4>EPA
United States
Environmental Protection
Agency
d Water Issue
An Approach for Developing Site-Specific Lateral and Vertical Inclusion
Zones within which Structures Should be Evaluated for Petroleum Vapor
Intrusion due to Releases of Motor Fuel from Underground Storage Tanks
John T. Wilson1, James W. Weaver2, Hal White3
Abstract
1.0 Introduction
2.0 The Lateral Inclusion Zone
2.1 Process to Define the Lateral Inclusion Zone
2.2 Dissolved Contaminant Plumes in the Lateral Inclusion Zone
2.3 Steps to Apply a Lateral Inclusion Zone
2.3.1 Map and Estimate the Extent of Contamination
2.3.2 Define an Inclusion Zone
2.3.2.1 A Definition that Does Not Consider Ground water
Flow
2.3.2.2 A Definition that Considers Ground water Flow
2.3.2.2.1 Find the Average Direction of Ground water
Flow
2.3.2.2.2 Assign a Weight to the Extent of the Inclusion
Zone for the Direction of Ground water Flow
2.3.3 Determine if Additional Monitoring Points Would
Reduce the Extent of the Refined Inclusion Zone
2.3.4 Test the Inclusion Zone against Simple Transport
Calculations
3.0 The Vertical Inclusion Zone
3.1 Steps to Apply a Vertical Separation Distance to Core Samples
3.1.1 Acquire Core Samples for Screening
3.1.2 Screen Core Samples for Subsequent Laboratory
Analysis
3.1.3 Compare the Distribution of Contamination in
Sediment to the Vertical Separation Criteria
3.2 Steps to Apply a Vertical Separation Distance to Core Samples
4.0 Next Steps
5.0 Summary
Notice
6.0 References
Appendix A. Recommendations for Sampling and Analysis
Appendix B. Quality Assurance
Appendix C. Equations for Steady State Plume Calculations
'U.S. EPAORD, wilson.johnt@epa.gov
2U.S. EPA ORD, weaver.jim@epa.gov
3 U.S. EPA OUST, white.hal@epa.gov
ABSTRACT
Buildings may be at risk from Petroleum Vapor Intrusion (PVI)
when they overlie petroleum hydrocarbon contamination in the
unsaturated zone or dissolved contamination in ground water.
The U.S. EPA Office of Underground Storage Tanks (OUST) is
preparing Guidance for Addressing Petroleum Vapor Intrusion
at Leaking Underground Storage Tank Sites. The OUST
guidance provides general screening criteria that can be used
to identify structures that are at risk from PVI. The criteria are
used to determine if a structure is included within a lateral or
vertical zone where proximity to the contaminant might make the
building vulnerable to PVI. If the structure is within a lateral or
vertical inclusion zone, then additional investigation is necessary
to evaluate and manage exposure to the vapors.
This Issue Paper contains technical suggestions and
recommendations proposed by the U.S. EPA Office of Research
and Development for applying the criteria provided in the OUST
guidance. The Issue paper provides a graphical approach
to define a lateral inclusion zone based on the proximity of a
structure to the presumed maximum extent of contamination.
The presumed maximum extent of contamination is defined by a
perimeter of clean monitoring locations that are arranged around
the known source of contamination. The lateral inclusion zone is
extended past the presumed maximum extent of contamination
to allow for uncertainty of the concentrations of contaminants
in the space between monitoring locations. The Issue Paper
provides instructions and suggestions to use knowledge of
ground water flow to refine the lateral exclusion zone, and
reduce the area where additional investigation is necessary.
The Issue Paper provides recommendations on collecting and
analyzing core samples to determine the vertical extent of
contamination in the unsaturated zone, and water samples to
determine the extent of contamination in ground water. The
Issue Paper provides illustrations of the appropriate comparison
of the field data to the criteria in the OUST Guidance. In
combination, definition of lateral and vertical inclusion zones
makes the best use of site characterization data for assessing
the risk of PVI to structures at a LUST site. The procedures
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outlined in this Issue Paper provide a realistic data-
driven approach to screen buildings for vulnerability
to PVI.
1.0 INTRODUCTION
Vapor intrusion is a process whereby vapors of
hazardous substances move through unsaturated
soil and enter buildings. Occupants of the buildings
are exposed to the hazardous substances as
vapors in indoor air. The vapors may originate
from contaminated ground water or from light non-
aqueous phase liquids (LNAPLs). Underground
storage tanks (USTs) are regulated under Subtitle
I of the Solid Waste Disposal Act. Most USTs are
used to store motor fuel (e.g., gasoline, diesel
fuel) that is composed primarily of petroleum
hydrocarbons (PHCs). Releases of motor fuel from
a leaking UST may result in generation of PHC
vapors and can result in petroleum vapor intrusion
(PVI).
The U.S. EPA is developing Guidance for
Addressing Petroleum Vapor Intrusion at Leaking
Underground Storage Tank Sites (U.S. EPA,
2013a). The guidance provides general screening
criteria that can be used to identify structures that
are at risk from PVI. In general, structures are at
risk from PVI when they overlie masses of residual
LNAPL in the unsaturated zone, accumulations
of liquid LNAPLs at the water table, or petroleum
contamination dissolved in ground water at levels
that have the potential to pose a risk to receptors
through the vapor intrusion pathway.
