United States Office of Air Quality EPA - 450/4-90-014
Environmental Protection Planning and Standards July 1990
Agency Research Triangle Park NC 27711
Air/Superfund
«rEPA AIR / SUPERFUND
NATIONAL TECHNICAL
GUIDANCE STUDY SERIES
Development of Example Procedures
for Evaluating the Air Impacts of Soil
Excavation Associated With Superfund
Remedial Actions
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DEVELOPMENT OF EXAMPLE PROCEDURES FOR EVALUATING
THE AIR IMPACTS OF SOIL EXCAVATION ASSOCIATED
WITH SUPERFUND REMEDIAL ACTIONS
by
PEI Associates, Inc.
11499 Chester Road
Cincinnati, Ohio 45246
Contract No. 68-02-4394
Work Assignment No. 38
PN 3759-38
James Durham, Technical Representative
U.S. ENVIRONMENTAL PROTECTION AGENCY
OFFICE OF AIR QUALITY PLANNING AND STANDARDS
RESEARCH TRIANGLE PARK, NORTH CAROLINA 27711
July 1990
U.S. Environments! Pm'-ecucr.
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DISCLAIMER
This report was prepared for the U.S. Environmental Protection Agency by
PEI Associates, Inc., Cincinnati, Ohio, under contract No. 68-02-4394, Work
Assignment No. 38. The contents are reproduced herein as received from the
contractor. The opinions, findings, and conclusions expressed are those of
the author and not necessarily those of the U.S. Environmental Protection
Agency.
ii
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CONTENTS
Figures iv
Tables y
Acknowledgment vii
1. Introduction 1
2. General Approach and Procedures 3
Site characterization 3
Selection of remedial alternative 4
Estimation of emission rates 4
Dispersion modeling 5
Risk assessment 6
3. Site Descriptions and Remedial Approaches 8
Site A 8
Site B 12
4. Emission Rate Calculations 15
General approach for emission rate calculations 15
Emission rate results for Site A 19
Emission rate results for Site B 30
Summary of emission estimates 40
5. Ambient Dispersion Modeling 43
General approach to dispersion modeling 43
Dispersion modeling for Site A 46
Dispersion modeling for Site B 51
Summary of dispersion modeling results 57
6. Risk Assessment 59
General approach to risk assessment 59
Risk assessment for Site A 61
Risk assessment for Site B 62
Summary of risk assessment results 63
7. Conclusions and Recommendations 64
References 70
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FIGURES
Number Pace
1 Site A configuration 9
2 Summary of emission points from soil handling operations. . . 11
3 Site B configuration 13
4 Daily emission profile for Site A 29
5 Emissions profile for total VOCs from Zones 1 and 2
remediation—Site B 40
6 Relative location of emission sources and fence!ine receptor
for Site A 48
7 Site B source configuration and receptor locations 53
iv
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TABLES
Number Page
1 Soil Contaminant Concentrations for Site A 10
2 Soil Contaminant Concentrations at Site B 12
3 Representative Soil Porosity 17
4 Properties of Chemicals Used in Site A Example Problems ... 20
5 Emissions From Soil Cap Removal--Site A 20
6 Emissions From Soil Cap in Bucket—Site A 21
7 Emissions From Truck Filling (Soil Cap)--Site A 21
8 Emissions From Transport of Soil Cap—Site A 22
9 Emissions From the Exposure of the Contaminated Waste
Layer-Site A 23
10 Emissions From Excavation of Contaminated Soil Layer—Site A. 23
11 Emissions From Excavation Bucket—Site A 24
12 Emissions From Truck Filling With Contaminated Soil
Layer—Site A 24
13 Emissions From Transport of Soil From the Contaminated
Waste Layer—Site A 25
14 Species Contributions to Overall VOC Emissions—Site A. ... 26
15 Remedial Activity Contribution to VOC Emissions Fractional
Contribution of Activity—Site A 27
16 Fraction Contributions of VOCs to Overall Emission
Rates—Site A 28
17 Emissions From Soil Cap Removal--Site B 31
18 Emissions From Excavation Bucket (Soil Cap)—Site B 31
19 Emissions From Truck Filling (Soil Cap)—Site B 32
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TABLES (Continued)
Number Page
20 Emissions From Transport of Soil Cap Material--Site B . . . . 33
21 Emissions From Dumping and Temporary Storage of Soil Cap at
the Incinerator Feed System—Site B 33
22 Emissions From the Exposed Contaminated Soil Layer—Site B. . 34
23 Emissions From Excavation of the Contaminated Soil Layer—
Site B 35
24 Emissions From Contaminated Soil Layer White in Excavation
Bucket-Site B 36
25 Emissions From Truck Filling With the Contaminated
Soil Layer-Site B 36
26 Emissions From Transport of the Contaminated Soil Layer--
Site B 37
27 Emissions From Dumping of Contaminated Soil Layers—Site B. . 37
28 Species Contributions to Overall VOC Emissions—Site B. . . . 38
29 Remedial Step Contribution to VOC—Site B 39
30 Fenceline VOC Concentrations From ISCLT Model Run for Site A. 49
31 Ambient Concentrations for Chemical Species at Specified
Receptors 51
32 Annual Ambient VOC Concentrations at Fenceline for Site B . . 55
33 Ambient Concentration of Each Chemical at Fenceline
Receptors—Site B 55
34 Peak 1-Hour Concentrations for Each Compound 57
35 Remediation Emission Factor Multipliers for Site A 66
vi
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ACKNOWLEDGMENT
This report was prepared for the U.S. Environmental Protection Agency,
Office of Air Quality Planning and Standards, by PEI Associates, Inc.,
Cincinnati, Ohio. The project was directed by Mr. David Dunbar. The project
was managed by Mr. Gary Saunders, who was also the principal author. The
author would like to acknowledge Mr. James Durham, who served as the
Environmental Protection Agency's Technical Representative, for his overall
guidance and direction.
vii
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SECTION 1
INTRODUCTION
Numerous remedial technologies can be used to clean up Superfund sites.
Many of them are identified in the Cost Of Remedial Alternatives (CORA) model
developed for the Superfund program. Obviously not all of the approximately
40 different remedial alternatives are applicable to every site. Also all
remediation technologies do not have an air pathway impact. The ambient air
impacts of each applicable remedial alternative for a Superfund site should
be considered in the evaluation performed during the Feasibility Study (FS)
phase. Estimating the effects from a remedial alternative requires the
development and use of some relatively simplified computational techniques.
The purpose of this project was to identify and define the computation
requirements for estimating the air impacts from the remediation of Superfund
sites. Two example sites employing soil excavation were selected because
they represent a complex emission source. The estimation of air impacts from
these sites include factors such as source type (point, area, or volume),
location, and movement of the sources. These factors are more complex for
soil excavation when compared to air stripper or in situ soil vapor
extraction. For example, one very important assumption in soil excavation is
the degree of homogeneity in the soil contamination. It was assumed that
large volumes contained homogeneous soil and contamination characteristics.
In actual Superfund site applications, the site characteristics may be quite
complex and necessitate a subdivision of the site into numerous smaller
"homogeneous" volumes. Calculation procedures for the individual homogeneous
volumes may then be performed much as they would be for a less complex site.
The procedures for the evaluation of the ambient impacts were divided
into several subtasks. These included site characterization, selection of
remedial alternatives, definition of remedial activities, estimation of
emission rates for each remedial activity, determination of ambient
concentrations from dispersion modeling, and evaluation of carcinogenic and
noncarcinogenic risks based on dispersion modeling results. Existing
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mathematical models were to be used to the extent possible, and key input
data were to be identified. All assumptions necessary to complete the
calculations were identified and documented.
The calculation of emission rates were used to estimate ambient impacts
through dispersion models. The ambient concentration at various receptors of
interest were compared to health based risk data and used to estimate an
increased risk value at these receptors. Carcinogenic and noncarcinogenic
effects were considered. The purpose of this effort, however, was not to
produce a risk assessment at each site. Rather, it was to outline a set of
procedures that could be used, with existing tools, to assist in the
evaluation of air-pathway effects. Simplification procedures were to be
identified for the calculations that might lend themselves to simpler
calculations or perhaps graphical procedures. In addition, a computerization
procedure was to be explored that might assist in the air pathway analysis.
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SECTION 2
GENERAL APPROACH AND PROCEDURES
Although each Superfund site has unique characteristics, a general
approach was defined that was applicable to a wide variety of sites and yet
allowed the flexibility to accommodate a limited number of site-specific
factors. These procedures are outlined below.
2.1 SITE CHARACTERIZATION
Both example sites were loosely based on actual Superfund sites in terms
of size and volume of the contaminated area. In each case, the actual size
and depth of contamination were used. The contaminants considered in these
examples, however, differed from those actually present at the sites. For
example, although these sites also include both semi volatile and nonvolatile
contaminates for the purpose of this exercise, only volatile organic
compounds (VOCs) contaminants present were considered for emission
calculations. In actual applications, the nonvolatile and semivolatile
compounds (particulates) may have to be considered.
Five VOCs were selected for emission rate calculations: benzene,
1-butanol, dichloromethane (methylene chloride), methyl ethyl ketone, and
o-xylene. These compounds were selected because they represent a wide range
of vapor pressures, solubilities, molecular weights, and Henry's Law
constants for VOCs. These physical characteristics of VOCs are important to
the emission rate calculation as they are the concentrations in the soil.
The concentrations of the various VOCs represented ranges of several
hundred ppm up to 27,500 ppm, which might "typically" be encountered at
Superfund sites. The contaminant levels also were assumed to be homogeneous
within a specified contaminant zone. Site A represents a simple site with a
single homogeneous contaminated zone. Site B represents two separate and
different homogeneous zones. At an actual Superfund site, the contamination
could be very heterogeneous. At an actual site, however, the contaminated
zone can be subdivided into discrete homogeneous zones for the
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purpose of calculation. Large and more complex sites could follow this
subdivision process for calculation purposes.
Other factors, such as soil characteristics and the presence of a soil
"cap" should be considered in the site characterization. These factors
affect the volatilization rates for the various compounds, as the effective
soil porosity controls the diffusion rates of the various compounds. These
other factors" or site characteristics were assumed for the example sites.
n
2.2 SELECTION OF REMEDIAL ALTERNATIVE
Excavation was selected as an option to be examined for each site for
several reasons. First, emission rate models have been developed and have
limited field validation data. Second, excavation is often considered at
both remedial and removal sites. Finally, excavation activities represent a
relatively complex series of activities that produce both area and volume
sources for ambient dispersion modeling purposes. The procedures outlined in
this report focus on the ambient impact associated with soil excavation.
Similar procedures could be used for other remedial options.
Following the selection of the remedial alternatives, a set of
remediation steps was outlined. The excavation alternative included
excavating of the soil cap, dumping the soil into a truck, excavating the
highly contaminated soil zone, dumping it into a truck, transporting the
contaminated soil, dumping the contaminated soil, and providing temporary
storage of the soil prior to its ultimate disposal. With the site
characteristics defined and remedial alternatives selected, the next step was
to estimate emissions from each operation for each chemical.
2.3 ESTIMATION OF EMISSION RATES
Emission rates of each of the five compounds were estimated for the
remedial activities. The emission calculations for excavation were based on
the use of the RTI Landtreatment equations modified to accommodate the
various activities associated with excavation. The ratio of each chemical to
the total VOC emissions was examined for each excavation activity, as well as
its contribution to overall VOC emissions. These emission rates served as an
input into the dispersion modeling analysis for determining ambient
concentration estimates.
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2.4 DISPERSION MODELING
The locations of remedial activities and their emission characteristics
were input into the selected computer-based dispersion models to estimate
both long-term and short-term ambient concentrations at numerous offsite
receptor locations. Industrial Source Complex (ISC) models were selected
initially to determine long-term (ISCLT) and short-term (ISCST) ambient
concentrations. The meteorological conditions selected for the analysis were
those from the Raleigh-Durham, North Carolina, airport.
The emission rates for the ISCST model were estimated for each hour of
the day. Emissions from excavation and truck filling were combined to form
one area "source." Emissions from hauling trucks were considered a line
source, whereas the dumped contaminated soil was considered another area
source. Air strippers, incinerators, and in situ soil vapor extraction units
would be considered point sources, whereas lagoons or surface impoundments
would be considered area sources. The principal advantage of the ISCST model
is the large number and types of sources that can be evaluated simultaneously
to estimate an overall ambient concentration over a variety of time periods
(i.e., 1, 3, 8, and 24 hour averages). The highest 100 predicted ambient
concentrations and their locations were listed for each averaging period.
The disadvantage of the ISCST model is its long run-time requirements for
execution.
The SCREEN model can be used to estimate 1-hour ambient contributions
from individual sources. This model is limited to a single-source evaluation
(point, area, or flare), can accept the input of atmospheric stability class
and windspeed, and requires very short run times. Its principal
disadvantages are that it cannot consider multiple sources, actual
meteorological data, and averaging periods other than 1 hour. Because of
these disadvantages the results must be manually converted to other averaging
times and the contributions from multiple sources must be added.
Long-term concentrations were computed with the ISCLT model. Emission
rates from each source were converted from their hourly values over the
remediation period to an annual average. The ISCLT model permits
simultaneous computation of contributions from multiple sources to the
ambient concentration at specified receptors. Meteorology from the site may
be used to compute annual averages. In the example site calculations for
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long-term averages, meteorological data from the Raleigh-Durham, North
Carolina, airport were used. The computational requirements for the
execution of the ISCLT model required relatively short run times. Ambient
2
concentrations for all models were computed in ^g/m .
2.5 RISK ASSESSMENT
The dispersion model results for the site remediation activities can be
used to compute risks associated with the alternatives selected. Both
short-term and long-term ambient concentrations of VOCs were used in the
evaluation of the risks. Short-term values are useful in evaluating acute
effects, whereas the long-term values are most useful in evaluating chronic
effects. Both carcinogenic and noncarcinogenic effects were considered.
