xvEPA
United States
Environmental Protection
Agency
EPA/600/R-14/277
Engineering Issue
Challenges in Bulk Soil Sampling and Analysis for
Vapor Intrusion Screening of Soil
TABLE OF CONTENTS
1 INTRODUCTION AND PURPOSE 1
2 BACKGROUND 2
3 RELATIONSHIPS AMONG VAPOR
AND OTHER PHASES IN SOIL
(PHASE PARTITIONING) 2
4 CHALLENGES IN BULK SOIL
SAMPLING AND ANALYSIS 4
4.1 Volatilization and Degradation
Losses 4
4.2 Sensitivity of Analytical Methods
for Bulk Soil Samples 4
4.3 Heterogeneity of Soil and
Contaminant Distribution 5
5 ALTERNATIVE OPTIONS FOR
MONITORING VOCS IN BULK SOIL 7
5.1 Visual Inspection for Black
Stains (PHCs) or Sudan IV Dye
Testing (PHCs and Chlorinated
Solvents) 7
5.2 Field Headspace Screening of
Soil Samples 7
5.3 Soil Gas Monitoring 7
6 OPTIONS FOR ENHANCING SOIL
EXCAVATION REMEDIES TO
REDUCE VAPOR INTRUSION
RISK 8
7 SUMMARY AND CONCLUSIONS 9
8
ACRONYMNS AND
ABBREVIATIONS
9 ACKNOWLEDGMENTS
10 REFERENCES
9
10
10
1 INTRODUCTION AND PURPOSE
This Engineering Issue Paper (EIP) discusses the benefits and
limitations of using bulk soil samples to assess vapor intrusion (VI)
risks from soil containing volatile organic compounds (VOCs).
Analyses of factors controlling the VOC concentration distribution
in soil and the sensitivity of current laboratory methods are used to
show that while bulk soil sampling and analysis may help delineate
source areas and determine the gross mass of contamination
present in a source area, they cannot adequately assess potential VI
exposures for most VOCs in undisturbed soil or in soil remaining
after excavation. To address this information gap, this EIP also
describes alternatives for monitoring soil VOCs and for enhancing
remedies at sites where soil excavation is being considered or used
for VOC-contaminated soils.
The U.S. Environmental Protection Agency (EPA) EIPs are a series
of technology transfer documents that summarize the latest
available information on selected treatment and site remediation
technologies and related issues. EIPs are designed to help remedial
project managers, on-scene coordinators, contractors, and other site
managers understand the types of data and site characteristics
needed to evaluate a technology for potential applicability to their
specific sites. Each EIP is developed in conjunction with a small
group of EPA scientists and with outside consultants and relies on
peer-reviewed literature, EPA reports, web sources, current ongoing
research, and other pertinent information.
Information in this document is for technical support and does not
represent EPA policy or guidance. The reader is expected to have a
basic technical background on the VI exposure pathway and how to
use groundwater and soil gas data in the context of a VI
investigation. For more information on the VI pathway, please refer
to the EPA VI webpage.1
1 http://www.epa.gov/oswer/vaporintrusion/.
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2 BACKGROUND
The 2002 draft EPA VI guidance and the 2015 EPA
Technical Guide for Assessing and Mitigating the Vapor
Intrusion Pathway recommend against using bulk soil
VOC concentrations for screening the VI pathway or
for deciding when no further action is needed at VI
sites (U.S. EPA, 2002b; 2015). The reasons for this
guidance include VOC losses during bulk soil
sampling and analysis and uncertainties associated
with soil partitioning calculations.2 However, both
documents note that bulk soil samples are useful for
determining the chemical composition and general
location of contamination in soil including whether
nonaqueous phase liquid (NAPL) is present.
VOC losses during sampling and analysis may be
minimized by submersing bulk soil samples in
methanol (U.S. EPA, 1996a; 2002c), but methanol's
presence in the sample leads to higher analytical
detection and reporting limits (i.e., lower sensitivity).
Heterogeneity in soil properties poses additional
challenges for bulk soil sampling because of the
difficulty obtaining representative samples given their
size (usually about 50 grams) relative to the scale of
contaminant concentration heterogeneities and the
amount of soil mass to be evaluated (Interstate
Technology & Regulatory Council [ITRC], 2012).
Another difficulty is that VOC bulk soil
concentrations corresponding to soil vapor
concentrations protective of the VI pathway can be
lower than typical bulk soil analytical method
detection and reporting limits for several common
VOCs, including trichloroethylene (TCE) and
perchloroethylene (PCE).