The potential for human exposure from PVI may
be limited because of the biodegradability of
PHCs. The PVI Guidance provides recommended
screening levels for petroleum constituents above
which the potential for PVI should be considered. If
the available data on the distribution of petroleum
components in soil and ground water suggest
a reasonable possibility that PVI may impact
a structure, that structure is considered to be
contained within an inclusion zone, which implies
that additional investigation is necessary to evaluate
and manage exposure to the vapors.
As discussed in detail later in this document, the
inclusion zone considers both lateral and vertical
proximity to the vapor source (i.e., mobile LNAPL,
residual LNAPL, and dissolved contamination). All
structures in the immediate vicinity of the source
area are first evaluated to determine if they are
within the lateral inclusion zone. This approach
logically follows the typical site investigation as it
progresses over time from the source area outward
in the direction of ground water flow to the edges
of the dissolved plume. As more site-specific
information is compiled, the extent of the inclusion
zone may change. If any structure is within the
lateral inclusion zone, then it is further evaluated to
determine if it is in the vertical inclusion zone.
The lateral inclusion zone is discussed in Section 2.
The vertical inclusion zone is discussed in
Section 3. As described and illustrated in these
sections, it may be necessary to acquire additional
site characterization data before this approach can
be used with confidence to screen structures and
determine whether they are within the inclusion
zone for PVI.
Both lateral and vertical inclusion zones should be
delineated using site-specific data. A conceptual
site model (CSM) that integrates all available data
and information about a particular site should be
developed and continually refined as new data
become available. Especially near the beginning
of an investigation at a leaking UST site, there is
typically much uncertainty due to the lack of site-
specific data and information. To compensate
for uncertainty due to lack of data, the screening
criteria produce a larger inclusion zone. As more
data are integrated into the CSM, the degree
of uncertainty progressively diminishes. Thus,
the extent of the lateral inclusion zone can often
be reduced. However, improved understanding
necessarily takes time and resources.
If inhabited buildings or sites for future buildings
are not located within one or the other of these
inclusion zones, the vapor intrusion pathway may
be considered to be incomplete and no further
consideration of the pathway should be necessary
for these buildings. This assumes that there are no
preferential pathways for contaminant migration at
the site. This also assumes that conditions at the
site do not change. Factors to consider in deciding
whether to exclude sites from further evaluation
of PVI may include future land use, construction
of utility trenches through or near previous
contamination, increased ground water usage that
might change the direction of ground water flow,
and additional releases of contaminants.
Ground Water Issue
Developing Site-Specific Lateral and Vertical Inclusion Zones
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This Issue Paper contains technical suggestions
and recommendations proposed by the U.S. EPA
Office of Research and Development for applying
the criteria provided in OUST's PVI Guidance (U.S.
EPA, 2013a). The material in this Issue Paper is not
guidance from the U.S. EPA Office of Underground
Storage Tanks (OUST).
The federal UST program delegates authority to
implement an UST program to the states. Most
of the state agencies use a risk based approach
to manage vapor intrusion of PHCs and other fuel
constituents (U.S. EPA 1995, 2002). The staff of the
state agencies or the Indian nations that implement
the UST program may choose to implement another
approach to apply screening criteria recommended
by U.S. EPA (2013a). If they choose to implement
this approach, they may modify this approach to
make it more appropriate to their particular needs.
2.0 THE LATERAL INCLUSION ZONE
This section discusses methods to determine
whether proximity of a structure to a source of
contamination puts the structure at risk for PVI.
Contamination can be mobile LNAPL, residual
LNAPL, or a dissolved plume. It is important
to define a lateral inclusion zone based on the
separation distance between the structure and
monitoring locations that are known to be clean
instead of the distance from known contamination.
This is especially critical if the extent of subsurface
contamination is not well-defined, as there is no way
to know how far the contaminated material actually
extends from the source of contamination toward
the receptor.
Typically at the beginning of a leaking UST
investigation the full extent and location of
contamination and the direction of ground water
flow are not well-defined. An illustration of these
uncertainties is presented in Figure 1. Here a
leaking UST has impacted the five monitoring
wells initially installed to assess the extent of
contamination. Because all of the wells are
contaminated, the actual extent of contamination
cannot be determined. Because sufficient ground
water monitoring data have not yet been collected,
the direction of ground water flow has not been
determined. Given the uncertainty in the direction
of ground water flow, a contaminant plume could
conceivably migrate away from the source in a
variety of directions as shown.
