SOD

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, or FOD: As defaults, these parameters are independent of site specific characteristics and can be used for any assessment. Also, 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, or SOD: 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. SOD parameters of principal note are the bioconcentration parameters specific to the chemicals, such the Biota Sediment Accumulation Factor, or BSAF. This parameter translates a bottom sediment concentration to a fish tissue concentration. The science is evolving for this parameter, including thought on the extent to which BSAFs generated for one species at one site can be generalized to other sites and/or species, the differences in BSAF between column and bottom feeders, the differences between past and ongoing contamination, and so on. Users should carefully review the justification for the SOD values selected for the demonstration scenarios before using the same values.

3) Site Specific, or SS: 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 were not fully included in Table 6-1 above are the source strength terms - the soil concentrations, effluent discharge rates, and stack emission rates. There are likely to be site-specific applications of this methodology for which detailed information is unavailable. Often the midrange values selected for the demonstration scenarios are suitable for site specific applications when data is unavailable.

The exposure parameters have not been 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. 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, 1992). Some of the exposure parameters of Table 2-1, Chapter 2, are appropriately described as FOD. 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 SOD or SS. Attaining site-specific information is recommended for them. However, this is often difficult for site specific assessments and impractical if the procedures in this assessment are used in general assessments. In the absence of site specific information, the following parameters can be considered SOD: adult exposure durations of 9 years for central scenarios (whether they be modeled after "residential" settings or not) and 20 years for high end scenarios (whether "farming" be the model for high end exposures or not), childhood soil ingestion rates, the fruit/vegetable food ingestion rates, the fraction of fruit/vegetable consumption that comes from a home garden, and the fractions of time spent at home (which are applied to inhalation and water ingestion pathways). The remaining exposure parameters pertain to the exposure pathways evaluated as most critical to dioxin exposures. For this reason, users should either pursue site specific information or carefully justify parameter selections in the absence of site specific information. These include the rate of beef, milk, and fish ingestion and the fraction of these food products which are impacted by the source. Fish ingestion rates for the demonstration of methodologies in this assessment were 1.2 g/day for central scenarios and 4.1 g/day for high end scenarios. These were developed using an approach recommended in the Handbook when site specific data was unavailable. Specifically, a meals per year of fish recreationally caught from the impacted water body was assumed, and then this was translated to a grams per day consumed. These rates are both less than a national average estimate of fish consumption that was published in an water quality criteria document for 2,3,7,8-TCDD, 6.5 g/day (EPA, 1984). The setting for the demonstration scenarios was a rural setting which contained farm and non-farm residences, but which did not contain a major water body for frequent recreational or subsistence fishing purposes. Rather, a smaller size water body which allowed for more occasional recreational fishing was assumed. In a setting where more substantial water bodies exist which do supply fish for commercial and recreational use, fish ingestion rates from these water bodies would be higher. The other parameters are the ingestion rates and contact fractions for beef and milk ingestion. The ingestion rates for these food products were 50% ingestion rates given in the Handbook. The contact fractions assigned for the high end scenarios were developed from a USDA survey (USDA, 1966) of rural farm households, some of which home produced. For home producers only, the percent of their total ingestion of beef and milk which was homegrown was 44% and 40%, respectively.

In addition to the above qualifications, the parameters of this methodology

have been categorized in terms of their role in the methodology. The following is a brief description of three principal categories.

Category 1. Human behavior exposure parameters

These are the contact rates, contact fractions, exposure durations, lifetime and body weights used in the following equation for lifetime average daily dose:

Lifetime Average Daily Dose (LADD) = (exposure media concentration x

contact rate x contact fraction x exposure duration ) /

(body weight x lifetime) (6-1)

Category 2. Fate, transport, and transfer parameters

These parameters are all the parameters required to estimate exposure media concentrations, except those specifically associated with a contaminant - chemical-specific parameters are included in Category 3 below. All fate, transport, and transfer parameters are listed, defined, and further subcategorized in Table 6-1. Not included in the discussions in Section 6.3 are perhaps the most important terms in this category, and these are critical source strength terms: the concentrations of dioxin-like compounds for the soil source categories (onsite and offsite source categories), and the release quantities of dioxin-like compounds into the air for the stack emission source category and into the surface water for the effluent discharge source category. A general comment that can be made for fate and transport parameters is that values for the demonstration scenarios were selected to be midrange and plausible, and that this document provides information on selecting alternate values for site-specific applications. Most of the parameters in this category fall under the SS qualification. Subcategories within the fate and transport category include:

- Contaminated and exposure site characteristics: These are areas, soil properties, and depths of tillage (which are depths to which residues transported by erosion or deposition are mixed in conditions of tillage such as agriculture or gardening, and no tillage). Like the soil concentration term, the area of contamination is a site-specific and critical parameter. Soil properties were assigned to be midrange and typical of agricultural soils. Depths of mixing for tilled and untilled circumstances are not known with certainty, and these two parameters were characterized as SOD.

- Soil and sediment delivery parameters: These include parameters associated the erosion of contaminated soil from a site of contamination to a nearby site of exposure and/or to a nearby surface water body. All but one of the parameters in this subcategory are physical, site-specific parameters which should be evaluated for site specific applications. The one parameter not of this description is the enrichment ratio, which describes the enrichment of eroded soil with dioxin-like compounds, and was assigned a rating of SOD. Geometric parameters include watershed drainage area, water body volumes, and distances. Physical parameters include soil loss estimates, organic carbon contents, water body suspended solids, and background watershed contaminant concentrations.

