United States
Environmental Protection
Agency
Office of Air Quality
Planning and Standards
Research Triangle Park NC 27711
EPA-450/1 -92-004
March 1992
Air/Superfund
AIR/SUPERFUND
NATIONAL TECHNICAL
GUIDANCE STUDY SERIES
Estimation of Air Impacts
for the Excavation of
Contaminated Soil
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EPA - 450/1-92-004
AIR/SUPERFUND NATIONAL TECHNICAL
GUIDANCE STUDY SERIES
Estimation of Air Impacts
for the Excavation of
Contaminated Soil
Prepared by:
Bart Eklund
Sandy Smith
Al Hendler
Radian Corporation
Austin, Texas
EPA Contract Number 68-D1-0031
Work Assignment 013
Prepared for:
James F. Durham
U.S. Environmental Protection Agency
Office of Air and Radiation
Office of Air Quality Planning and Standards
Research Triangle Park, North Carolina 27711
March 18,1992
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DISCLAIMER
This report has been reviewed by the Office of Air Quality Planning
and Standards, U.S. Environmental Protection Agency, and has been
approved for publication. Mention of trade names or commercial
products does not constitute endorsement or recommendation for use.
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TABLE OF CONTENTS
Page
INTRODUCTION 1
PROCESS DESCRIPTION 2
ESTIMATION OF AIR EMISSIONS 5
ESTIMATION OF AMBIENT AIR CONCENTRATIONS 12
ESTIMATION OF HEALTH EFFECTS 15
EXAMPLE 25
CONCLUSIONS 28
ACKNOWLEDGEMENTS 30
REFERENCES 30
APPENDIX A: MODEL DERIVATION
APPENDIX B: PHYSICAL AND CHEMICAL CONSTANTS FOR SELECTED
COMPOUNDS
11
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LIST OF FIGURES
1 Idealized Excavation Scenario 3
2 One-Hour Average Downwind Dispersion Factor Versus Distance
for Excavation With No Air Emission Controls 13
LIST OF TABLES
1 Input Parameters for Emission Estimation Equations 9
2 Example Scenarios for Excavation of Contaminated Soil 14
3 Long-Term and Short-Term Health-Based Action Levels for
Ambient Air 17
•
4 Estimated Emission Rates and Ambient Air Concentrations
for Example Problem 29
5 Action Level Concentrations for Example Problem 29
in
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INTRODUCTION
The U.S. Environmental Protection Agency's Office of Air Quality
Planning and Standards and the Regional Air Offices have been given the responsibility
to evaluate air impacts from Superfund sites. An important part of this program is the
analysis of air impacts from various alternatives for cleaning up Superfund sites. Since
these analyses are frequently required for planning purposes prior to actual cleanup they
depend on estimated emissions and ambient concentrations rather than on field
measurements.
This report provides procedures for roughly estimating the ambient air
concentrations associated with the excavation of contaminated soil. These procedures
are analogous to procedures for air strippers and soil vapor extraction systems that have
previously been published1'2. Excavation is an integral part of any Superfund site
remediation that involves removal or ex-situ treatment such as incineration, thermal
desorption, bioremediation, or solidification/stabilization. Procedures are given to
evaluate the effect of concentration and physical properties of the contaminants in the
soil on the emission rates and on the ambient air concentrations at selected distances
from the the excavation site.
Health-based ambient air action levels are also provided for comparison to
the estimated ambient concentrations. Many of the health levels have not been verified
by EPA or are based on extrapolations of oral exposures or occupational exposures.
Their indiscriminate use could either under or over estimate the potential health effects.
The statements and conclusions presented in this report are those of the authors and do
not reflect U.S. EPA policy.
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PROCESS DESCRIPTION
Excavation and removal of soils contaminated with Volatile Organic
Compounds (VOCs) is a common practice at Superfund sites. Excavation and removal
may be the selected remediation approach or it may be a necessary step in a remediation
approach involving treatment. If removal is the preferred approach, the excavated soil is
typically transported off-site for subsequent disposal at a landfill. If the soil contains
large amounts of fuel or highly toxic contaminants, the soil may need to be treated off-
site prior to final disposal. Excavation activities are also typically part of on-site
treatment processes such as incineration, thermal desorption, batch biotreatment,
landtreatment, and certain chemical and physical treatment methods. The soil is
excavated and transported to the process unit and the treated soil is typically put back
into place on the site.
VOC emissions from handling operations result from the exchange of
contaminant-laden soil-pore gas with the atmosphere when soil is disturbed and from
diffusion of contaminants through the soil. There are multiple potential emission points
for each of the various soils handling operations. For excavation, the main emission
points of concern are emissions from:
• exposed waste in the excavation pit;
• • material as it is dumped from the excavation bucket; and
• waste/soil in short-term storage piles.
An idealized excavation scenario is shown in Figure 1 and assumes that
each scoop of excavated soil has dimensions of 1m x 2m x 1m and that the soil is
removed as a series of blocks that retain their shape and are stacked in a temporary
storage pile.
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Time Between Scoops is
Approximately 40 Seconds.
Figure 1. Idealized Excavation Scenario
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The magnitude of VOC emissions depends on a number of factors,
including the type of compounds present in the waste, the concentration and distribution
of the compounds, and the porosity and moisture content of the soil. The key
operational parameters are the duration and vigorousness of the handling, and the size
of equipment used. The longer or more energetic the moving and handling, the greater
likelihood that organic compounds will be volatilized. The equipment size influences
volatilization by affecting the mean distance a volatilized molecule has to travel to reach
the air/solid interface at the surface of the soil. In general, the larger the volumes of
material being handled per unit operation, the lower the percentage of VOCs that are
stripped from the soil.
The success of excavation for a given application depends on numerous
factors with the three key criteria being: 1) the nature of the contamination; 2) the
operating practices followed; and 3) the proximity of sensitive receptors. Each of these
criteria is described below.
The magnitude of emissions from soils handling operations will vary with
the operating conditions. Add-on control technologies are available for minimizing VOC
emissions, but they are relatively ineffective and costly to implement. VOC emission
control can also be achieved by controlling the operating conditions within preset
parameters. The rate of excavation and dumping, the drop height, the amount of
exposed surface area, the length of time that the soil is exposed, the shape of the storage
piles, and the dryness of the surface soil layers will all influence the levels of VOC
emissions. Large reductions in emissions can be achieved by identifying, and operating
within, acceptable ranges of operating conditions.
Since some release of volatile contaminants is inevitable during excavation
and removal unless extreme measures are taken (e.g. enclose the remediation within a
dome), the proximity of downwind receptors (i.e. people) will influence whether or not
excavation is an acceptable option. Excavation of contaminated areas that abut
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residential areas, schoolyards, etc. may require more extensive controls, relocation of the
affected population, or remediation only during certain periods (e.g. summertime for
school sites).
ESTIMATION OF AIR EMISSIONS
Only limited guidance is currently available for estimating the air emissions
from soils handling operations. The emissions of concern from soils handling operations
such as excavation, dumping, grading, transport, and storage are typically volatile organic
compounds (VOCs), though emissions of particulate matter and associated metals and
semi-volatile compounds may be of concern at some sites.
There are several alternative approaches for estimating the emissions from
excavafion. The best method is to directly measure the emissions during full-scale or
pilot-scale soils handling activities. The next best method is to estimate the emissions
using predictive equations with site-specific inputs. If site-specific inputs are not
available, a very conservative estimate can be made by using default values for the input
parameters. Equations are given below for estimating an average long-term emission
rate and a short-term emission rate.
Average Long-Term Emission Rate
k
A simple check of the total emissions potential for the site should be made
by dividing the total mass of a given contaminant to be removed by the expected
duration of the clean-up:
ER = (Sv)(C)(fi)(l) / tR (Eq. 1)
where: ER = Average emission rate (g/sec);
Sv = Volume of contaminated soil to be excavated (m3);
C = Average contaminant concentration (ug/g);
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B = Bulk density of soil (g/cm3);
1 = Constant (g/106ug * 106cm3/m3); and
tR = Duration of remediation (sec).
The volume of contaminated soil and the total mass of each contaminant of concern
present are typically determined during the remedial investigation (RI) of the site, while
the fraction of contaminated soil that must be removed or treated is typically determined
during the feasibility study (FS) of the site. Final clean-up criteria should also be
considered when calculating the volume of soil to be excavated. The duration of the
clean-up will usually be limited by the rate at which contaminants can be transported off-
site or treated on-site. For Equation 1, a typical default value for bulk density of
uncompacted soil is 1.5 g/cm3. The following paragraphs discuss the key variables
influencing air emissions from the excavation of contaminated soil and present an
empirical equation for estimating a short-term emission rate.