To better understand the difficulties described above
of bulk soil sampling, this document provides
information on:
• how contaminants may be distributed among
the solid, liquid, and gaseous phases in bulk
soil;3
• how partitioning equations can be used to
calculate bulk soil concentrations from soil
vapor screening levels;
• typical laboratory method detection limits
(MDLs) for bulk soil analysis; and
3 HOW HETEROGENEITY IN SOIL
PROPERTIES AFFECTS CONTAMINANT
DISTRIBUTION IN SOIL.
RELATIONSHIPS AMONG VAPOR AND
OTHER PHASES IN SOIL (PHASE
PARTITIONING)
VOCs in vadose zone soils partition among the solid,
aqueous, and gaseous (vapor) phases and may also be
present as a fourth, separate NAPL (Feenstra et al.,
1991; Feenstra, 2003; U.S. EPA, 2012a). Methods for
calculating bulk soil concentrations that correspond
to soil vapor concentrations for VI assessment can be
developed by applying commonly accepted
equilibrium partitioning relationships. Although
equilibrium between phases may not exist in the field,
for example, where biologically degradable
compounds such as petroleum hydrocarbons (PHCs)
are present or where processes induce relatively fast
contaminant transport (e.g., a soil vapor extraction
[SVE] system), equilibrium partitioning is a widely
used simplification in subsurface investigations and
modeling studies.
2 Phase partitioning calculations are used to calculate
groundwater vapor intrusion screening levels (VISLs);
however, VOC losses during ground-water sampling are
less likely than losses during bulk soil sampling and
analysis (Maskannec et al., 1989; U.S. EPA, 2002a).
3 In this paper, bulk soil concentration refers to the total
mass of a contaminant in a specific mass of dry soil,
most often with units of mg/kg (parts per million or
ppm) or |ig/kg (parts per billion or ppb). Also known as
whole or total soil concentration, a bulk soil
concentration includes contaminants that are sorbed to
or within the soil mass (solid phase), dissolved in soil
moisture (aqueous phase), present as vapors in soil gas
(vapor phase), and present as a pure liquid (nonaqueous
phase).
'hway Screening for Soil Excavation Remedies
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The mass of a VOC associated with the soil solid
phase is controlled by the amount of organic carbon
in the soil to which VOCs adsorb,4 and the VOC's
affinity for soil organic carbon, which is typically
expressed in terms of the VOC's organic carbon
partition coefficient (JC). The mass of VOCs
associated with the soil aqueous and vapor phases is
determined by the soil moisture content, the air-filled
soil porosity, and the VOC's equilibrium partitioning
relationship between vapor and water, as expressed by
its Henry's Law constant (HLC) (Thomas, 1990).
If NAPLs are not present and the VOC
concentrations in the soil phases are at equilibrium,
the bulk soil concentration (Cuk) corresponding to a
particular soil vapor phase concentration (Cmpo^) of
interest can be derived from the soil-water partition
equations and default soil properties provided in
EPA's Soil Screening Guidance Technical Background
Document (U.S. EPA, 1996b) as follows:
^vapor
HLC pb '
OC foe pb+ ew + HLC ej
where
bulk soil concentration (mass/mass), site
specific
soil vapor concentration (mass/volume),
site specific
dimensionless Henry's Law constant,
chemical specific
dry soil bulk density (mass /volume),
default = 1,500 kg/m3
organic carbon partition coefficient
(volume/mass), chemical specific
fraction of organic carbon in soil
(mass/mass), default = 0.006 (0.6%)
water-filled soil porosity (volume/volume),
default = 0.15
air-filled soil porosity (volume/volume) =
n - total porosity (volume/volume),
default = 0.43
These equilibrium partitioning relationships are also
used in calculating the EPA Regional Screening
Levels (U.S. EPA, 2014a).