The procedure described to define the lateral
inclusion zone is based on the assumption that
the closer together the monitoring points, the less
uncertainty there is about the extent and location of
contamination. Conversely, with fewer monitoring
points spaced farther apart the uncertainty is
greater. As monitoring points are placed closer
together and additional monitoring points are
installed to fill in the gaps in the monitoring network,
the extent of contamination is determined more
accurately. This concept is depicted schematically
in Figure 2, which shows a simplified relationship
between the location of clean monitoring points
and the extent of contamination. In this example,
contamination extends from leaking USTs in the
direction of a potential receptor, which has been
established by determining the ground water flow
direction. The extent of contamination is bounded
laterally by two clean monitoring points, but no well
is available to provide a boundary to the plume
in the direction of ground water flow. In Figure
2(a), contamination extends between two clean
monitoring points for an unknown distance and
may, therefore, impact a down-gradient dwelling.
This scenario may occur even if the clean wells
are closer together, as shown in Figure 2(b). In
Figure 2(c), an additional monitoring location has
been installed and determined to be clean, which
eliminates the illustrated building from consideration
for additional PVI investigation assuming that there
are no preferential transport pathways present
that could lead to PVI. This example illustrates
that ground water flow directions and monitoring
well locations should be carefully considered when
defining the lateral inclusion zone. Section 2.3.2.2
provides a methodology to account for ground water
flow direction and locating monitoring wells.
Extending the inclusion zone by a distance equal
to the distance between monitoring wells is an
arbitrary choice. This ratio is recommended as a
starting point. If a caseworker has local knowledge
that justifies either a greater or lesser ratio, that
local knowledge should be applied and the ratio
adjusted accordingly. The ratio should be based on
local regulatory policy and the distribution of existing
and potential receptors around the release site.
An Approach for Developing Site-Specific Lateral and Vertical Incli
Ground Water Issue 3
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(a) A plume produced by a
constant ground water flow
direction in uniform porous
media
A plausible extent
of contamination
II
o
^
^
Leaking
USTs
Contaminated
Monitoring Point
. Potential
Receptor
Subsurface
Contamination
Direction
k Ground Water
Flow
An equally
plausible extent
of contamination
(b) A plume produced under the same
conditions as in (a) above, except the
direction of ground water flow differs by
90 degrees.
(c) Potential extent of
contamination due to variable
ground water flow conditions.
Figure 1. Examples of Plausible Extent of Contamination for Hypothetical Petroleum Release
Ground We
for Developing Site-Specific Lateral and Vertical Inclusion Zones
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MP-1
MP-3
-i (b)
-2
Lea king USTs
Q Monitoring Point
^^ } Zone of Potential Contamination
Potential Receptor
Direction Ground Water Flow
MP-3
MP-2
Figure 2. Effect of the distance between clean monitoring points on the extent of the plausible zone of potential
contamination.
The ratio of one-to-one in the judgment of
the authors is a good point of departure for
unconsolidated media. In fractured consolidated
media, particularly if the hydraulic gradient is
aligned with fracture orientation, a larger ratio would
be appropriate.
Strictly speaking, no matter how close together
they are, the contaminant concentration between
two monitoring points is never known with absolute
certainty; it can only be extrapolated. Because there
is a practical limit to the number of monitoring points
that can be installed, there will always be some
degree of uncertainty. The techniques described in
this Issue Paper recognize the uncertainty inherent
in the site investigation process and represent
one approach for balancing between being
overly protective and not sufficiently protective.
Site-specific data regarding the actual extent of
contamination and its potential for migration are
necessary for defining the lateral inclusion zones.
2. ] Process to Define the Lateral Inclusion Zone
Figure 3 illustrates the process of defining the
inclusion zone. In this example, a first round
of sampling showed that the LIST resulted in
contamination of all five wells surrounding the
leaking LIST (red circles, e.g., representing borehole
locations). New monitoring locations were installed
to establish the extent of contamination (blue
circles). Soil samples and ground water samples
from the new location were found to be clean. In
this case, the maximum extent of contamination
may be presumed to be defined by the smoothed
shape bounding the clean monitoring points
(Figure 3(a)). EPA recommends that dwellings (e.g.,
House A) within the area of presumed maximum
extent of contamination are to be evaluated for
An Approach for Developing Site-Specific Lateral and Vertical lnclusi<
round Water Issue 5
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(a) The presumed maximum
extent of contamination is the
area bounded by clean
monitoring points: all
contaminated monitoring
points should be contained
within this area.
Presumed maximum extent of
known contamination
II Leaking
USTs
O Clean
Monitoring Point
p Contaminated
Monitoring Point
Potential
Receptor
Lateral
Inclusion zone
(b) Draw straight lines around
the perimeter of contamination.
These lines connect each
adjacent clean monitoring point.
Then draw perpendicular lines
(e.g., x', and y'), that are the
same length as the lines
connecting adjacent wells.
(c) To identify the lateral
inclusion zone, draw a line
connecting the endpoints of
the perpendicular lines and
the clean monitoring points,
Lateral
Inclusion Zone
D
Figure 3. Determination of lateral inclusion distance based on separation distance between clean monitoring
points
Ground We
for Developing Site-Specific Lateral and Vertical Inclusion Zones
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potential PVI impacts. While Houses B, C, and
D are outside the presumed maximum extent of
contamination, there is uncertainty about the extent
of contamination between monitoring locations,
particularly where monitoring points are separated
by a large distance.