Potency factors for carcinogens were available for two compounds--
benzene and methylene chloride. Of the remaining compounds, only 1-butanol
does not have health benchmark levels available. Both methyl ethyl ketone
and o-xylene are listed in the Health Effects Assessment Summary Tables
(HEAST) as noncarcinogenic toxicants. These values may be used to compute
the values for risks associated with ambient concentrations and to establish
ambient levels of concern for different levels of risk. Both procedures were
used to determine if any unacceptable risks were associated with the selected
remedial alternatives.
The carcinogenicity potency factors are extrapolated to 70-year
exposures, whereas the remedial alternatives at the example sites represent
relatively short-term exposures (2 to 6 months). Current Superfund guidance
calls for prorating the actual duration of exposure to a 70-year exposure. A
similar procedure that may be used is to prorate the 70-year exposure value
down to the period of remediation. The results obtained are identical and it
was this second procedure, that was used to compare the actual ambient
concentrations with health related values. In these examples the 70-year
values were prorated to 1 year and compared with annual ambient
concentrations from the ISCLT model (which used emission rates prorated to an
annual rate).
The noncarcinogenic hazard index is based on a long-term exposure. For
the two noncarcinogens, the annualized emissions and corresponding ambient
concentrations were compared with the hazard indices. Carcinogens may also
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have noncarcinogenic effects, but no hazard Index data were available for the
two carcinogens.
Much less guidance is available on the evaluation of short-term
exposures. Certainly, threshold limit values (TLVs) and permissible exposure
levels (PELs) can be used as guidance and are often used for worker
protection. The procedure for translating these values to the general public
that may be exposed to high concentrations for short periods of time is not a
well-defined, however. Work is currently underway within the Superfund
program to provide guidance on evaluating short-term exposures.
Based on the risk and health effects estimates, the remedial alternative
under consideration can be assessed and compared with other remedial
alternatives for their risk values in addition to their costs.
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SECTION 3
SITE DESCRIPTIONS AND REMEDIAL APPROACHES
This section presents descriptions of two sites that represent small to
moderate-sized contaminated soil volumes for which excavation may be
appropriate. Site A represents the least complex scenario and Site B
represents a somewhat more complex arrangement. The proposed approach for
large complex sites, however, would be similar to that used for smaller, less
complex sites.
3.1 SITE A
Site A is a 10-acre site on which the contaminated soil zone is located
near the center of the site. Soil in this area was contaminated by leakage
from drums stored above ground. The contaminated zone is an area
approximately 91 meters (300 ft) by 23 meters (75 ft), where the
contamination extended to approximately 2 meters (6.8 ft) below the surface.
This zone is considered homogeneous in soil type and contaminant
concentration. Figure 1 presents the site configuration.
The initial remedial steps included removing all the stored drums and
stopping the addition of more contamination to the site. A clay soil cap
approximately 0.5 meter deep was placed on top of the contaminated soil to
minimize rainwater penetration and infiltration through the soil.
As shown in Table 1, the contaminated soil contains five VOCs. The VOC
found in the highest concentration is methylene chloride, which represents
over 54 percent of the VOC contamination. The VOC found in the lowest
concentration is 1-butanol.
The remedial alternative selected for Site A was soil excavation. A
total of 3823 cubic meters (5000 yd ) of material would be removed. The soil
excavation would occur on an 8 hour/day, 6 day/week schedule. The average
removal rate would be 885 cm3/s (24-hour average) during the 2 month
remediation period. The excavated soil would be transported by truck to an
offsite Resource Conservation and Recovery Act (RCRA) site approximately 10
miles away, where it would be thermally desorbed of the VOCs. The "cleaned"
8
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201.2 m
Contaminated
Soil Area
N
t
Contaminated Zone Dimensions: 91.4 m x 22.9 m
Haul Road Length: 100.6 m
Figure 1. Site A configuration.
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TABLE 1. SOIL CONTAMINANT CONCENTRATIONS FOR SITE A
Compound
Benzene
1-Butanol
Methyl ene chloride
Methyl ethyl ketone
o-xylene
Total
Soil
concentration,
mg/kg
2,700
1,440
27,500
6,880
12.000
50,770
Soil
concentration,
g/cm3
0.0049
0.0026
0.0495
0.0124
0.0216
Fractional
percentage of
VOC in soil
0.0542
0.0284
0.5417
0.1355
0.2403
soil would be returned to the site to fill in the excavated area and to
restore the site to a "clean" condition.
The excavation procedure proposed for removing the soil is a vertical
column removal procedure. In this procedure, a backhoe is used to dig up the
soil and place it in a truck for transport. The soil cap is removed first to
expose the contaminated soil below the cap. As the soil is dumped into the
truck, VOC emissions are generated from the soil gases incorporated in the
soil cap. The exposure of the contaminated soil layer and the subsequent
exposure of each new area as a bucketful of material is removed from above
also generates VOC emissions. The movement, dumping, and truck transport of
the contaminated soil layer also generates emissions. Once remediation has
begun, constant VOC emissions from the exposed contaminated soil zone
contribute to both short- and long-term emissions. Figure 2 shows the
procedures involved and the emission points.
Based on its characteristics, the soil was treated as being in two
distinct zones. The soil cap was treated as a relatively clean,
low-moisture, and moderately compacted soil. The contaminated soil was
treated as a low-moisture compacted subsoil. These definitions and
assumptions help to define diffusion rates for the various VOCs through the
soil as the soil is excavated. The soil cap is assumed to contain a
concentration of VOC that is 1 percent of the value for the contaminated soil
layer. The contaminated soil layer is assumed to contain the initial
concentrations specified in Table 1.
10
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3.2 SITE B
Site B is a 15-acre site (approximately 60,000 m ) with two separate
contaminated zones. Each zone has a distinct chemical composition, but each
is considered homogeneous within itself. The site plot plan in Figure 3
indicates the approximate location of each contaminated zone.
Zone 1 of Site B encompasses an area of approximately 930 m (10,000
n
ft ) in which contamination reaches an average depth of 4.11 m (13.5 ft).
The contaminated soil volume totals approximately 3823 m3 (5,000 yd ), and
the contaminants are 1-butanol, methyl ethyl ketone, and benzene.
Zone 2, the larger of the two zones, encompasses an area and includes a
volume of contaminated soil twice as large as those in Zone 1. The
2 2
approximate area is 1860 m (20,000 ft ), and contamination reaches an
average depth of 4.11 m (13.5 ft). The total contaminated soil volume is
o o
approximately 7646 m (10,000 yd ). This zone is contaminated with methyl
ethyl ketone, methylene chloride, and o-xylene. The contaminant level within
Zone 2 is considered homogeneous. Although both zones contain one identical
compound (methyl ethyl ketone), each zone was considered separately because
of the contamination level. Ground water has been contaminated by the VOCs
in the soil and will require remediation. Table 2 presents the
concentrations of the VOCs within each zone.
TABLE 2. SOIL CONTAMINANT CONCENTRATIONS AT SITE B
Location/compound
Soil
concentration,
mg/kg
Soil
concentration,
g/cm3
Fractional
percentage of
VOC in zone
Zone 1
Benzene
1-Butanol
Methyl ethyl ketone
Zone 2
Methylene chloride
Methyl ethyl ketone
o-Xylene
1770
1885
3175
8330
2930
6440
0.0032
0.0034
0.0057
0.0150
0.0053
0.0116
0.2760
0.4649
0.2592
0.4693
0.1651
0.3628
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Contaminated Zone 2
Site Center
Contaminated Zone
245 m
Contaminated Area Dimensions
Zone 1: 91.4 m x 22.9 m
Zone 2: 91.4 m x 45.7 m
Figure 3. Site B configuration.
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The remedial alternative investigated for this site includes excavation
3 3
and onsite incineration of all 11,470 m (15,000 ft ) of contaminated soil.
The two contaminated zones will be remediated sequentially, not
simultaneously starting with Zone 1. The total time for soil remediation is
estimated to be approximately 6 months--2 months for Zone 1 and 4 months for
Zone 2. The soil excavation procedures for each zone would be similar; a
vertical column method with a backhoe will be used for removal. The
excavation schedule would call for excavation 8 hours/day, 6 days/week. The
average soil removal rate would be 885 cm3/s (24-hour average).
A relatively uncontaminated layer of soil is assumed to be on top of the
contaminated zone. The concentration of the three VOCs are assumed to be in
the same proportions, but at only 1 percent of the level of the
concentrations in the contaminated zone. This soil cover depth extends to
approximately 0.5 meter below the surface. The soil in the contaminated zone
extends below this level.
The soil cover is to be removed by backhoe excavation, loaded into
trucks, and hauled to the incinerator feed bin. The incinerator systems and
buildings are designed to capture VOC emissions from the bin after the
material is dumped and to feed to the primary and secondary air systems for
destruction in the incinerator. Little onsite storage capability will be
provided for unprocessed soil beyond what the feed bins can hold (24 hours of
feed) so as to minimize VOC emissions and additional spreading of
contaminated soil. The treated soil will be stored on site until it is ready
to be used to refill the excavated soil area. The incinerator is designed to
remove and destroy 99.99 percent of the benzene from the soil. Similar
destruction removal efficiency is expected for the other VOCs. Only VOC
emissions from the soil handling around the incineration were considered in
the example calculations. Additional impacts would have to be considered for
particulate and acid gases associated with incineration.
Emissions to be considered for Site B are those associated with
excavation, transport, and dumping of contaminated soil and incinerator
emissions. Emissions from the boiler stack of the high-temperature air
stripper were not included in this example site analysis.
14
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SECTION 4
EMISSION RATE CALCULATIONS
4.1 GENERAL APPROACH FOR EMISSION RATE CALCULATIONS
Numerous mathematical models are available for describing emission rates
from an undisturbed Superfund site and from some remedial processes. Few
models, however, have been designed with remedial activities in mind,
especially when soil excavation is involved. Most mathematical models and
procedures are adaptations of existing models for undisturbed soil. This
also applies to the calculation of emissions resulting from excavation work
at the two example sites.
One such approach was to modify the RTI Landtreatment Model to account
for emissions generated by soil excavation. The RTI Landtreatment Model and
the Thibodeaux-Hwang Models, which are very similar, predict a time variant
emission rate that is a function of the diffusion rate of the VOC through the
air-filled pore spaces in the soil. The time-dependent equation was
considered applicable to soil excavation. Radian conducted a survey of
Superfund sites and proposed several sites that would typify soil excavation
2
activities. Typical equipment sizes, operating cycles, and removal rates
were also defined so the equations would be as applicable to the excavation
activities as possible. These values and modifications are discussed in
Reference 2. The results of the equations produce an emission rate of an
organic species per unit volume of soil exposed and handled.
Several basic equations are used in the calculation of emissions
associated with excavation activities. For the most part, the equations for
each excavation step follow a similar format, but they must be performed
sequentially because each succeeding step accounts for VOC losses that have
occurred in all previous steps. Detailed computations are not presented
here, but the equation formats are presented for illustration purposes.
Once the general soil removal procedure has been outlined, the equations
may be applied sequentially to estimate emissions from each soil removal step
by inputting the appropriate values. (These equations are currently under
15
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review and may be modified). The same values for removal rate, equipment
cycle times, and equipment dependent characteristics were used in the
emission rate estimates for each chemical at each step. Typical values for
the remediation characteristics were also defined for each step.
Two values used consistently in each equation are Keq, the partition
coefficient between the quantity of a specific VOC in the gas phase and VOC
in the liquid phase, and De, the effective diffusity of the compound through
the soil pore spaces. These values were calculated first.
The value of K can be calculated by
or
*
v Pi MWORGEa (2)
"S>q = RTL
3
where H^ = Henry's Law constant of species i (atm.m /gmol)
E * Air porosity of soil (vol/vol)
R = 8.21 x 10"5 atm'm3/gmoTK in Equation 1
R =82.1 atm'cm /gmoTK in Equation 2
T = Temperature (K)
E.. = Volume fraction occupied by the waste in the soil
w
*
(vol/vol)
Vapor pressure of species i (atm)
ORG = Average molecular weight of organic waste phase (g/mol)
L = Organic waste loading (g organic/cm soil)
Equation 1 is used when soil concentrations are low and the soil gas
concentration is assumed to be in equilibrium with the soil moisture. For
this exercise, the soil cap or surface soil was assumed to be characterized
by this equation. Equation 2 is used for higher soil concentrations, and the
soil gas concentration is governed by Raoult's Law. The subsurface
contaminated soil was assumed to be characterized by Equation 2. It is
possible that the contaminated soil layer may be governed by Equation 1
(Henry's Law) for some highly soluble VOC's, but no effort was made to
determine if Equation 1 should be applied for these examples and it was
assumed that soil concentrations were sufficiently high to use Raoult's Law.
16
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For some compounds that are both highly volatile and very soluble, this could
result in a significant difference in the estimated emission rate.
The effective diffusion rate is calculated by the following equation:
3.33
(3)
where Da - Diffusity in air (cm /s)
O
E, = Air porosity of soil (vol/vol)
a
Ej = Total porosity of soil (vol/vol).
Several values (shown in Table 3) are recommended based on soil conditions.
TABLE 3. REPRESENTATIVE SOIL POROSITY
Soil type/
moisture content
Surface
Low moisture
High moisture
Compacted subsoils
Low moisture
High moisture
Porosity.
Total porosity (Ej)
0.61
0.61
0.35
0.35
vol/vol
Air-filled porosity (Eft)
0.50
0.35
0.30
0.15
Whereas the values of K and Dfi could change as a result of the soil
excavation activities, they are assumed to remain constant for the sake of
simplicity.