- Volatilization and dust suspension parameters: These parameters are associated with suspension, dispersion, and transport of contaminants from contaminated soils. One parameter included in this category is the exposure duration, which appears to be misplaced. In fact, the exposure duration is used to determine the average vapor phase air concentration - this is further discussed in Section 6.3 below. Parameters in this category are site-specific and should be evaluated for specific methodology applications.

- Bioconcentration and biotransfer parameters: These include parameters describing the biota and the media surrounding the biota which influence the transfer of dioxin-like compounds from the media to the biota. Some of these parameters are site-specific, although obtaining values may be difficult. Included here are annual rainfall, fish lipid contents, a fresh to dry vegetable weight conversion factor, and yields and intercept fractions for vegetation categories. Others are theoretical; values for these were determined from the literature and can be used for other assessments if better information is unavailable. Included here are atmospheric deposition velocities of particles, washout of wind-suspended particles from the atmospheric, the retention of wet particle depositions on vegetations, empirical correction factors for atmospheric to plant and soil to plant transfers, and the bioavailability of soil as compared to vegetation as a vehicle of transfer of dioxin-like compounds to cattle. These were given a rating of SOD. A third group describes exposure of cattle to dioxin-like compounds through their diet. These include fractions of cattle diet which are soil, pasture grass, and cattle feed, and the extent to which these three are impacted by the source of contaminant. Sensitivity analysis below shows how beef concentrations are impacted by changes in assumptions of how cattle are exposed to dioxin-like compounds through their diet. Since beef and milk dietary exposures are most critical for human exposure, the cattle exposure assumptions made for demonstrating the methodologies of this assessment should be carefully considered before using them for other assessments.

- Effluent discharge source category: These are three physical parameters that can be determined on a site-specific basis, and include flow rates of the effluent and receiving water body, organic carbon contents of suspended solids in the effluent and the receiving water body, and suspended solids content of the effluent and the receiving water body.

- Stack emission source category: In fact, most of the parameters required to evaluate the impact of stack emissions to a nearby site of exposure have been included in other categories. Sensitivity analysis only focuses on parameters and issues unique to this category. One set of input values are contaminant wet and dry deposition rates. Three depositions are required: one for the site of exposure, one to represent depositions on watershed soils which drain into the water body, and one to represent direct deposition onto the water body. These were all generated using the COMPDEP model, as described in Chapter 3. Two other key inputs generated by the COMPDEP model are the ambient air vapor phase and particle phase concentrations of contaminant at the site of exposure. All such quantities are a function of that model's algorithms and parameter input requirements, particularly the release rate from the stack. Information on the COMPDEP model and its application is given in Chapter 3 and not discussed further in this chapter. Users can determine air concentrations and contaminant deposition rates in other ways, and use those in the methodologies to determine impacts and exposures. The no-till depth of mixing at the site of exposure, d_not, is required for the off-site soil source algorithm as well. It's selected value for the stack emission source category was 1 cm in contrast to 5 cm assumed for the off-site soil source category; hence its impact is examined twice in Section 6.3. below. The only other unique parameters not included in other subcategories are the average watershed mixing depth (used for determining watershed soil concentrations, which are then used to determine impacts to water bodies) and the fraction of particles depositing on water bodies which remain in suspension. These are both theoretical values and can be used in other assessments lacking better information.

Category 3. Chemical properties of dioxin-like compounds

The ten chemical-specific parameters required for the algorithms of this assessment fall under two categories, FOD and SOD. As such, they are all independent of the specifics of the site. The parameters deemed FOD are chemical fate and transport parameters, some of which are common and often determined in laboratory conditions. These include the Henry's Constant, the organic carbon partition coefficient, molecular diffusivity in air, a plant weathering rate constant for contaminated particles, and the soil dissipation rate for eroding or depositing contaminants. The selected values for these parameters are, in the authors' opinion, the best values derivable from current data. A second set of chemical specific parameters are associated with bioconcentration/biotransfer algorithms. Some of them are determined from field data (data on dioxin-like compounds or other compounds), and others are determined by experimentation and with that experimentation, development of empirical relationships between a critical transfer factor and the chemical's octanol water partition coefficient. The authors cannot be definitive in a judgement that values given to these parameters be considered default, hence the SOD rating. For these compounds, field/experimental data is conflicting or there simply is a lack of appropriate data. Parameters included in this category are a soil to below ground vegetation transfer factor, two air to plant factors: the air-to-leaf vapor phase transfer coefficient and the plant washoff rate constant, two water body to fish parameters: the biota to sediment accumulation factor and the related biota to suspended solids accumulation factor, and a beef/milk bioconcentration factor.

Sensitivity analysis was undertaken in order to evaluate the impact of model results with changes in model parameters. The following sections describe the limitations, methodology and parameter selections, and results.

The exercises were not comprehensive and/or definitive. Following are some key limiters:

• The COMPDEP model was not evaluated in this section. Chapter 3 describes the COMPDEP model. No sensitivity analysis runs were performed on COMPDEP model output for this chapter. This section does evaluate the impact of different deposition rates and modeled ambient air concentrations on exposure sites soils, surface water, and biota.

• Sensitivity to changes in exposure parameters was not evaluated. The basic equation for evaluating lifetime average daily dose was given above as Equation (6-1).

Chapter 2 described all terms in this equation except the exposure media concentration, which was the focus of Chapter 4. Because LADD estimates are a linear function of all exposure parameters, sensitivity analysis was not performed on LADD exposure estimates. The focus of this section instead is on the fate, transport, and bioconcentration/biotransfer algorithms used to estimate the exposure media concentration term in Equation (6-1).