The uncertainty in the presumed maximum
extent of contamination may be accounted for by
extending the inclusion zone beyond the presumed
maximum extent of contamination (defined by
the blue line in Figure 3(a)). This concept is
illustrated in Figures 3(b) and 3(c). Any building
within the inclusion zone defined in this manner is
recommended for further evaluation. If any portion
of a structure falls within the inclusion zone, all of
the structure is considered to be within the inclusion
zone. With this concept of inclusion zone, Houses B
and C, in addition to House A, are recommended to
be investigated for potential vapor intrusion impacts.
This example illustrates that more closely spaced
monitoring locations allow for greater certainty in
defining the areas likely to be impacted by vapor
intrusion and, generally, will reduce the areal extent
of the inclusion zone. This example also illustrates
that it is important to carefully consider the
placement of monitoring points relative to receptors,
so that portions of a building are not unnecessarily
included in the inclusion zone.
The lateral inclusion zone is defined by bounding
the plume with clean monitoring points. However,
defining the boundary of the plume is less important
in those parts of the site with no occupied buildings.
To minimize expense, monitoring points should be
located so they provide the most usable information
for both the initial site characterization effort and
any follow-up assessment of vapor intrusion. Be
sure to place monitoring points between the source
of contamination and any potentially impacted
buildings. This approach is followed in the example
presented below in Section 2.3. In the example, a
new well is placed in front of buildings that might
be down gradient of the source, but where the
edge of the plume is not well defined. In contrast,
no additional work is suggested in areas that were
upgradient of the source, or that did not have
structures that would be vulnerable to PVI. For
a new case the selection of the initial monitoring
locations should be related to the locations of
buildings. These locations can be chosen to
minimize the number of monitoring points installed.
2.2 Dissolved Contaminant Plumes in the Lateral
Inclusion Zone
Contaminant plumes are dynamic features and
generally necessitate three-dimensional monitoring
to assess the transient behavior of ground
water flow and the transport of contaminants. In
unconsolidated deposits, the contaminant plume
should extend down gradient in the direction
of ground water flow. However, a variety of
hydrological phenomena can change the direction
of ground water flow, including aquifer recharge
following rainfall or snow melt, changes in the
pumping of ground water, and tides or changes
in the stage of a nearby river. Heterogeneity of
geologic materials comprising the upper-most water
bearing zone also may influence the direction of
migration and extent of contaminant plumes. Plume
behavior in heterogeneous materials may be quite
different from that anticipated for homogeneous
materials. In some cases plumes may be either
narrower or broader, or bifurcated with lobes moving
in different directions.
Changes in the direction of ground water flow are
common at leaking LIST sites (see Goode and
Konikow, 1990; Mace et al., 1997; Wlson, 2003;
Wlson et al. 2005a; Wilson et al., 2005b). Figure 4
illustrates variability of ground water flow directions
at two leaking LIST sites. In Figure 4(a), the flow
direction as indicated by the cluster of arrows varies
by more than 90 degrees. The fluctuation of ground
water flow directions in Figure 4(b) ranges over
nearly 180 degrees. Determination of flow direction
may require periodic sampling over more than
one annual cycle to understand the ground water
flow regime at a given site. As the plume migrates,
appropriate adjustments to the sampling plan
should be made to ensure that potential receptors
continue to be protected.
EPA recommends that ground water elevations be
measured when the wells are sampled so that the
direction of ground water flow can be determined for
that particular sample round.
An Approach for Developing Site-Specific Lateral and Vertical Incli
Ground Water Issue "J
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Sampling Point
Approximate Scale in Meters
(a) An MTBE site in Elizabeth City,
NC. The arrows represent the
distance that water would move
in one year, based on the
direction and hydraulic gradient
present in a particular round of
sampling. The origin of the
arrows is the center of the LNAPL
source area. The black dots are
locations of monitoring wells.
The shaded area includes all the
monitoring wells with
concentrations of MTBE above 20
ug/l. Reprinted from Figure 3.1.
of Wilson etal. (2005a).
MW-12
MW-11
MW-10
MW-6
MW-9«
20 meters
MW-7 «MW-13
xJVlW-1
" Tank Pit
MW-4
MW-3
0.00 0.01 0.02 0.03 0.04
I I I I I I I
Hydraulic
Gradient
(b) A leaking UST site in New
Jersey. The arrows represent the
distance that water would move
in one year, based on the
direction and hydraulic gradient
present in a particular round of
sampling. The origin of the
arrows is the center of the LNAPL
source area (shaded gray).
(Wilson etal., 2005b)
Figure 4. Relationship between the distribution of contamination in ground water and the variation in direction and
magnitude of ground water flow
Ground We
for Developing Site-Specific Lateral and Vertical Inclusion Zones
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2.3 Steps to Apply a Lateral Inclusion Zone
There are four general steps in defining a lateral
inclusion zone:
IMap and estimate the extent of contamination at
the site with existing monitoring points.
2. Define an inclusion zone. Consider ground water
flow direction.
3. Determine if additional monitoring points could
be used to reduce the extent of the inclusion
zone.