4.1.1 Excavation of Soil Cap and Contaminated Waste Laver
The following equation is used to calculate the mass of material emitted
from excavation of the soil cap and the contaminated waste layer:
0.72
0.5
f Keq "e A 1
4
C01
"L
(4)
17
-------
where
Mi
eq
Emission rate of species i, (g/cm soil handled)
Partition coefficient of species i (previously defined)
Effective diffusity of species i (previously defined)
Cycle time of soil exposure (s)
oi
Initial concentration of species i (g/cm
waste layer
Depth of layer removed (cm).
soil) in the
4.1.2 Exposure of Contaminated Waste Layer
Equation 5 is used to estimate VOC emissions from the exposed soil waste
layer:
H,.
0.72
\q De '2 '
4d2 J
0.5
(5)
where d = Depth of soil waste layer under the soil cap (cm).
K = Partition coefficient of species i (previously defined)
De = Effective diffusity of species i (previously defined)
t = Time to remove the soil cap.
4.1.3 Emissions From Soil in the Excavation Bucket, Truck Filling.
Truck Transport, and Truck Dumping
An equation of identical form can be used for each of these operations,
but with different values for d, depth of each soil layer, and t, exposure or
cycle time. The equation is as follows:
0.72
Keq De
4d
0.5
(Coi • MTOTi:
(6)
where
M
TOT
i
2 M. of all previous steps prior to current
remediation step.
The equation takes into account the losses from the previous steps.
This equation is evaluated separately for both the soil cap and the
contaminated waste layer.
18
-------
4.1.4 Total Emissions for Excavation
The results of the preceding equations permit the estimation of the VOC
emissions for each operation and for each species. The results of each
equation are expressed in grams of emissions per cubic centimeter of soil.
Total emissions of VOCs from each step or each species may be obtained by
summing the results of the calculations. Hourly, daily, monthly, and annual
emission rates can be estimated by using soil removal rates (cm /s). A
summary of these results was computed for each site. Values for exposure
time, depth of exposed layers from various operations, were obtained directly
from values reported by Radian.
4.2 EMISSION RATE RESULTS FOR SITE A
The characterization data and the "typical" values for removal of the
soil layers was input into the equations to estimate the emission rates for
the site. The steps for which emissions were calculated were excavation of
the soil cap, emissions from the soil cap in the excavation bucket, emissions
from the soil cap material as the truck is filled, emissions from the trans-
port of the soil cap offsite, emissions from exposure of the contaminated
waste layer, excavation of the contaminated waste layer, emissions from the
contaminated soil in the excavation bucket, emissions from contaminated waste
layer during truck filling, and emissions from transport of the contaminated
waste layer. Once the material was offsite, emissions were no longer
considered. Emissions for all five contaminants were calculated. Table 4
lists the pertinent physical constants for the five compounds. The overall
24-hour average soil removal rate was 885 cm /s. This value was divided
according to the volume proportion of the soil cap and contaminated zone to
calculate the emission rate for each component (g/s). The soil cap removal
rate was 292 cm /s and the contaminated soil zone removal rate was 593 cm /s
(24-hour average).
19
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TABLE 4. PROPERTIES OF CHEMICALS USED IN SITE A EXAMPLE PROBLEMS
Compound
Benzene
1-Butanol
Methylene chloride
Methyl ethyl ketone
o-Xylene
Molecular
weight,
g/gmol
78
74
85
72
106
Vapor3
pressure,
mmHg
101
6.5
427.8
77.5
2.77
Henry's Law
constant,
atm'mVgmol
5.50 x 10"3
8.01 x 10"6
3.19 x 10"3
2.72 x 10"5
5.27 x 10'3
Diffusivity
in air,
cm2/s
0.0932
0.081
0.100
0.095
0.0628
Properties at 25°C.
Estimated from vapor pressure/solubility characteristics at 25°C.
4.2.1 Emissions From Removal of Soil Cap
The soil cap is assumed to be equivalent to a low moisture surface soil
with a total porosity value of 0.61 and an air-filled porosity of 0.50. Soil
concentrations were assumed to be 1 percent of the value for each contaminant
in the contaminated waste layer, and Henry's Law was assumed to apply. Table
5 presents the results. The emission rate value in g/s represents a 24-hour
average.
TABLE 5. EMISSIONS FROM SOIL CAP REMOVAL-SITE A
Compound
Benzene
1-Butanol
Methylene chloride
Methyl ethyl ketone
o-Xylene
Total VOC
Emission,rate,
g/cnr
2.22 x 10'6
5.48 x 10'8
6.79 x 10"6
2.94 x 10"7
3.75 x 10"6
1.30 x 10'5
Emission rate,
9/s
6.46 x 10'4
1.60 x 10"5
1.98 x 10"3
8.59 x 10"5
1.09 x 10"3
3.82 x 10"3
Fractional VOC
percentage
0.1700
0.0042
0.5216
0.0226
0.2878
20
-------
4.2.2 Emissions From Soil Cap In Excavation Bucket
Soil characteristics and K were assumed to be constant. Equation 6
was used with a value of t«30 seconds for soil exposure, d=150 cm for depth
of exposed material in the bucket and the losses of material accounted for
from the excavation action. Table 6 presents the results.
TABLE 6. EMISSIONS FROM SOIL CAP IN BUCKET-SITE A
Compound
Benzene
1-Butanol
Methyl ene chloride
Methyl ethyl ketone
o-Xylene
Total VOC
Emission rate,
g/cm3
1.26 x 10~6
3.28 x 10"8
4.00 x 10"6
1.42 x 10"7
1.39 x 10"6
6.83 x 10"6
Emission rate,
g/s
3.67 x 10"4
9.58 x 10"6
1.17 x 10"3
4.15 x 10"5
4.06 x 10"4
1.99 x 10"3
Fractional VOC
percentage
0.1842
0.0048
0.5867
0.0208
0.2034
4.2.3 Emissions From Dumping Soil Cap Into Trucks
Equation 6 was used to account for previous losses and to estimate
emission rates for each of the five compounds. The values used for exposure
time and depth were 60 seconds and 51 cm, respectively. Table 7 presents the
results.
TABLE 7. EMISSIONS FROM TRUCK FILLING (SOIL CAP)--SITE A
Compound
Benzene
1-Butanol
Methyl ene chloride
Methyl ethyl ketone
o-Xylene
Total VOC
Emission,rate,
g/cnr
5.07 x 10"6
1.36 x 10"7
1.65 x 10"5
5.91 x 10"7
5.71 x 10'6
2.80 x 10"5
Emission rate,
q/s
1.48 x 10"3
3.98 x 10"5
4.82 x 10"3
1.73 x 10"4
1.07 x 10"3
7.34 x 10"3
Fractional VOC
percentage
0.1810
0.0049
0.5893
0.0211
0.2038
21
-------
4.2.4 Emissions From Transport of Soil Cap
Equation 6 was used to estimate emission rates from offsite transport of
the soil. The values used for exposure time and soil depth in the truck were
720 seconds and 151 cm, respectively. Table 8 presents the results.
TABLE 8. EMISSIONS FROM TRANSPORT OF SOIL CAP-SITE A
Compound
Benzene
1-Butanol
Methyl ene chloride
Methyl ethyl ketone
o-Xylene
Total VOC
Emission3rate,
g/cm
5.15 x 10"6
1.58 x 10"7
1.85 x 10"5
6.87 x 10"7
6.34 x 10"6
3.08 x 10"5
Emission rate,
g/s
1.50 x 10'3
4.62 x 10"5
5.41 x 10"3
2.01 x 10"4
1.85 x 10"3
9.01 x 10"3
Fractional VOC
percentage
0.1668
0.0051
0.6002
0.0223
0.2055
4.2.5 Emissions From Exposure of Contaminated Waste Laver
The values for K were calculated by using
The results are presented in Table 9. The
Equation 6 was used to estimate the emissions from exposure of the
contaminated waste layer. The value for exposure time (t) was 300 seconds
and the soil depth was 122 cm.
the Raoult's Law relationship.
emission rate (g/s) is based on the 24-hour average removal rate of the
contaminated soil layer.
4.2.6 Emissions From Excavation of Contaminated Soil Laver
Equation 4 was used to estimate the emissions from the excavation of the
contaminated waste layer. This equation was modified slight by replacing the
term for concentration with (CQi - MjOTi) to accommodate losses due to
exposure of the contaminated soil layer prior to actual excavation
(Subsection 4.1.5). A value of 60 seconds was used for the exposure and a
value of 90 cm for dL (soil depth). Table 10 summarizes the results.
22
-------
TABLE 9. EMISSIONS FROM THE EXPOSURE OF THE CONTAMINATED WASTE LAYER-SITE A
Compound
Benzene
1-Butanol
Methyl ene chloride
Methyl ethyl ketone
o-Xylene
Total VOC
Emission-rate,
g/cnr
3.36 x 10"6
4.22 x 10"7
7.21 x 10"5
7.49 x 10"6
2.05 x 10"6
8.54 x 10"5
Emission rate,
g/s
1.99 x IO"3
2.50 x IO"4
4.27 x IO"2
4.43 x IO"3
1.21 x IO"3
5.06 x IO"2
Fractional VOC
percentage
0.0393
0.0049
0.8441
0.0877
0.0240
TABLE 10. EMISSIONS FROM EXCAVATION
OF CONTAMINATED SOIL
LAYER-SITE A
Compound
Benzene
1-Butanol
Methylene chloride
Methyl ethyl ketone
o-Xylene
Emission-rate,
g/cm
2.04 x 10~6
2.56 x 10~7
4.36 x 10"5
4.54 x iO"6
1.24 x IO"6
Emission rate,
g/s
1.21 x IO"3
1.51 x IO"4
2.58 x IO"2
2.69 x IO"3
7.36 x 10"4
Fractional VOC
percentage
0.0394
0.0049
0.8439
0.0878
0.0240
Total VOC
5.17 x 10
-5
3.06 x 10
-2
23
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4.2.7 Emissions From Contaminated Soil Laver in Excavation Bucket
Equation 6 was used to estimate emissions generated while the
contaminated waste soil layer was in the bucket. A value of 30 seconds was
selected for the exposure time and 150 cm for soil depth. Table 11 presents
the results.
TABLE 11. EMISSIONS FROM EXCAVATION BUCKET-SITE A
Compound
Benzene
1-Butanol
Methylene chloride
Methyl ethyl ketone
o-Xylene
Total VOC
Emission3rate,
g/cm
8.26 x 10"7
1.07 x 10"7
1.68 x 10"5
1.85 x 10"6
5.24 x 10"7
2.00 x 10"5
Emission rate,
g/s
4.89 x 10"4
6.36 x 10"5
9.87 x 10"3
1.10 x 10"3
3.10 x 10"4
1.18 x 10"2
Fractional VOC
percentage
0.0413
0.0054
0.8344
0.0926
0.0262
4.2.8 Emissions From Truck Filling With Contaminated Soil Laver
Equation 6 was used to estimate emissions generated from the filling the
trucks with soil from the contaminated waste layer. The time value of 60
seconds and a depth of 51 cm were selected. Table 12 presents the results.
TABLE 12. EMISSIONS FROM TRUCK FILLING WITH CONTAMINATED SOIL LAYER-SITE A
Compound
Benzene
1-Butanol
Methylene chloride
Methyl ethyl ketone
o-Xylene
Total VOC
Emission3rate,
g/cnr
3.44 x 10"6
4.46 x 10"7
6.97 x 10"5
7.69 x 10"6
2.18 x 10"6
8.35 x 10"5
Emission rate,
g/s
2.03 x 10"3
2.64 x 10"4
4.13 x 10"2
4.56 x 10"3
1.29 x 10"3
4.94 x 10"2
Fractional VOC
percentage
0.0411
0.0053
0.8352
0.0922
0.0261
24
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4.2.9 Emissions From Transport of Soil From the Contaminated Waste Layer
The final step for emissions generation at this site involves the
transport of the contaminated waste layer soil from the site. Equation 6 was
used to estimate the emissions from truck transport of the contaminated soil.
The exposure time was 720 s with a depth of 151 cm. Table 13 presents the
results.
TABLE 13. EMISSIONS FROM TRANSPORT OF SOIL FROM THE CONTAMINATED
WASTE LAYER-SITE A
Compound
Benzene
1-Butanol
Methyl ene chloride
Methyl ethyl ketone
o-Xylene
Total VOC
Emission3rate,
g/cnr
1.19 x 10"5
1.55 x 10"6
2.41 x 10~4
2.66 x 10"5
7.55 x 10"6
2.89 x 10"4
Emission rate,
g/s
7.04 x 10"3
9.15 x 10"4
1.43 x 10"1
1.58 x 10"2
4.47 x 10"3
1.71 x 10"1
Fractional VOC
percentage
0.0412
0.0054
0.8351
0.0922
0.0261
4.2.10 Summary of Emission Estimates for Site A
A review of the preceding tables indicates that the fractional
percentage of the total VOC emissions for each species from each step is
relatively constant within a soil layar although not precisely identical for
each step. The emission rate for each chemical species is largely a function
of the initial concentration, the method of partition coefficient calculation
(Henry's or Raoult's Law), and the physical characteristics of the compound.
The percentage of each compound lost compared with its initial concentration
also depends on these factors, the excavation practices, and the soil
characteristics.
The predicted VOC emission rates during the excavation process steps are
generally higher for the contaminated waste soil layer than for the soil cap.
This is not surprising given the lower concentrations assumed for the soil
cap and the use of Henry's Law constants to calculate emission rates. The
25
-------
predicted VOC emission rates are generally influenced by the larger volumes
of soil and higher concentrations found in the contaminated subsoil zone.
Table 14 presents the average fractional percentage of the individual VOCs
for the soil cap, the contaminated zone, and the overall average value.
TABLE 14. SPECIES CONTRIBUTIONS TO OVERALL VOC EMISSIONS-SITE A
Fraction per- Fraction per- Fraction per-
centage for cent age for centage for over-
Compound Soil Cap contaminated zone all excavation
Benzene
1-Butanol
Methyl ene chloride
Methyl ethyl ketone
o-Xylene
0.1737
0.0049
0.5815
0.0218
0.2181
0.0407
0.0052
0.8374
0.0911
0.0256
0.0495
0.0052
0.8191
0.0857
0.0404
The overall excavation value is a weighted average percentage based on
emission rates from both the soil cap excavation and the contaminated
subsurface layer. On any given day, the remediation would be working to
remove both layers. The overall fractional percentage was the value used to
interpret ambient concentration data.
