&EPA
United States Office of Air Quality EPA-451/R-93-003
Environmental Protection Planning and Standards April 1993
Agency Research Triangle Park, NC 27711
Air/Superfund
AIR/SUPERFUND
NATIONAL TECHNICAL GUIDANCE
STUDY SERIES
ESTIMATION OF AIR IMPACTS
FOR BIOVENTING SYSTEMS
USED AT SUPERFUND SITES
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EPA 451/R-93-003
ESTIMATION OF AIR IMPACTS
FOR BIOVENTING SYSTEMS
USED AT SUPERFUND SITES
Report ASF - 33
U.S. Environmental Protection Agency
Stegion 5, Library (PL-12J)
77 West Jackson Boulevard, 12th Floor
L 60604-3590
Prepared for:
Office of Air Quality and Standards
U.S. Environmental Protection Agency
Research Triangle Park, North Carolina 27711
April 1993
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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 as received from
the contractor. The contents reflect the
views and policies of the Agency, but any
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 1
ESTIMATION OF VOC AIR EMISSIONS 6
Average Long-Term Uncontrolled VOC Emission Rate 6
Short-Term Uncontrolled VOC Emission Rate 7
•
ESTIMATION OF AMBIENT AIR CONCENTRATIONS 10
ESTIMATION OF HEALTH EFFECTS 15
Cancer Effects Due to Long-Term Exposure • 15
Non-Cancer Effects Due to Long-Term Exposure 25
Short-Term Exposure 26
EXAMPLE 27
CONCLUSIONS 31
REFERENCES 31
APPENDIX A PHYSICAL PROPERTY DATA FOR SELECTED ORGANIC
COMPOUNDS
APPENDIX B PHYSICAL PROPERTY DATA FOR SELECTED SEMI-VOLATILE
ORGANIC COMPOUNDS
ll
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LIST OF TABLES
Page
1 Default Values for VOC Emissions Model for Bioventing Systems 9
2 Example Scenarios for Bioventing Systems 12
3 Example Scenarios for an Area Source Bioventing Systems 12
4 Long-Term and Short-Term Health-Based Action Levels for Organic
Compounds in Ambient Air ; 16
5 Long-Term and Short-Term Health-Based Action Levels for Selected
Semi-Volatile Organic Compounds in Air 24
6 Estimated Emission Rates and Ambient Air Concentrations 30
7 Action Level Concentrations 30
LIST OF FIGURES
1 Bioventing System with Extraction Wells and an Exhaust Stack 3
2 Bioventing System without Extraction Wells 5
3 One-Hour Average Downwind Dispersion Factor Versus Distance for
Bioventing Systems using an Extraction System 13
4 One-Hour Average Downwind Dispersion Factor Versus Distance for
Bioventing Systems with Area Source Emissions 14
111
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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 assessment of air
impacts from various alternatives for cleaning up Superfund sites. Since these assessments
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 bioventing. These procedures are analogous to procedures for air strippers,
soil vapor extraction systems, and excavation that have previously been published1'2-3.
Bioventing is an in situ remediation process where oxygen is introduced to the contaminated
subsurface area to enhance biodegradation rates of the organic contaminants. Procedures are
given to evaluate the effect of exhaust gas flow rate and contaminant concentration on the
emission rates and on the ambient air concentrations at selected distances from the treatment
area.
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 use could
either under or over estimate the potential health effects. The use of action levels that are
neither EPA-verified nor EPA-approved should be considered on a case-by-case basis. The
statements and conclusions presented in this report are those of the authors and do not reflect
U.S. EPA policy.
PROCESS DESCRIPTION
Bioventing is the process of supplying oxygen and other essential nutrients in situ to
microorganisms in the subsurface to enhance biodegradation of organic contaminants in the
soil. Bioventing is a combination of bioremediation and soil vapor extraction technology.