4. If information is available, test the inclusion zone
against simple transport calculations, and adjust
the inclusion zone as required.
Srinivasan et al. (2004) used a site in South
Carolina as a case study to illustrate the
implementation of a software application that can be
used to identify the optimum locations of monitoring
wells. The Optimal Well Locator (OWL) is further
described in Section 2.3.2.2.1. This Issue Paper will
use the same site as a case study to define a lateral
inclusion zone for ground water contamination.
The data used in the case study are provided
as an illustration. They do not necessarily
reflect current conditions at the site, and have no
bearing on past or current regulatory action taken
by the South Carolina Department of Health and
Environmental Control.
v /
2.3.1 Map and Estimate the Extent of
Contamination
The first step is to obtain a map showing the
distribution of contamination and the location of
potential receptors at the site. Figure 5 in this
paper is a reproduction of Figure 5 originally
presented by Srinivasan et al. (2004). The source
of contamination is located in a commercial area
extending along an arterial highway. On the other
side of the contaminated area are four residential
houses. The contours on the map showing the
general distribution of contamination do not include
the houses that may potentially be impacted by
PVI; however, there are no clean wells between
the source of contamination and these potential
receptors.
Close examination of the contours shows that
the boundaries of the plume, even if based on an
interpolation scheme, are arbitrary; the location
of the 10 ug/L, 100 ug/L and 1,000 ug/L contours
are unsupported by data over most of their length.
There are no wells that bound the lateral extent of
contamination between the 10 ug/L contour and
the houses. The location of the toe of the plume
(i.e., the longitudinal extent of the plume) beneath
Circus Donuts is similarly unsupported by data by
any wells that define the longitudinal extent of the
plume. The contours present a highly subjective
depiction of the extent of contamination, limiting it to
the commercial area without justification based on
the data. As a result, the available data for this site
does not support understanding of potential impacts
to the neighboring houses.
2.3.2 Define an Inclusion Zone
It is not necessary for the first definition of the
inclusion zone to consider the direction of ground
water flow. At recent petroleum release sites,
this information may not be available. The most
conservative assumption is that contamination
can move in any direction, and that movement in
any particular direction is equally plausible. This
approach to define the inclusion zone is described
in Section 2.3.2.1.
If data are available that can be used to infer the
direction and magnitude of ground water flow,
then information on ground water flow can be
used to refine the Inclusion Zone. Approaches to
accomplish this are described in Section 2.3.2.2. In
addition, it may be necessary to install additional
monitoring wells to adequately define the
lateral exclusion zone. Approaches for selecting
appropriate well locations are described in
Section 2.3.3.
2.3.2.1 A Definition That Does Not Consider
Ground water Flow
The general approach was illustrated schematically
in Figure 3. Clean monitoring locations are used
to establish a boundary around the presumed
maximum extent of contamination. Then segments
are drawn that extend the lateral inclusion zone past
the presumed maximum extent of contamination.
The extension of the inclusion zone compensates
for the uncertainty in the true limit of contamination
in the space between the monitoring points. The
approach is applied to the case study beginning
An Approach for Developing Site-Specific Lateral and Vertical Incli
Ground Water Issue 9
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DORCHESTER ., Q Bnnl
PAWN & GOLD I1 9.800J
NOT SAMPLED
WELLN
3T FOUND
..
o
cc
cc
LU
LU
I
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cc
o
o
LJ
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3
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Figure 5. Distribution of benzene in ground water at a LIST release in South Carolina. The red arrows are the
distance that ground water would be expected to move in three years based on the hydraulic conductiv-
ity and porosity of the aquifer and the hydraulic gradient that pertained in a particular round of sampling.
The heavy blue arrow is the distance water would move under average conditions in five years. Circled
wells have concentrations of benzene less than the detection limit.
I 0 Ground We
for Developing Site-Specific Lateral and Vertical Inclusion Zones
-------
with Figure 6, which shows segments connecting
clean monitoring points to establish the maximum
presumed extent of contamination.
The figures in this Issue Paper were created
using an accompanying EXCEL spreadsheet titled
Inclusion Zone Calculations. The spreadsheet
can facilitate the calculations necessary to apply the
approach to other sites. The spreadsheet contains
two tabs that facilitate finding the angle between
well pairs. Use the following process to define an
inclusion zone on a map of a site. Using M.S. Word,
PowerPoint or some similar computer application,
"insert" a straight line over the line between two
monitoring wells on the map of the site. Then cut
the line and paste it onto the chart in the tab Angle
Comparison. Select the line segment and move it
around on the chart until the axis of rotation of the
line segment passes through the point (0,0). Then
open the tab Data Angle Comparison, and change
the value for the direction of a test angle (Cell D21)
by trial and error until the line in Angle Comparison
labelled "test angle" converges with the line pasted
into Angle Comparison. The value of the angle
where the lines converge is the direction of the line
segment.
Evaluation continues for well pairs moving clockwise
around the perimeter as defined by the clean wells.