If a pure-phase NAPL is present in the soil and the
soil is considered to be a closed system, the vapor
phase concentration (Cmpo^), which is typically
expressed as the mass of VOC divided by the total
volume of the vapor phase, can be related to the pure
or NAPL phase vapor pressure (Pv) by rearranging
the ideal gas law to the following:
-'vapor.NAPL
MW -Pv
R-T
Eq. 2
where
R
T
MW — molecular weight (g/mol)
Pv = vapor pressure (mmHg)
= universal gas constant (62.36367 x 10~3 m3 •
mm Hg • 1C1 • mol4)
= temperature (298.15°K = 25°C)
The saturated bulk soil concentration (G»/^™/) above
which NAPL is likely to be present in the soil is
calculated by substituting Cmpor,NAPL for Cmpor in
Equation 1 (U.S. EPA, 2002c):
[(Koc fo
(HLC 0a)] Eq. 3
HLL pb
where all other parameters are as defined above. In
this case, the solid, liquid, and gas phases contain the
maximum possible mass (at equilibrium) and are
considered saturated. In other words, Cw^/is the
bulk soil concentration that corresponds to the
maximum (i.e., saturated) dissolved, sorbed, and
vapor VOC concentrations in a soil. Any additional
VOC mass would necessarily be present as a NAPL
phase and will not result in higher vapor, dissolved, or
sorbed concentrations. Note that at or below the
saturated bulk soil concentration, most (>85%) of the
4 Organic carbon is usually the dominant sorbent in a soil
down to an organic carbon content of about 0.1%
(Brusseau, 1994; Rorech, 2001). In very dry soils, VOCs
can adsorb to mineral surfaces (Chiou and Shoup, 1985),
but this is not usually a significant fraction of the total
soil VOC mass because most natural soils are sufficiently
moist.
Vapor Intrusion Pathway Screening for Soil E>
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VOC mass is present in aqueous and sorbed phases,
whereas at concentrations above the saturated bulk
soil concentrations, most of the VOC mass is present
in the NAPL.
These relationships are used in Section 4 to calculate
bulk soil concentrations corresponding to soil vapor
concentrations of interest for VI assessments (i.e., soil
vapor screening levels and soil vapor concentrations
indicating NAPL is present), which will illustrate both
the benefits and limitations of bulk soil sampling for
the VI pathway.
4 CHALLENGES IN BULK SOIL SAMPLING
AND ANALYSIS
4.1 Volatilization and Degradation Losses
There is a potential for significant losses of VOCs by
volatilization, degradation, or a combination of the
two when collecting bulk soil samples in the field,
during storage prior to analysis, and during
subsampling and sample preparation in the
laboratory. Studies evaluating soil sampling and
analysis protocols have shown that VOC
concentrations can be biased low by a factor between
10 and 1,000 when methods to minimize VOC losses
are not employed (e.g., U.S. EPA, 1993; 2002a).
Soil sampling using EPA's SW-846 Method 5035
specifies immediate immersion of the soil sample in
methanol to minimize volatilization losses and
degradation (U.S. EPA, 1996a), but a field technician
needs to be vigilant and must work quickly (Indiana
Department of Environmental Management, 2012).
Additionally, the presence of methanol in the sample
reduces analytical sensitivity and elevates MDLs5 by
5 The MDL statistically defines the minimum
concentration of a substance that can be detected, with
99% confidence that the analyte concentration is greater
than zero. The reporting limit is set by the analytical
laboratory above the MDL to accommodate day-to-day
variation in laboratory instrument sensitivity. In general,
values between the MDL and the reporting limit
represent true detections whose concentration cannot be
reliably quantified.
one or two orders of magnitude. Methanol also is
flammable and can be dangerous to transport.
Alternative Method 5035a provides for field sampling
with certain sampling devices such as the EnCore and
Associated Design & Manufacturing samplers5-7 that
allow analysis using purge-and-trap sample extraction
techniques and do not involve methanol preservation
(U.S. EPA, 2002a). These devices minimize VOC loss
by confining the sample in a sealed zero headspace
chamber, with storage for up to 48 hours before
laboratory preservation and preparation for analysis.
They function well for cohesive, uncemented soils but
are not suitable for noncohesive or cemented soils
that can cause headspace to develop within the device
during or after sampling. The sensitivity of this
method is greater than that of the methanol
preservation approach, providing lower detection and
reporting limits. However, the sample size is small for
these devices,8 so problems of representative
sampling remain a limitation.
4.2 Sensitivity of Analytical Methods for Bulk
Soil Samples
Bulk soil analytical methods have MDLs typically
around 35 |J.g/kg or higher when methanol is used as
a field preservative (e.g., EPA Method 5035; U.S.