Short-Term Emission Rate
A number of assumptions were made to develop a typical scenario for soil
excavation. It is assumed that an infinite, homogeneous body of waste or contaminated
soil exists under a cap of clean soil. The cap is removed and then contaminated
soil/waste is excavated for 50 min/hour. Each scoop of soil contains 2 m3 of soil and 75
scoops moved per hour (= 150 m3 of soil moved per hour). Each scoop has dimensions
of 1m x 2m x 1m and adds 2 m2 of surface area to the pile of excavated material. The
pit, after one hour has dimensions of 10m x 15m x 1m. Furthermore, each scoop of
dumped soil is assumed to maintain its 1x2x1 dimensions (the pile of dumped soil is
equivalent to a series of stacked blocks). After one hour, a pile 5m x 10m x 3m is
established. The total exposed surface area is 140 m2 for the pile and another 150 m2
for the pit. The pile is assumed to thereafter be covered with some type of impermeable
cover that acts as a barrier to further emissions. Both soil and air temperatures are
assumed to be near 25°C
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Since it is rarely feasible or efficient to dig soil and immediately transfer
the soil directly to transport vehicles or treatment systems, the equations presented below
must be applied to each event in which the soil is handled. In most cases, soil will be
excavated and placed in a temporary holding area and then moved one to two more
times on-site. Elevated levels of VOC emissions are possible each time the soil is
handled. When estimating emissions from sequential soil handling steps, it may be
important to adjust the starting concentrations for each step to account for contaminants
emitted during prior steps.
The detailed equation (model) for estimating emissions from excavation is
given below followed by a simple screening equation to estimate excavation emissions.
Appendix A presents the derivation of the simple screening equation, contains a
discussion of the various input variables, and has an example calculation. The more
detailed equation should be used in place of the screening equation whenever there are
significant deviations from the assumptions used for air-filled porosity, air temperature,
or the time that the soil is exposed to the atmosphere before being covered with
additional soil. Field data should be used whenever possible and default values used
only when no valid data are available.
Average Emission Rate (Detailed Model)
The average emission rate (ER, with units of g/sec) from excavation is
equal to the 'sum of emission rates from the soil pore space (ERpj, g/sec) and from
diffusion (ERDIFF, g/sec):
ER = ERps + ERDffF > 2)
P MW 10* E, Q ExC
ER__ = i
PS RT
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_ (0(10,000X5A) V~M" 4^
DOT
The term ExC in Equation 3 is the fraction of the VOC in the pore space that is emitted
to the atmosphere during excavation. All variables in Equations 2, 3, and 4 are defined
in Table 1. Also shown in Table 1 are the units of each variable and a typical default
value to use if valid field data are not available. Values of molecular weight, vapor
pressure at 25°C, and diffusivity in air at 25°C are given in Appendix B. Equation 3 is
based on the assumption that the soil pore gas is saturated with the compound of
interest. If this is not the case, then Equation 3 may overpredict the emission rate. The
output from Equation 3 should be multiplied by the duration of excavation and
compared to the total mass of contaminants present in the soil:
M = C * Sv " 106cm3/m3 (Eq. 5)
where: M = Total mass of contaminant in a given volume of soil (g).
If Equation 3 gives a value that exceeds one-third of M, then the following equation
should be substituted for Equation 3:
= M * 0.33/tsv • (Eq. 6)
where: t^ = Time to excavate a given volume, S^ of soil (sec).
Average Emission Rate (Simplified Model)
The average emission rate from excavation is again equal to the sum of
emission rates from the soil pore space and from diffusion:
ER = ERPS + ERDirf (Eq. 2)
8
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Table 1
Input Variables for Emission Equations
Default Variable
P
MW
R
T
Ea
Sv
Q
106
ExC
C
10,000
SA
K«
^
U
I
De
0.98
1.22 x 106
1.79 x 109
Definition
Vapor pressure
Molecular weight
Gas constant
Temperature
Air-filled porosity
Volume of soil
moved
Excavation rate
Conversion factor
Soil-gas to
atmosphere
exchange constant
Concentration in
soil
Conversion factor
Emitting surface
area
Equilibrium
coefficient
Gas-phase mass
transfer
coefficient
Pi
Timea
Effective
diffusivity in air
Conversion factor
Conversion factor
Conversion factor
Units
mm Hg
g/g-mol
mm Hg-cm3/g-mol
°K
Degrees Kelvin
Dimensionless
m3
m3/sec
cm3/rn3
Dimensionless
g/cm3
•5 , 1
cm/nr
m2
Dimensionless
cm/sec
Dimensionless
sec
cm2/sec
g/mm Hg-m3
cm2-sec-mmHg/g
sec2-cm-mmHg/g
Default Value
35
100
62,361
298
0.440
150
0.042
—
0.33
1.35X10"4
—
290
0.613
0.15
3.14
60
0.0269
—
~
—
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Table 1 (Continued)
Default Variable
M
C
Definition
Total mass of
contaminant
Concentration in
soil
Units
g
ug/g
Default Value
—
100
Other Variables Required to Calculate Certain Variables Listed Above
tsv
6
P
Da '
U
n*
Pa
de
Time to excavate
a given volume of
soil
Bulk density
Particle density
Diffusivity in air
Wind speed
Viscosity of air
Density of air
Diameter of
excavation
sec
g/cm3
g/cm3
cnr/sec
m/sec
g/ cm-sec
g/cm3
m
—
1.5
2.65
0.1
2.0
l.SlxlO-4 .
0.0012
24
aSee Page 11 of Appendix A for discussion of time term.
10
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ERPS = P * Q * 0.91 (Eq>
(Eq. 8)
(C)(10,000)(SA) M
f cl
1.22xl06 _
1 PJ
-
/ \
1.79xl09 -
Variables are defined in Table 1. The derivation of these equations is presented in
Appendix A (Equation 7 equals Equation A-13 in Appendix A and Equation 8 equals
Equation A-20). Assuming a typical bulk density of undisturbed soil, C can be modified
to a soil concentration term: C = C * Icm3/1.5g * 106 ug/g; where: C =
Concentration of species i in soil (ug/g). The emission rate obtained using Equation 7
should be compared to the total mass of contaminant present in the volume of soil
excavated - M. If Equation 7 gives a value that exceeds 1/3 of M, then Equation 6 should
be substituted for Equation 7.
Worst-Case Emission Rate
The worst-case (i.e. maximum) instantaneous emission rate, ER^^x, for
contaminated soil occurs when the exposed surface area is at a maximum and
immediately'after a bucket load of soil is dumped onto the storage pile. This emission
rate can be approximated by considering the case where a pure chemical is exposed to
the atmosphere. This emission rate can be determined from Equation 6 (there is no
need to consider pore space gas concentrations and diffusion since the pure chemical is
already exposed to the atmosphere). Set the time term, t, equal to zero and replace the
Keq term with the equivalent expression: P*MW*Ea/R*T*C. Equation 6 then reduces
to:
11
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(kg)(P)(MW)(SA)(10,000) ' 9)
=
ESTIMATION OF AMBIENT AIR CONCENTRATIONS
Estimates of short-term, worst-case ambient concentrations should be
obtained by using site specific release parameters in the EPA's TSCREEN model3.
Estimates of long term concentrations should be obtained by using EPA's Industrial
Source Complex (ISCLT) model. Here, for simplicity, the annual average estimates are
derived by multiplying the short term estimate obtained from the TSCREEN model, by a
conversion factor to account for variations of wind direction over time. This approach
results in a higher estimate of the annual average concentration than if the ISCLT
model,, with site specific data, is used.
Table 2 presents three excavation scenarios that vary in excavation rate
and physical dimensions. The scenarios were developed based on a review of the existig
literature5 and field experience. The worst-case, short-term downwind dispersion of
emitted gases from each of these scenarios for an emission rate of 1 gram per second, is
illustrated in Figure 2. Of the variables listed in Table 2, only the physical dimensions of
the excavation pit and storage pile factor into the estimated downwind dispersion. Two
additional curves in Figure 2 indicate the downwind dispersion for excavation areas of
larger dimensions (500 m2 and 1,000 m2, respectively). The curves were calculated
according to 'the following assumptions: 1) the combined emission rate for the
excavation pit and storage pile is 1 gram per second; 2) the excavation pit and storage
pile are sufficiently close to one another so that the size of the area emission source is
equal to the combined horizontal areas of the pit and storage pile; 3) a flat terrain
without any structures near the excavation site was assumed; and 4) downwash was not
applicable. The emission source and the receptors were assumed to be at ground level.
Downwind concentration estimates for emission rates other than 1 gram per second can
be extrapolated from Figure 2 by multiplying the indicated y-axis value (dispersion
factor) for the applicable downwind distance by the actual emission rate.
12
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fl
CD
Q.
CO
o
•2
C
o
0)
CL
co
Q
100,000
30,000
10,000
3,000
1,000
300
100
30
10
50 100 200 500 1,000 2,000
Downwind Distance (meters)
5,000
10,000
Figure 2. One-Hour Average Downwind Dispersion Factor Versus Distance for
Excavation With No Air Controls
13
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Table 2.