See Table 1 and Figure 7. The direction of the line
segment between wells in Table 1 is presented in
Degrees from North with the first well named in the
line segment as the axis of rotation. A clockwise
rotation is a positive direction and a counter
clockwise rotation is a negative direction. The
direction of the new line segment associated with
each well pair is simply 90° less than the direction
of the segment between wells. The resulting lateral
inclusion zone is depicted in Figure 8.
Figure 6. Area enclosed by the perimeter of clean monitoring wells (shaded red).
An Approach for Developing Site-Specific Lateral and Vertical lnclusi<
round Water Issue ] ]
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Table 1. Calculations to correct the length of a new line segment for the probability that ground water will flow in
that direction. See Figure 6 for the line segments.
Line
between
Wells
MW-1 3 to
MW-16
MW-1 6 to
MW-1 7
MW-1 7 to
MW-1 4
MW-1 4 to
MW-1 3
Direction
Line Segment
between
Wells
Degrees right of
North
129
240
291
33
Direction of
New Line
Segment
Degrees right
of North
39
150
201
-57
Distance
between
Clean Wells
Feet
444
260
328
344
Weight on
New Line
Segment
0.0000
0.9451
0.1408
0.0000
Ratio New Line
Segment to
Distance Between
Wells
1
1
1
1
Length of
New Line
Segment
Feet
0
246
46
0
-57° = 33°-90°
39° = 129° - 90°
MW-14
150° = 240° - 90°
Figure 7. The area enclosed by the perimeter of clean monitoring wells (shaded red) with angles of line segments
that connect the monitoring wells measured clockwise from North. For the line from MW-14 to MW-13,
the angle is 33P past a complete circle. Extensions of the inclusion zone are directed 90° from the lines
connecting the monitoring wells. For example, between MW-13 and MW-16 the outward extension is
129° - 90° = 39°, and between MW-14 and MW-13 the outward extension is 33°- 90° = - 57°.
1 2 Ground We
for Developing Site-Specific Lateral and Vertical Inclusion Zones
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t
North
I
inclusion zone
based on existing
clean wells.
MW-14
MW-16
Figure 8. Lateral inclusion zone defined without using information on ground water flow directions.
2.3.2.2 A Definition that Considers Ground water
Flow
At sites where the flow field is primarily
unidirectional and the aquifer can be said to
be homogeneous and isotropic, contaminant
plumes tend to be elongated in the down gradient
(longitudinal) direction and extend to a smaller
degree in the lateral (transverse) direction. If
historical ground water monitoring data are
sufficient to provide a high degree of confidence
in defining the extent of the plume, then it may be
reasonable to reduce the extent of the inclusion
zone in the lateral direction in proportion to the ratio
of the longitudinal to the transverse extension of the
plume. To make the comparisons between lateral
and transverse extension of a plume, it is best to
have data describing the seasonal variability in flow
direction and velocity, and data from wet years and
dry years. Note: this information is not typically
available at the beginning of an investigation of a
leaking LIST. Therefore, more conservative criteria
are generally used, which results in a larger lateral
inclusion zone to compensate for the uncertainty
and variability in the ground water flow direction.
Panel (a) of Figure 9 depicts a situation in which
the plume is roughly circular, with extension in the
longitudinal direction (x) equal to extension in the
transverse direction (y). Though a circular plume
is not common, this situation may be encountered
when the ground water flow field is highly variable
throughout the year or when a ground water mound
forms beneath a tank excavation. In such a case,
the inclusion zone could extend outward from
clean monitoring points to the same distance as
the spacing between the monitoring points. Note
that the inclusion zone may also extend some
distance in a direction that may later (after sufficient
data have been collected) be considered to be
upgradient from the source.
Panel (b) of Figure 9 depicts a plume which
extends twice as far in the longitudinal direction as
it does in the transverse direction (or, to state this
differently, the plume only extends half as far in the
transverse direction as it does in the longitudinal
direction). In this situation, the lateral inclusion zone
could reasonably be extended in the transverse
direction half the distance of the spacing between
monitoring points along the sides of the plume.
In the longitudinal direction, the inclusion zone
would extend outward the same distance as the
spacing between clean monitoring points. Panel (c)
of Figure 9 is similar to Panel (b) except that the
An Approach for Developing Site-Specific Lateral and Vertical lnclusi<
round Water Issue 13
-------
(a)Y = X
(b) Y = % X
(c) Y = % X
%T\
(d) Adjustment in Lateral
Inclusion Zone
when Y = % X
Figure 9. Adjusting the Lateral Inclusion Zone to compensate for variations in flow directions.
longitudinal extension is four times greater than
extension in the transverse direction.
Panel (d) of Figure 9 applies this concept to define
an inclusion zone for flow in one predominant
direction, where the longitudinal extension is four
times greater than extension in the transverse
direction. The inclusion zone in the transverse
direction would extend outward only one-quarter
of the distance between the clean monitoring
locations. These adjustments to the lateral inclusion
zone can be made for a real site if additional ground
water monitoring data are available on the changes
in the hydraulic gradient and the flow direction for
several rounds of sampling.