1
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Like bioremediation, this treatment method relies on microorganisms to degrade the organic
contaminants to water and carbon dioxide. These microbes are either indigenous to the site
or are introduced into the contaminated area.
Figure 1 presents a general process diagram for bioventing systems with extraction
wells and an exhaust stack. Bioventing is a developing technology, so there are many
process configurations currently in use. Many bioventing systems utilize extraction wells to
provide oxygen to the subsurface by applying a vacuum to the soil and thereby pulling
ambient air through the unsaturated or vadose zone. In bioventing, as opposed to soil vapor
extraction, the purpose of the extraction is not to volatilize and remove the contaminants but
to provide an oxygen-enriched environment to enhance in situ biodegradation of the
contaminants. Oxygen transfer can be enhanced with passive ambient air wells or with air
injection wells which provide a positive pressure for oxygen transfer. The induced air flow,
however, whether under positive or negative pressure, can cause the transfer (partitioning) of
a significant amount of the more volatile compounds present in the soil to the gas phase.
Extraction wells are typically positioned towards the perimeter of the contaminated area to
increase transport time and thereby minimize volatilization of the organic contaminants (i.e.,
to give biodegradation more time to occur). Injection wells are typically put in the areas of
highest concentration of contaminants to provide maximum oxygen transfer to these areas.
Bioventing systems are composed of the same type of equipment used in conventional
soil vapor extraction (SVE) systems. The purpose of this equipment and the basis for this
technology is quite different. SVE is used to collect contaminants in the subsurface by
volatilization and removal. Bioventing systems operate under an entirely different principle:
the induced air flow is used to create an oxygen-enriched environment to encourage
bioremediation of the contaminants. For this reason, the total gas flow rates are significantly
lower for bioventing systems than SVE systems. Typical flow rates for bioventing systems
are 50 to 600 cfm°-6'7; SVE systems have exhaust gas flow rates ranging from 500 to
15,000 cfm2. The air flow rate for bioventing systems is dependent on the size of the site.
Sites with larger volumes of soil to be treated will require a relatively larger amount of air
flow to ensure adequate oxygen transfer to the soil.
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Clean Flue Gas
Figure 1. Bioventing System with Extraction Wells and an Exhaust Stack
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In order to provide an environment suitable to the microorganisms, moisture and
nutrients (nitrogen and phosphorus) are sometimes applied to the subsurface. This is usually
achieved by applying the nutrients in an aqueous solution to the subsurface using drip
irrigation hoses or to the soil surface above the contaminated area with water sprays. The
addition of nutrients ensures that the biodegradation rate is limited only by the amount
(concentration) of the organic contaminants present.
The specific point source of air emissions from bioventing systems is the off-gas
collected by the vapor extraction system. The exhaust gas flow rate is small compared to
soil vapor extraction (SVE) and other remediation processes. Also, the off-gas
concentrations are lower than for SVE systems since biodegradation (destruction of the
organic contaminants) is an integral part of the remediation process. Due to this, in many
cases, the off-gas from the bioventing system is not treated. Collection and control equipment
such as activated carbon or afterburners can be used to prevent the release of contaminants to
the atmosphere. If control devices are present and working properly, the stack will vent
small concentrations of the original VOC contaminants, as well as any products of
combustion that might occur from the afterburner. The estimates of uncontrolled emissions
obtained from this manual can be used to help estimate the required removal efficiency of an
emission control system or the size and cost of a given control system.
When injection wells are used to supply oxygen and no gas extraction system is used,
the injected air results in a positive pressure gradient in the soil, and the emissions are
diffused over a wide area. An example of such a system is shown in Figure 2. Bioventing
systems without extraction wells and an exhaust stack are less common than systems with
these components. Similarly, area-wide emissions can occur with an extraction well system
if it does not adequately collect the injected air. Typically, the air injection rates are low
enough that emissions at the surface are thought to be minimal. Emissions from leaking
components of the bioventing system and control devices are possible as well, but these
emissions are usually considered to be negligible and are not considered in this document.