Example Scenarios for Excavation of Contaminated Soil
Parameter
Soil Moved Per Scoop
No. Scoops Per Hour
Total Volume of Soil Moved
Excavation Pit:
Dimensions
Area
Storage Pile:
Dimensions
Area
Units
m3
#/hr
m3/hr
m
m2
m
m2
Scenario
Small
1
50
50
10x5x1
50
5x5x2
65
Medium
2
75
150
10x15x1
150
5x10x3
140
Large
4
60
24Q
10x12x2
120
8x10x3
188
14
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Figure 2 can be used to estimate the maximum hourly ambient air
concentration for an emission rate of 1 gram per second at selected distances downwind
from an excavation pit. If the excavation rate is not known, a medium rate scenario
should be assumed. The dispersion factor, in micrograms/m3 per g/sec, obtained from
Figure 2 can be substituted into Equation 10 to estimate the maximum hourly ambient
concentration and into Equation 11 to estimate the annual average ambient air
concentration for a given downwind distance. Since TSCREEN provides maximum
short-term estimates, the factor of 0.05 in Equation 11 is used to convert the short-term
estimate to a maximum annual average estimate. A conservative factor of 0.05 assumes
that the wind blows downwind 5% of the time over one year and that the terrain is
relatively flat. This assumption has been recently revised by EPA; it is still under review
by EPA, however, and is subject to further change.
Cm = (ER)(F) (Eq. 10)
Ca = (ER)(F)(0.05) (Eq. 11)
where: Cm = Maximum hourly ambient air concentration^g/m3);
Ca = Annual average ambient air concentration (^g/m3);
ER = Emission rate (g/sec); and
F = Dispersion Factor from Figure 2 (//g/m3/g/sec).
ESTIMATION OF HEALTH EFFECTS
Cancer Effects Due to Long-Term Exposure
Potential cancer effects resulting from long-term exposure to substances
emitted to the air can be evaluated using inhalation unit risk factors. Inhalation unit risk
factors are a measure of .the cancer risk for each ,ug/m3 of concentration in the ambient
air. They are available on EPA's Integrated Risk Information System (IRIS), the
Agency's preferred source of toxicity information. User Support can be contacted at
(513) 569-7254. Table 3 provides inhalation unit risk factors listed in IRIS as of
January 1991 for selected organic compounds.
15
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The next best source of inhalation unit risk factors is EPA's Health Effects'
Assessment Summary Tables (HEAST) which are updated quarterly.5
4
Equation 12 can be used to estimate the cancer risk at a specified distance
downwind of the excavation area. Cancer risk is a measure of the increased probability
of developing cancer in a lifetime as a result of the exposure in question. Equation 12
assumes continuous exposure (24 hours/day, 365 days/year for 70 years) to the estimated
annual average concentration in air.
R = (CJ(IUR) (Eq. 12)
R is the cancer risk from long-term exposure to a specific VOC in air,
dimensionless; Ca is the annual average ambient concentration estimated from Equation
11, /zg/m3; IUR is the inhalation unit risk factor, (^g/m3)'1 obtained from Table 3.
If the source operates for less than 70 years, multiply Ca by x/70, where x
is the expected operating time of the source in years before using Equation 12. If more
than one VOC is present, the cancer risks for each VOC can be summed to derive the
total cancer risk at a specified distance downwind of the source.
Non-Cancer Effects Due to Long-Term Exposure
Non-cancer effects can be evaluated by using chronic inhalation reference
concentrations (RfCs). An inhalation RfC is the an estimate (with uncertainty spanning
perhaps an order of magnitude) of continuous exposure to the human population that is
likely to be without appreciable risk of deleterious effects during a lifetime. RfCs for a
limited number of compounds are available in IRIS and HEAST.
16
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If inhalation RfCs were not available from either IRIS or HEAST, then
chronic oral reference dose (RfD) data (in mg/kg/day) were multiplied by 70 kg
(average body weight of an adult), then divided by 20 m3/day (average adult inhalation
rate), and finally multiplied by 1000 ,wg/mg to derive a value in //g/m3.
Ambient air action levels based on extrapolated oral data should be used
cautiously. Before extrapolating data an array of factors should be assessed on a
compound by compound basis to determine the feasibility of route-to-route
extrapolations. Important factors include the absorption, distribution, metabolism and
excretion of the compound; portal of entry effects; acute and chronic toxicities, and other
information.
For compounds lacking RfC or RfD values, action levels were based on
occupational exposure levels recommended by the Occupational Safety and Health
Administration (OSHA)6 and the American Conference of Governmental Industrial
Hygienists (ACGIH)7. .The action levels were estimated by using the lower of the OSHA
Permissible Exposure Limit-Time Weighted Average (PEL-TWA) level (or ceiling value)
or the ACGIH Threshold Limit Value - Time Weighted Average (TLV-TWA) level (or
ceiling value). The lower value was divided by 1000 to compensate for differences
between occupational and residential exposures.
Long-term ambient air action level concentrations for non-carcinogens
based on RfCs, extrapolated RfDs and occupational exposure levels for 168 compounds
are also listed in Table 3. The action levels are in units of /
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Short-Term Exposure
The short term (one hour) action levels, in yUg/m3, are presented in the last
column of Table 3. The listed values were obtained by dividing the lowest of (1) the
OSHA PEL-TWA or (2) the ACGIH TLV-TWA (or ceiling limits if 8-hour averages are
not available) by 100. Division by 100 accounts for variations in human sensitivity
(occupational levels are designed to protect healthy adult workers) and for uncertainties
in using occupational exposure levels to derive ambient air action levels.
The occupational exposure levels on which the short-term action levels are
based are subject to change. To check the values in Table 3 (or to derive values for
compounds not listed in Table 3), determine the current OSHA PEL-TWA values by
consulting 29 CFR Section 1910 and the most recent edition of the ACGIH publication
entitled Threshold Limit Values and Biological Exposure Indices.
The short-term action levels listed in Table 3 can be compared directly
with the estimated maximum hourly ambient air concentrations obtained by using
Equation 10 and Figure 2. Use of the short term action levels should consider that no
EPA accepted method exists to determine the short-term concentrations of airborne
chemicals acceptable for community exposure.
EXAMPLE
The following steps illustrate the use of the estimation procedures
presented in this document. The goal is to estimate the maximum hourly and annual
average ambient air concentrations at the nearest receptor to an excavation area and
compare these values to the action level concentrations listed in Table 3.
25
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Step 1 For this example, assume a site that has approximately 10,000 m3
of soil contaminated with chloroform, 1,1,1-trichloroethane, and
trichloroethylene at concentrations in the soil of 0.1, 10, and 1.0
ug/g, respectively. The volume of contaminated soil is not known
with any certainty. The bulk density of the soil at the site
averages about 1.5 g/cm3. The rate of excavation has not yet been
determined, nor has the need for air emission controls, so a
medium excavation rate of 150 m3/hour and no air emission
controls is assumed. The removal is expected to be in continual
operation for 20 days (1.728 x 106 seconds). The nearest off-site
downwind receptor is 400 meters away.
Step 2 Estimate the total emissions potential for the site. Using Equation
1, the average long-term emission rate of chloroform would be:
ER = (10.000X0.1X1.5X1) =
(1.728 x 106)
io-
The average long-term emission rate for 1,1,1-trichloroethane is
8.68xlO'2 g/sec, and for trichloroethylene is 8.68xlO'3 g/sec.
Step 3 Estimate the emission rate of each compound. The data are
plugged into Equations 7 and 8 along with the assumed excavation
rate of 0.042 m /sec. For chloroform, the emission rate would be:
ERPS = (208)(0.042)(0.98) = 8.56 g/sec
ER
DffF
(1.5 x 10'7)( 10,000X290)
(1.22 x
106)
(l
I
.5
x I
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The overall emission rates for all three compounds are given
below. In all cases, Equation 6 was used in place of Equation 7.
Step 5 Compare the estimated emission rates from Step 3 and 4 to those
from Step 2. The comparison is:
Step 6
Compound
Chloroform
1, 1, 1-Trichloroethane
Trichloroethylene
Equation 1
Emission Rate
(g/sec)
0.000868
0.0868
0.00868
Equation 6
Emission Rate
(g/sec)
0.38
3.1
0.74
Given the excavation is not to be performed continuously over the
twenty day period, it is expected that the short-term emission rates
exceed the long-term emission rates. Each rate will be used to
calculate the downwind risk over the appropriate time period.
Estimate the downwind ambient air concentrations. From Figure
2, the maximum hourly ambient air concentration at a distance of
400 meters is approximately 2800 ug/m3 per g/sec emission rate.
This corresponds to an annual average dispersion factor of 140
ug/m3 per g/s (2900 x 0.05 = 140). The ambient air
concentrations estimated from Equations 10 and 11 are presented
in Table 4. Using Equation 10, the hourly average ambient air
concentration for chloroform would be:
Cm = (0.38)(2800) = 1100 ug/m3
Using Equation 11, the annual average air ambient concentration
for chloroform would be:
Ca = (0.000868)(140) = 0.12 ug/m3
Step 7 Compare the downwind concentrations to the action level ambient
air concentrations. The short-term and long-term action levels
from Table 3 for the compounds of interest are presented in Table
5. Of the estimated maximum hourly ambient concentrations, only
chloroform exceeds the applicable action levels. The estimated
value is about one order of magnitude greater than the action
level. The annual average ambient concentrations show
exceedances of the long-term action levels for both chloroform
and trichloroethylene, by a factor of 2 to 3.