The transverse extension of a plume is generally
presumed to be a consequence of transverse
dispersion in flowing ground water. Because
transverse dispersion coefficients are low (Gelhar
et al., 1992), as a practical matter, the transverse
extension of a plume more likely results from
variations in ground water flow direction over
time (Wilson et al., 2005a). Mace et al. (1997)
collected data on the variation in flow direction at
132 gasoline stations in Texas. The median of the
standard deviation of the direction of ground water
flow was 36 degrees. This extent in variation in the
direction of ground water flow can easily account
for the transverse extension of most plumes. At a
site in North Carolina, Wilson et al. (2005a) used
the elevation of water in wells to calculate the
direction of ground water flow for thirteen separate
monitoring events. The space occupied by the
plume of contaminated ground water was the same
as the space swept out by the variation in ground
water flow direction (see Figure 4(a)).
2.3.2.2.1 Find the Average Direction of Ground
water Flow.
The U.S. EPA provides a software application
that can be used to estimate ground water flow
directions. It was originally intended to guide the
placement of additional monitoring wells at a site
(Srinivasan et al., 2004). The Optimal Well Locator
(OWL) uses linear regression to fit a plane to the
elevation of ground water in wells during a particular
round of sampling. The slope of the plane provides
the best estimate of the overall hydraulic gradient
and direction of ground water flow during that round
of sampling. The OWL software is available at no
cost on an EPA web site (see http://www.epa.gov/
ada/csmos/models/owl. html).
1 4 Ground We
for Developing Site-Specific Lateral and Vertical Inclusion Zones
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The OWL computer application (Srinivasan et al.,
2004) was used to analyze data on water elevations
and fit a slope to the water table in each of seven
rounds of sampling. Data were available on the
hydraulic conductivity of ground water at the site,
and a value for the aquifer porosity was estimated
(Srinivasan et al., 2004). This information was used
to estimate how far and which direction ground
water would move under the conditions observed
during each particular round of samples.
^ An EXCEL file titled Inclusion Zone
Calculations is supplied with this issue
paper. The vectors are presented in the tab
Flow Vectors. The file contains the additional
calculations used for the case study. The file can
be used as a template to apply the calculations to
another site. Data entry is in the tab data Flow
Vectors.
*. x1
The seven vectors estimate the distance that
ground water would move at the site if it moved
for three years following the hydraulic gradient
in each of the seven rounds of sampling. The
seven flow vectors are presented in red (see
Figure 5); the average is represented by the blue
vector. In general, ground water flow was not
toward the residential houses, but some of the
vectors indicated that there might be a concern that
contamination might reach some of the houses.
Notice that the flow vectors vary in both direction
and length. Simply taking the mean and standard
deviation of the flow directions would give equal
weight to short vectors and long vectors. If we
assume that the variation in flow direction at the
site is random, we can use the normal frequency
distribution to estimate the fraction of the time that
flow might be in a particular direction. To do that,
we need to scale the variation in flow direction to
the probability distribution. As an approximation,
the flow direction was weighted by the lengths of the
vectors.
The magnitude of the hydraulic gradient at the site
varied from 0.01184 on 12/12/1995 to 0.02818 on
1/11/1999 (Table 2). Calculations of the Weighting
Multiplier for each sampling period are presented
under tab Weight Multipliers the Excel file
Inclusion Zone Calculations. The gradient in
each sampling period was divided by the smallest
gradient, and then the quotient was multiplied by ten
to calculate the Weighting Multiplier (expressed to
the nearest whole number). Results are presented
in Table 2.
The weighting is accomplished in the tab Weight
Calculator in the Excel file Inclusion Zone
Calculations. The flow direction for each particular
round of samples was entered multiple times into
a column of data. The number of times a direction
was entered was proportionate to the magnitude
of the hydraulic gradient on that date. The number
of times a flow direction is entered becomes the
weight assigned to the data from that particular
sampling date. The mean of all of the multiple
entries of flow direction is an estimate of the
average direction of flow, and the standard deviation
Table 2. Hydraulic gradients and flow directions were extracted for each round of sampling using OWL (red arrows
in Figure 4). For each round of sampling the hydraulic gradient was used to select a weighting multiplier
to be used to calculate an average flow direction and the standard deviation of the flow direction. The
weighting multipliers are the number of times a direction was entered in tab Weight Calculator of the
EXCEL file Inclusion Zone Calculations.
Date
1/25/1994
12/12/1995
10/30/1998
12/4/1998
12/21/1998
1/11/1999
3/29/1999
Gradient
0.0153
0.01184
0.01335
0.01186
0.02156
0.02818
0.0198
Gradient/Smallest
Gradient
1.29
1.00
1.13
1.00
1.82
2.38
1.67
Weighting Multiplier
(number of times to enter value in spreadsheet)
13
10
11
10
18
24
17
An Approach for Developing Site-Specific Lateral and Vertical Incli
Ground Water Issue ] 5
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of all of the multiple entries of flow direction is an
estimate of the variability in the direction of flow.
The proportionality factor is arbitrary. However, to
make the calculated mean and standard deviation
a reasonable approximation of the "true" mean and
standard deviation, the smallest hydraulic gradient
should be entered at least ten times.