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Air Inlet Wells
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Figure 2. Bioventing System without Extraction Wells
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ESTIMATION OF VOC AIR EMISSIONS
There are several alternative approaches for estimating the emissions from bioventing
processes. The best method is to directly measure the emissions during full-scale or
pilot-scale operations. The next best method is to estimate the emissions using predictive
equations with site-specific inputs. If site-specific information is not available, a
conservative estimate can be made using default values for the input parameters. Equations
are given below for estimating an average long-term emission rate (Equation 1) and a
short-term emission rate (Equation 2). Equation 2 is the recommended equation for
estimating volatile organic compound (VOC) emissions; Equation 1 can be used as a gross
check of total emissions.
Average Long-Term Uncontrolled VOC Emission Rate (Worst Case)
A simple check of the total emissions potential for VOCs 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:
(Sy)(Q(P)d)
tR
where ERlvg = Average worst case emission rate (g/sec);
Sv = Volume of contaminated soil to be treated (m3);
C = Average contaminant concentration (/*g/g);
0 = Bulk density of soil (g/cm3);
1 = Constant (g/106 jug * 106 cnrVm3); and
tR = Duration of remediation (sec).
The volume of contaminated soil and the total mass of each contaminant of concern present
typically are determined during the remedial investigation (RI) of the site, while the fraction
of contaminated soil that must be treated typically is determined during the feasibility study
(FS) for the site. Final clean-up criteria also should be considered when calculating the
volume of soil to be treated since remediation to background levels is usually not required.
The duration of the clean-up will be limited by the biodegradation rate of the less
biodegradable contaminants. Biodegradation rates for various bioventing field programs have
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been documented8. For Equation 1, a typical default value for bulk density of uncompacted
soil is 1.5 g/cm3.
Equation 1 assumes that 100% of the organic contaminants present in the soil will
eventually be volatilized. However, this may not be the case for all of the contaminants,
especially semi-volatile organic compounds. This is apparent in one field study where up to
80% of the hydrocarbons were removed by biodegradation (versus 20% removed by
volatilization) when the air flow rates were optimized to minimize the volatilization and
maximize the biodegradation of the contaminants at the site4-6-7.
Short-Term Uncontrolled VOC Emission Rate
The primary factors affecting the emission rate of a given compound from a
bioventing system are the concentration of the contaminant in the soil and the volatility of the
contaminant. Uncontrolled VOC emissions from a bioventing system can be estimated by
using the following mass balance approach:
ER = (C.\(-Q-
where ER = Emission rate for contaminant of interest (g/sec);
Cf = Concentration of the contaminant in the soil gas Qig/m3);
Q = Exhaust gas flow rate (nrVmin);
1/60 = Conversion factor (min/sec); and
10"* = Conversion factor (g/Mg).
Equation 2 does not take into account emissions, if any, from the ground surface.
Site-specific field data must be collected (e.g., during the RI/FS) to provide the input
data necessary to generate reasonably accurate estimates of air emissions. The minimum
field data required are knowledge of the specific contaminants present in the soil to be treated
and the average contaminant concentrations.
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The preferred source of input data for Equation 2 is field measurements for the
bioventing system configuration. The exhaust gas flow rate may be available from design
specification documents. The properties of the contaminated soil, such as air-filled porosity,
moisture content, and bulk temperature can affect the volume and composition of the
collected off-gas. Field test data should be obtained to estimate these parameters when
possible. Once the bioventing system is in operation, stack sampling of emissions from the
system can be performed to confirm the emission estimates.
If the exhaust gas flow rate is not available from the sources listed above, it can be
estimated from the following equation:
Q = f-12_)/Sv)(EJ
' (l,440jl VA *
where 1.0 = Estimated flow rate for maximum biodegradation and minimum
volatilization (pore volume/day);
1/1440 = Conversion factor (day/min); and
E, = Air-filled porosity (fraction).