27
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Step 8 Document the results of the air pathway analysis and define a
future course of action. Based on these screening level results, a
more rigorous analysis of the air impacts is warranted. This would
most likely involve refining the emission rate, dispersion, and
health risk estimates. The emission rate estimate could be
improved by using the actual or proposed operating conditions or
by making field measurements at the excavation site. The
dispersion estimates could be improved by using a less
conservative model (e.g. EPA's Industrial Source Complex model)
and site-specific meteorological conditions. The health risk
estimate could be improved by using the expected operational
lifetime of the SVE system rather than assuming a 70-year
exposure. If the more rigorous analysis still indicates that adverse
air impacts may occur, then the addition of air emission controls
or altering the operating conditions to control emissions (e.g.
limiting the excavation rate and the total exposed surface area)
should be considered.
CONCLUSIONS
The procedures presented here are not intended to negate the need for
rigorous analyses that consider site specific meteorological conditions and the health
effects of the specific compounds involved. Although the procedures are based on what
is typical and reasonable for cleaning up Superfund sites, the underlying assumptions
need to be kept in mind. Emission models assume steady-state conditions, dispersion
models assume Gaussian distribution of the plume contaminant concentration, and many
of the health levels are not endorsed by the Environmental Protection Agency. EPA's
Regional Toxicologist should be contacted for general toxicological information and
technical gufdance on evaluation of chemicals without established toxicity values.
28
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Table 4.
Estimated Emission Rates and Ambient Air Concentrations
Chloroform
1, 1, 1-Trichloroethane
Trichloroethylene
Soil
Concentration
For Example
Problem
(g/cm3)
1.5 x ID'7
1.5 x 10'5
1.5 x 10^
Emission Rate (g/s)
Long Term: 8.7 x 10^
Short Term: 0.38
Long Term: 8.7 x KT2
Short Term: 3.1
Long Term: 8.7 x 10°
Short Term: 0.74
Ambient Concentrations
(^g/m3)
Maximum
Hourly
1100
9000
2100
Annual
Average
0.12
12
1.2
Table 5.
Action Level Concentrations
Chloroform
1,1,1-Trichloroethane
Trichloroethylene
Table 3 Action Levels ^g/m3
Short-Term
98
19,000
2,690
Long-Term
0.0431
^OOO2
0.591
on 10^, 70-year risk.
2Based on reference dose concentrations (RfCs).
29
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ACKNOWLEDGEMENTS
Jawad Touma and Norman Huey of EPA contributed to the overall
direction of this project. The health effects sections were prepared in consultation with
Fred Hauchman of EPA.
REFERENCES
1. Eklund, B., S. Smith, and M. Hunt. Estimation of Air Impacts For
Air Stripping of Contaminated Water. EPA-450/1-91-002. May
1991.
2. Eklund, B., S. Smith, P. Thompson, and A. Malik. Estimation of
Air Impacts For Soil Vapor Extraction (SVE) Systems. EPA
Contract No. 68-D1-0031, WA13. December 2, 1991.
3. U.S. EPA, A Workbook of Screening Techniques for Assessing
Impacts of Toxic Air Pollutants. EPA-450/4-88-009. September
1988.
4. Church, H. Excavation Handbook. McGraw-Hill, 1981,
5. Health Effects Assessment Summary Tables (HEAST). U.S.
Environmental Protection Agency, Wash. D.C., Fourth Quarter,
1990, OERR 9200.6-303(91-1), NTIS No. PB91-92199, January
1991.
6. 29 CFR ch. XVII. Subpart Z. Section 1910.1000. July 1, 1990.
7. 1990-1991 Threshold Limit Values for Chemical Substances and
Physical Agents and Biological Indices. American Conference of
Governmental Industrial Hygienists, Cincinnati, Ohio, 1990.
30
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APPENDIX A
MODEL DERIVATION
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APPENDIX A
MODEL DERIVATION
Derivation of a Screening Model for
VOC Emissions From Soils Handling Activities
Bart Eklund
Radian Corporation
8501 N. Mopac Blvd.
Austin, TX 78759
March 11, 1992
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Screening Model for VOC Emissions from Soils Handling Activities
APPENDIX A - MODEL DERIVATION
A.1 INTRODUCTION
Background information about the modeling problem is presented in this
appendix followed by a presentation of an emission model for estimating VOC emissions
from the excavation of contaminated soil. A simplified version of the model is
developed, then the models are evaluated.
Objective
Develop simple predictive model for estimating VOC emissions from soils
handling activities, such as excavation.
Intended Use
The model will be used for assessing potential emissions during
remediation of Superfund sites. At a minimum, the model should provide an emission
factor to estimate emissions per unit time or unit operation. Ideally, it should also be
appropriate for evaluating the effect of different remediation scenarios, e.g. starting
waste concentrations, excavation rates, and control efficiencies.
Requirements
1. Model should be conservative, since the data may be used in some
cases for health risk assessment.
2. Model should require as few input parameters as is feasible for ease
of use.
Assumptions
1. During excavation, the surface area of soil in contact with the
atmosphere is greatly increased. This results in up to one-third of
the soil gas being released to the atmosphere. In dry soils
containing very low levels of VOCs, most of the contaminants are
present in the soil pore spaces, thus the percentage of the VOCs
emitted is relatively high.
2. Once the soil has been dumped into place, the organic liquid to soil
gas equilibrium is quickly re-established. The emissions can be
estimated by a modification of the RTI landtreatment model.1
!p. 5-14 and 5-15 of EPA-450/3-87-026, Review Draft, November 1989.
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3. The freshly dumped soil is soon covered by relatively deep layers of
subsequently excavated soil. These layers of soil result in longer-
term emissions from the deeper layers being diffusion controlled,
i.e., low. Therefore, the significant period for emissions is during
excavation and the first six minutes or so afterwards. Subsequent
(i.e. t > 6 min) emissions from this material are assumed to be zero.
4. The total exposed surface area of contaminated soil is assumed to
remain constant. New material is exposed at the same rate that
previously exposed material is covered.
5. The emissions from the pit are approximately equivalent to the
emissions from the pile of excavated soil. The emissions from the
soil in the backhoe bucket are negligible.
6. Wet soils are assumed to have relatively low levels of VOC
emissions, even if the soil VOC concentrations are high. Wet soils
may have little air-filled porosity and therefore the rate of diffusion
of VOCs through wet soils is relatively low.
Possible Excavation Scenarios
Two general scenarios are followed during excavations at waste sites.
1. Soil is excavated using a backhoe and placed into a short-term
storage pile. The soil is later picked up from the pile and dumped
directly into transport vehicles (e.g. trucks or railcars) that are
subsequently covered to minimize further emissions. Overall, each
m3 of soil is excavated and dumped two times.
2. Soil is excavated using a backhoe and placed into a temporary
storage pile. The soil is moved from the pile using a front-end
loader (and/or backhoe) to a staging area where a large storage pile
is established. The pile is typically covered to minimize leaching
and air emissions. The soil is eventually re-excavated and dumped
into transport vehicles (e.g. trucks or railcars) that are subsequently
covered to minimize further emissions. Alternatively, the soil may
be re-excavated and fed to an on-site treatment system. Overall,
each m3 of soil is excavated and dumped three times.
It is rarely feasible or efficient to dig soil and immediately transfer the soil directly to
transport vehicles or treatment systems. The excavation scenario and the emission
equations shown below are designed to predict the emissions from a single soil handling
event. To predict the total emissions from excavation, the equations must be
sequentially applied to each event where the soil is handled (i.e., two or three times in
most cases). The values for certain input parameters to the equations, such as the
concentration of the contaminant in the soil and the bulk density of the soil, will be
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altered by the act of excavation and a separate (different) value will be required for
these parameters when modeling each soil handling event of the overall excavation
process.
Details of Excavation Scenario
Soil is excavated for 50 min/hour2. Each scoop of soil contains 2 m3 of
material and has dimensions of 1m x 2m x 1m. The cycle time is 40 seconds3, so 75
scoops are moved per hour (= 150 m3 of soil moved per hour). The excavation pit, after
one hour of operation, has dimensions of 10m x 15m x 1m.
Each scoop of dumped soil is assumed to maintain its 1x2x1 dimensions, so
that the pile of dumped soil is equivalent to a series of stacked blocks. After one hour, a
pile 5m x 10m x 3m high is established. The total exposed surface area of the pile is 140
m2 and the bottom of the pit has another 150 m2 of exposed area (the sides of the
excavation pit are assumed to be clean overburden). The exposed surface areas are
assumed to remain constant during further hours of operation with any additional area
being covered with some type of impermeable cover that acts as a barrier to further
emissions.