• The analysis was not exhaustive in its coverage. Principal algorithms in the fate, transport, and transfer of dioxin-like compounds were evaluated, and all parameters required for algorithms were tested at least once. However, not all possible tests were conducted. Before noting those, following is a list of algorithms which were tested:

- Volatilization/suspension and transport of vapor/particle phase airborne residues from a site of soil contamination to a nearby site of exposure (using algorithms developed for the off-site soil source category);

- Volatilization/suspension and dispersion of vapor/particle phase contaminants for the circumstance where soil contamination is at the site of exposure (on-site soil source category);

- Transport via erosion of contaminants at a site of soil contamination to a nearby site of exposure to impact exposure site soils (off-site soil source category);

- Transport via erosion of contaminants at a site of soil contamination to a nearby surface water body, to impact bottom sediments, water, and fish (off-site soil source category);

- Transfers of contaminants from soils to below ground vegetables and from air to above ground vegetations (on-site soil source category);

- Transfers of contaminants from soils and vegetation to beef (on-site soil source category);

- Direct discharges of dioxin-like compounds into surface water bodies, and the effect of surface water and effluent parameters on fish and water concentration estimation (effluent discharge source category); and

- Particle depositions and ambient air concentrations, which result from stack emissions, onto exposure site soils, watershed soils, surface water bodies, and biota (stack emission source category);

The exercise was purposefully limited since several possible exercises would have been duplicative. For example, impacts to beef and milk in the off-site soil source category are, of course, modeled within this assessment, but specific sensitivities to beef and milk concentration predictions with parameter changes within the off-site soil source category are not explicitly evaluated below. Parameters required for the beef bioconcentration algorithm are evaluated in the context of the on-site source category, and these are the same ones required for all three source categories which include beef impacts (the effluent discharge source category does not include beef and milk impacts). Further, only impacts to the beef algorithms were tested. The milk bioconcentration algorithm was not tested because principal conclusions from the beef exercise are generally true for the milk algorithm. Any generalizations from the on-site source category exercises are transferable to the other two source categories.

A related limitation has to do with the cascading effect of certain parameters. For example, a key contaminant parameter is the organic partition coefficient, Koc, which impacts (among other concentrations) vapor phase air concentrations. Air concentrations are used to estimate above ground vegetation concentrations, including those of grass and cattle feed. Beef concentrations are a function of concentrations in grass and cattle feed. What is not done for this example (and many others like it) is to evaluate the impact of changes in Koc to beef concentrations. What is done, however, is as follows. The sensitivity of air concentration predictions to changes in the partition coefficient are evaluated. Then, the sensitivity to grass and cattle feed concentrations to plus and minus one order of magnitude differences in estimated vapor phase air concentrations are evaluated. In this way, any possible parameter change(s) which influences air concentrations within a plus/minus order of magnitude range is evaluated for grass and feed concentrations. Finally, beef concentration estimations are evaluated within a similar plus/minus order of magnitude change for grass and feed concentrations. With some examination, therefore, the effect of cascading impacts can be determined.

The impact of changing soil concentrations to estimates of exposure media concentrations (air, water, biota) is linear and direct in all cases - i.e, increasing soil concentrations by a factor of five increases all impacted exposure media by the same factor of five. For this reason, soil concentrations are not displayed in the sensitivity graphs displayed in the next section, with one exception. This was in the estimation of beef concentrations from soil contamination. Beef concentrations are a function of concentrations in the dry matter diet of the cattle, including soil, grass, and cattle feed. Therefore, if soil concentrations were to change and concentrations on the other intakes were to not change, than beef concentrations would not be a linear and direct function of soil concentrations. However, and in the context of this sensitivity analysis, when changing only soil concentrations, vegetative concentrations are linearly and directly impacted by the same order of magnitude change. Therefore, beef and milk concentrations turn out to be linearly related to soil concentrations.

A final limitation to note is that this exercise does not evaluate the multiple effects of changing more than one independent parameter simultaneously. Other numerical methods, particularly Monte Carlo, can be used to evaluate the impact of simultaneous changes to model parameters. Applications of this technique to dioxin exposure assessments are discussed in Chapter 7 of this volume.

There are instances where parameters were evaluated as dependent and changes were made simultaneously. One example is in three parameters which are related to the size of a watershed (also termed the "effective drainage area" since such an area might be smaller than a surrounding river system watershed), and which are important in determining the impact of a bounded area of soil contamination to a nearby surface water body. These three include the watershed size, the watershed sediment delivery ratio (which decreases as watershed size increases), and the surface water body volume (which increases as watershed size increases, assuming sources of water - surface runoff, interflow, and groundwater recharge - remain the same on a per unit area basis). To test the impact of watershed size to surface water and sediment concentrations, all three parameters were changed simultaneously in modeling a small and a large watershed. One set of parameters which might not be independent, but which were treated as such in the sensitivity testing, are the chemical specific parameters. For example, a higher organic carbon partition coefficient might be associated with a lower Henry's Constant - tighter binding to soils means less of a tendency to volatilize. Empirical relationships between such chemical specific parameters have not been established, and since there is uncertainty in precise values selected for the dioxin-like compounds, chemical specific parameters were treated as independent parameters.

• Only a high and a low value for model parameters were tested; no discussions of likelihood for parameter values or distributions of parameter values are included. Certainly the identification of all model parameters and the justification for assignment of high and low values will be helpful to others using the methodology. Assignment of parameter values for purposes of demonstrating the methodologies in Chapter 5 should be carefully considered when users apply this methodology for specific purposes or specific sites.