Battelle has found that flow rates equivalent to 0.25 to 2.0 pore volumes per day are typical
for bioventing systems [soil pore volume = (SV)(EJ]4. A flow rate of 1.0 pore volumes per
day is thought to maximize the amount of biodegradation and minimize the amount of
volatilization. Table 1 presents default values for air-filled porosity, if field data for this
parameter are not available. If the soil type is not known, then a sandy silty soil type can be
assumed.
The best values for the contaminant concentration in the off-gas, Cg, are field
measurements from a pilot-scale system. The second best approach is to estimate Cg by
analyzing the headspace vapors above the contaminated soil for the contaminants in the soil.
If field data is not available, however, a very conservative estimate for Cg can be made by
assuming that the soil-gas is saturated. The maximum vapor concentration of any
compound in the collected vapor is its equilibrium or saturated vapor concentration9:
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Table 1.
Default Values for VOC Emissions Model for Bioventing Systems
Parameter
Exhaust Gas Flow Rate*
Air Filled Soil Porosity
Clayey Soil
Silty Soil
Sandy Soil
Symbol
Q
E.
Units
mVmin
Fraction
Default
Value
5.7
0.28
0.31
0.39
Expected
Range
1.4- 142
0.22 - 0.34
0.25. - 0.37
0.34 - 0.39
Reference
—
Author's estimate
Other Parameters of Possible Interest
Equivalent Flow Rate
--
Pore
volume/day
1.0
0.25 - 2.0
4
'Assumes dry standard conditions (20°C, 1 atm)
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(Eq. 4)
where Pvip = Pure contaminant vapor pressure at the soil temperature (mm Hg);
MW = Molecular weight of the contaminant (g/gmol);
R = Gas constant = 62.4 L-mm Hg/gmol-°K;
T = Absolute temperature of soil (°K); and
10' = Conversion factor 0*g-L/g-m^.
•
Values of molecular weight and vapor pressure and saturated vapor concentration at 25 °C for
various contaminants are given in Appendices A and B. Equation 4 gives the theoretical
maximum soil-gas concentration. This is an overprediction for compounds present in the soil
at relatively low concentrations. Equation 4 will also overpredict the long-term average
value of C( since the soil-gas concentration tends to drop over time. Also, biodegradation is
not accounted for in this model. In addition, if all other factors are constant, Cg will
decrease as Q increases.
VOC control devices generally will reduce the emissions by one to two orders of
magnitude. Equation 2 can be modified to account for the effectiveness of any control
device by adding the following term:
(1 - CE/100)
where CE = Control efficiency (%).
More information on VOC control devices and their design, applicability, and cost is
available in an EPA manual on controlling air emissions at Superfund sites.10
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 model11. 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
10
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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.
Typical remediation scenarios for bioventing systems that are point sources are given
in Table 2. Table 3 presents the dimensions of a typical area source for a bioventing system
where the vapors are not collected with an extraction system. The scenarios are based on
information obtained from a review of the existing literature and conversations with vendors
of bioventing systems. Figures 3 and 4 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 a Bioventing system with and without extraction wells. Of the variables listed in Table
2, the stack height and the exhaust gas velocity and temperature are used to estimate the
downwind dispersion in Figure 3. For Figure 4, the surface area and release height are used
to estimate the downwind concentrations. The dispersion model used is not reliable for
estimating air concentrations close-in to area sources. Estimates should not be made for any
downwind location that is closer to the source than the side length of the source (30 m in this
case).
Figures 3 and 4 illustrate the uncontrolled downwind concentrations for the example
bioventing systems. These curves were calculated according to the following assumptions: 1)
the combined emission rate is 1 gram per second; 2) a flat terrain without any structures
near the bioventing system; 3) the emission plume is of low, positive buoyancy; 4) the stack
is the only downwash structure (Figure 3 only); and 5) the receptors are at ground level.