A.2 DERIVATION OF EMISSION MODELS
The models are based on adding the emissions resulting from the release
of soil-gas (pore space gas) to the atmosphere when excavation soil is dumped onto a
storage pile to the emissions resulting from diffusion from contaminated soil present in
the excavation pit and in the storage pile. A discussion of the input parameters and .
typical input values are given in Sections A.4 and A.5. Limitations of the models are
also given in those sections.
Pore-Space Gas Model
The general form of the equation used to estimate the emission rate from
the pore space gas for any given compound is the ideal gas law:
P V = nR T (Eq. A-l)
where: P = Vapor pressure of compound i (mm Hg);
V = Volume (cm3);
n = Number of moles of gas;
2Page 8-35 of the Excavation Handbook by H.K. Church (MCGraw-Hill, 1981) states
that excavation equipment can be assumed to be in use for 30 to 50 minutes per hour.
3Page 12-38, op cit, gives a cycle time of 0.67 minutes for a 25 foot hoist distance and
a 90° angle of swing return.
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R = Gas constant; and
T = Temperature (°K).
The mass of contaminants present in the pore space of soil can be determined as follows.
First substitute MPS/MW for n and then solve for MPS:
M PVMW - A'2)
Mp- = -
PS RT
where: MPS = Mass of pore space contaminants (g); and
MW = Molecular weight of species i (g/g-mole).
Then substitute soil volume and air-filled porosity terms for V to account for the volume
of air within a given volume of soil. Air-filled porosity is the fraction of the total soil
volume that is air. A factor of 106 to convert from cm3 to m3 is also needed:
P MW (Eq' A'3)
MPS - Z--
where: Ea = Air-filled porosity (dimensionless);
106 = Conversion factor (cm3/m3);
Sv = Volume of soil moved (m3); and
R = Gas constant, 62,361 (mm Hg - cm3/g-mole °K).
To derive an emission rate, Equation A-3 must be modified to account for
the rate at which soil is being moved and to account for the percentage of soil gas that is
released or exchanged with the atmosphere:
P MW * (Eq" A'4)
ERPS = -^T1 (10s)(Et)(Q)
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Diffusion Model
The general form of the equation used to estimate the emission rate from
the contaminated soil in the excavation pit and in the storage pile is the RTI
landtreatment model:
M
EF = —
(Eq. A-5)
where:
EF
O
K
•eq
= Emission flux through the soil at some time t (g/cm2-sec);
= Initial loading of contaminant in soil (g/cm2);
= Depth to which contaminant is mixed in soil (cm);
= Weight fraction of VOC in air space (dimensionless);
= Gas-phase mass transfer coefficient (cm/sec);
De = Effective diffusivity (cm2/sec);
t = Time since start of excavation of soil of interest (sec); and
tb = Time constant for biological decay of contaminant i (sec).
Several modifications to the model were made to make it applicable to
excavation. First, the biological expoential decay term (e"t/tb) was set equal to one since
the timeframes of interest are very short. Second, the initial loading term (M0) and the
depth to which the waste is mixed term (1) were combined into a waste loading term,
designated C. Third, a factor of 10,000 was added to convert the emission units from
mass per cm2 to mass per m2. Fourth, a term was added to account for the surface area
of the emitting soil. The resulting equation is:
ERDiff ~
(Q(10,000)
[SA]
(Eq. A-6)
where:
ERD!ff
C
10,000
SA
Instantaneous emission rate from diffusion through the soil
(g/sec);
Soil concentration of species of interest (g/cm3);
Conversion factor (cnr/m2); and
Surface area of emission source (m2).
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The surface area term, SA, includes the area of the exposed contaminated
soil for both the excavation pit and the storage pile. It is assumed that the surface area
of the emission source remains constant, i.e., excavation was already underway before the
particular soil being modeled was handled and excavated soil is moved off-site or
covered to reduce emissions at the same rate that new soil is being uncovered and
excavated. To model the case where no contaminanted soil is initially exposed, the
surface area term in Equation A-6 can be divided by a factor of two to yield an average
amount of exposed surface area.
A.3 EMISSION MODELS
The overall emission rate equation is formed by adding Equations A-4 and
A-6. Note that the timeframes of the two equations as shown are not equivalent.
Equation A-4 describes the emissions over the course of excavating and dumping one
scoop of soil (40 seconds in the assumed scenario), while Equation A-6 gives an
instantaneous emission rate at some time t since the contaminated material was first
exposed to the air. An average value for t is discussed in Section A.4 and the timeframe
of the two models are reconciled so that they yield an average emission rate.
The general form of the emission models for estimating an "average"
emission rate for. the excavation of contaminated soil is given as Equation A-7 and a
worst-case emission rate is given as Equation A-8. It is a simple matter to modify either
of these equations to calculate an emission flux (i.e., rate per area) or total emissions for
a given period of time.
Emission Rate
An emission rate in g/sec for excavation was derived in the previous
section and is:
(Eq. A-7)
ER = -L-^l (106)(Ea)(Q)(ExC) + V^A^—; (SA)
RT
Worst-Case Emission Rates
The worst-case (i.e., maximum) instantaneous emission rate, ER^x* for
contaminated soil occurs when the exposed surface area is at a maximum and
immediately after a bucket load of soil is dumped onto the storage pile. This emission
rate can be approximated by considering the case where a pure chemical is exposed to
the atmosphere. This emission rate can be determined from Equation A-6 (there is no
need to consider pore space gas concentrations and diffusion since the pure chemical is
already exposed to the atmosphere). Set the time term, t, equal to zero and replace the
Kgq term with the equivalent expression: P*MW*Ea/R*T*C. Equation A-6 then reduces
to:
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(kg)(P)(MW)(SA)( 10.000) > A'8)
=
A.4 SIMPLIFIED EMISSION MODELS
The first half of Equation A-7 is simplified first, followed by simplification
of the second half of Equation A-7.
following:
Simplified Pore-Space Gas Model
The first half of Equation A-7 can be simplified as follows. Assume the
R = 62,361;
MW = 100;
T = 298;
ExC4 = 0.33.
Substituting these values into the first half of Equation A-7 yields an emission rate for
pore space gas, ERPS, of:
(Eq. A-9)
P MW , (P)(Ea)(Q)(100)(106)(0.33)
ERPS = -L^::(106)(Ea)(Q)(ExC) = V A aA A —-
RT (62,361)(298)
(Eq. A-10)
5-4g/m1 « P * Ea . Q * 0.33
mm Hg
4Assume ExC = 0.33 for dry, sandy soils and ExC = 0.10 for wet soils or those with
a high clay content.
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Vapor pressures for most VOCs of interest are available in tabluated
physical constants in Appendix B. These values are for 25°C, but P can be estimated at
other temperatures5. According to SEAMs, the air-filled porosity (Ea) can be assumed
to be:
E,
0.35
0.55
Soil Conditions
Wet, or compacted soil
Dry, uncompacted soil
Ea can be assumed to be 0.05 for sludges, tarry wastes, and saturated soils.
Alternatively, Ea can be calculated as follows:
Ea=
(fi)(MFRAC)
(Eq. A-11)
where:
6
Bulk density of soil (g/cm3);
Moisture fraction in soil (Wt.% Moisture/100); and
Particle density (g/cm3).
Default values are as follows. Bulk density (6) usually is in the range of
1.0 to 2.0 and can be assumed to be about 1.5 for uncompacted soils prior to excavation.
After excavation, the bulk density is lower and a value of 1.2 may be assumed. Particle
density (p) is typically about 2.65 ± 5% for soils. These default values yield an Ea for
dry soil of 0.43 before excavation and 0.55 after excavation.
Vapor pressure can be roughly estimated at temperatures other than 25°C by the
following equation:
* / v v
-21TB ! _ j_
1.987 T T0
i / i
(Eq. A-12)
where:
P = P° e
P
po
1.987
21
Vapor pressure of compound i at temperature T (mmHg);
Vapor pressure of compound i at temperature T0 (mmHg);
Normal boiling point of compound i (°K);
Temperature (8K);
Reference Temperature (°K) - Usually 298°K;
Gas constant (cal/g-mol °K); and
Heat of vaporization constant (cal/g-mol °K).
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Using the SEAMS value for Ea (0.55), Equation A-10 for dry soil then
reduces down to:
(Eq. A-13)
ERPS - P * Q * 0.98 g/mmHg-m3
Equation A-13 is the simple screening model. If desired, it can be further
reduced. Using the excavation scenario described above, Q can be assumed to be
150 m3/3600 sec. Equation A-13 for dry soil then reduces down to:
ERPS = (0.04 g/mm Hg)*P (Eq. A-14)
Simplified Landtreatment Model
The second half of Equation A-7 can be simplified as follows. The
following equations6'7 can be used to describe the terms Keq and De, which appear in
Equation A-7:
(Eq. A-15)
PMWEa M
eq " RT C
(Eq.A-16)
Da (Ea)3-33
where: Da = Diffusivity in air of species i (cm2/sec); and
Ej- = Total porosity (dimensionless).