EPA, 1996a) and about one to two orders of
magnitude lower when methanol is not used (e.g.,
EPA Method 5035a; U.S. EPA, 2002a). Table 1
compares typical MDLs for both analytical methods
with bulk soil concentrations corresponding to target
subslab soil gas concentrations (i.e., subslab vapor
screening levels) from EPA's Vapor Intrusion
Screening Level (VISL) Calculator9 (U.S. EPA, 2014e;
2015). The bulk soil concentrations corresponding to
5 http: // www.ennovativetech.com/non-methanol-
sampling/en-core-sampler-information
7 http://www.associateddesign.com/catalog.pdf
8 For example, the EnCore device collects a single 5- or
25-g soil sample, and Associated Design and
Manufacturing's device produces 5- and 10-g samples.
9 EPA's VISL Calculator is available online at
www.epa.gov/oswer/vaporintrusion/documents/VISL-
Calculator.xlsm.
lhway Screening for Soil Excavation Remedies
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these subslab vapor screening levels were calculated
using Equation 1, default values for soil properties
from EPA's Supplemental Soil Screening Guidance
(U.S. EPA, 2002c), and the chemical-specific Henry's
Law constants (HLC) and soil/organic carbon
partition coefficients (JC) taken from the VISL
Calculator and shown in Table 1.
The calculated bulk soil concentrations corresponding
to the subslab vapor screening levels are 30 to 80,000
times lower than the Method 5035 methanol MDLs
for many VOCs, except for some VOCs that are not
very toxic and have high screening levels. These bulk
soil concentrations are also below the nonmethanol
MDLs for constituents of most concern for VI,
including TCE and PCE. Thus, for the VOCs that are
typically of concern for the VI pathway, bulk soil
samples analyzed using the currently available
analytical methods cannot adequately assess VI risks.
Either the nonmethanol or methanol method is
capable of evaluating the presence of NAPL sources
in soil. Table 1 includes the saturated bulk soil
concentrations (in column 6) calculated using
Equations 2 and 3, above which NAPL would be
expected to be present in the soil; in all cases, these
concentrations are orders of magnitude higher than
the MDLs. Soil with NAPL generally represents the
bulk of the mass of VOCs in the vadose zone
sources, so delineating and excavating the NAPL-
containing soil represents the greatest opportunity for
mass removal and is an appropriate application for
bulk soil VOC analysis. The remaining VOCs in
unexcavated soil can often be more cost-effectively
remediated by polishing steps that may include natural
attenuation, SVE, or bioventing; see Section 6 for
additional information.
The calculated bulk soil concentrations listed in Table
1 were derived assuming equilibrium partitioning
among phases, which is a widely used simplification
when evaluating subsurface contaminant distribution,
fate, and transport. Equilibrium partitioning is
expected when concentrations are steady over time
and sufficient time is available for equilibration
among phases. In actuality, equilibrium conditions
may not exist in the field because the kinetics of
phase-transfer mechanisms may be slower than the
rate of change in VOC concentrations in response to
changes in atmospheric temperature and pressure,
infiltration of rainwater, and water table fluctuations.
Furthermore, phase partitioning may not be perfectly
linear and reversible as the equations assume. For
these reasons, the bulk soil concentrations
corresponding to screening-level vapor
concentrations presented in Table 1 are approximate
and may be uncertain (i.e., generally lower than field
measured values) by an order of magnitude or more
(Garret al., 2010).
4.3 Heterogeneity of Soil and
Contaminant Distribution
Collecting bulk soil samples that represent the bulk
VOC concentration in soil can be challenging because
of heterogeneity in the soil properties (fraction of
organic carbon, porosity, and moisture content) that
control the mass of VOCs that can be held by a soil.
The spatial scale of soil heterogeneities can vary from
a few centimeters to a few meters depending on the
origin and composition of the soil. Soil moisture can
also vary temporally (Boulding and Barcelona, 1991;
Payne et al., 2008). This heterogeneity poses a
challenge for estimating average soil concentrations in
unexcavated soil, given the typical size (40 g or less)
and typical sampling density (often spaced meters
apart) of bulk soil samples. VOC vapor
concentrations in discrete soil samples may or may
not be representative of larger-scale average VOC
concentrations. Incremental sampling techniques such
as those developed by Hewitt et al. (2008) can
Vapor Intrusion Pathway Screening for Soil E>
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o>
Table 1. Calculated Example Bulk Soil Concentrations Corresponding to Generic Subslab Vapor Screening Levels, NAPL Saturation, and Detection Limits for Selected VOCs
Chemical Name
Benzene
Carbon Tetrachloride
Chloroform
Dichlorobenzene, 1 ,2-
Dichlorobenzene, 1 ,4-
Dichloroethane, 1,1-
Didiloroethane, 1,2-
Dichloroethylene, 1,1-
Ethylbenzene
Tetrachloroethane, 1,1,2,2-
Tetrachloraethylene
Toluene
Trichloroethane, 1,1,1-
Trichloroethane, 1,1,2-
Trichloroethylene
Vinyl Chloride
Xylenes
Target
Indoor Air
Cone. @
TCR =
1E-06 or
THQ = 1
(ug/m3)
3.6E-01
4.7E-01
1.2E-01
2.1E+02
2.6E-01
1.8E+OQ
1.1E-01
2.1E+02
1.1E+00
4.8E-02
1.1E+01
5.2E+03
5.2E+03
1.8E-01
4.8E-01
1.7E-01
1.0E+02
Toxicity
Basis
C/NC
C
C
C
NC
C
C
C
NC
C
C
C
NC
NC
C
C
C
NC
Target
Subslab Soil
Gas Cone.