Four of the six example scenarios of Chapter 5 served as "baselines" in the sensitivity analysis exercises. The single scenario for the off-site soil source category, example scenario #3 in Chapter 5, served as the basis for testing on these algorithms: 1) transport of vapor and particulate phase airborne contaminants from a site of contamination to a nearby site of exposure, and 2) transport of soils via erosion to nearby sites of exposure and to surface water bodies to impact bottom sediments, fish, and water. The source strength for this scenario, in summary, was a 40,000 m² (4 ha, 10 ac) area of soil concentrations of 1 m g/kg (ppb) within a watershed of size 4,000 ha (40,000,000 m²; 10,000 ac; 15.5 mi²) with soils otherwise at 0.0 ppb. The high end scenario for the on-site soil source category, example scenario #2 in Chapter 5, served as the baseline for testing on these algorithms: 1) suspension and dispersion of vapor and particle-phase contaminants at a contaminated site, which was also the exposure site for the on-site source category, 2) impacts of soil concentrations and other parameters to below ground vegetations, and air concentrations and other parameters to above ground vegetations, and 3) impacts of soil, grass, and feed concentrations, and other parameters, to beef concentrations. The source strength for this scenario, in summary, were soil concentrations within a 4,000 ha small farm of 1 ng/kg (ppt). The high end example scenario for the stack emission source category, example scenario #5, served as the basis for the testing the impact of particle depositions and ambient air concentrations on soils and biota. The ambient air concentrations and deposition rates at the site of exposure 500 meters from the stack served as the baseline source strength terms. The single scenario for the effluent discharge source category, example scenario #6, was used to evaluate the impact of parameters required for that source category on fish and water concentrations. The source strength in that case was a discharge of 0.0315 mg/hr into a surface water body with a harmonic mean flow rate of 4.7x10⁸ L/hr. Assignment of that baseline discharge was based on data from the 104 pulp and paper mill study, and then considering reductions is discharges which have occurred in these pulp and paper mills since the 104 mill study in 1988.

The baseline chemical for all these sensitivity runs was 2,3,7,8-TCDD; i.e., all the chemical specific parameters were those assigned to this example compound. The high and low values for parameter testings were determined starting with the 2,3,7,8-TCDD assignments. Care was not taken to encompass a range of possible values for all dioxin-like compounds. However, the ranges that were tested are mostly inclusive of the dioxin-like compounds. What will be noted and discussed below is that mostly the model response to chemical-specific parameters is linear or nearly linear, so that model responses to values outside the ranges tested can be evaluated easily.

All the initial parameter values required for all four source categories, and the values selected for high and low sensitivity analysis were listed above in Table 6-1. Following are brief discussions on the selection of these high and low values. Longer discussions on all parameter values can be found in Chapter 4, which included justifications for all parameter values selected for the demonstration of the methodologies in Chapter 5. Often, ranges of possible values were discussed in Chapter 4; those ranges were the basis of high and low parameter values selected below. Discussions in Chapter 4 are not repeated here, but are referenced below. The summaries below are organized in the same order as the parameter listings in Table 6-1.

• Contaminated and exposure site characteristics: These are the area and distance parameters, and the soil characteristic parameters of the site of contamination and the site of exposure. The "site of contamination" refers to the bounded area of high soil concentration for the off-site source category. The "site of exposure" for these sensitivity runs is the small farm which was the basis for the definition of the "high end" example scenarios demonstrated in Chapter 5. The area of the site of exposure, AES, and site of contamination, ASC, are both 40,000 m² in the demonstration scenarios, which is equal to 4 ha or 10 ac. Low and high values tested were 4,000 m² (0.4 ha, 1 ac) and 400,000 m² (40 ha, 100 ac). The soil description parameters include soil porosity, ESLP, particle bulk density, Psoil, soil bulk density, Bsoil, and the organic carbon fraction, OCsl. The assignment of high/low values to these parameters were developed from Brady (1984) and cover a reasonable range of agricultural field soils. The no-till and tillage depths, d_not and d_t, refer to the depth to which eroded soil or depositing particulates mix at the site of exposure. The no-till depth was set at 5 cm and was varied between 1 and 20 cm, and the tilled depth was varied between 10 and 30 cm. The no-till concentrations were used to estimate soil concentrations for soil related exposures: soil ingestion and soil dermal contact, and also for the beef and milk bioconcentration algorithm. The tilled concentrations were used only to estimate the concentration in below ground vegetations.