6The equation shown for calculating Keq assumes that the contaminant is an oily
waste. For dilute aqueous wastes, Keq = H/RT, where H = Henry's Law constant in
mm Hg-cm3/g-mol.
7Strictly speaking, the concentration term, C, in Equations A-15 and A-7 should be
adjusted to account for the mass of contaminant lost with the pore-space gas. This
adjustment has not been included in the model for the sake of simplicity.
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K represents the relative saturation of the soil-gas with respect to a given
compound and cannot realistically exceed 1. Calculated values of K:q using Equation
A-15 will exceed 1 if the soil-gas is below saturation with respect to that compound. If
the output of Equation A-15 is Keq > 1, then a value of Keq = 1 should be used in all
equations having a Keq term. Alternatively, K,.q could be determined by field
measurements of the pore space concentration in the soil ratioed to the total
concentration of the contaminant in the soil.
zero.
can be calculated by Equation A-11 if the moisture fractir is set to
Assume the following:
R
MW
T
62,361;
100;
298;
0.1;
0.55;
0.625;
Substitute these values into Equations A-15 and A-16 to yield:
(Eq. A-17)
K.
C 332,200
De = 0.035
(Eq. A-18)
The second half of Equation A-7 can then be simplified by inserting
Equations A-17 and A-18, and by assuming that Ea = 0.55 and that kg = 0.15. Equation
A-7 then reduces to:
ERDiff =
(C)(10,000)
1.22xl06
>
C
p,
*
2.98xl07
C t
P ,
(Eq. A-19)
(SA)
Equation A-19 provides an instantaneous emission rate at time = t. It is
assumed that emissions from freshly excavated soil are significant for a period of 360
seconds, after which the soil is covered by subsequent layers of excavated material. The
emission rate versus time over this 360 second period for a given scoop of soil will
generally exhibit an exponential decay. The exact shape of this decay curve will vary as
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the input parameters such as vapor pressure and air-filled porosity vary. Therefore, it is
necessary to determine at what time t the instantaneous emission rate approximates the
average emission rate over the 360 second period. This can be done by calculating the
instantaneous emission rates at t = 0 second, t = 15 seconds, t = 30 seconds, and so on.
The emission rate is calculated for every 15 second period up to t=360 and the results
plotted. The average emission rate is calculated by summing the instantaneous emission
rates and dividing the sum by the number of data points (in this example, 24). The value
for the average emission rate is then found on the plot of emission rate versus time, and
the corresponding time found on the x-axis. This time t is then used in Equation A-19.
For the typical case, the instantaneous rate at t = 60 seconds is a good approximation of
the overall emission rate for the first 360 seconds. Using this value Equation A-19 yields
the simple screening equation:
(Eq. A-20)
ERDlff. ™°m (SA)
/
1.22 x
\ /
106 —
P
+
1.79 x
\
109 C
P
Equation A-20 assumes that the emission flux arising from diffusion is equal for both the
excavation pit and the excavated soil in the storage pile. Equation A-20 will overpredict
emissions if Kcq>l. P at temperatures other than 25°C can be estimated using Equation
A-12. From the excavation scenario described earlier, SA can be assumed to be 290 m2.
Assuming a typical bulk density of undisturbed soil, C can be modified to a
weight basis as follows:
(Eq. A-21)
* 1 0 LL 0 / 2
1-5 g
where: C = Concentration of species in soil (wg/g).
The overall emission rate is determined by adding Equations A-13 and
A-20. This estimated value should be checked to see whether or not it exceeds the total
mass of contaminants present in the soil that is moved, which is equal to the theoretical
maximum emissions (not considering emissions from the un-excavated soil in the pit).
To do this, the emission rate should be multiplied by 3,600 seconds to get the total
emissions over a reasonably long period of time, one hour. The mass of contaminants
present in the soil can be determined by:
= C • Sv * 106 cm3/m3 (Eq. A-22)
where: CTOT = Total starting mass of contaminant in excavated soil (g).
Equations A-4 and A-13 are based on the assumption that the soil pore gas
is saturated with the compound of interest. If this is not the case, then Equations A-4 or
A-13 may overpredict the emission rate. The output from Equations A-4 or A-13 should
be multiplied by the duration of excavation and compared to the total mass of
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contaminants present in the soil. If Equations A-4 or A-13 gives a value that exceeds
one-third of CTQ^, then they should be replaced with the following equation:
where:
A.5
ERpS = CTOT * 0.33/t^
Time to excavate a given volume of soil (sec).
(Eq. A-23)
MODEL EVALUATION
The emission model was evaluated to determine the sensitivity of the
model to various input parameters. All the independent variables in Equation. A-7 are
listed in Table A-l. For each variable a typical value is given along with the range of
values likely to be encountered at Superfund site excavations. The uncertainty associated
with measuring each variable is also estimated in Table A-L The range of physical
properties was based on n-butane being the lightest VOC likely to be encountered at a
site and naphthalene being the heaviest compound likely to be of concern. Typical
physical property values were based on C6 to C8 compounds (e.g. benzene to xylene).
The soil volume -term was kept constant to show the variability in surface area for a
given volume of soil. The gas-phase mass transfer coefficient (kg) was estimated using
the correlations given with the RTI landtreatment model and the following input values:
Parameter
Wind Speed
Viscosity of air
Density of air
Diffusity. in air
Diameter of excavation
Units
m/sec
g/ cm-sec
g/cm3
cnr/sec
m
Minimum
Value
1.0
Maximum
Value
4.47
Typical Value
2.0
l.SlxlQ-4
1.2xW3
0.25
0.059
0.1
24
The minimum and maximum values for the independent input parameters
from Table A-l were combined to generate a best-case and worst-case set of emission
scenarios. These are shown in Table A-2 along with the case using the typical input
parameters. As seen in Table A-2, the three cases shown differ greatly in the estimated
average emission rate.
To identify which parameters had the greatest effect on the overall
emissions, a set of calculations were performed using the base or typical case as the
starting point. The effect of each parameter was examined by substituting the minimum
and maximum value for each into the base case conditions. The results of this first-order
sensitivity analysis are shown in Table A-3. The two independent variables having the
largest effect on the overall emission rate are the starting concentration of the
contaminant in the soil and the vapor pressure of the contaminant. Note that
temperature has a small effect, but that emissions are inversely proportional to
temperature. This is, of course, contrary to the overall effect of temperature on
emissions: emissions increase as temperature increases. This seeming anomaly is due to
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main effect of temperature being to increase the vapor pressure and diffusivity terms. If
these terms are not corrected for temperature, then the model will become less accurate
as the temperature deviation from 25°C increases.
Equation A-7 requires the input of the time after the start of excavation
(t). It was assumed earlier that the emission rate at t = 60 seconds was equal to the
average emission rate over t = 0 to t=360 seconds. It was further assumed that after 360
seconds, the excavated soil would be covered with additional layers of soil and the
diffusion of further material (emissions) would be minimal. The effect of time (t) was
examined by substituting a range of times into the base case conditions. The results of
these trials are given in Table A-4 and depicted in Figure A-l and A-2.
The effect of the initial soil concentration of the contaminant on the
predicted emission rate was examined by using the same base case assumptions and
varying the concentration from 1 ppbw to 10,000 ppmw. These results are shown in
Table A-5 and are plotted in Figure A-3. As the concentration increases, the percentage
of the total mass of material emitted decreases. Also, the relative contribution of pore-
space gas to the total emissions also decreases. The effect of vapor pressure (and
molecular weight) was examined by inserting the values for vapor pressure and molecular
weight for several common organic species into the base case. All compounds were
assumed to be present at 100 ppmw in the soil. These results are shown in Table A-6.
A final check of the models was made by comparing model predictions to
field data (Eklund, et al. Field Measurement of VOC Emissions From Soils Handling
Operations at Superfund Sites. EPA Contract No. 68-02-4392, Work Assignment 64.
September 1990). Comparisons of both the detailed (Equation A-7) and simple models
(Equations A-13 and A-20) to field data are shown in Table A-7. Total emissions for
twenty minute sampling periods are shown for two different field sites. The detailed
model using site-specific input data agrees with the field measurements within a factor of
five in all but two cases. The simplified model shows equally good agreement.
The equations presented here are a first attempt to model emissions from
soils handling operations. The equations are limited by a lack of laboratory or field data
to define certain key relationships between the variables. For example, the excavation
rate and the total exposed area are assumed in the equations to have a direct linear
relationship with the emission rate. No data, however, exist to support this assumption.
Similarly, the effects of temperature, scoop size, and surface area to volume ratio on
emissions have not been investigated. Another limiting assumption is that 33% of the
pore space gas is exchanged with the atmosphere. This value is arbitrary and was
selected since it fit reasonably well with the very limited field data that are available.