for AF = 0.03
@TCR =
1E-06or
THQ = 1
(ug/m3)
1.2E+01
1.6E+01
4.1E+00
7.0E+03
8.5E+00
5.8E+01
3.6E+QO
7.0E+03
3.7E+01
16E+00
3.6E+02
1.7E+05
1.7E+05
5.8E+00
1.6E+01
5.6E+00
3.5E+03
Bulk Soil Cone.
Corresponding
to Target
Subslab Soil
Gas Cone.
(Eq. 1)
(Mi/kg)
5.4E-02
7.9E-03
8.7E-03
2.1E+02
2.0E-01
8.5E-02
2.6E-02
3.2E+00
3.3E-01
7.2E-02
4.0E-01
1.0E+03
1.2E+02
8.2E-02
2.1E-02
2.2E-03
4.0E+01
NAPL
Phase
Vapor Cone.
@25°C
(Eq.2)
(ug/m3)
4.0E+08
9.5E+08
1.3E+09
1.2E+07
1.4E+07
1.4E+09
4.2E+08
3.3E+09
5.5E+07
1.2E+08
1.7E+08
1.4E+08
8.9E+08
1.7E+08
4.9E+08
1.0E+10
4.8E+07
Saturated
(Eq.3.3)
Bulk Soil
Cone.
(Eq.3)
(ug/kg>
1.8E+06
4.8E+05
2.7E+06
3.6E+05
3.3E+05
2.1E+06
3.0E+06
1.5E+06
4.8E+05
5.4E+06
1.8E+05
8.1E+05
6.3E+05
2.3E+06
6.5E+05
3.9E+06
5.5E+05
Typical Bulk Soil
Method Detection
Limit
Methanol
(Mi/kg)
3.4E+01
3.4E+01
3.4E+01
1.7E+01
4.7E+01
3.4E+01
3.4E+01
2.9E+01
3.4E+01
5.0E+01
3.4E+01
3.7E+01
4.6E+01
7.0E+01
5.0E+01
5.5E+01
3.4E+01
Non-
methanol
(M8*g)
3.7E-01
9.4E-01
1.1E+00
5.4E-01
6.5E-01
7.2E-01
1.3E+00
9.2E-01
4.0E-01
4.5E+00
2.1E-01
3.4E-01
1.3E+00
5.9E-01
5.2E-01
9.4E-01
4.2E-01
Mol.
Weight
(g/mol)
78
154
119
147
147
99
99
97
106
168
166
92
133
133
131
63
106
Vapor
Pressure
(mm Hg)
95
115
197
1.5
1.7
272
79
634
9.6
13
19
28
124
23
69
2980
8.4
Koo
(m3/kg)
0.15
0.044
0.032
0.38
0.38
0.032
0.040
0.032
0.45
0.095
0.095
0.23
0.044
0.061
0.061
0.022
0.38
HLC
(-)
0.23
1.1
0.15
0.078
0.10
0.23
0.048
1.1
0.32
0.015
0.72
0.27
0.70
0.034
0.40
1.1
0.21
TCR = target cancer risk; THQ = target hazard quotient; AF = subslab to indoor air attenuation factor; C = cancer; NC = noncancer; NAPL = nonaqueous phase liquid; KOC = organic
carbon/water partition coefficient; HLC = dimensionless Henry's Law constant.
NOTE: The target indoor air subslab soil gas concentrations were calculated using EPA's Vapor Intrusion Screening Level Calculator, which is available at
www.epa.gov/oswer/vaporintrusion/docunients/VISL-Calculator.xlsni. The bulk soil concentrations corresponding to the target subslab soil gas concentrations were calculated using Equation 1.