As shown in Table 14, the VOC with the highest emission rate is
methylene chloride, based on its fractional contribution. This is as
expected because its concentration is substantially greater than that of the
other compounds. In addition, methylene chloride has a high vapor pressure
and moderate Henry's Law constant. When input into the equations, these
factors all act to generate a higher predicted emission rate. Also a
substantial variation exists for several compounds, depending on which form
of the partition equation is used to calculate the emission rate.
As shown in Table 15 the relative contribution from each remedial
activity was examined. Again the relative contribution of each activity is
influenced by the methodology used to calculate the partition coefficient and
the assumptions used to compute the emission rates for each compound. For
26
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TABLE 15. REMEDIAL ACTIVITY CONTRIBUTION TO VOC EMISSIONS
FRACTIONAL CONTRIBUTION OF ACTIVITY-SITE A
Remedial activity
Excavation
Bucket
Truck filling
Transport
Exposure of contaminated
zone
Soil cap
0.1652
0.0867
0.3554
0.3917
Contaminated zone
0.0977
0.0377
0.1577
0.5455
0.1614
Combined
0.1154
0.0449
0.0670
0.6031
0.1695
this site, however, the most substantial source of VOC emissions was
determined to be the transport of soil off the site, followed in order by
exposure of the contaminated waste layer (removal of the soil cap) and the
actual excavation of the soil. Emissions from the soil while it is in the
bucket and during the actual filling of the truck account for the smallest
contributions based on the emission rate equations. These predictions assume
no controls (tarps or foam) are used to control VOC emissions from any of the
remedial process steps.
Equation 7 can be used to estimate emission rates and to establish an
emission profile for the period of remediation. Assuming that the site was
previously undisturbed except for soil sampling and removal of the leaking
drums, a baseline VOC emission rate was estimated by using the RTI equation
for instantaneous, time-dependent emissions.
1
«t vl/2
K9Keq
(7)
where
d
E.
K
eq
t
D..
Flux rate of component i, g/cm per second
Initial mass of i, g/cm soil
Depth of contaminated layer, cm
Soil air porosity, dimensionless
Gas-phase mass transfer coefficient, cm/s
Partition coefficient, g vapor/g liquid
Time, seconds
Effective diffusion rate of i, cm2/s.
27
-------
These variables are further defined in Reference 2. Assuming a 9-month
period between sampling and remediation, the average VOC emission rate prior
to remediation was 1.125xlO"2 g/s, or 972 g VOC/day. The baseline emissions
were calculated for the five individual compounds. Table 16 presents the
relative contribution of each. During remediation, the estimated VOC
emission rate increased to 2.988X10"1 g/s, or 25,816 g VOC/day. This
represents an increase of 26.6 times the estimated baseline emission rate.
Table 16 presents the relative contribution for the remedial alternative from
Table 14 for comparison purposes. Although the process of remediation does
increase total VOC emissions, it also changes the emission characteristics
and contributions of the VOC species. Figure 4 is the short-term emissions
profile for the site. This emissions profile is plotted on a semi logarithmic
scale because of the larger increase in the VOC emission rate. The slight
decrease in daily emissions is due to a decrease in emissions from the
baseline area, as the unremediated area gradually decreases in size. At the
end of remediation, emissions fall to zero for the site.
TABLE 16. FRACTIONAL CONTRIBUTIONS OF VOCS TO OVERALL EMISSION RATES--SITE A
Fractional percentage
Compound
Benzene
1-Butanol
Methyl ene chloride
Methyl ethyl ketone
o-Xylene
Baseline
0.1014
0.0168
0.7086
0.1362
0.0366
Remediation
0.0495
0.0052
0.8191
0.0857
0.0404
The VOC emission rates and the site characteristics form the basis for
dispersion modeling at the site. These are discussed in Section 5.
28
-------
Figure 4. Daily emission profile for Site A.
29
-------
4.3 EMISSION RATE RESULTS FOR SITE B
Site B is similar to Site A with regard to excavation. Some notable
differences, however, include a much larger volume of soil to be remediated,
onsite incineration of the contaminated soil, and lower concentrations of
VOCs than on Site A. Also, the contaminated soil was divided into two
distinct homogeneous zones of differing chemical composition. The excavation
procedures are assumed to be similar to those for Site A including the use of
the same equations and the addition of VOC emissions from truck dumping into
the incinerator feed system. The two homogeneous zones, however, will be
treated in sequence rather than simultaneously. The average rate of
remediation for the site is 885 cm /s (based on a 24-hour average and on the
same work schedule as for Site A). The relative removal rates, however, for
the soil cap and contaminated soil zone differ because their relative volume
percentages are different. The average rates for soil cap and contaminated
soil removal are 131 and 754 cm /s, respectively, based on a 24-hour
averaging period. These factors are used to convert the values for g/cm
soil handled to g/s values. The emission rates for each zone are presented
separately in each table for the appropriate remedial step. The initial
concentrations of contaminants have been presented previously in Table 2.
4.3.1 Emissions From Removal of Soil Cap
The surface soil was assumed to be equivalent to a low-moisture
compacted soil with a total porosity value of 0.30 and an air filled porosity
of 0.35. Surface soil concentrations were assumed to be 1 percent of the
contaminated subsurface soil values in the same relative contributions as the
contaminated zone. Henry's Law was assumed to apply in the computation of
emission rates. The results are presented in Table 17.
4.3.2 Emissions From Soil Cap in Excavation Bucket
Equation 6 was used with a value of 30 seconds for the exposure time and
150 cm for the depth. Table 18 presents the results for the two zones.
4.3.3 Emissions From Truck Filling With Soil Cap
The emission rate for truck filling was estimated by Equation 6, with
the exposure time, t, equals 60 seconds, and the depth equals 51 cm. Table
19 presents the results.
30
-------
TABLE 17. EMISSIONS FROM SOIL CAP REMOVAL-SITE B
Compound
Zone 1
Benzene
1-Butanol
Methyl ethyl ketone
Total (Zone 1)
Zone 2
Methylene chloride
Methyl ethyl ketone
o-Xylene
Total (Zone 2)
Emission3rate,
g/cm
1.32 x 10"S
4.67 x 10 7
1.20 x 10"'
1.49 x 10"6
2.16 x 10"5
8.99 x 10"?
1.57 x 10"b
3.81 x 10"6
Emission rate,
g/s
1.73 x 10"4
6.13 x ID"?
1.57 x 10"b
1.95 x 10"4
2.83 x 10"J
1.18 x 10"J
2.06 x 10
5.00 x 10"4
Fractional VOC
percentage
0.8880
0.0314
0.0805
0.5655
0.0236
0.4110
TABLE 18. EMISSIONS FROM EXCAVATION BUCKET (SOIL CAP)--SITE B
Compound
Zone 1
Benzene
1-Butanol
Methyl ethyl ketone
Total (Zone 1)
Zone 2
Methylene chloride
Methyl ethyl ketone
o-Xylene
Total (Zone 2)
Emission3rate,
g/cnr
7.54 x 10~7
2.80 x ID'S
7.17 x 10"B
8.53 x 10"7
1.27 x I0~l
5.38 x 10"?
9.26 x 10"'
2.25 x 10"6
Emission rate,
g/s
9.88 x 10"!
3.67 x 10"?
9.40 x 10"a
1.12 x 10"4
1.67 x 10"J
7.06 x 10"?
1.21 x 10"4
2.95 x 10"4
Fractional VOC
percentage
0.8832
0.0328
0.0840
0.5650
0.0239
0.4111
31
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TABLE 19. EMISSIONS FROM TRUCK FILLING (SOIL CAP)--SITE B
Emissiorurate, Emission rate, Fractional VOC
Compound
Zone 1
Benzene
1-Butanol
Methyl ethyl ketone
Total (Zone 1)
Zone 2
Methyl ene chloride
Methyl ethyl ketone
o-Xylene
Total (Zone 2)
g/cnr
3.04 x 10~5
1.16 x 10";
2.98 x 10''
3.46 x 10'6
5.24 x 10"§
2.24 x 10";
3.81 x 10"°
9.28 x 10"6
g/s
3.99 x 10"J
1.53 x 10"c
3.90 x 10"D
4.53 x 10"4
6.87 x 10"J
2.93 x 10"J
5.00 x 10~*
1.22 x 10"3
percentage
0.8802
0.0337
0.0861
0.5648
0.0241
0.4111
4.3.4 Emissions From Transporting Soil Cap Material
Unlike Site A, the soil excavated at this site is transported to a
thermal desorption/incinerator for soil treatment and dumped into the
incinerator feed system. Time constants and depth of material assumptions,
however, were the same as Site A (t = 720 seconds, d = 51 cm). Table 20
presents the results.
4.3.5 Emissions From Dumping Soil Cap Material
Equation 6 was used to estimate VOC emissions from the dumping of soil
into the thermal desorption/incinerator system. The temporary storage pile
is exposed as the feed system accepts the soil. An exposure time value of
1900 seconds was used in Reference 2 and is also used here. The depth of
soil used is 151 cm. Table 21 presents the results.
32
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TABLE 20. EMISSIONS FROM TRANSPORT OF SOIL CAP MATERIAL-SITE B
Compound
Zone 1
Benzene
1-Butanol
Methyl ethyl ketone
Total (Zone 1)
Zone 2
Methyl ene chloride
Methyl ethyl ketone
o-Xylene
Total (Zone 2)
Emission3rate,
g/cm
3.13 x 10"?
1.36 x 10";
3.46 x 10"7
3.61 x 10"6
5.87 x lO"?
2.60 x 10";
4.28 x 10"b
1.04 x 10"5
Emission rate,
g/s
4.10 x 10"5
1.78 x 10"i
4.54 x 10"D
4.73 x 10"4
7.69 x 10"J
3.41 x 10";
5.62 x 10"*
1.37 x 10"3
Fractional VOC
percentage
0.8664
0.0376
0.0960
0.5636
0.0250
0.4114
TABLE 21.
SOIL
EMISSIONS FROM DUMPING
CAP AT THE INCINERATOR
AND TEMPORARY STORAGE OF
FEED SYSTEM--SITE B
Compound
Zone 1
Benzene
1-Butanol
Methyl ethyl ketone
Total (Zone 1)
Zone 2
Methylene chloride
Methyl ethyl ketone
o-Xylene
Total (Zone 2)
Emission3rate,
g/cm
4.35 x 10"5
2.19 x 10";
5.58 x 10"7
5.13 x 10"6
9.06 x 10"5
4.20 x 10";
6.63 x 10"b
1.61 x 10"5
Emission rate,
g/s
5.70 x 10"{
2.87 x 10'ij
7.32 x lO"*
6.72 x 10"4
1.19 x 10"3
5.51 x 10";
8.70 x 10
2.11 x 10"3
Fractional VOC
percentage
0.8484
0.0427
0.1088
0.5623
0.0261
0.4116
33
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4.3.6 Emissions from Exposure of Contaminated Soil Laver
The soil characteristics (low moisture, compacted soil) were considered
identical to those of the soil cap. The emissions generated from exposure of
the contaminated soil were based on the bulk concentrations reported in Table
2. Equation 4 was used to estimate the emissions for each contaminant in
each zone; Raoult's Law was used to calculate Ke_. Table 22 summarizes the
results.
TABLE 22. EMISSIONS FROM THE EXPOSED CONTAMINATED SOIL LAYER-SITE B
Compound
Zone 1
Benzene
1-Butanol
Methyl ethyl ketone
Total (Zone 1)
Zone 2
Methyl ene chloride
Methyl ethyl ketone
o-Xylene
Total (Zone 2)
Emission3rate,
g/cnr
1.91 x ID'S
4.70 x 10 ~(
3.01 x 10"b
5.39 x 10"6
1.31 x IQ'l
1.94 x 10"?
6.46 x 10"7
1.57 x 10"5
Emission rate,
9/s
1.44 x 10~3
3.55 x 10"5
2.27 x lO""3
4.06 x 10"3
9.91 x IQ'l
1.46 x lO'f
4.87 x 10~*
1.19 x 10"2
Fractional VOC
percentage
0.3546
0.0872
0.5581
0.8357
0.1232
0.0411
4.3.7 Emissions From Excavation of the Contaminated Soil Layer
Equation 4 was used to estimate excavation emissions and to account for
losses from exposure of the contaminated waste layer because of removal of
the soil cap and subsequent layers. An exposure time of 60 seconds and depth
of 90 cm was used in the emissions calculations. Table 23 presents the
results.
34
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TABLE 23. EMISSIONS FROM EXCAVATION OF THE CONTAMINATED SOIL LAYER-SITE B
Compound
Zone 1
Benzene
1-Butanol
Methyl ethyl ketone
Total (Zone 1)
Zone 2
Methyl ene chloride
Methyl ethyl ketone
o-Xylene
Total (Zone 2)
Emission3rate,
g/cm
3.32 x 10"!
8.18 x lO'i
5.23 x 10"°
9.37 x 10"6
2.28 x 10"5
3.37 x ID"?
1.12 x 10"b
2.73 x 10"5
Emission rate,
g/s
2.51 x IQ'l
6.17 x 10"3
3.94 x lO"5
7.07 x 10"3
1.72 x IQ'l
2.54 x 10"i
8.47 x 10~*
2.06 x 10"2
Fractional VOC
percentage
0.3546
0.0873
0.5581
0.8356
0.1233
0.0411
4.3.8 Emissions From Contaminated Soil Layer in the Excavation Bucket
Equation 6 was used to estimate VOC emissions from the contaminated soil
layers in each zone while they were in the excavation bucket. An exposure
time of 30 seconds and depth of 150 cm were used in the computations. Table
24 presents the results.