Measurements of emission rates from dynamic processes such as excavation
are very difficult to perform and are of limited accuracy. Limitations exist for dispersion
models used in indirect approaches (e.g., transect) and in the sampling and analytical
precision when attempting to determine emission rates using a mass balance approach.
Emerging measurement technologies, such as remote optical sensing, may allow more
detailed evaluation of the effect of these parameters in the future.
-------
Table A-4.
Effect of Time (t) on Emissions
Time (sec)
0
5
10
20
30
40
50
60
90
120
180
240
300
360
420
480
.540
600
1200
1800
2400
3000
3600
Diffusion Emission Rate
(mg/sec)
81.9
11.0
8.09
5.89
4.87
4.25
3.83
3.51
2.89
2.51
2.06
1.79
1.61
1.47
1.36
1.28
1.20
1.14
0.81/
0.66
0.58
0.51
0.47
Total Emission Rate
(mg/sec)
83.1
12.1
9.23
7.03
6.01
5.39
4.96
4.65
4.02
3.65
3.20
2.93
2.74
2.61
2.50
2.41
2.34
2.28
1.95
1.80
1.71
1.65
1.61
-------
T G. • I O | ! r~p
.- --^ ^ —' . , I I i
15 30 45 60 75 90 120 150 180 210 240 270 300 330 360
Time i sec;
Figure A-l. Emission Rate vs. Time for Base Case Conditions for 0 to 360 seconds.
-------
Emission Rate Vs. Time (0 to 60 min)
0
X
rotai tmiss.ons
8 9 10 20 30 40 50 60
Diffusive Emissions
Figure A-2. Emision Rate vs. Time for Base Case Conditions for 0 to 60 Minutes.
-------
Table A-5.
Effect of Cone. (C) on Emissions
Cone (ug/Kg)
1
10
100
1000
10000
100000
1000000
10000000
Log Cone
(ug/Kg)
1
2
3
4
5
6
7
8
Pore Gas
Emission
Rate (g/sec)
1.88 x 10"5
1.88 x lO^1
1.87 x 10'3
0.019
0.188
1.138
1.138
L 1.138
Diffusive
Emission
Rate (g/sec)
4.52 x 10'5
4.52 x 10"
4.52 x lO'3
0.045
1.14
3.51
10.15
25.32
Total Emission
Rate (g/sec)
6.40 x 10'5
6.40 x 10"*
6.40 x 10°
- 0.06
1.33
4.65
11.29
26.46
Emissions*
Vs. Total
Mass (%)
114
114
114
114
236
82.6
20.1
4.7
* Includes only mass of contaminants in excavated soil
-------
irne
il'i '- jncentrofon v-"es
•>?
~otal Emissions
tog Concentration (ug/Kg)
•t- Diffusive Emissions
Figure A-3. Emission Rate vs. Time as Soil Concentration Increases.
-------
Table A-6.
Effect of Molecular Weight (M\V) + Vapor Pressure (P) on Emissions
Cone (ug/Kg)
Molecular
Weight (g/g-mol)
Vapor Pressure
(mm Hg)
Diffusive
Emission Rate
(g/sec)
Total Emission
Rate t 'sec)
Alkanes
butane
pentane
hexane
heptane
octane
nonane
58.12
72.15
86.18
100.2
1 14.23
128.26
1820
513
150
46
17
4.3
4.52
4.52
4.52
4.05
2.57
1.30
6.40 '
6.40 '
6.40 *
5.55
3.21
1.48
Aromatics
benzene
ethylbenzene
o-xylene
78.12
106.16
106.2
95.2
10
7.0
5.18
1.87
1.54
7.06
2.21
1.78
* Pore space emissions equal the total mass of contaminant present divided by 3.
------- An error occurred while trying to OCR this image.
-------
-------
APPENDIX B
PHYSICAL AND CHEMICAL CONSTANTS
FOR SELECTED COMPOUNDS
-------
-------
APPENDIX B - PHYSICAL PROPERTY DATA
No.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
• 34
35
36
37
38
39
40
41
42
43
Organic Compound
Acettldehyde
Acetic acid
Acetic anhydride
Acetone
Acetonitnle
Acrotein
Acrylic acid
Acrylonitrile
Ally! alcohol
Ally! chloride
Aniline
Anthracene
Benzaldehyde
Benzene
Benzoic acid
Benzyl alcohol
Benzyl chloride
Bromoform
1,3-Butadiene
N-Butane
2-Butanol
N-Butanol
N-Butyl-Acetate
Tert-Butyl-Alcohol
Carbon disulfidc
Carbon tetrachloride
Carbonyl sulfidc
Catcchol
Chlorine
Chlorobenzene
Chlorodifluoromethane
Chloroform
Chloromethyl methyl ether
Chloropentafluoroethane
Qiloroprene
M-Cresol
O-Cresol
P-Cresol
Cyanogen
Cyclohexane
Cyclohexanol
Cyclohexanone
Cyclohexcne
CAS NO.
75-07-0
64-19-7
108-24-7
67-64-1
75-05-8
107-02-8
79-10-7
107-13-1
107-18-6
107-05-1
62-53-3
120-12-7
100-52-7
71-43-2
65-85-0
100-51-6
10O44-7
75-25-2
106-99-0
106-97-8
15892-23-6
71-36-3
123-86-4
75-65-0
75-15-0
56-23-5
463-58-1
120-80-9
7782-50-5
108-90-7
75^*5-6
67-66-3
107-30-2
76-15-3
126-99-8
108-39-4
95-48-7
10&44-5
460-19-5
110-82-7
108-93-0
108-94-1
110-83-8
Formula
C2H4O
C2H4O2
C4H6O3
C3H6O
C2H3N
C3H40
C3H4O2
• C3H3N
C3H6O
C3H5CL
C6H7N
C14H10
C7H6O
C6H6
C7H6O2
C7H8O
C6H5CH2C1
CHBr3
C4H6
C4H10
C4H10O
C4H10O
C6H12O2
C4H10O
CS2
CCL4
COS
C6H4(OH)2
C12
C6H5CL
CHCLF2
CHCL3
C2H5QO
C2CLF5
CH2CHCH2C1
C7H8O
C7H8O
C7H8O
C2N2
C6H12
C6H12O
C6H10O
C6H1O
Molecular
Weight
(g/g-mo»)
44.00
60.06
10X09
58.08
41.06
56.1
72.1
53.06
58.08
7&S3
93.13
178.23
106.12
78.12
122.12
108.14
126.6
252.77
54.09
58.12
74.12
74.12
116.16
74.12
76.13
153.82
60.1
110.1
70.9
112.56
86.47
119.38
80.51
154.47
76.53
108.14
108.14
108.14
52.04
84.16
100.16
98.14
82.15
Vapor
Pressure
(mm Hg)1
760
15.41
5.266
266
90
244.2
5.2
114
23.3
368
1
1.3E-06
1
95.2
0.00704
0.15
1.21
5.6
2100.00
1820
10
6.5
15
0.17
366
113
-
-
-
11.8
-
208
-
-
273
0.08
0.24
0.11
3980
100
1.22
4.8
-
Diffusiviry in
Air
(cm2 /sec)
0.1240
1.1300
0.2350
0.1240
0.1280
0.1050
0.0908
0.1220
0.1140
0.0700
0.0932
0.0750
0.2490
0.2490
0.1040
0.0632
0.0730
0.0888
0.1040
0.0740
0.0740
0.0740
0.0839
0.2140
0.0784
-------
Appendix B. (Continued)
No.
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
Organic Compound
Cyclopenune
Diazomethane
Dibutyl-O-Phthalate
O-Dichlorobenzene
P-Dichlorobenzene
Dichloroethylether
Dichlorodifluoromethane
1,1-Dichloroethane
1,2-Dichloroethane
1,1-Dichloroethylene
cis-l,2-Dichloroethylene
trans-U-Dichloroethylene
Dichloromeihane
Dichloromonofluoromethane
1,2-Dichloropropane
1,3-Dichloropropene
l,2-Dichloro-l,1.2.2-Tetralluoroethane
Diethanolamme
Diethyl amme
N,N-Dimethylanihne
Diethyl ether
Dimethytamine
Dimethyl formamide
1,1-Dimethyl hydrazine
2,4-Dinitrophenol
1,4-Dioxane
Diphenyl
Epichlorohydnn
1,2-Epoxybulane
Ethanol
Ethyl acetate
Ethyl acryiate
Ethyl amine
Ethyl benzene
Ethyl Bromide
Ethyl carbamale
Elhyl Chlonde
Ethylenediamme
Ethylene dibromide
Ethytene glycol
Ethylene imine
Ethylene oxide
CAS NO.