The NAPL phase vapor concentrations were calculated using Equation 2. The saturated bulk soil concentrations were calculated using Equation 3. All bulk soil concentrations in Table 1 were
calculated as example values only for the purposes of this document. They were calculated using default values from U.S. EPA (1996b; 2002c) for the variables listed in Equations 1 through 3,
which may vary from actual site-specific conditions. The resulting values should not be applied to specific sites without a thorough review of the assumptions and defaults on which they are based.
1*
I
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minimize the influence of heterogeneity (ITRC, 2012)
but are subject to the same challenges as other soil
samples preserved in methanol, including detection
limits that are not low enough to fully assess VI risk.
In summary, representative average bulk soil
concentrations can be difficult to achieve for a given
volume of soil because of the spatial heterogeneity of
VOC concentrations in the soil, the volume of typical
soil samples relative to the soil volume that needs to
be characterized, and issues with VOC losses during
sampling and analysis.
5 ALTERNATIVE OPTIONS FOR
MONITORING VOCS IN BULK SOIL
Despite its limitations, bulk soil sampling can help
delineate heavily contaminated (e.g., source) areas and
confirm that high-VOC soil has been removed prior
to other management options such as polishing
remediation techniques or redevelopment with
appropriate building mitigation systems. Other
methods to define soil source area or excavation
limits include visual inspection, field headspace
screening, and soil gas monitoring.
5.1 Visual Inspection for Black Stains (PHCs)
or Sudan IV Dye Testing (PHCs and
Chlorinated Solvents)
PHCs associated with crude oil can readily be
identified by a characteristic black staining of soil. The
presence of chlorinated solvents and free-phase fuel
products that do not exhibit black staining can be
detected by testing with Sudan IV dye. Soil is placed
in a clear glass jar with water, and Sudan IV dye is
added to color the hydrocarbons red, which allows
them to be distinguished from water (U.S. EPA,
2004).
5.2 Field Headspace Screening of Soil Samples
Field headspace screening can be conducted by
placing 0.5 kg to 2 kg of excavated soil (or soil from
sidewalls or floors of excavated area) to about half fill
a sealed container and measure the VOCs in the
headspace over time (Fitzgerald, 1993; U.S. EPA,
1997; South Dakota Department of Environment and
Natural Resources, 2003). If pure-phase hydrocarbons
are present and the VOC concentration in the
headspace drops well below the NAPL vapor
concentration shown in Table 1, one can check for
rebound by closing the container, waiting a few hours
or agitating it for a few minutes, and retesting. If after
a few hours the VOC concentrations are back up
above the NAPL vapor concentration, the soil likely
has NAPL source material, and excavation of such
material will significantly reduce the overall VOC
mass in the soil. Temperature, soil and container
volume, equilibration time, and inertness of the
container material all need to be specified and kept as
consistent as possible during field headspace
screening to minimize error and obtain consistent and
comparable results (Fitzgerald, 1993; U.S. EPA, 1997;
South Dakota Department of Environment and
Natural Resources, 2003).
Field screening of headspace and soil gas probe
samples for total hydrocarbons can be conducted
using a photoionization detector (PID) for
chlorinated VOCs or a flame ionization detector
(FID) for PHCs. A field gas chromatograph can be
used to measure individual VOCs. In the vicinity of a
NAPL, PID or FID readings will likely go off-scale
(e.g., > 10,000 ppmv on a PID), or the FID may
flame out because there is not enough oxygen. The
range of these portable instruments is several orders
of magnitude, so they are easily sensitive enough for
source delineation. For example, the ITRC guidance
on petroleum VI recommends a value of 500 ppmv as
a PID/FID level indicative of a NAPL source and
provides additional useful indicator criteria for
identifying the presence of PHCs in soil (ITRC,
2014), as does Mass DEP (1996).
5.3 So/7 Gas Monitoring
Soil gas monitoring probes installed into undisturbed
soil or the intact soil in excavation sidewalls and
floors is commonly used as a line of evidence to
assess VI risks (U.S. EPA, 2015) and can be used to
determine whether further excavation is needed. The
Vapor Intrusion Pathway Screening for Soil E>
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probes need to be installed at a sufficient depth to
pass a leak test to ensure they are truly sampling soil
gas. The soil gas samples collected with this method
can be analyzed with methods (such as TO-15 and
TO-17) that have MDLs sufficiently low to directly
assess VI potential. Soil gas samples can be analyzed
in the field with a mobile laboratory/instrument or
shipped for analysis in a fixed laboratory.