• Soil and Sediment Delivery Parameters: Contaminated soil erodes from a site of contamination, a 4 ha site in the demonstration scenarios, to a nearby site of exposure and also to a nearby stream. The distance to the site of exposure from a site of contamination, DL_e, was set at 150 meters for the example scenarios, and varied between 50 and 1000 meters in this exercise. The same initial distance of 150 meters was the distance to the nearby stream, DL_w, and it was also varied between 50 and 1000 meters. The unit amount of soil eroding off the site of contamination, SL_s, was initialized at 21520 kg/ha-yr, equal to 9.6 Eng. ton/ac-yr (abbreviated t/ac-yr hereafter). Assumptions inherent in this estimate include: midcontinent range of annual rainfall erosivity (which is also the middle of the range of rainfall intensities of the US), midrange agricultural soil erosivity, a gentle 2% slope, no man-made erosion protection (ditches, etc.), and bare soil conditions. A doubling of this amount to 42,000 kg/ha-yr (19 t/ac-yr) was used as a high erosion estimate off the site of contamination. This could reflect any number of different assumptions, such as more erosive soil, more erosive rainfall, steeper slopes, and so on. A low estimate of one-tenth the default value, at 2100 kg/ha-yr (1 t/ac-yr), could reflect all the same assumptions except a dense cover of grass or weeds, which changes the bare soil assumption leading to a "C" (cropping management factor) of 1.0 to a C of 0.1. The erosion amount of 2152 kg/ha-yr was the initial amount assumed for a second unit erosion term needed in this assessment, a unit erosion typical of land area between the contaminated and the exposure site, SL_ec. The critical assumption in this initialization was that all conditions for this land area were similar to the contaminated site, except that the ground was densely covered with grass or weeds. The value of SL_ec was reduced to 0 kg/ha-yr for the low, which is unrealistically low but might give a sense of how the algorithm would perform if mixing with soil between the contaminated and exposure site were not considered. The high value was 21,000 kg/ha-yr, which is similar to the initial assumption for the contaminated site, could reflect similar erosion conditions between the contaminated site and the exposure site. The third unit soil loss parameter required is one which reflects average erosion conditions within the watershed draining into the water body, SL_w. This was initialized at 6455 kg/ha-yr (2.88 t/ac-yr) which reflects similar erosion conditions as the contaminated site (soil erosivity, rainfall intensity, average slopes, lack of support practices) except some erosion protection due to vegetation - C equal to 0.3 instead of 1.0. It was reduced to 2100 kg/ha-yr, which might translate to C equal to 0.1, and increased to 21,000, which was equal to the initial higher erosion from the contaminated site. The range of the enrichment ratio, ER, was noted at between 1 and 5 for its application in agricultural runoff field data and model simulations (Chapter 4, Section 4.3.1), and was given an initial value of 3 in this application. High and low values tested were 5 and 1. An average watershed concentration of contaminant was set at 0 for the off-site demonstration scenarios, where the soil concentration of 2,3,7,8-TCDD (and the other example compounds) was set at 1 ppb. This was selected so that the off-site impact to surface water bodies could be demonstrated as an incremental impact. A concentration of 2,3,7,8-TCDD of 1 ppt was, however, justified as a "background" concentration for demonstrating the on-site source category. The value was used to evaluate the impact of a bounded site at 1 ppb when a background concentration of 1 ppt is also assumed to exist. Three parameters reflect watershed size. These include the effective drainage area, A_w, the watershed sediment delivery ratio, SD_w, and the volume of the receiving water body, VOL_w. These are related and should therefore be changed in tandem. The initial watershed size of 4000 ha (15.4 mi²) was reduced to 400 ha (1.5 m²) and increased to 400,000 ha (1540 mi²). Since the water body volume was estimated using a in/yr runoff times an area, it was concurrently reduced 1 order of magnitude for the small watershed test and increased two orders of magnitude for the large watershed. The values of SD_w were estimated using Figure 4-5 (Chapter 4), which shows watershed delivery ratios as a function of watershed area. The remaining three parameters further described the water body, and were the total suspended solids, TSS, and the organic carbon contents of suspended and bottom sediments, OC_ssed and OC_sed. The initial value of TSS of 10 mg/L is typical of a moving water body (stream, river) supportive of fish and other aquatic life. It was reduced to 2 mg/L, which is typical of a stationary water body (pond, lake, reservoir) and increased to 50 mg/L, which begins to be high for a water body expected to be supportive of fish. The organic carbon contents were initialized at 0.05 for OC_ssed and 0.03 for OC_sed. The premise was that they were related - that sediments in suspension were lighter and likely to be higher in organic carbon content than bottom sediments. They were also changed in tandem to 0.02 (OC_ssed) and 0.01 (OC_sed) for a low organic carbon sensitivity test and 0.10 and 0.05 for a high organic carbon test.

• Volatilization and Dust Suspension Parameters: Distances and areas are pertinent to estimating vapor-phase and particulate-phase air concentrations, and these have been discussed above in the first two categories. One parameter included for sensitivity testing in this category is the exposure duration, ED. It is included in these exercises because the estimation of average volatilization flux over a period of time is a function of that period of time. The derivation of the flux model assumed contamination originates at the soil surface at time zero, and over time, originates from deeper within the soil profile. Therefore, the flux decreases over time (because residues have to migrate from deeper in the profile), and the average flux over a period of time will decrease as that period of time increases. This is further discussed in Chapter 4, Section 4.3.2., and in the original citation for the volatilization flux algorithm, Hwang, et al. (1986). The exposure duration assumed in the high end scenarios was 20 years, this was changed to 1 and 70 years in sensitivity tests. A range of average windspeeds, U_m, around the U.S. was noted at 2.8 and 6.3 m/sec, and these two values were used around the selected value of 4.0 m/sec. The frequency with which wind blows from a site of contamination to a site of exposure, FREQ, was set at 0.15, which is appropriate if one assumes that wind blows in all directions roughly equally. It was changed to 0.05 and 0.50, which might translate to an assumption of a prevailing wind direction, either away from or towards a site of exposure. The remaining parameters, fraction of vegetative cover, V, threshold wind speed, U_t, and model specific function, F(x), all refer to the wind erosion algorithm which suspends contaminated particulates into the air. Sensitivity tests were applied to this trio for the on-site and the off-site source categories. V for the off-site scenario was initialized at zero, implying bare ground cover; it was increased to 0.9 reflecting dense ground cover in the single sensitivity test here. It was set at 0.5 for the on-site small farm demonstration scenario, reflecting some bare ground conditions (in the agricultural fields, e.g.) as well as some dense vegetation (in other grassed areas of the farm property). It was decreased to 0 and increased to 0.9. The parameters U_t and F(x) reflect intrinsic erodibility of the soil and were varied together. Values were selected to reflect a high and low wind erodibility soil, following guidance in EPA (1985), the primary reference for the wind erosion algorithm.