287-92-3
334-88-3
84-74-2
95-50-1
106-46-7
Hl^M
75-71-8
• 75-34-3
107-06-2
75-35-4
156-59-2
156-60-5
75-09-2
75^3-4
78-87-5
542-75-6
76-14-2
111-42-2
109-89-7
121-69-7
60^29-7
124-40-3
68-12-2
57-14-7
51-28-5
123-91-1
92-52-*
106-89-8
106-88-7
64-17-5
141-78-6
140-88-5
75-04-7
100-41-4
74-96-1
51-79-6
75-00-3
107-15-3
106-93-4
107-21-1
151-5^4
75-21-8
Formula
C5H10
CH2N2
C16H22O4
C6H4CL2
C6H4CL2
C4H8C12O
CCL2F2
C2H4CL2
C2H4CL2
C2H2CL2
C2H2CL2
C2H2CL2
CH2CL2
CHCL2F
C3H6CL2
C3H4C12
C2CL2F4
C4H11N02
C4H11N
C8H11N
C4H100
C2H7N
C3H7NO
C2H8N2
C6H4N2O5
C4H802
C12H10
C3H5C1O
C4H8O
C2H6O
C4H8O2
C5H8O2
C2H7N
C8H10
C2H5Br
aH7NO2
C2HSC1
C2H8N2
C2H4Br
aH602
C2H5N
C2H4O
Molecular
Weight
(g/g-moO
70.13
4X04
278.35
147.00
147.00
143.02
120.91
98.96
98.96
96.94
96.94
96.94
84.93
102.92
112.99
110.98
170.92
105.14
73.14
121.18
74.12
45.08
73.09
60.10
184.11
88.11
154.21
92-53
72.0
46.07
88.11
100.12
45.08
106.16
108.97
89.09
64.51
60.10
187.88
6107
43.07
44.06
Vaoor
Pressure
(mm Hit'1
317.44
-
l.OOE-05
1
1.2
1.4
4870
234
80
600
208
324
362
1360
42
43
-
-
350@35C
-
440@20C
563 @OC
4.0
157
53.8
37
-
17
-
50
100
40
1057
10
-
10
1200
10.7
14
0.13
-
1250
Diffusmty in
Air
(cm2 /sec)
0.0439
0.0690
0.0690
0.0919
0.0907
0.1040
0.0782
0.0939
0.1060
0.2290
0.0860
0.1230
0.0770
0.0750
0.2710
0.1080
0.1040
-------
Appendix B. (Continued)
No.
86
87
88
89
90
91
92
93
94
95
%
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117'
118
119
120
121
122
123
124
12S
126
127
Organic Compound
Formaldehyde
Formic acid
Furan
Glycerol
N-Heptane
N-Hexane
Hydrazine
Hydrochloric acid
Hydrogen cyanide
Hydrogen sulfide
Isobuunoi
Isobutyl acetate
Isopropyi alcohol
Isopropyl amme
Isopropylbenzene
Methanol
Methyl acetate
Methyl acrylate
Methyl amine
Methyl bromide
Methyl-ten-butyl-ether
Methyl chloride
Methyl cyclohexane
Methyl-ethyl-ketone
Methyl formate
Methyl hydrazine
Methyl iodide
Methyl-lsobutyl-Ketone
Methyl isacyanate
Methyl-Isopropyl-Ketone
Methyl me reap tan
Methyl methacrylate
Methyl-N-Propyl-Ketone
Alpha-Methyl-Styrene
Monoethanolarmne
Morpholme
Naphthalene
2-Nitropropane
N-Nitrosodimethylamine
N-Nitrosomorpholine
N-Nonane
N-Octane
CAS NO.
5040-0
64-18-6
110-00-9
56-81-5
142-82-5
110-54-3
302-01-2
76474)1-0
74-90-8
77834)64
78-83-1
110-19-0
67-63-0
75-31-0
98-82-8
67-56-1
79-20-9
96-33-3
74-89-5
74-83-9 .
1634-04-4
74^7-3
108-87-2
78-93-3
107-31-3
60-344
74-88-4
108-10-1
624-83-9
563-80-4
74-93-1
80-62-6
107-87-9
98-83-9
14143-5
110-91-8
91-20-3
7946-9
62-75-9
59-89-2
111-84-2
111-65-9
Formula
CH2O
CH2O2
C4H4O
C3H803
C7H16
C6H14
H4N2
HO
CHN
H2S
C4H10O
C6H12O2
C3H8O
C3H9N
C9H12
CH40
C3H6O2
C4H7O2
CH5N
CH3BR
C5H120
CH3CL
C7H14
C4H8O
C2H4O2
CH6N2
CH31
C6H120
C2H3NO
C5H10O
CH4S
C5H8O2
C5H10O
C9H10
aH7NO
C4H9NO
C10H8
C3H7N02
C2H6N20
C4H8N2O
C9H20
C8H18
Molecular
Weight
(g/g-mot)
30.03
46.03
68.08
92.09
100.2
86.18
32.05
36.46
27.03
34.08
74.12
116.16
60.1
59.11
120.19
32.04
74.08
86.09
31.06
94.94
88.15
50.49
98.19
72.11
60.05
46.07
141.94
100.16
57.05
86.13
48.1
100.10
86.13
118.18
61.08
87.12
128.19
89.09
74.08
116.11
128.26
114.23
Vapor
Pressure
(mm Hg)1
3500
42
5%
1.60E-04
46
150.3
14.4
32,450
-
15,200
10
-
42.8
460
10.9@40C
114
235
-
770@-6C
-
245
3830
43
100
500
49.6
91
19.31
348
15.7
-
39
-
0.076
-
10.08
0.023
12.9
-
-
4.28
17
DifTusiviry in
Air
(cirf/sec)
0.1780
0.0790
0.1040
0.2000
0.1760
0.0860
0.0980
0.1500
0.1040
0.0806
0.1260
0.0808
0.0750
0.0770
0.2640
0.0590
-------
Appendix B. (Continued)
No.
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
Organic Compound
N-Pentane
Phenanthrcne
Phenol
Phosgene
Phosphine
Phthalic anhydride
Propane
1,2-Propancdiol
1-Propanol
beta-Propiolactone
Propionaldehyde
Propionic acid
N-Propyl-Acetate
Propylene oxide
1,2-Propytenimmc
Pyndine
Quinone
Styrene
l,l,l,2-Tetrachloro-2,2-Difluoroeihane
l,lA2,-Tetrachloroethanc
Tetrachloroethylene
Tetrahydrofuran
Toluene
P-Toluidine
1.1.1-Trichloroethane
1 . '. .2-Trichloroethane
; nchloroethylene
Trichloronuoromethane
1,2,3-Tnchioropropane
l,l,2-Trichloro-l,2,2-Trifluoroethane
Triethytamme
Thfluorobromomethane
1,23-Trimethylbenzene
1,2,4-Trimeihylbenzene
1,3,5-Trimethylbenzene
Vinyl Acetate
Vinyl bromide
Vinyl-Chloride
M-Xylene
O-Xylene
P-Xylene
CAS NO.
109-664
85-01-8
108-95-2
75-44-5
7803-51-2
85^4-9
74-98-6
57-55-6
71-23-«
57-57-8
123-38-7
79-09-t
109-60-*
75-56-9
75-55-8
110-86-1
106-5 1-4
KXM2-5
76-11-9
79-34-5
127-18-4
109-99-9
108-88-3
106-49-0
71-55-6
79-00-5
79-01-6
75-69^*
96-18-4
76-13-1
121-14-8
75-63-8
526-73-8
95-63-6
108-67-8
108-05-4
593-60-2
75-OM
108-38-3
95^*7-6
106-42-3
Formula
C5H12
C14H10
C6H60
CQ20
H3P
C8H4O3
C3H8
C3H8O2
C3H8O
C3H402
C3H6O
C3H6O2
C5H10O2
C3H6O
C3H7N
C5H5N
C6H4O2
C8H8
C2CL4F2
aH2CL4
C2CL4
C4H80
C7H8
C7H9N
C2H3CL3
C2H3CL3
C2HCL3
CCL3F
C3H5CL3J
C2CL3F3
C6H15N
CBRF3
C9H12
C9H12
C9H12
C4H6O2
C2H3Br
aH3CL
C8H10
C8H10
C8H10
Molecular
Weight
(g/g-mol)
72.15
178.23
94.11
98.92
34.00
148.11
44.1
76.11
60.1
72.06
58.08
74.08
102.12
58.08
54.1
79.1
108.09
104.15
203.83
167.85
165.83
72.11
92.14
107.16
133.41
133.41
131.4
137.37
147.43
187.38
101.19
148.91
120.19
120.19
120.19
86.09
107.0
62-5
106.2
106.2
106.2
Vapor
Pressure
(mm Hg)1
513
2.00E-04
0.0341
1,394
2,000
0.0015
760
0.3
20.85
3.4
300
10
35
524.5
112
20
-
7.3
-
6.5
19
72.1
30
0.3
123
25
75
667
3.1
300
400
-
-
-
1.86
115
89
2660
8
7
9.5
Diffusrvicy in
Air
(en? /sec)
0.0820
0.1080
0.0710
0.1040
0.0910
0.0710
0.0720
0.0980
0.0870
0.0780
0.0792
0.0790
0.0850
0.0900
0.0700
0.0870
1 All vapor pressures are at 25* C unless otherwise indicated.
------- |