However, pausing excavation operations to sample
and analyze soil gas may not be practicable. Leaving
an excavation open for a long period may pose safety
concerns and affect the excavation schedule and cost.
Additionally, soil vapor concentrations measured in
an excavation may not be representative of those that
may arise when buildings are constructed. There are
also complications in obtaining representative soil gas
samples if the soils are wet; soils under buildings are
likely to be drier than surrounding open areas subject
to recharge. Furthermore, soil vapor concentration
profiles in the absence of buildings may differ
considerably from profiles that may develop after a
building is constructed.
6 OPTIONS FOR ENHANCING SOIL
EXCAVATION REMEDIES TO REDUCE
VAPOR INTRUSION RISK
Soil excavation commonly is used to remediate VOCs
and can be the most practical and cost-effective
remedy where high concentrations of VOCs are
present and accessible at shallow depths (~20 ft or
less). A key parameter in designing an excavation
remedy is the bulk soil concentration that can remain
unexcavated, which is used to define the extent of
excavation. For example, EPA has developed and
published bulk soil screening levels for the
groundwater, direct ingestion, outdoor air inhalation
(of VOCs and fugitive dust), and dermal contact
exposure pathways (e.g., U.S. EPA, 1996b; 2002a).
As previously described, however, bulk soil sampling
and analysis is not sufficiently sensitive for most
VOCs to adequately evaluate the protectiveness of an
excavation remedy for indoor air exposures through
the VI pathway. An alternative approach for soil may
be to excavate as much source material (e.g., NAPL-
containing soil) as possible and employ polishing
remedies to bring the unexcavated soil concentrations
down to levels that will not lead to a VI concern. The
following examples of subsurface remediation
technologies can be used to augment or polish soil
excavation remedies (Suthersan, 1997; Nyer et al.,
2001; Van Deuren et al., 2002):
• SVE (U.S. EPA, 2013) to remove VOCs from
permeable soils above the water table, especially
after NAPL is removed. If the soil permeability
is too low for SVE, excavation to the extent
possible may be the best option for
contaminants that do not readily biodegrade
(Suthersan, 1997; Nyer et al., 2001; Van Deuren
etal.,2002);
• natural attenuation, bioventing, or both to
aerobically biodegrade PHCs in place (U.S.
EPA, 1994; 2012b;2014c);
• enhanced degradation to anaerobically degrade
chlorinated hydrocarbons in place (U.S. EPA,
2014b);
• backfill areas of soil excavation with low-
permeability, fine-grained material or other
material to create a barrier or decrease the
effective mass flux rate from the unexcavated
soil to the surface; and
• building mitigation systems such as active
subslab depressurization or ventilation, or vapor
barrier systems (U.S. EPA, 2008; 2014d).
PHC vapor concentrations in shallow aerobic
(oxygenated) soils will biodegrade naturally after
excavation of NAPL, especially if the soil surface is
left open to the atmosphere for a few years prior to
redevelopment (Trombetta, 2008). PHCs in
somewhat deeper soils or soil below buildings or
other low-permeability ground cover where oxygen
may be limited may benefit from a bioventing system
to bring additional oxygen to the subsurface.
Chlorinated hydrocarbons like TCE and PCE are
more difficult to biodegrade naturally in the vadose
>hway Screening for Soil Excavation Remedies
-------
zone because they usually require anaerobic (low
oxygen) conditions that can be difficult to maintain
above the water table. Biodegradation of chlorinated
hydrocarbons often requires adding nutrients,
catalysts, reducing agents, and other such
supplements to maintain anaerobic conditions and
enhance biodegradation.
In some cases, polishing techniques may not be
needed if sufficient VOC mass is removed and the
remaining mass can be shown to yield vapor
concentrations that decline over time sufficiently that
long-term VI risks become acceptable. This approach
entails estimating the VOC mass remaining (e.g.,
through soil concentration profiling) and evaluating
(e.g., through modeling or monitoring) the expected
decrease in mass transport rates that occurs when
only limited source material is available.
7 SUMMARY AND CONCLUSIONS
This EIP discusses the benefits and limitations of
bulk soil sampling for assessing VI risks from
contaminated soil and describes alternatives for
monitoring and enhancing soil remedies at sites where
soil excavation is being considered or used as part of
the remedy for VOC-contaminated soils. Topics
discussed include how VOCs may be distributed
among the solid, liquid, and gaseous phases in bulk
soil; how to calculate bulk soil concentrations
corresponding to soil vapor screening levels; how
those bulk soil concentrations compare with typical
laboratory detection limits for bulk soil analysis; and
the usefulness of those bulk soil concentrations for
assessing VI risks.