For the area source, the emissions are assumed to be uniformly distributed over the emitting
area and constant over time. The third-order inflection points in the curves are an artifact of
the model. Ideally, the curves should follow a smooth decay from peak values.
If a given area source is larger than the example scenario, the curve in Figure 4 may
still be used. The dispersion factor, in /ig/Wper g/sec, obtained from Figure 3 or 4 can be
substituted into Equation 5 to estimate the maximum hourly ambient concentration and into
Equation 6 to estimate the annual average ambient air concentration for a given downwind
distance. ' .
11
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Table 2.
Example Scenarios for Bioventing Systems
Parameter
Gas Volume"
Stack Height
Stack Diameter
Exit Gas Velocity1*
Exit Gas Temperature0
Units
mVmin
cfm
m
m
m/sec
°C
Typical System
5.7
200
4.6
0.1
12.1
25
Large Stack1 System
5.7
200
4.6
0.5
0.5
25
'Refers to a system with a larger stack diameter, i.e., 18 inches (0.5 m) compared to 4
inches (0.1 m).
b Assumes dry standard conditions (20°C, 1 atm).
c Assumes either no off-gas treatment or treatment with activated carbon; if an afterburner is
used to treat the off-gas, the temperature will be much higher.
Table 3.
Example Scenario for an Area Source Bioventing System
Parameter
Treatment Area:
Dimensions
Area
Release Height
Units
m
m2
m
Scenario
30x30
900
0
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Cm = (ER)(F) (Eq. 5)
C. = (ER)(F)(0.08) (Eq. 6)
where Cm = Maximum hourly ambient air concentration
C. = Annual average ambient air concentration (/tg/m3);
ER = Emission rate (g/sec); and
F = Dispersion factor from Figures 3 or 4 0*g/m3 / g/sec).
Since TSCREEN provides maximum short-term estimates, the factor of 0.08 in Equation 6 is
used to convert the short-term estimate to an annual average estimate. The 0.08 factor in
Equation 6 was developed by the U.S. EPA for point sources. This factor has recently been
revised and it is still under review by EPA and may be subject to further change. There is
no EPA policy for converting from hourly to annual estimates for area sources. Various
factors have been published that range up to 0.0812. Factors up to 0.20 have been suggested
for certain specific locations.13 Graphical estimation tools in lieu of these factors also have
been proposed.14'13 It is assumed that this point source factor can be applied to area sources
without greatly increasing the overall uncertainty of the air impact estimate. As previously
stated, the recommended approach is to use ISCLT to estimate long-term concentrations.
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 pg/m3 of concentration in the ambient air. They are
available on EPA's Integrated Risk Information System (IRIS), the U.S. EPA's preferred
source of toxicity information. User Support can be contacted at (513) 569-7254. The next
best source of inhalation unit risk factors is EPA's Health Effects Assessment Summary
Tables (HEAST) which are updated annually.16 Inhalation unit risk factors listed in IRIS as
of January 1993 or in HEAST (FY 1992) are given in Table 4 for 168 volatile and semi-
volatile organic compounds. This is an updated version of the same table contained in
References 2 and 3. Similar information is given in Table 5 for selected semi-volatile
organic compounds.
15
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Equation 7 can be used to estimate the cancer risk at a specified distance downwind of
the bioventing system. Cancer risk is a measure of the increased probability of developing
cancer in a lifetime as a result of the exposure in question. Equation 7 assumes continuous
exposure (24 hours/day, 365 days/year for 70 years) to the estimated annual average
concentration in air.
R = (CJ(IUR) (Eq. 7)
where R = Cancer risk from long-term exposure to a specific compound in air
(dimensionless);
C, = Annual average ambient concentration, from Equation 6 (/xg/m3); and
IUR = Inhalation unit risk factor, from Table 4 or 5 (/xg/m3)'1.