• Bioconcentration and Biotransfer Parameters: The only such factor for fish concentration estimation was the fraction of fish lipid, f_lipid. The brief discussion on this parameter in Chapter 4 (Section 4.3.4.1) indicated a range of around 5 to over 20%. Considering that the lipid content of edible portions of fish are less than whole fish lipid contents, a value less than 5%, 3% (0.03), was chosen as the low value, and also considering edible lipid content considerations, an upper value of 20% (0.20) was selected.

Several parameters are required for the vegetation concentration algorithm, most of which were associated with the algorithm for dry plus wet deposition of particulates. One parameter not associated with fate and transport was the dry to fresh weight conversion factor, FDW. The algorithm calculates vegetative matter concentrations on a dry weight basis, which is appropriate for the role of vegetation in the beef/milk bioconcentration algorithm. However, ingestion rates of fruits and vegetables are on a fresh weight basis, so dry weight concentrations have to be converted to a fresh weight basis. The initial value of 0.15 assumes that fruits and vegetables are 85% liquid. The high and low values tested for this parameter were 0.30 (70% liquid) and 0.05 (95% liquid). Four parameters are described as empirical correction factors for the air-to-leaf algorithm adopted for vapor phase transfers to vegetation (three of the parameters), and for the soil-water-to-root algorithm adopted for below ground vegetation. There is one each for the four principal vegetations considered: below ground vegetables/fruits - VG_bg, above ground vegetables/fruits - VG_veg, grass - VG_gr, and feed - VG_feed. The concept for assignment of values to these parameters was the same, and briefly is as follows. The principal biotransfer factors (air-to-leaf and soil-water-to-root) were developed in laboratory experiments where relatively thin vegetations (azalea leaves for air-to-leaf transfers and barley roots for soil-water-to-root transfers) were used. Concurrently, there is evidence that the strongly hydrophobic/lipophilic dioxin-like compounds are found only in outer portions of vegetations and not inner portions of bulky vegetation; there is very little translocation of dioxin-like compounds into and within vegetation. Therefore, the full vegetation concentrations of thin vegetations measured in the laboratory experiments (and the laboratory experiments did use dioxin-like compounds among the several used) would most likely mirror only the outer surface concentrations found for dioxin-like compounds in bulky vegetations, and not full vegetation concentrations of bulky vegetations. As such, an empirical correction factor, based on a surface area to volume calculation, was introduced to arrive at full vegetation concentrations for bulky vegetations. These were principally the fruits and vegetables and the surface area to volume calculations led to assignments of VG_bg and VG_veg of 0.01. These were reduced to 0.001 and increased to 0.10 in sensitivity testing. The VG_gr was set at 1.00 since grass was thought to be analogous to the azalea leaves. Although there is insufficient justification to change VG_gr, a lower value of 0.50 was chosen. The VG_feed was set at 0.5, recognizing that some cattle feed is unprotected and thin vegetation such as hay, while others are protected grains such as corn grain. That value was changed to 0.25 and 0.75 in sensitivity testing. There is one required parameter for the dry deposition algorithm, and this is the particle deposition velocity by gravity settling, V_p, in m/yr. The initial value of 3.2x10⁵ m/yr, from a velocity assumption of 1 cm/sec, was given by Seinfeld (1986) as the gravitational settling velocity for 10 m m particles. This is the appropriate size to consider since the wind erosion algorithm was developed only for inhalable size particulates, those less than 10 m m (EPA, 1985). This was reduced to 0.5 cm/sec and 2 cm/sec (transformed to m/yr) for sensitivity testing. Three of the vegetation bioconcentration parameters are associated with the particulate wet deposition algorithm. These are the atmospheric washout ratio, W_p, the retention of particles on vegetation, R_w, and the annual rainfall amount, R. The definition, derivation, and ranges for these values are described in Chapter 4, Section 4.3.4.2, and are not repeated here (the ranges are given in Table 6.1). The remaining bioconcentration parameters are the yield and crop intercept values for the three above ground vegetations: vegetables/fruits (Y_veg, INT_veg), grass (Y_gr, INT_gr), and cattle feed (Y_feed, INT_feed). Again, discussions of chosen, and high and low, values for these quantities are given in Chapter 4, Section 4.3.4.2 (and displayed in Table 6.1). It is noted that these two terms are correlated - high yields are correlated with high interception amounts. In sensitivity testing, therefore, these parameters were changed in tandem.