4.3.9 Emissions From Truck Filling With the Contaminated Soil Laver
Equation 6 was used to estimate VOC emissions from truck filling with
the contaminated soil layer from each zone. An exposure time of 60 seconds
and depth of 51 cm were used in the equation to estimate the emission rates.
Table 25 presents the results.
4.3.10 Emissions From Soil Transport of the Contaminated Soil Laver
Equation 6 was used to estimate VOC emissions from the transport of the
contaminated soil layer. An exposure time of 720 seconds and depth of 51 cm
were used to estimate emission rates for each compound. Table 26 presents
the results.
35
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TABLE 24. EMISSIONS FROM CONTAMINATED SOIL LAYER WHILE IN
EXCAVATION BUCKET-SITE B
Compound
Zone 1
Benzene
1-Butanol
Methyl ethyl ketone
Total (Zone 1)
Zone 2
Methyl ene chloride
Methyl ethyl ketone
o-Xylene
Total (Zone 2)
Emission3rate,
g/cm
1.41 x 10"6,
3.47 x 10";
2.22 x 10"b
3.97 x 10"6
9.67 x 10"6-
1.43 x 10 "?
4.77 x 10"'
1.16 x 10"5
Emission rate,
g/s
1.06 x 10~3
2.62 x 10'?
1.67 x 10"J
2.99 x 10"3
7.29 x IQ'l
1.08 x 10";
3.57 x 10"*
8.73 x 10"3
Fractional VOC
percentage
0.3545
0.0874
0.5581
0.8353
0.1234
0.0412
TABLE 25. EMISSIONS FROM TRUCK FILLING WITH THE CONTAMINATED
SOIL LAYER-SITE B
Compound
Zone 1
Benzene
1-Butanol
Methyl ethyl ketone
Total (Zone 1)
Zone 2
Methylene chloride
Methyl ethyl ketone
o-Xylene
Total (Zone 2)
Emission, rate,
9/cm
5.85 x 10"!
1.44 x 10"?
9.22 x 10"b
1.65 x 10"5
4.02 x ID";?
5.94 x 10"?
1.98 x 10"°
4.81 x 10"5
Emission rate,
g/s
4.41 x IQ'l
1.09 x 10",
6.95 x lO""3
1.24 x 10"2
3.03 x 10"|
4.48 x 10"X
1.50 x 10"J
3.63 x 10"2
Fractional VOC
percentage
0.3545
0.0874
0.5581
0.8353
0.1234
0.0412
36
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TABLE 26. EMISSIONS FROM TRANSPORT OF THE CONTAMINATED SOIL LAYER-SITE B
Compound
Zone 1
Benzene
1-Butanol
Methyl ethyl ketone
Total (Zone 1)
Zone 2
Methyl ene chloride
Methyl ethyl ketone
o-Xylene
Total (Zone 2)
Emission3rate,
g/cm
2.02 x 10'5
5.00 x 10"i
3.19 x 10"D
5.71 x 10"5
1.39 x 10"J
2.05 x 10"5
6.87 x 10~°
1.66 x 10"4
Emission rate,
g/s
1.53 x 10~2
3.77 x 10,
2.40 x 10"^
4.30 x 10"2
1.05 x 10"i
1.55 x ID'S
5.18 x 10"J
1.25 x 10"1
Fractional VOC
percentage
0.3544
0.0875
0.5581
0.8350
0.1236
0.0413
4.3.11 Emission From Dumping of Contaminated Soil Layer
Equation 6 was used to estimate VOC emissions from truck dumping into
the incinerator feed system. An exposure time of 1900 seconds and depth of
51 cm were used in the emission rate estimation. Table 27 presents the
results.
TABLE 27. EMISSIONS FROM DUMPING OF CONTAMINATED SOIL LAYERS-SITE B
Compound
Zone 1
Benzene
1-Butanol
Methyl ethyl ketone
Total (Zone 1)
Zone 2
Methylene chloride
Methyl ethyl ketone
o-Xylene
Total (Zone 2)
Emission3rate,
g/cm
3.26 x lO'l
8.10 x ID"?
5.14 x 10°
9.21 x 10"5
2.23 x 10"J
3.32 x lO'J
1.16 x 10~*
2.74 x 10"4
Emission rate,
g/s
2.46 x 10"?
6.11 X 10 y
3.88 x 10"^
6.95 x 10'2
1.68 x 10'J
2.51 x 10~£
8.41 x 10"J
2.07 x 10"1
Fractional VOC
percentage
0.3540
0.0879
0.5580
0.8350
0.1236
0.0413
37
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4.3.12 Incinerator Emissions
Estimates of VOC emissions from the incinerator stack were based on a
99.99 percent destruction removal efficiency of the concentration remaining
in the contaminated soil. This was equivalent to 1.41 x 10"5 g/s for the
Zone 1 soils, and 7.86 x 10"4 g/s for Zone 2 soils. Additional computations
would be required to estimate the products of combustion but these were not
calculated because this study concerns VOCs. The incinerator emissions were
modeled as a point source in subsequent steps.
4.3.13 Summary of Emissions Estimates for Site B
A review of the preceding tables for Site B reveals that no substantial
variation occurs in the fractional percentage contribution of any given VOC
within soil layers (i.e., soil cap or contaminated soil zone) or within
homogeneous zones (Zone 1 or 2). This illustrates that once other parameters
are set (e.g., remediation steps, time for any given step, the depth of
contamination, and the soil characteristics) the relative emission rate is
controlled by the concentration and physical/chemical characteristics of the
contaminant. Thus, a relatively fixed ratio of chemical constituents is
calculated for VOC emissions from either zone. In addition, the contaminated
soil layers provide the largest contribution to the overall emission rate.
This is illustrated in Table 28.
TABLE 28. SPECIES CONTRIBUTIONS TO OVERALL VOC EMISSIONS-SITE B
Compound
Fraction per-
cent for
Type I soil
Fraction per-
cent for
Type II soil
Fraction per-
cent for overall
excavation
Zone 1
Benzene 0.8665
1-Butanol 0.0376
Methyl ethyl ketone 0.0959
Zone 2
Methylene chloride 0.5637
Methyl ethyl ketone 0.0250
o-Xylene 0.4113
0.3544
0.0877
0.5579
0.8354
0.1234
0.0412
0.3613
0.0870
0.5516
0.8310
0.1217
0.0473
38
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The contribution of each remedial step to the VOC emissions was
examined. Table 29 presents the results for each step. Although different
chemical constituents and concentrations are present in each zone, the
contribution of each remedial step to the VOC emissions in the overall
excavation process is relatively constant. This contribution appears to be
independent of the concentration of chemical species; rather, it depends more
on the parameters of the soil and the remedial activity pattern. At this
site, dumping and temporary storage at the incinerator feed account for 50
percent of the VOC emissions. Transport from the excavation zone is the
second highest contributor of emissions. All activities are assumed to be
uncontrolled. The use of tarps and/or foam suppressants could substantially
reduce these emissions from transport and storage.
TABLE 29. REMEDIAL STEP CONTRIBUTION TO VOC--SITE B
Remedial activity
Excavation
Bucket
Truck filling
Transport
Dumping
Incinerator
Exposed soil
Zone 1
0.0515
0.0220
0.0912
0.3085
0.4979
0.0001
0.0288
Zone 2
0.0507
0.0217
0.0902
0.3039
0.5029
0.0019
0.0286
Overall site
0.0509
0.0218
0.0905
0.3051
0.5016
0.0014
0.287
Because remediation of the two zones occurs in sequence, the VOC
emissions characteristics and the relative contribution of each constituent
change depends on which zone is being remediated first. For illustration
purposes, Zone 1 is assumed to be remediated first. Figure 5 is an emission
profile for total VOC emissions during the total remediation period of 6
months. The emissions from Zone 2 are higher primarily due to higher
concentrations of compounds with greater volatility than occur in Zone 1.
39
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Figure 5. Emissions profile for total VOCs from Zones 1 and 2
remediation—Site B.
40
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4.4 SUMMARY OF EMISSION ESTIMATES
Because the procedures for estimating emissions are intensive,
repetitive, and sequential, the use of a computer program is encouraged for
performing a detailed analysis. For Site A, approximately 100 calculations
were required to provide an estimate of the emissions rate for each step of
the excavation process for all five chemicals. For Site B, where two
separate homogeneous zones containing three chemicals each were identified,
approximately 160 calculations were required. The estimates of fractional
contributions required additional calculations. For a large heterogeneous
site where subdivision into smaller homogeneous areas would likely be the
preferred choice, the calculation requirements could be substantial.
The calculation procedures suggest that, although site-specific factors
can be input into the equation for equipment-related factors (e.g., bucket
size or remediation cycle time), standardized values for the remediation
rates and equipment should be used. Factors correlating with low, medium,
and high rates of remediation perhaps could be selected for analysis
purposes. For these example sites, mid-range remediation rates should be
selected.
The results indicate that the relative emission contributions of each
chemical do not change substantially with each remediation step during
excavation. This was true within a given soil layer (i.e., the soil cap or
the contaminated soil layer). The emissions from the contaminated soil zone
accounted for the dominant proporation of the VOC emissions compared with the
soil cap. Thus, calculations could be simplified substantially by the
elimination of soil cap emissions unless the soil cap contains a substantial
quantity of VOCs and represents a substantial percentage of the soil to be
remediated. In the example sites, the assumed VOC content of 1 percent of
the contaminated soil layer resulted in minor emissions compared with those
from the contaminated soil layer.
Results for the Site B example indicate that the VOC emissions for each
remediation step occur at a relatively fixed ratio regardless of the VOC
characteristics and concentration. This indicates that these ratios are
governed by the parameters used to describe the remediation steps (i.e.,
cycle time, depth of contaminated layer, and remediation rate) and not by the
chemical or physical characteristics of the chemical constituents. If this
41
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Is true, certain scenarios could be proposed for site remediation and VOC
emissions allocated on the basis of remediation steps, initial concentrations
in the contaminated soil zone, and rates of remediation (e.g., low, medium,
or high).
The emission rates computed through these equations served as the basis
of dispersion modeling to determine the ambient impact of remediation.
Determining that the relative contributions of each VOC to the overall VOC
emission rate remained constant was helpful in describing the emission rate
and determining the modeling process, which is the next step in this
procedure.
42
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SECTION 5
AMBIENT DISPERSION MODELING
The ambient Impact associated with remedial activities and the emissions
they generate can be estimated through the use of dispersion models. Three
computer-based models were used to estimate short- and/or long-term ambient
concentrations associated with remediation of the example sites. The
procedures used and the results obtained are summarized in this section.
5.1 GENERAL APPROACH TO DISPERSION MODELING
When the emission rates were defined for each chemical and remediation
step, the ambient concentration of each compound could be estimated. The
procedure, however, was not as simple as inputting the emission rate into the
appropriate dispersion model and waiting for the results. Because the
dispersion models used are not specifically designed for some of the
remediation activities at Superfund sites, some adaptions and simplifications
are required to obtain "reasonable" results. For example, the dispersion
models contain algorithms for estimating ambient concentrations associated
with point, area, and, depending on the model, volume sources. The models do
not, however, address excavation characteristics or dumping and transporting
of soil directly and some effort must be expended to adapt these processes to
the model input requirements.
The Industrial Source Complex Models (both the ISCLT and ISCST versions)
and the SCREEN Model were used for this project. The ISCLT model computes
the values necessary for the annual ambient concentrations used for chronic
exposure risk assessment, whereas the ISCST computes the predicted values for
short-term (acute) exposure. Output results for ISCST can be calculated for
several averaging periods, including highest 1-hour, 3-hour, 8-hour, and
24-hour ambient concentrations. The SCREEN Model uses stability class and
windspeed data to compute 1-hour ambient concentration at discrete distances.
Each model has its strengths and weaknesses as well as its own input and
output format. These are discussed briefly in this section.
43
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The ISC Models were considered for use first because they can estimate
ambient Impacts from multiple source locations and source types
simultaneously. The ISC Models can estimate ambient contributions at
discrete receptor sites from point sources, area sources, volume sources, and
line sources. Each of these source types can be present at a Superfund site.
A.point source may be present as an incinerator, air stripper, or in situ
soil vapor extraction stack. An area source may be present as a contaminated
soil layer or pile. Volume sources may be represented by truck filling, and
dumping, whereas line sources may be representing emissions from truck
transport down a haul road. The contribution of each source or group of
sources to the ambient concentration may be displayed in the model output for
discrete receptor sites as well as the total contribution of all sources.
The ISC Models also use actual historical meteorological data to predict
ambient concentrations.
The capability of the ISC Model to model many sources and source types
adds a degree of complexity to the overall procedure. The onsite location,
dimensions, source type, and emission rate (in the proper format for the
model input) must be calculated for each source and input into the model.
The receptor locations also must be selected. Specific points oriented to
the site or a grid of concentric receptor points located at various distances
from the "site center" may be selected. As the number of sources on site
increases, the input requirements become more substantial and more computer
time is needed to estimate the contribution of each source at each receptor.
In addition, increasing the number of receptors to be evaluated increases
computer run time. The high level of specificity has a price, particularly
for the ISCST Model. It is quite easy to define 20 or more emission sources
at a typical Superfund site where excavation is used. This large number of
sources presents more opportunity for data input errors. More important for
the ISCST Model, however, are the computer run-time requirements. Run times
could exceed 20 hours (when an AT class PC running at 6 MHz with a math
coprocessor is used). The ISCLT Model does not demand the same run time;
typical run time for the ISCLT Model was approximately 10 minutes with the
same computer. Clearly, some simplification is justified unless
site-specific factors dictate the need for such detail.
The SCREEN Model is very simple to use for short-term estimates. The
SCREEN Model is limited to area and point sources, does not consider combined
44
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source impacts at discrete receptors (these have to be manually calculated),
does not use site-specific meteorological data, and calculates only 1-hour
average concentrations at the receptor of interest. The greatest strengths
of the SCREEN model, however, are the ease of inputting data by using screen
prompts and the ability to obtain the results in a matter of seconds.