If the source operates for less than 70 years, multiply C, by x/70, where x is the
expected operating time of the source in years before using Equation 7. If more than one
contaminant is present, the cancer risks for each contaminant 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 an estimate (with uncertainty spanning perhaps
an order of magnitude) of continuous exposure of the human population to contaminants in
the air that is likely to be without appreciable risk of deleterious effects during a lifetime.
During the past three years, the EPA has become increasingly active in the development of
chemical specific RfCs.
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 nWday (average adult inhalation rate), and finally multiplied by
1000 /ig/mg to derive a value in ng/m3. This methodology was selected as the best available
approach for this screening document. The EPA, however, does not condone derivation of
RfCs from data for other routes of exposure on a chemical specific basis.
25
-------
The EPA considers the minimum basis for the derivation of an RfC to be a properly
conducted inhalation study that examines portal of entry effects. Portal of entry effects are
particularly important with respect to inhalation. There are many known cases where
respiratory effects due to inhalation exposure are much more severe than effects resulting
from equivalent oral doses. Therefore, ambient air action levels based on extrapolated oral
data should be used cautiously, since there may be significant differences between the oral
and inhalation exposure pathways.
For compounds lacking RfC or RfD values, action levels were based on occupational
exposure levels recommended by the Occupational Safety and Health Administration .
(OSHA)17 and the American Conference of Governmental Industrial Hygienists (ACGIH).18
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. It should be noted that occupational exposure levels are not intended for
evaluation or control of community air pollution or in estimating the toxic potential of
continuous, uninterrupted exposures. Therefore, ambient air action levels based on
occupational exposure limits are not precise distinctions between safe and dangerous ambient
air concentrations, nor are they necessarily indices of toxicity.
Long-term ambient air action level concentrations for non-carcinogens based on RfCs,
extrapolated RfDs and occupational exposure levels are also listed in Tables 4 and 5. The
action levels are in units of /ig/m3 to facilitate comparison to the ambient air concentrations
estimated from Equation 6.
Short-Term Exposure
The short-term (one hour) action levels, in /ig/m3, are presented in the last column of
Tables 4 and 5. 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
26
-------
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 Tables 4 and 5 (or to derive values for compounds
not listed in the tables), 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 Tables 4 and 5 can be compared directly with
the estimated maximum hourly ambient air concentrations obtained by using Equation 5 and
Figures 3 and 4. 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 a bioventing system and compare these values to the
action level concentrations listed in Tables 4 and 5.
Step 1 First, collect all necessary information. For this example, assume a
site that has approximately 10,000 m3 of soil contaminated with
benzene and benzo(a)pyrene at concentrations in the soil at 100 /*g/g
and 10.0 /*g/g, respectively. A bioventing system will be used to treat
the silty soil. The bulk density of the soil at the site averages about 1.5
g/cm3. The exhaust gas flowrate is unknown. The stack diameter is
assumed to be around 0.1 m. For this initial screening evaluation, it is
assumed that no air pollution controls are used.
Pilot-scale testing showed soil gas concentrations of benzene and
benzo(a)pyrene at 100,000 Mg/m3 and 10 /tg/m3, respectively. The only
source of emission is assumed to be the stack (point source). The
bioventing system is expected to be in continual operation for 6 months
27
-------
Step 2
(1.58 x 107 seconds). The nearest off-site downwind receptor is 400
meters away.
Estimate the maximum average emission rate for the six-month
operation period. Using Equation 1, the average long-term emission
rate of benzene would be:
•vg
(1.58 x 107)
= 9 49 x 10.2
Step 3
The average long-term emission rate for benzo(a)pyrene is 9.49xlO'3
g/sec. These rates assume that 100% of the contaminant is lost to the
atmosphere. This is obviously an overly conservative assumption for
the VOCs if control devices are used or if the VOCs are biodegraded
and for semi-volatile organic compounds.
