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Federal Highway Administration Research and Technology
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| This report is an archived publication and may contain dated technical, contact, and link information |
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| Publication Number: FHWA-HRT-14-021 Date: January 2014 |
Publication Number: FHWA-HRT-14-021 Date: January 2014 |
Parameters that are metal specific are used to quantify exposure mechanisms that are dependent on the properties of the contaminant under evaluation. The exposure equations reflect the particular rates of uptake into humans and/or food products, based on the magnitude to which constituents may move through the human body or migrate from soil to other media. Site-specific parameters represent hydrogeology or climate conditions that affect the calculation of screening levels.
The following subsections describe contaminant-specific and site-specific parameters used in the assessment, including the gastrointestinal absorption factor, particulate emission factor (PEF), groundwater transport factors, bioavailability factors, environmental half-life, and toxicity factors.
The gastrointestinal absorption factor describes the absorption rate of contaminants that are ingested in beads through the gastrointestinal system. The bioaccessible fraction of arsenic and lead in glass beads reflects a wide range of values and is a source of uncertainty in the proposed modeling methodology. In the presented model, a gastrointestinal adsorption factor of 0.003 is used.([21])
The PEF is used to determine the dust load in the air and the resulting air concentration for the exposed receptors. The dust load is dependent on soil, weather, and activity factors. The equations used in the PEF calculation are presented in figure 24 and figure 25.
Figure 24. Equation. Formula to calculate PEF.
Where:
Q/C = inverse of mean concentration at center of source (g/m2-s per kg/m3).
V = fraction of vegetative cover (unitless).
Um = mean annual wind speed (m/s).
Ut = equivalent threshold value of wind speed at 7m height (m/s).
F(x) = experimentally derived function dependent on Um/Ut (unitless).
Figure 25 presents the formula for deriving the value of Q/C.
Figure 25. Equation. Formula for deriving the inverse of the mean concentration at the center of the source (Q/C).
Where:
A, B, C = experimentally derived constants based on air dispersion modeling for specific climate zones.
Asite = area of contamination (acres).
Table 19 presents the results of the PEF calculation, which describe the emission rate of particles that have a diameter of 10 µm or less. (18)
The groundwater transport parameters are used to describe the lateral migration of contaminants that have been leached to groundwater. In general, it is assumed that the concentration of contaminants in the leachate is the concentration that reaches the groundwater. This assumption results in conservative groundwater concentrations because some contaminants will likely not reach groundwater but will remain sorbed to particles in the vadose zone. Once they reach the groundwater, contaminant transport may be retarded relative to groundwater flow within the aquifer or may disperse significantly relative to the advective motion of the aquifer. Both of these mechanisms would tend to reduce the contaminant concentration in groundwater. Groundwater transport factors also have a site-specific component related to the site hydrogeology and weather conditions.
To be conservative, a DAF of 10 is used to evaluate contaminant migration through groundwater. Calculated DAFs that range over several orders of magnitude are likely for sites throughout the United States. The DAF is linearly related to the screening level for ingestion of groundwater in the residential exposure scenarios, and a tenfold increase in DAF would increase the screening level by an order of magnitude. Therefore, the DAF is a source of a significant and key uncertainty that needs to be carefully considered for each specific site when using the current modeling methodology. In addition, if a residential groundwater well was screened across both a contaminated and uncontaminated zone in a bead-impacted aquifer, further dilution of groundwater contaminants would occur within the well.
Table 19. Calculation of particulate emission factor.