Guidelines are available for converting the 1-hour averages to 3-hour,
8-hour, and 24-hour averages. Conversion of a 1-hour value to an annual
average is somewhat more complex and was not performed during this project.
For the long-term annual concentrations, the ISCLT Model was used.
The fact that the excavation sources are "moving" sources (i.e., the
location of the excavation changes daily) is a problem to both models. For
these example sites, this would have a greater effect on short-term values
than on long-term values. Under certain meteorological conditions,
excavation at locations near the fence! ine and closest to the receptors may
generate high short-term concentrations. Changing location by a distance of
100 meters may reduce short-term concentrations by a factor of two or more,
depending on wind and stability conditions.
Two simplifying assumptions were made to help in the modeling. First,
the emissions from the actual excavation, the material in the bucket, and
truck filling operations were combined and considered as one area source.
The emissions associated with exposure of the contaminated soil can be
combined with the excavation emissions for the short-term emissions. For
long-term emissions the exposed area source was modeled as being immediately
adjacent to the excavation source. Truck-hauling emissions and truck-dumping
emissions were considered as separate sources. The second simplifying
assumption was that the excavation location was on the edge of the
contaminated zone (midline) in each case. Calculations of the area exposed
daily were based on the area of contamination and the number of estimated
days required for remediation. The following equation was used:
H£- - Area of contamination (m2)
- -
day - Day$ required for excavation
Other options are available for estimating the location of the emissions
on site, but the preceding assumptions concerning location and quantities of
VOC emissions seemed reasonable.
45
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The use of the SCREEN Model required the estimation of maximum 1-hour
concentrations by manually summing the impacts from all sources. The
computed maximum concentration value from the SCREEN Model was based on
worst-case meteorological data from an array of atmospheric stability classes
and windspeeds contained within the SCREEN Model. The fact that the relative
contribution of each chemical constituent remained constant throughout the
remedial process within a homogeneous zone simplified the model runs
considerably. If variable contributions were found, a modeling run for each
chemical would be required to determine the ambient concentration of each
VOC.
One substantial difference exists between the long-term and short-term
modeling. The emission rates for the long-term model reflect the annual
average emission rate. The emissions generated during 2 months of excavation
at Site A and 6 months of excavation at Site B were converted to an annual
average emission rate. This can be performed by calculating the total
emissions generated during the remediation for each step and dividing by the
number of seconds in a year (31,536,000 s/yr). The ISCLT Model was run with
the VOC emissions "annualized" to produce an annual ambient concentration.
The SCREEN Model, on the other hand, was run with actual maximum hourly
emission rates to determine the short-term ambient concentrations associated
with the remediation steps at each site. All remediations taking less than 1
year would require similar adjustment. The results from the ISCLT model are
used in the risk assessment step, whereas the SCREEN Model results can be
used for contingency monitoring and for defining short-term "action level"
detection limits or identifying the potential for acute toxicity problems.
5.2 DISPERSION MODELING FOR SITE A
The locations of the sources and emission rates were determined for Site
A and input into the ISCLT Model and the SCREEN Model. The combined
emissions from excavation, material in the bucket, and truck filling were all
classified as one area source for the ISCLT Model. The size of this source
o
was estimated as a square with a side of 5.9 m (34.84 m ). The emissions
from the exposed contaminated waste layer due to soil removal were included
in the ISCLT model as a separate but adjacent source of identical size. The
46
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total annualized VOC emission rate for excavation was estimated at 3.25 x
M A
10 g/s per m , and the exposed area source VOC emissions were estimated at
2.43 x 10"4 g/s per m2. The locations for these two area sources were
established (grid coordinates: 0,0) at the center of site. Estimated
ambient contributions of VOCs from the trucks that used the haul road were
estimated by representing the VOC emissions from the trucks as series of 10
volume sources along the 100-meter haul road. The annualized VOC emission
2-3
rate for the length of the haul road was 3.00 x 10 g/s or 3.00 x 10 g/s
from each of the 10 segments. A release height of 3 meters was selected.
These data and the coordinates for the receptor locations were input into the
ISCLT Model. The meteorological data selected for the example represented
the Raleigh, North Carolina, area. The output was selected to show the
relative contribution of each source group at each receptor. Receptors at
the fenceline of the site were also selected. Figure 6 shows the relative
position of sources and receptors.
The ISCLT program estimated the annual ambient concentration for all
receptors in increments of 22.5-degree sectors around the site at the
fenceline and at distances ranging from 250 to 20,000 meters from the site
center. As expected from these types of sources, maximum ambient impact
occurred at the fenceline because the close proximity and low level release
heights provide little time for dispersion. The highest annual concentration
3
(7.23 pg/m ) was found due north (0 degrees) of the site center at the
fenceline (100.6 meters). Table 30 presents the fenceline VOC values. In
general, the highest annual concentrations are predicted for the north and
east of the site. The relative contribution of each source to the ambient
concentration receptors is not constant, primarily because of the wind data,
emission rates, ISC algorithms, and relative location of each source within
the site.
Short-term ambient concentrations may be determined with the SCREEN
Model. For this site, the worst condition would logically occur when winds
were lined up with all the sources (north-south orientation). The combined
effects from excavation, exposed soil, and truck-hauling emissions would be
the greatest with these wind conditions.
47
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N
Exposed
Area Sources
xcavation
Emission
Source
Haul Road
Emission
Sources
enceline
Receptors
Figure 6. Relative location of emission sources and
fenceline receptor for Site A.
48
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TABLE 30. FENCELINE VOC CONCENTRATIONS FROM ISCLT MODEL RUN FOR SITE A
Azimuth,
degrees
0
22.5
45.0
67.5
90.0
112.5
135.0
157.5
180.0
202.5
225.0
247.5
270.0
292.5
315.0
337.5
Range,
meters
100.6
112.5
142.3
112.5
100.6
112.5
142.3
112.5
100.6
112.5
142.3
112.5
100.6
112.5
142.3
112.5
Annual average ambient
concentration,
^g/ms
7.23
7.20
5.16
6.84
5.48
3.80
2.55
3.98
5.11
3.40
2.13
2.24
3.23
2.88
2.01
3.03
The short-term ambient concentrations of concern would result from
short-term emissions from the various excavation processes. The model input
requires the short-term emission rate during actual excavation hours. The
exposed area source and excavation emissions could be simplified by adding
them together and treating them as one area source. The truck-hauling
emissions would be handled separately as 10 separate elevated area sources.
The results would then be combined manually.
The short-term VOC emission rates were estimated by taking the VOC
emission rate (g/cm ) and multiplying it by the quantity of soil actually
moved during the 8-hour remediation period each day. Because the 24-hour
average rate was 885 cm soil/s, the average hourly rate during remediation
must be increased by a factor of three (24/8) or 2665 cm3/s during the 8-hour
shift. Therefore, the short-term VOC emissions are 2.04 x 10"1 g/s from
excavation and 5.07 x 10 g/s from the exposed soil. Short-term emissions
from soil transport were estimated to be 5.41 x 10"1 g/s. The SCREEN Model
does not handle volume sources, but the ambient effects from the truck
emissions could be modeled as a series of 10 small elevated area sources at
49
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discrete distances from the site boundary and adding their effects together
at the receptor of interest. This works well for a wind from the south
because all distances from the source to the receptor would exceed the
accepted minimum of 100 meters for modeling results. Winds form the north
would place receptors very close to the emission "points" from the truck
hauling, and results would be questionable. Thus, only winds from the south
were considered.
The SCREEN Model predicted the highest 1-hour concentration under F
class stability (moderately stable atmospheric conditions) and a windspeed of
1.0 m/s from the SCREEN Model's array of stability class and windspeed data.
Under these conditions, the ambient impact from the combined excavation
sources at the fenceline was predicted to be 4621 pg VOC/m . The
truck-hauling emissions were modeled as 10 elevated area sources (each a
square source with a side dimension of 3 meters) with an area of 9 m each.
Each of these sources was located 10 meters apart along down the haul road,
starting at 5 meters from the site center (105 meters from the northern
fenceline). The total contribution of all the truck hauling sources was
3 3
estimated to be another 5577 ^g/m , for a total of 10,198 ng/m for the
highest 1-hour value.
The fractional percentage of each compound in the ambient air at a given
receptor is identical to the fractional emission rate for each compound
emitted from a source. The contribution of multiple sources to the
concentration at a given receptor would have to be weighted. This is
particularly true when different fractional contributions of VOCs are
involved from multiple sources. Because these fraction emission rate
relationships stay relatively constant for all excavation remedial steps
within a homogeneous zone, ambient concentrations can be determined by
multiplying the ambient concentrations by the average fractional percentage
emission of each VOC. Table 31 presents the values for each of the five
compounds by applying the factors presented in Table 16. The concentrations
estimated for methyl ene chloride represent the most prevalent compound
expected followed by methyl ethyl ketone and benzene. These values are used
in the risk assessment for this site (Section 6) for both long- and
short-term exposure.
50
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TABLE 31. AMBIENT CONCENTRATIONS FOR CHEMICAL SPECIES AT SPECIFIED RECEPTORS
Azimuth,
degrees
Range,
meters
Benzene, 1-Butanol,
Methylene
chloride,
Methyl
ethyl
ketone,
o-Xylene,
Annual concentrations
0
22.5
45.0
67.5
90.0
112.5
135.0
157.5
180.0
202.5
225.0
247.5
270.0
292.5
315.0
337.5
100.6
112.5
142.3
112.5
100.6
112.5
142.3
112.5
100.6
112.5
142.3
112.5
100.6
112.5
142.3
112.5
0.36
0.36
0.26
0.34
0.27
0.19
0.13
0.20
0.25
0.17
0.11
0.11
0.16
0.14
0.10
0.15
0.04
0.04
0.03
0.04
0.03
0.02
0.01
0.02
0.03
0.02
0.01
0.01
0.02
0.01
0.01
0.02
5.92
5.90
4.23
5.60
4.49
3.11
2.09
3.26
4.19
2.78
1.74
1.83
2.65
2.36
1.65
2.48
0.62
0.62
0.44
0.59
0.47
0.33
0.22
0.34
0.44
0.29
0.18
0.19
0.28
0.25
0.17
0.26
0.29
0.29
0.21
0.28
0.22
0.15
0.10
0.16
0.21
0.14
0.09
0.09
0.13
0.12
0.08
0.12
Maximum 1-hour concentration
100.6 504.8
53.0
8353.2
874.0
412.0
5.3 DISPERSION MODELING FOR SITE B
The mathematical approach used for Site B was similar to that used for
Site A except that the two distinct contaminated zones were modeled
separately. Also Site B includes more sources because an onsite thermal
desorber/incinerator is used in the soil remediation (Figure 7).
The emissions from excavation, material in the bucket, and truck filling
were all combined into one area source for input into the ISCLT Model. The
exposed contaminated soil layer was considered a second adjacent area source
for the ISCLT Model. The emissions from the onsite truck transport were
modeled as a segmented series of elevated volume sources. Dumping into the
incinerator feed system was also considered a volume source. The incinerator
stack was modeled as a point source. The ISCLT Model was run twice—once for
each zone—because the emissions profile of chemical constituents for the two
51
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zones differs. A single model run could have been made, but It would have
required the model output to display the contribution of each source to each
receptor and the chemical constituent makeup of each source. By assuming two
separate remediation zones and the remediation of each performed sequentially
rather than simultaneously, the ambient concentration of each compound at
each receptor could be determined by simple multiplication of the fractional
contribution of each compound. Only for methyl ethyl ketone (present in both
zones) would the annual ambient concentration results have to be added
together.
The ISCLT modeling results for Site B included results from 12 defined
sources in Zone 1 and 16 defined sources in Zone 2. As expected, the maximum
ambient concentrations for remediation of each zone occurred at the fenceline
of the site due to low release heights of neutral buoyancy plumes.
Meteorological data representing Raleigh, North Carolina, were used in the
model runs. Emission rates for each source were converted to annual average
emission rates.
For Zone 1, the annualized VOC emission rates for excavation activities,
material in the bucket, and truck filling were estimated to be 1.10 x 10
2
g/s per m . The annual emissions from the exposed contaminants were
-5 2
estimated to be 1.920 x 10 g/s per m. Each area source was estimated to
2
cover 34.86 m , and they were located adjacent to each other at the midline
point and edge of the Zone 1 contamination for modeling purposes. The
emissions from truck hauling were divided into eight volume source segments,
-4
each emitting at an annual average of 8.94 x 10 g/s. Truck dumping into
the incinerator was also treated as a volume source with an annual average
o
emission rate of 1.15 x 10 g/s. The incinerator stack was treated as a
point source with an annual average emission rate of 1.41 x 10 g/s. The
coordinates (location) for each of these sources were also input into the
model.
For Zone 2, similar source data were calculated except that truck
hauling was divided into 12 volume segments rather than 8 as in Zone 1
because of a longer haul road. Also, the location of the sources (except for
the incinerator feed system and the incinerator system) is different in Zone
2. The input emission rates for the combined excavation sources were 6.39 x
10" g/s per m and 1.12 x 10 g/s per m for the exposed contaminated waste
layer. The emission rate for transport was 3.47 x 10 g/s for each segment
52
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Zone 1 Exposed Area
Source and Excavation
Area Emissions
Stte--»J_Ll
Center^*44
N
t
Zone 2
Exposed Are;
Source and
Excavation
Area Source
Emissions
:enceline
Receptors
Incinerator
Feed System
/Incinerator/Building
YB
5tack
-f
Figure 7. Site B source configuration and receptor locations.
53
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and 6.70 x 10"2 g/s for dumping emissions. The incinerator emission rate was
7.86 x 10"4 g/s.
The modeling results for each run indicated that the highest
concentrations would occur at the fenceline for the remediation of either
Zone 1 or Zone 2. The ambient concentration at each differ because of
differences in emission rates and the relative position of the various
sources. Table 32 presents the annual concentrations for each zone. All
receptor positions are relative to the site center.