Estimate the exhaust gas flow rate for the bioventing system. The air-
filled porosity is assumed to be 0.31 from Table 1 for silty soils. The
gas flow rate can be estimated using Equation 3:
(-M-) (10,000) (0.31) = 2.2 m3/min
^ 1440;
Step 4 Estimate the VOC emission rate of each compound from the bioventing
system. The appropriate data are inserted into Equation 2. For
benzene, the emission rate would be:
(100,000) (—1(10-') = 3.67 x lO'3 g/:
sec
StepS
The VOC emission rate for benzo(a)pyrene is 3.67xlO"7 g/sec.
Compare the estimated emission rates from Step 4 to those from Step
2. The comparison is:
Compound
Benzene
Benzo(a)pyrene
Equation 1 Emission Rate
(g/sec)
9.49xlO'2
9.49xlO'3
Equation 2 Emission
Rate (g/sec)
3.67xlO-3
3.67xlO-7
28
-------
The benzene emission rate estimated using Equation 2 is nearly the
same as the benzene emission rate (in g/sec) estimated using Equation 1
based on the total mass of benzene present at the site. This is expected
since most of the VOCs may be volatilized to the atmosphere. The
estimated emission rate from Equation 2 for benzo(a)pyrene is much
lower than the maximum long-term emission rate because
benzo(a)pyrene is not very volatile at ambient temperatures.
Step 6 Estimate the downwind ambient air concentrations. From Figure 3, the
maximum hourly ambient air concentration at a distance of 400 meters
for a bioventing system is approximately 1420 pg/m3 per g/sec
emission rate. This corresponds to an annual average dispersion factor
of 114 ^g/m3 per g/s (1420 x 0.08 = 114). Using Equation 5, the
hourly average ambient air concentration for benzene would be:
Cm = (3.67xlO-3)(1420) = 5.2 /ig/m3
Using Equation 6, the annual average air ambient concentration for
benzene would be:
C. = (5.2)(0.08) = 0.42 /zg/m3
The ambient air concentrations estimated from Equations 5 and 6 are
presented in Table 6.
Step 7 Compare the downwind concentrations to the action level ambient air
concentrations. The short-term and long-term action levels from Tables
4 and 5 for the compounds of interest are presented in Table 7. Of the
estimated maximum hourly ambient concentrations, the benzene
concentration is above the action level. The action level is based on a
70-year risk. Since, this system will be operated for much less than 70
years, the action level (AL) can be adujusted by the following:
AL.
'adjoted
(AL)
70
Length of
operation
in years /
(0.12)
J70
0.5
16.8 jig/m3
Step
The benzene concentration is not above the adjusted action level.
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 not necessary.
29
-------
Table 6.
Estimated Emission Rates and Ambient Air Concentrations
Benzene
Benzo(a)pyrene
Soil Concentration
For Example
Problem G*g/g)
100
10.0
Emission Rate
(g/s)
3.67 x 10-3
3.67 x 1C'7
Ambient Concentrations Gtg/m3)
Maximum 1 Hourly
5.2
5.2 x 10A
Annual Average
0.42
4.2 x 10-5
Table 7.
Action Level Concentrations
Benzene
Benzo(a)pyrene
Action Levels ng/m3
Short-Term
320
2.0
Long-Term
0.121
5.9 x 1O4 '
'Based on 10^, 70-year risk.
30
-------
The air impacts should be re-examined, however, when the site-specific
data become available, if the inputs differ from the default values.
Also, it would be advisable to perform an ambient air monitoring
program during remediation to document the actual worker and
community exposures.
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. For example, emission models assume typical operating conditions, dispersion models
assume Gaussian distribution of the plume, and many of the health levels are not endorsed by
the Environmental Protection Agency. EPA's Regional Toxicologist should be contacted for
general lexicological information and technical guidance on evaluation of chemicals without
established toxicity values.
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. Eklund, B., S. Smith, and A. Hendler. Estimation of Air Impacts for the
Excavation of Contaminated Soil. EPA-450/1-92-004. March 1992.