|
Scenario |
Parameters for Dispersion Factor Calculation |
Q/C Dispersion
Factor (g/m2-s per kg/m3) |
Measured Average
Wind Speed at Elevation of 7 m (m/s) |
Threshold
Friction Velocity (m/s) |
Equivalent Value
of Threshold Wind Speed at 7 m (m/s) |
Parameter for
Determination of Equation Describing f(x) (unitless) |
Function f(x)
Derived from Um/Ut (unitless) |
Particulate
Emission Factor (m3/kg) |
||
|
A |
B |
C |
||||||||
|
Nationwide |
16 |
19 |
216 |
82.9 |
4.69 |
0.21 |
11.36 |
2.10 |
0.22 |
5.39E+08 |
|
Northeast |
13 |
19 |
215 |
65.4 |
3.80 |
0.17 |
9.20 |
2.10 |
0.22 |
4.25E+08 |
|
Southeast |
15 |
18 |
204 |
71.6 |
4.10 |
0.19 |
9.93 |
2.10 |
0.22 |
4.65E+08 |
|
Central |
16 |
19 |
216 |
82.9 |
4.70 |
0.21 |
11.38 |
2.10 |
0.22 |
5.39E+08 |
|
Northwest |
11 |
20 |
225 |
63.0 |
3.90 |
0.18 |
9.45 |
2.10 |
0.19 |
4.61E+08 |
|
Southwest |
10 |
19 |
212 |
53.5 |
2.80 |
0.13 |
6.78 |
2.10 |
0.19 |
3.92E+08 |
Notes: Calculation based on equations presented in the EPA Soil Screening Guidance and supporting sources.(14, 15)
Values for Minneapolis, MN, were found to represent the 90th percentile of results derived from air dispersion modeling for 29 cities in the United States.
Variables for site area, fraction of vegetative cover, sand percentage in soil, and measured annual average wind speed are user-entered values. All other parameters are calculated as described in EPA.(14) Parameters A, B, and C for the Dispersion Factor Calculation were generated by EPA from atmospheric modeling simulations for sites across the country and best fit regression analysis of the results.
Table assumes a 1-acre source area with no ground cover and a roughness height of 0.1 cm (open ground).
The bioavailability, biotransfer, and bioaccumulation factors are used to describe the contaminants’ availability to biological organisms and the potential for concentration into food items such as vegetables, livestock, or fish. Recent laboratory analysis of the bioaccessibility of arsenic and lead in glass beads, as presented in table 9, indicate an average for bioaccessible lead of 1.8 ppm compared with a total lead average of 71 ppm, or an average bioaccessible fraction of approximately 2.5 percent. Table 20 presents the calculated bioaccessible fraction, which ranged from 0.06 to 16 percent, indicating significant variability in the parameter. No extractable or bioaccessible arsenic was observed.
Table 20. Extractable and bioaccessible fraction of arsenic and lead in bead samples expressed as a percentage of the total content.
|
Bead Group |
Percentage of Total Lead as Extractable |
Percentage of Total Lead as Bioaccessible |
|
AA |
0.48 |
0.13 |
|
AC |
3.36 |
16.36 |
|
BD |
0.31 |
0.15 |
|
BE |
0.79 |
0.11 |
|
BI |
3.29 |
1.70 |
|
DA |
0.14 |
0.06 |
|
DB |
NA |
NA |
|
DC |
NA |
NA |
|
DD |
NA |
NA |
|
EA |
NA |
NA |
|
FH |
0.43 |
0.26 |
|
GA |
NA |
NA |
|
GB |
NA |
NA |
|
GC |
NA |
NA |
|
GD |
NA |
NA |
|
Overall Average |
1.3 |
2.7 |
NA = not available because results were below detection limits
The environmental half-life is a parameter that describes the persistence of glass beads once they are released to the environment and is considered site specific. It is assumed that the glass beads are extremely stable and remain intact under typical environmental conditions, with the exception of beads removed by mechanicals means that might result in crushing of beads. Beads removed from the base material (paint, thermoplastic) by vehicle traffic are assumed to be intact, as are beads released into a storage yard.
The toxicity factors are used to define, for each constituent, the potential for adverse health effects based on the magnitude of exposure for each receptor and pathway for each constituent. Contaminant speciation and complexation are a significant consideration in assigning a value for toxicity parameters.
Arsenic speciation in leachate from beads was measured and
indicated that the dominant species is arsenic (V) in laboratory experiments (table 13). Although specific toxicity data are not available for particular
arsenic species, studies have indicate that arsenic (III) is a generally more
toxic form of arsenic. The full toxicological profile for arsenic is available
from the Agency for Toxic Substances and Disease Registry (ATSDR) at http://www.atsdr.cdc.gov/toxprofiles/
tp2.pdf. The ATSDR profile for lead is available at
http://www.atsdr.cdc.gov/toxprofiles/
tp13.pdf. Table 21 summarizes the toxicological parameters used in this study’s
modeling approach for arsenic.