The remaining bioconcentration/biotransfer parameters are for the beef/milk bioconcentration algorithm. One of the parameters relates the bioavailability of soil relative to the bioavailability of vegetation, where bioavailability refers to the efficiency of transfer of a contaminant attached to a vehicle. Fries and Paustenbach (1990) developed the bioconcentration factor, BCF, from studies where cattle were given contaminated feed. The studies of McLachlan, et al. (1990), from which BCFs for dioxin congeners were derived and used for this assessment, also used standard cattle feeds. This feed is assumed to be analogous to the vegetation in cattle diet; therefore, the experimental BCFs can be directly applied to vegetation in cattle diets. However, Fries and Paustenbach also hypothesized that soil is less bioavailable than feed, based on some rat feeding studies, and therefore the BCF developed from feed cannot directly be used on a soil concentration - it should be reduced. Information in Fries and Paustenbach led to an assignment of 0.65 for the soil bioavailability factor, B_s. This was reduced to 0.30 and increased to 0.90 in sensitivity testing. Three parameters describe the proportion of the dry matter in the diet of beef cattle that is soil, BCSDF, grass, BCGDF, and feed, BCFDF. The sum of these three terms, by definition, equals 1.00. Beef cattle are principally pastured (where incidental soil ingestion occurs), with supplemental feeds including hay, silages, and grain, particularly in cooler climates where they are housed during the winter. Values of 0.04 for BCSDF, 0.48 for BCGDF, and 0.48 for BCFDF were used in the demonstration scenarios. The same three parameters are required for cattle raised for dairy products: DCSDF for soil, DCGDF for grass, and DCFDF for feed. The dairy cattle model was one of very little pasturing, principally being fed high-quality grain indoors while they were in lactation: DCSDF of 0.02, BCGDF of 0.08, and DCFDF of 0.90. A final set of four parameters describes the proportion of these dietary intakes that are contaminated. Two are defined as the fraction of grazing land that is contaminated - BCGRA for beef cattle and DCGRA for dairy cattle. The initial assumption of 1.00 for both these parameters meant that all the vegetations as well as all the soil in the cattle diets was contaminated (since soil was assumed to be ingested during grazing). The last two similarly are defined as the proportion of feed that is contaminated - BCFOD for beef cattle and DCFOD for dairy cattle. They were also set at 1.00, perhaps indicating that feed was grown on-site. Rather than change these diet fraction assumptions and extent of contamination assumptions individually or in tandem (if necessary), what is done instead is to model four different scenarios relating to cattle exposures. Also, what is done here is to model only the beef cattle exposure. Generally, the trends that result from changes in the diet pattern will be analogous between the beef and dairy cattle. These four scenarios and the parameter changes made are:

1) High and low soil ingestion Low: BCSDF = 0.01

BCGDF = 0.50

No changes to BCGRA or BCFOD; BCFDF = 0.49

diet assumptions changed to

reflect high and low soil High: BCSDF = 0.15

ingestion patterns BCGDF = 0.43

BCFDF = 0.42

2) Low exposure conditions BCSDF = 0.01

Grazing is under lush conditions, so BCGDF = 0.50

soil ingestion and diet pattern is BCFDF = 0.49

modeled as "low" soil ingestion above; BCFOD = 0.25

also, most feed is purchased externally

and uncontaminated; BCFOD reduced

from 1.00 to 0.25

3) Low extent of contamination BCGRA = 0.25

Diet assumptions are unchanged from BCFOD = 0.25

initial assumptions; only it is assumed

that 25% instead of 100% of dry matter in

cattle diet is contaminated

4) High/low lifetime pasturing Low: BCSDF = 0.02

Tests for beef cattle only assuming BCGDF = 0.08

heavy lifetime pasturing, 90% grass, and BCFDF = 0.90

light lifetime pasturing, 08% grass High: BCSDF = 0.08

BCGDF = 0.90

BCFDF = 0.02

• Effluent Discharge Source Category: Section 4.6, Chapter 4, discusses briefly how data from the 104-mill pulp and paper mill study (EPA, 1990b) were used to develop initial parameters required for this source category in its demonstration in Chapter 5. The use of the 104-mill data in a model evaluation exercise is expanded upon in Chapter 7, Section 7.2.3.6. The data is also used here to assign high and low values for four of the seven required parameters for this source category. Two have to do with flow rates: Q_e which is the effluent flow rate, and Q_u which is the receiving water flow rate. The range of Q_e is from 10⁵ to 10⁷ L/hr, which are the low and high surrounding the 4.1x10⁶ rate used in the demonstration scenario in Chapter 5. The range of Q_u is 10⁷ to 10⁹ L/hr (excluding the top ten receiving water bodies, which were in the 10¹⁰ L/hr range and for which model did not appear to perform adequately), and these were the low and high around the 4.7x10⁹ L/hr rate used in Chapter 5. Two parameters describe the suspended solids content of the effluent, TSS_e, and the suspended solids content of the receiving water body, TSS_u. TSS_e ranged from 10 to 250 mg/L in the 104-mill study, so this was the range around the 70 mg/L used as the initial value. Data from STORET used to develop TSS_u led to an average of 9.5 mg/L and a range of less than 1 to 50 mg/L; a range of 2 (a reasonable value for a stationary water body such as a pond or lake) to 50 mg/L was tested. One required parameter was, of course, the rate of contaminant discharge, LD, in units of mg/hr. The assumed value was 0.0315 mg/hr, and this decreased and increased an order of magnitude for low and high testing. The remaining two parameters are the organic carbon contents of effluent solids, OC_e, and upstream river suspended solids, OC_u. A range based on data was not available for these parameters. OC_e was assigned a value of 0.36 based on the fact that solids in effluent discharges are primarily biosolids, and this value was one cited for surface water algae; values of 0.15 and 0.50 were tested. The value of 0.05 for OC_u was the value assumed for demonstration of other source categories, where the parameter was called OC_ssed. The same range of 0.02 to 0.10 for OC_ssed was used for OC_u.