4. Miller, R., C. Vogel, and R. Hinchee. A Field-Scale Investigation of
Petroleum Hydrocarbon Biodegradation in the Vadose Zone Enhanced by Soil
Venting at Tyndall AFB, Florida. Presented at the AWMA and HWAC
seminar entitled "Bioventing and Vapor Extraction: Uses and Applications in
Remedation Operations." April 15, 1992.
5. Hinchee, R. and R. Miller. Bioventing for In Situ Remediation of Petroleum
Hydrocarbons. Presented at the AWMA and HWAC seminar entitled
"Bioventing and Vapor Extraction: Uses and Applications in Remedation
Operations." April 15, 1992.
31
-------
6. Dupont, R., W. Doucette, and R. Hinchee. Assessment of In Situ
Bioremediation Potential and the Application of Bioventing at a Fuel-
Contaminated Site. Presented at the AWMA and HWAC seminar entitled
"Bioventing and Vapor Extraction: Uses and Applications in-Remedation
Operations." April 15, 1992.
7. Dupont, R. Application of Bioremediation Fundamentals to the Design and
Evaluation of In-Situ Soil Bioventing Systems. Presented at the AWMA 85th
Annual Meeting and Exhibition. Kansas City, Missouri. June 21-26, 1992.
8.- Hinchee, R. and S. Ong. A Rapid In Situ Respiration Test for Measuring
Aerobic Biodegradation Rates of Hydrocarbons in Soil. Journal of the Air &
Waste Management Association. 42 (10). October 1992.
9. Johnson, et al. A Practical Approach to the Design, Operation, and
Monitoring of In-Situ Soil-Venting Systems. Ground Water Monitoring
Review, pp 159-178. Spring 1990.
10. Eklund, B., et al. Control of Air Emissions from Superfund Sites.
EPA/625/R-92-012. U.S. EPA Center for Environmental Research
Information, Cincinnati, Ohio. November 1992.
11. U.S. EPA. A Workbook of Screening Techniques for Assessing Impacts of
Toxic Air Pollutants. EPA-450/4-88-009. September 1988.
12. Huey, N.A. and G.J. Schewe. Empirical Factor Emission of Air Toxic
Source Impacts. In: Proceedings of HMC/Superfund '92. Published by
Hazardous Materials Control Resources Institute, Greenbelt, MA. 1992.
13. Huey, N.A. (U.S. EPA, Region VII). Personal communication to Bart Eklund
(Radian Corporation). April 1993.
14. Guinnup, D.E. A Tiered Modeling Approach for Assessing the Risks Due to
Sources of Hazardous Air Pollutants. EPA-450/4-92-001. March 1992.
15. Cowherd, C., P. Englehart, G. Muleski, and J. Kinsey. Hazardous Waste
TSDF Fugitive Paniculate Matter Air Emissions Guidance Document. EPA-
450/3-89-019. May 1989.
16. Health Effects Assessment Summary Tables (HEAST). U.S. Environmental
Protection Agency, Wash. D.C., 1990, OERR 9200.6-303(92-1), NTIS No.
PB91-92199, March 1992.
17. 29 CFR ch. XVYI. Subpart Z. Section 1910.1000. July 1, 1990.
18. 1992-1993 Threshold Limit Values for Chemical Substances and Physical
Agents and Biological Indices. American Conference of Governmental
Industrial Hygienists, Cincinnati, Ohio, 1992.
32
-------
APPENDIX A
PHYSICAL PROPERTY DATA
FOR SELECTED ORGANIC COMPOUNDS
(For compounds in Table 4 of the report)
Source: Eklund, B. and C. Albert. Models for Estimating Air Emission Rates from
Superfund Remedial Actions. EPA-451/R-93-001. March 1993.
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PHYSICAL PROPERTY DATA
FOR SELECTED SEMI-VOLATILE ORGANIC COMPOUNDS
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