The annual VOC concentrations at each receptor could be added together,
but it is more advantageous to determine the fractional contribution of each
chemical from each zone and then to add the appropriate VOCs. Only one
compound, methyl ethyl ketone, fits this additive category. Table 33
presents the annual concentrations of each compound. These values are based
on the fractional emission contribution from each zone (Table 28).
The emissions from Zone 2 produce higher ambient VOC concentrations at
the designated receptors than do the emissions from Zone 1 for two reasons.
First, emission rates from Zone 2 are higher than those from Zone 1. Second,
the length of time required to remediate Zone 2 is 4 months. This longer
period of emissions results in a higher annual ambient impact.
Short-term modeling with SCREEN is more complicated for this site
because of site orientation. Information presented in the site orientation
map for emission sources suggests that the highest emission potential differs
for remediation of the two zones. For Zone 1 the highest potential would
appear to occur when winds were aligned in a northwest-southeast direction.
During remediation of Zone 2, the highest potential would appear to occur
from a N-S or NNE-SSW wind orientation. This is one situation where the
ISCST Model would be helpful because of the complex site orientation. The
run-time penalties associated with the ISCST Model (20 to 24 h per run),
however, are substantial, and the speed associated with the SCREEN model make
it a better choice for estimating short-term ambient concentrations.
The actual predicted emission rates during the daily remediation
activities were used as input for the short-term SCREEN Model. The results
of the model runs for the individual source groups indicate that emissions
during dumping at the incinerator feed system have the highest potential for
ambient impacts, followed by losses during truck hauling and emissions during
excavation activities. This is consistent with the results of the emission
54
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TABLE 32. ANNUAL AMBIENT VOC CONCENTRATIONS AT FENCELINE FOR SITE B
Azimuth,
degrees
0
22.5
45.0
67.5
90.0
112.5
135.0
157.5
180.0
202.5
225.0
247.5
270.0
292.5
315.0
337.5
Range,
meters
122.5
137.4
173.2
137.4
122.5
137.4
173.2
137.4
122.5
137.4
173.2
137.4
122.5
137.4
173.2
137.4
Zone 1
concentration,
A»g/m3
2.65
1.92
1.10
1.20
1.08
0.76
0.59
0.99
1.80
1.38
0.85
1.09
1.38
1.29
0.83
1.44
Zone 2
concentration,
**g/m3
12.47
17.33
9.28
11.26
8.49
5.56
3.78
6.97
10.22
6.71
4.14
4.37
5.66
5.18
3.95
5.56
TABLE 33. AMBIENT CONCENTRATION OF EACH CHEMICAL AT
FENCELINE RECEPTORS--SITE B
Azimuth,
degrees
0
22.5
45.0
67.5
90.0
112.5
135.0
157.5
180.0
202.5
225.0
247.5
270.0
292.5
315.0
337.5
Range,
meters
122.5
137.4
173.2
137.4
122.5
137.4
173.2
137.4
122.5
137.4
173.2
11375
122.5
137.4
173.2
137.4
Benzene,
A»g/m3
0.96
0.69
0.40
0.43
0.39
0.27
0.21
0.36
0.65
0.50
0.31
0.39
0.50
0.47
0.30
0.52
1-Butanol,
A»g/m3
0.23
0.17
0.10
0.10
0.09
0.07
0.05
0.09
0.16
0.12
0.07
0.09
0.12
0.11
0.07
0.13
Methyl ene
chloride,
/ig/m3
10.36
14.40
7.71
9.36
7.06
4.62
3.14
5.79
8.49
5.58
3.66
3.63
4.70
4.30
3.28
4.62
Methyl
ethyl
ketone,
^g/m3
2.98
3.17
1.74
2.03
1.63
1.10
0.79
1.39
2.24
1.58
1.01
1.13
1.45
1.34
0.94
1.47
o-Xylene,
^g/m3
0.59
0.82
0.44
0.53
0.40
0.26
0.18
0.33
0.48
0.32
0.21
0.21
0.27
0.25
0.19
0.26
55
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rate calculations. Although the visual assessment of the site plot would
suggest that the greatest alignment comes with a wind from the northwest when
Zone 1 is remediated, the high emission rate and the proximity of the
incinerator dump and feed system to the southern property line suggest the
highest 1-hour concentration would occur under F-stability class when the
winds are from the north. An ambient concentration (1-h average) of 3633
Mg/m3 was predicted by the SCREEN Model.
A similar situation occurs with the remediation of Zone 2. Alignment of
sources is greater with the winds from the NNW direction. The SCREEN Model,
however, predicts that the highest impact occurs when winds are from the west
because of the dumping area's proximity to the fence!ine. The highest
3
predicted 1-hour ambient concentration was 10,676 /*g/m during F-stability
conditions.
The SCREEN Model will not calculate the effect of multiple sources, and
a manual method had to be developed. When the SCREEN Model is run, the user
selects an array of discrete distances to estimate ambient concentrations at
various distances from the source. Through a visual procedure (a ruler
helps), different combinations of sources are estimated for different wind
directions. As the wind "crosses" each emission source, the distance to the
appropriate fenceline point is estimated and the ambient contribution from
that source to that receptor is determined. The ambient contribution from
each source is added to determine the total ambient concentration at the
receptor of interest. This procedure is similar to the ISC Model's receptor
estimation technique.
The sequential remediation of Zones 1 and 2 means that distinct 1-hour
maximum concentrations can be associated for each chemical during remediation
as a fraction of the total VOC release rate from each zone. Table 34
presents the peak 1-hour concentrations.
56
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TABLE 34. PEAK 1-HOUR CONCENTRATIONS FOR EACH COMPOUND
Compound
Benzene
1-Butanol
Methylene chloride
Methyl ethyl ketone
o-Xylene
Maximum 1-hour con-
centration, /*g/m3
1312
316
8871
2004.7
495
Zone
1
1
2
1
2
5.4 SUMMARY OF DISPERSION MODELING RESULTS
The dispersion model results for each site require several simplifying
assumptions. First, the emission rates at the excavation area were grouped
together into a single emission rate to reduce the complexity of the
modeling. Second, the position of the excavation was placed at one point
within the contaminated area. The midpoint of the long side of the
contaminated zone at the edge nearest the haul road was selected. Other
options are available and can be explored further. The model limitations,
however, prevent accurate modeling of the moving excavation source or
changing truck-hauling patterns in either version of the ISC Models.
Finally, each homogeneous area was modeled separately because it appeared
easier to determine the ambient effects separately and then to combine them
where appropriate.
As of now, the run-time penalties associated with the ISCST Model appear
to be too great to include the use of this model in a remediation site
evaluation. If use of the SCREEN Model indicates there may be problems with
short-term ambient concentrations, use of the ISCST Model may then be
warranted to provide a more detailed evaluation. The SCREEN Model produces
results quickly although they have to be added manually to determine the
effects at a receptor of interest. The ISCLT and SCREEN Model results also
indicate that where the receptor with the maximum ambient concentration will
57
-------
occur is not explicitly obvious because of differences in source strengths
and site geometry; however, most interested parties should be able to make a
good estimate.
Although these predicted ambient concentrations are often perceived and
used as absolute values, they really should be viewed as probabilities. The
ISC Models use historical meteorology data, and the predicted ambient
concentrations depend not only on the algorithms used but also on
meteorological characteristics. The implied assumption is that the
historical meteorology is representative of the actual or expected future
meteorology. The SCREEN Model is a little more specific in predicting
ambient concentrations if certain stability class and windspeeds are present.
It does not, however, use wind direction patterns to predict where, the
specific ambient concentrations will occur relative to the site orientation.
These limitations were considered in the use of these predicted ambient
concentrations in the site risk assessment.
58
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SECTION 6
RISK ASSESSMENT
The procedures outlined in this section are used to predict the risk
associated with exposure to the various VOCs. This risk assessment, which
depends on predicted ambient concentrations and risk values assigned for each
chemical, represents the last step in the process of evaluating the effects
of remediation.
6.1 GENERAL APPROACH TO RISK ASSESSMENT
Several elements must be considered in the risk assessment of a remedial
activity. These include acute and chronic exposure effects from exposure to
each chemical released during remediation and the carcinogenic and
noncarcinogenic effects. Although there is much debate as to the
appropriateness of such estimation procedures, guidance does exist for the
evaluation of chronic exposures to both carcinogens and noncarcinogens and
work is currently underway within the EPA to provide guidance in the
evaluation of acute (short-term) exposures.
The risk assessment procedures differ for carcinogens and
noncarcinogens. Data for evaluating risks were found in the Health Effects
Assessment Summary Tables (HEAST) manual for carcinogens and noncarcinogens.
Of the chemicals in this analysis only benzene and methylene chloride are
listed as carcinogens in the HEAST manual. Methyl ethyl ketone and o-xylene
are listed as noncarcinogenic toxicants, and 1-butanol is not currently
listed. Methylene chloride is also listed for having other noncarcinogenic
toxic effects. Risk assessments were conducted only for the compounds for
which data were available.
The noncarcinogenic toxicity effects are expressed as a function of
reference doses (RfDs). For the compounds noted, the chronic RfDs are
3
reported as mg/m for continuous, 24 h/day exposure over any given 1-year
period. A hazard index for each compound can be determined by the following
equation:
59
-------
UT (Annual concentration^
HIi ' RfD1 1 (9)
where HI^ - Hazard Index of compound i
RflL - Reference dose for compound i
(Annual concentration)j - Annual average ambient concentration of i
at the location of interest.
If more than one compound is involved, the hazard indices for each should be
calculated and then summed. In general, concerns about noncarcinogenic
toxicity effects would be expected only if
SH^ > 1.0
For the purposes of noncarcinogenic toxic effects, the ambient
concentration found from the ISCLT annual average may be compared directly
with the RfO values. When compounds have both noncarcinogenic and
carcinogenic effects, the RfD refers only to the noncarcinogenic endpoint. A
hazard index of less than 1.0 for any compound or groups of compounds may not
be protective for carcinogenic effects. The RfDs for the compounds used in
this study are 1) methylene chloride * 3x10 mg/m , 2) methyl ethyl ketone =
3x10 mg/m , and 3) o-xylene - 7x10 mg/m . Standard body weight and
breathing rates are assumed in the derivation of these values.
The carcinogenic potency factors listed in the HEAST manual are based on
chronic exposure extended over a 70-year lifetime. The unit cancer risk
values reported in the HEAST manual may be converted to a specific increased
lifetime cancer risk as follows for each carcinogen by the following
equation:
Increased Lifetime Cancer Risk « Ambient concentration (»g/m ) x
Unit Cancer Risk (^g/m3)"1 (10)
A limitation exists for remedial activities. The increased lifetime
risk values are based on 70 years of continuous exposure, whereas most
remediation activities will be completed in much less time. In the case of
the remediation scenarios presented, herein, the time required to complete
excavation is less than 1 year. The annual average ambient concentrations
determined by the ISCLT Model were based on conversion of the short-term
60
-------
emission rates to an annual average (as if the emission occurred over the
entire year at a reduced rate rather than in the 2- or 4-month time period
actually estimated). After remediation is completed, however, the ambient
concentration from remediation falls to zero. Thus, the annual ambient
concentration value for the cancer risk assessment must be adjusted by the
number of years required for remediation.
Adjusted ambient concentration * Annual ambient concentration x
No. of years for remediation/70 (11)
The number of years for remediation will always be greater than or equal
to 1 because the annual ambient concentration for remediation periods of less
than 1 year already reflect adjustment to an annual average emission rate.
The annual ambient concentrations estimated by the ISCLT Model for the
example sites must be divided by a factor of 70 because the remediation time
is less than 1 year. This methodology assumes cancer risk is the same
whether the dose is received over a short time period at high levels or over
a lifetime at very low levels. Although the correctness of the approach is
debatable, it is nevertheless the current policy of the Superfund program.
The National Contingency Plan (NCR) calls for the increased acceptable
cancer risk (aggregated for each chemical for each pathway) to be less than
1 x 10"4 (less than 1 in 10,000). The NCR also states that this acceptable
risk should be achieved for the maximum exposed individual (MEI). The MEI
has not been defined for this project. Fence!ine values were used for risk
comparison because they represent the highest annual concentrations.
6.2 RISK ASSESSMENT FOR SITE A
A quick comparison of the maximum ambient concentrations of methylene
chloride, methyl ethyl ketone, and o-xylene indicates that the
noncarcinogenic hazard index for each chemical and the chemicals in total is
very small; therefore, the chronic noncarcinogenic risks associated with
these compounds during remediation are small.
The carcinogenic risks associates with the remediation scenario are also
very small, well below the 10 threshold defined in the NCR. The highest
concentration values were taken from the fence!ine receptors (Table 31 in
Section 5) and converted to cancer risk values as follows:
61
-------
Benzene cancer risk - (0.36/70)(8.3 x 10"6) - 4.3 x 10"8 (12)
Methylene chloride cancer risk - (5.92/70)(4.7 x 10"7) - 4.0 x 10"8 (13)
-8
The aggregate cancer risk for these two compounds is 8.3 x 10 , which is
well below the acceptable increased cancer risk range.
Less guidance is available on how to evaluate the short-term acute
exposure that may occur. One method would be to compare the short-term
ambient concentration potential (8-hour average) with the Threshold Limit
Value (TLV) recommended by American Conference of Governmental and Industrial
Hygenists (AGCIH) or with the Permissible Exposure Limit (PEL) as regulated
by the Occupational Safety and Health Administration (OSHA). Site monitoring
contingency plans frequently reference these values with some safety margin,
but the selection of these values is relatively arbitrary at this time. None
of the short-term ambient concentrations approaches the TLV or PEL values.
6.3 RISK ASSESSMENT FOR SITE B
As in the case of Site A, the annual concentrations of the compounds
with noncarcinogenic toxic effects from Site B are very small when compared
with the RfD values; thus, the total hazard index would be considerably less
than 1.0. Chronic noncarcinogenic risks associated with the remediation of
this site are small.