• Stack Emission Source Category: The parameters in this category listed in Table 6-1 are the only ones which are unique to this source category (one parameter, the no-till mixing depth at the exposure site, d_not, is also used for the off-site soil source category, but its assigned value was 5.0 cm for that source category, and 1.0 cm for the stack emission source category; that is why it is listed for both source categories). As seen, there are only a very few unique parameters. Most of these are associated with surface water impact, and one series of tests evaluated the impact of parameter changes to surface water concentrations and fish concentrations. These include the contaminant deposition rates, RDEP_wat and RDEP_sw, which are depositions onto the watershed draining into the surface water body and the surface water body itself (units are m g/m²-yr). The initial values for these were those modeled to occur 500 meters from the stack. This assignment for the stack emission demonstration scenarios, #4 and #5 in Chapter 5, assumes that the stack is located essentially next to the water body. These depositions rates are specific to 2,3,7,8-TCDD. Rates of 2,3,7,8-TCDD deposition at 200 meters and at 5000 meters were used as high and low values, respectively. It should be noted that depositions are higher at 200 meters and lower at 5000 meters as compared to 500 meters, but air concentrations are lower at 200 meters as compared to 500 meters. This trend occurs because wet deposition is highest nearest the stack. Total depositions are driven by these high wet deposition totals; hence total depositions at 200 meters exceed those at 500 meters. However, dispersion modeling shows that ambient air concentrations of contaminants in the vapor phase (given the wind data and all other parameters and assumptions in using the COMPDEP model for the demonstration scenarios) are highest 500-1000 meters from the stack. For sensitivity testing, differences in model performance as a function of distance from the stack will be evaluated. RDEP_p is the deposition of particles themselves and was supplied in order to maintain a mass balance of solid materials entering the water body. The default value of 0.03 g/m²-yr was taken from Goeden and Smith (1989) for a study on the impacts of a resource recovery facility on a lake. They estimated a total deposition of particles to the lake from all sources was 74.4 g/m²-yr. Assuming the stack is unlikely to contribute all sources of particles to a water body, a high value was chosen as 3 g/m²-yr, and a low value was given as 0.003. The fraction of depositing particles remaining in suspension, f_sd, was initialized as 1.00 (meaning that all directly depositing particles remain in suspension) based on an argument that the small particles emitted from the stack and transported directly to the surface water body would settle to surface water bottoms much more slowly than other solids entering water bodies. A low value of 0.00 was tested (meaning that all solids directly depositing within a year settle quickly to become bottom sediments). The average watershed mixing zone depth, d_wmx, was initialized at 0.10 m (10 cm) which is midway between the 1 cm assumed for non-tilled conditions and 20 cm assumed for tilled conditions. This assumption might translate to a rural watershed comprised equally of farmed and unfarmed land. It was reduced to 1 cm and increased to 20 cm in sensitivity testing. A second series of tests evaluated biota impacts at the site of exposure, vegetables/fruits and beef/milk. Parameter inputs for these tests include the ambient air concentration and depositions at the site of exposure, C_va and RDEP_e, and the no-till depth of mixing, d_not. The no-till depth of mixing was increased from 1 to 5 cm. Concentrations and depositions of 2,3,7,8-TCDD at 200 and 5000 meters were tested. The baseline quantities at 500 meters were varied to reflect different vapor/particle partitioning assumptions. Currently, the assumption is that 2,3,7,8-TCDD emissions are 55% in the vapor phase and 45% in the particle phase. Linear adjustments to the emissions in vapor and in particle form can be made to stack emissions. Concentrations and depositions at specific locations are then adjusted in the same linear manner to reflect different vapor/particle partitioning assumptions. Two assumptions tested include 10% vapor/90% particle and 90% vapor/10% particle.

• Contaminant Physical and Chemical Properties: The initial values for testing of this category of parameters were the ones used for 2,3,7,8-TCDD. Generally, the high and low values tested are those which may represent a range for this contaminant only, not all dioxin-like compounds. However, several of the ranges also encompass values that could be pertinent to other compounds. It should be remembered that this is simply a model performance exercise and nothing else. Also, it could be argued that some of the parameters should be changed in tandem - that there may be a relationship between soil/water adsorption, as modeled by Koc, and bioconcentration. Such relationships were not explored in these exercises. Notes on the parameters are as follows:

1. Henry's Constant, H - The value of 1.65x10^-5 atm-m³/mole was used for 2,3,7,8-TCDD. Except for a heptachloro-PCB, Henry's Constants for the dioxin-like compounds ranged from 10^-6 to 10^-4. Because of this, the initial value was reduced and then increased an order of magnitude for this test.

2. Molecular Diffusivity in Air, D_a - This parameter is needed for the volatilization flux algorithm. Because no values were available for the dioxin-like compounds, values were estimated based on the ratios of molecular between a dioxin-like compound of interest and a compound for which a D_a was available - in this case, diphenyl. The range of values tested are 0.005 cm²/s as a low and 0.10 cm²/s around the initial value of 0.047 cm²/sec.

3. Organic Carbon Partition Coefficient, Koc: The Koc is perhaps the single most influential parameter in this assessment, impacting surface water concentrations, vapor phase air concentrations, and directly or indirectly, all biomass concentrations (fish, vegetations, beef/milk). The literature for 2,3,7,8-TCDD shows a range of Koc under 10⁶ (from Schroy, et al., 1985) to over 2x10⁷ L/kg (Jackson, et al., 1986). The value selected for 2,3,7,8-TCDD was 2.69x10⁶ based on an empirical relationship between Koc and Kow developed by Karickhoff, et al. (1979) (see Section 4.3.1., Chapter 4). The values tested were one order of magnitude less (2.7x10⁵) and one order of magnitude more (2.7x10⁷)