Bulk soil concentrations corresponding to VI
screening levels for soil gas were calculated using
equilibrium partitioning relationships and compared
with typical bulk soil analysis MDLs. This evaluation
indicates that bulk soil sampling is useful for
identifying source areas with high concentrations of
VOCs, such as where NAPL is present, and for
estimating the total VOC mass that may be present in
soils at a site. However, available analysis methods are
not sufficiently sensitive to detect VOCs in bulk soil
concentrations corresponding to typical VI screening
levels. Other challenges with bulk soil sampling and
analysis include the potential for low bias
(underestimation) of VOC levels due to loss during
sampling and analysis and the difficulty characterizing
the heterogeneity in VOC concentration distributions
in the bulk soil mass of interest.
Soil excavation can be an appropriate part of a VOC
contamination remedy, particularly if focused on
shallow accessible source materials with relatively high
concentrations of VOCs that are readily measured
with bulk soil samples. But because of the limitations
described above, soil excavation alone is not likely to
be cost effective for soil with relatively modest VOC
concentrations that may pose a VI risk but cannot be
detected with current bulk soil analysis methods.
Remedies to augment soil excavation in such cases
include SVE, bioventing, and natural attenuation (for
PHCs); enhanced/accelerated bioattenuation (for
chlorinated hydrocarbons); building structure
mitigation; and backfilling excavated areas with low-
permeability barrier materials that will reduce the
concentrations reaching the surface. Improved
understanding (e.g., through modeling or monitoring)
of the role mass flux plays in VI will help with the
assessment of how much source mass needs to be
excavated and what additional activities may be
needed to manage post-excavation VI risk.
8 ACRONYMNS AND ABBREVIATIONS
EIP Engineering Issue Paper
EPA Environmental Protection Agency
FID Flame lonization Detector
HLC Henry's Law Constant
ITRC Interstate Technology & Regulatory Council
MDL Method Detection Limit
NAPL Nonaqueous Phase Liquid
PCE Perchloroethylene
PHC Petroleum Hydrocarbon
PID Photoionization Detector
Vapor Intrusion Pathway Screening for Soil E>
-------
SVE Soil Vapor Extraction
TCE Trichloroethylene
VI Vapor Intrusion
VISL Vapor Intrusion Screening Level
VOC Volatile Organic Compound
9 ACKNOWLEDGMENTS
This Engineering Issue Paper was prepared for the
EPA Office of Research and Development (ORD),
National Risk Management Research Laboratory
(NRMRL), Engineering Technical Support Center
(ETSC) by RTI International under Contract No. EP-
C-l 1-036. Doug Grosse served as the EPA Task
Order Manager and technical lead. Robert Truesdale
directed the project for RTI. Additional authors were
Dr. Helen Dawson of Geosyntec Consultants (lead
author), Dr. Todd McAlary of Geosyntec, Chris Lutes
of CH2MHILL, and Dan Carr of Sanborn | Head
Associates. Helpful direction and comments were
received from Dr. Henry Schuver of the EPA Office
of Resource Conservation and Recovery, Hal White
of the Office of Underground Storage Tanks, and
Rich Kapuscinski of the Office of Superfund
Remediation and Technology Innovation. Dr. Dave
Mickunas of EPA's Environmental Response Team
and Dr. Blayne Hartman of Hartman Environmental
Geosciences peer reviewed this paper. The authors
express their gratitude for all of these reviews.
As the technology and science advances, interested
parties should further consult the body of literature
and experience that constitutes the state of the art for
vapor intrusion and soil excavation monitoring. For
additional information, interested parties may also
contact EPA ORD/NRMRL/ETSC:
Dr. John McKernan, Director
U.S. EPA Engineering Technical Support Center
26 W. Martin Luther King Drive, Mail Code-190
Cincinnati, OH 45268
513-569-7415
Reference herein to any specific commercial products,
process, or service by trade name, trademark,
manufacturer, or otherwise does not necessarily
constitute or imply its endorsement,
recommendation, or favor by the United States
Government. The views and opinions of the authors
expressed herein do not necessarily state or reflect
those of the United States Government and shall not
be used for advertising or product endorsement
purposes.
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