The carcinogen risks associated with Site B are somewhat larger than
those for Site A, but well within the acceptable range defined by the NCP.
The highest concentration values were taken from fenceline receptors and
converted to increased cancer risk values as follows:
Benzene cancer risk = (0.96/70)(8.3 x 10"6) = 1.1 x 10"7 (14)
Methylene chloride cancer risk = (14.40/70)(4.7 x 10"7) = 9.7 x 10"8 (15)
The aggregate increased cancer risk for the two compounds is 2.1 x 10" ,
which is below the acceptable increased cancer risk range.
The short-term ambient concentrations are below the TLV and PEL values
in all cases except for methyl ethyl ketone. The ACGIH recommends a
not-to-be-exceeded ceiling value of 1500 /*g/m . The SCREEN Model results
62
-------
predicted that a value of 2005 ng/m could be expected under certain
meteorological conditions at the fenceline. The OSHA PEL for methyl ethyl
ketone for an 8-hour time-weighted average Is 590 mg/m . Again guidance is
not currently available to indicate whether this remediation approach would
present a problem.
6.4 SUMMARY OF RISK ASSESSMENT RESULTS
The results of the risk assessments for carcinogenic and noncarcinogenic
effects of the remediation and site scenarios indicate that the risks from
chronic exposure are within an acceptable range. This conclusion is based on
several assumptions, including site characterization, emission rate
estimates, ambient dispersion modeling, and the risk assessment procedures
themselves. Different assumptions or site characteristics might change the
conclusion. The risk assessment procedure is the least complicated because
straightforward guidance is provided for converting ambient concentrations to
risk.
63
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SECTION 7
CONCLUSIONS AND RECOMMENDATIONS
Analysis of remedial alternatives for a Superfund site depends on
several steps, including site characterization, emission rate calculations,
dispersion modeling analysis, and risk assessment. Each step in the process
clearly involves its own uncertainty factors, but the overall uncertainty
factor for the entire process is not clear.
It is assumed that Superfund site characteristics will be given,
including soil characteristics, extent and nature of the contamination, and
the location of the contaminated zones. It may be necessary to subdivide the
site into smaller homogeneous zones of contamination for remediation analysis
purposes.
The steps to be followed during remediation must be defined for the
analysis of ambient impacts. This requires some knowledge of the remedial
alternatives to be considered and a definition of the major subtasks within
the remedial alternative. Evaluating emissions generation on the basis of
exact equipment use or precise rates of remediation, may not be possible, but
typical equipment ranges could be selected for ranges of remediation rates.
Manually calculating the emission rates to estimate the individual VOC
rates from each step in the excavation is a time-consuming task. The several
basic formulas used to calculate emissions from each excavation step are
repetitive and sequential and each step requires a change of only three
variables. Emission rate calculations were performed for each of the five
chemicals identified in this project. These calculation procedures led to
two useful conclusions. First, the relative contribution after each
evacuation step to total VOC emissions is virtually constant throughout the
excavation effort. For example, if methylene chloride constitutes 88 percent
of the VOC emissions from the first step (excavation of the contaminated soil
zone), it will also constitute nearly 88 percent from the last step (dumping
to a storage pile). The equations predict that some slight decrease in
pollutant concentration of highly volatile compounds should occur because of
64
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losses at each step, however, the emission ratios between VOCs remain
essentially constant. The emission rate of any given compound depends on the
chemical's physical properties and on its initial concentration.
The second conclusion reached was that the release rate of each VOCs
from each remediation step in a homogeneous zone is constant with respect to
the overall VOC emission rate and is independent of the VOC concentration in
the soil. The equations depend more on the definition of the remediation
step characteristics than on the concentrations themselves. Thus, it may be
possible to determine the relative contribution of each VOC to the overall
VOC emission rate and to apply simple factors for the contribution of each
remediation step. For example, the excavation of the soil cap and the
contaminated soil zone produced 11.54 percent of the total estimated VOC
emissions for Site A (Table 15). Although the emission rate of each of the
five compounds varied (in g/cm soil handled or g/s), the percent
contribution for each individual compound to its total emission rate did not
vary. The emissions of each compound (benzene, methylene chloride, methyl
ethyl ketone, 1-butanol, and o-xylene) from the excavation step represented
11.54 percent of the total emissions for each compound (i.e., 11.54 percent
of all benzene emissions for the site came from excavation; 11.54 percent of
all methylene chloride emissions from the site came from excavation, etc.)-
If the ratios between emissions of various compounds are established from
their concentration in the soil (Table 14: 81.91 percent methylene chloride,
8.57 percent methyl ethyl ketone, 4.95 percent benzene, etc.) and the
relative contribution of each step in the remediation process is defined,
then simplified factors can be developed. The excavation step is the first
step of ths process and setting all emission rates relative to it produces a
series of multiplication factors for each compound in subsequent steps.
These are presented in Table 35. Thus, if the estimated emission rate for
_o
methylene chloride was 3.0x10 g/s from excavation, then the predicted
emission from the other step would be this value multiplied by the
appropriate factor. It should be noted that these factors would vary
depending on the remediation steps included (i.e., these factors differ when
onsite storage and incineration are included). But if well-defined scenarios
could be developed, then the appropriate multipliers could be developed.
Different ranges of remediation rates could be selected to represent typical
values.
65
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TABLE 35. REMEDIATION EMISSION FACTOR MULTIPLIERS FOR SITE A
Remedial activity Factor
Excavation 1.00
Bucket 0.39
Truck filling 0.58
Transport 5.23
Exposure of contaminated zone 1.47
The dispersion models themselves present some problems. Of necessity,
the position of a source cannot change between days, nor can a variable
emission rate be selected. These realities of Superfund remediation are not
adequately addressed by currently available dispersion models. These
limitations are probably more important for short-term modeling than for
long-term modeling. The SCREEN Model and the ISCLT Model were the ambient
dispersion models used to produce short-term and long-term results,
respectively. The ISCST Model was also considered, but computer run times
were too long for the scenarios modeled so it was dropped from consideration.
The strength of the ISC models is their ability to estimate impacts from
multiple sources at specific receptors of interest under site meteorological
conditions. Satisfactory run times are achieved for the ISCLT Model, and
short-term emissions were converted to annual averages for ISCLT.
The principal advantages of the SCREEN Model are that it is fast and
easy to use. For simple sites with one or two emission points, the SCREEN
Model should produce satisfactory results. As the sites become more complex
and contain more sources, the inability of the SCREEN Model to account for
multiple sources necessitates performing more manual calculations and
graphical determinations of orientation and impacts of specific sources on
specific receptors. In some situations, site orientation may be very
complex, and determination of the short-term ambient impacts may be
difficult. If complex sites are encountered, the use of ISCST may be
warranted with only a few receptors of interest (to shorten the computer run
time) to determine if potential problems exist with a proposed remediation
scenario.
66
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The risk assessment for each site involved both carcinogenic and
noncarcinogenic compounds. Values for reference doses for chronic exposure
to the noncarcinogens may be compared directly with the ambient
concentrations predicted by the ISCLT model. The hazard indices for the
noncarcinogens at both sites were well below the levels of concern for
chronic exposures. The values for carcinogens had to be adjusted for
comparison of predicted ambient concentrations with the unit cancer risk
values. The published unit cancer risk values are based on chronic 70-year
exposures, and remediation of the sites took considerably less time. This
adjustment increased the "allowable" concentration for a specific risk value
by a factor of 70 for both sites. With these adjustments, the computed
-4
increased cancer risks were well below the 10 threshold specified in the
NCR.
The approach used for this set of evaluations is not the only
methodology available. Judgment may be exercised in subdividing a site,
other computation methods may be used to estimate emission rates, and
different methods may be used for setting up dispersion models or for
selecting dispersion models.
The manual computations for the excavation of the two sites produced
detailed emission rate estimates for each chemical and each excavation step.
For a single chemical in a homogeneous zone, a minimum of 20 computational
steps are required to estimate emission rates for each excavation step.
Although the equations used are identical (requiring only changes in the
physical values that define the step) and sequential (requiring the
accounting of VOCs lost from all previous steps) the potential for
calculation errors becomes greater as the complexity of the site increases.
The addition of more chemicals or more homogeneous areas increases the number
of calculations substantially. Compared with other requirements for site
evaluation, the emission rate calculations took a substantial amount of time.
Two other approaches may be used to estimate emissions. The first
involves establishing a graphical procedure with simplified "look-up" tables.
Standardized remediation rates based simply on low, medium, and high rates
could be established with standardized equipment specifications. Because
emission rates are a function of chemical and physical characteristics and
chemical concentration, it may be possible to assign numerical values to
these more common compounds based on the vapor pressure, molecular weight,
67
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and diffusion coefficients of the compounds. Calculation procedures could be
provided for compounds not included in the listing, adjustment factors could
be provided for different soil types, and the emission rates could be
provided for the initial excavation step. All other emissions from hauling
and dumping would be multiplied factors of the initial excavation emissions.
A graphical procedure to evaluate ambient impacts from remediation could
be provided on the basis of exposed area and distance to receptors. From the
graphical determination of dispersion coefficients and emission rates, an
estimated ambient concentration for the compound(s) of concern could be
determined. This would then be compared with the unit cancer risk values to
estimate the risk associated with the excavation remedial option.
The second approach would involve computerizing the calculation
procedures. The emission rate equations certainly lend themselves to this
procedure because the equations require replacement of variables related to
the chemical and to the process step involved. A data base for chemicals
with appropriate physical values that are automatically selected could be
incorporated into the program. Factors such as soil characteristics could be
defined with default values or could allow for user changes.
Ideally, the computerized approach should allow the user to answer a few
questions about the site characteristics, site orientation, and
concentrations in the soil; calculations for emission rates, ambient
concentrations, and site risk assessments could then be automatically
performed. This may be possible but the recommended approach would be to
develop a modularized calculation system. One module would be a data base
for chemicals, another would be a module for calculating emission rates from
various remediation techniques, and a third could include health effects
data. Modules could be updated as new data become available. The major
limitation would appear to be the assembling of information into the proper
formats so that the ambient dispersion models would run properly in a
complete package.
The graphical approach might be used as a conservative, quick, manual
screening method for estimating impacts from remediation of a simple site. A
more complex site or one that indicates potential problems might call for
more refined modeling or for the more intensive and faster calculating
ability that can be incorporated into a computer program. Very large and/or
complex sites appear to require the abilities of a computer to estimate
68
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emission rates and the combined ambient effects from remediation because the
effects from multiple sources are not always Intuitive. Some simple method
of evaluating a remedial alternative is necessary to evaluate risk, as
required by the NCR.
69
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REFERENCES
U.S. Environmental Protection Agency. Superfund Exposure Assessment
Manual. EPA 540/1-88/001, April 1988.
Radian Corporation. Estimating VOC Emissions During Soil Handling
Operations at National Priority List (NPL) Sites, Technical Note.
November 13, 1989.
U.S. Environmental Protection Agency. Health Effects Assessment Summary
Tables.
U.S. Environmental Protection Agency. Risk Assessment Guidance for
Superfund Volume I. Human Health Evaluation Manual (Part A). Interim
Final. EPA/540/1-89/002, December 1989.
70
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TECHNICAL REPORT DATA
(fttast nod Instructions on the reverse be fort completing
1. REPORT NO.
EPA-450/4-90-014
3. RECIPIENT'S ACCESSION NO.
4. TITLE AND SUBTITLE
Air/Superfund National Technical Guidance Series Develop
ment of Example Procedures for Evaluating the Air ImpactjP
of Soil Excavation Associated With Superfund Remedial Ac
8. REPORT DATE
July 1990
>. PERFORMING ORGANIZATION CODE
ions
'.'AUTHOR(S)
Gary L.. Saund.ers
•. PERFORMING ORGANIZATION REPORT NO.
DCN 90-203-080-61-02
, PERFORMING ORGANIZATION NAME ANO ADDRESS
PEI Associates, Inc.
11499 Chester Road
Cincinnati, Ohio 45246
10. PROGRAM ELEMENT NO.
61
11. CONTRACT/GRANT WO.
68-02-4394
12. SPONSORING AGENCY NAME ANO ADDRESS
U.S. Environmental Protection Agency
Office of Air Quality Planning and Standards
Research Triangle Park, NC 27711
13. TYPE OF REPORT ANO PERIOD COVERED
Final
14. SPONSORING AGENCY CODE
16. SUPPLEMENTARY NOTES
16. ABSTRACT
The purpose of this project was to identify and define the computation requirements
for estimating the air impacts from the remediation of Superfund sites. Two example
sites employing soil excavation were selected because they represent a complex emission
source. The estimation of air impacts from these sites include factors such as source
type (point, area, or volume), location, and movement of the sources.
The procedures for the evaluation of the ambient impacts were divided into several
subtasks. These included site characterization, selection of remedial alternatives,
definition of remedial activities, estimation of emission rates for each remedial
activity, determination of ambient concentrations from dispersion modeling, and evalua-
tion of carcinogenic and noncarcinogenic risks based on dispersion modeling results.
The calculation of emission rates were used to estimate ambient impacts through
dispersion models. The ambient concentration at various receptors of interest were
compared to health based risk data and used to estimate an increased risk value at these
receptors. Carcinogenic and noncarcinogenic effects were considered. The purpose of
this effort, however, was not to produce a risk assessment at each site. Rather, it was
to outline a set of procedures that could be used, with existing tools, to assist in the
evaluation of air-pathway effects.
7.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS C. COSATI Field/Group
Air Pathway Analysis
Air Pollution
Superfund
Soil Excavation
Air Pathway Analysis
B. DISTRIBUTION STATEMENT
19. SECURITY CLASS (Thit Report)
21. NO. OF PAGES
20. SECURITY CLASS (Ttutpagt)
22. PRICE
F«m 2220-1 (R«v. 4-77) PREVIOUS COITION i> OBSOLETE
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