If many values are available for the magnitude and
direction of ground water flow, entering the weighted
values of flow direction into the Weight Calculator
tab can be tedious. The tab Weight Calculator (2)
automates the process to some extent. Enter data
on the magnitude of ground water flow starting with
cell A33 and data on the direction of flow in cell B33.
Sort the entered data from the smallest value of
magnitude of flow to the largest value for magnitude
of flow. Click on cell C33, and select the box at
lower right with the mouse, then pull down to extend
the formula in rowC across all the cells. Multiply
the ratio of the gradient to the weakest gradient by
ten, and then enter the nearest whole number that
corresponds to the ratio in the corresponding cells
starting with D33.
The spreadsheet uses nested IF statements
to populate the weighted flow directions (X) in
column G. The spread sheet then calculates the
square of the weighted flow directions (X2) in
column H. Copy the numbers that are greater than
zero from cells in column G and H, select paste
special, and paste them into cells in columns I and
J as values. Excel 7.0 only allows seven nested
IF statements. If there are data available from
more than seven dates, insert the data from the
first seven dates, copy and paste the data from
columns G and H into columns I and J. Then erase
the data in columns G and H and insert the data
from the second seven dates. Copy and paste the
numbers from columns G and H into columns I and
J, inserting the new numbers below the previous
numbers. Continue the process until columns I
and J contain the weighted flow directions (X) and
the square of the weighted flow directions (X2) that
correspond to all available values for the magnitude
and direction of ground water flow.
The angles extracted using OWL were then
entered into tab Weight Calculator in the Excel file
Inclusion Zone Calculations. The flow direction
on 12/12/1995 was entered 10 times and the
flow direction on 1 /11 /1999 was entered 24 times
(weighting multiplier in Table 2). Similar entries
were made for the other dates. By following this
procedure all the multiple entries for all of the
dates were used to calculate an overall mean and
standard deviation. For this data set, the overall
flow direction was 157 degrees clockwise from
North, with a standard deviation of 22 degrees
(cells H11 and H12 of tab Weight Calculator).
To find the weight for a particular direction, enter
the direction in cell H7. The weight relative to the
average direction of ground water flow appears in
cell H9.
2.3.2.2.2 Assign a Weight to the Extent of the
Inclusion Zone for the Direction of Ground
Water Flow
The probability that ground water will flow in a
particular direction is taken as the solution to the
probability density function <|)(z).
1
V/27I
-V
, 2
The value of a particular direction of flow is
entered in cell H7 of tab Weight Calculator. The
spreadsheet calculates a z score for that particular
direction by subtracting the particular direction from
the mean direction, then dividing the difference by
the standard deviation. The z score is reported
in cell H15. For the value of z, the spreadsheet
calculates a value of the probability density function,
-------
is entered into cell C1 of the calculator in the tab
Weight Calculator it returns a weight of 0.0000.
The probability that water will move upgradient
across the line segment between well MW-13 and
MW-16 is so small that it can be ignored (weighting
factor less than 0.01). Weighting factors for these
line segments are presented in the fifth column of
Table 1. The weighting factor for the line segment
between MW-13 and MW-14 is also 0.0000, thus
for both of these segments, it is not necessary
to extend the inclusion zone. For the segments
between MW-16 and MW-17 and between MW-17
and MW-14, the inclusion zone extends outward,
but in both of these cases the distance is less than
that separating the monitoring wells. Figure 10
shows the reduced inclusion zone.
Tab New Line Segment of the Inclusion Zone
Calculations spreadsheet uses the distance
between the clean monitoring wells and the ratio
between the length of the new line segment and the
distance between the clean wells to calculate the
length of the new line segment.
2.3.3 Determine if Additional Monitoring Points
Would Reduce the Extent of the Refined
Inclusion Zone
Wth the information on the direction and length
of the new line segments between well pairs,
draw a new perimeter that connects the clean
wells and the ends of the line segments that are
projected from the mid-points between clean well
(see Figure 10). Compare Figure 8 and Figure 10.
Although the inclusion zone is much reduced, the
four houses that are immediately to the West of
the contaminated area are still in the inclusion
zone. There may be benefit in installing additional
monitoring points.
In Figure 11, a hypothetical new well is located
approximately half way between the region with
known contamination and the houses under
consideration. If the well is clean, for the cost of
one monitoring well, the inclusion zone can be
redefined and no longer includes the four houses
under consideration. Selecting the best location
for a new well involves a trade-off. If the new
well is located too close to existing contaminated
MW-13
MW-14
MW-16
Figure 10. A Lateral Inclusion Zone defined using information on ground water flow
An Approach for Developing Site-Specific Lateral and Vertical lnclusi<
round Water Issue 17
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MW-13
MW-14
Figure 11. One possible outcome of the evaluation after a new well is installed to better define the Lateral Inclusion
Zone.
wells, there is a good chance that it will also
be contaminated and will not help to refine the
inclusion zone. If a new well is located too close to
a structure (e.g., directly adjacent to the structure
of concern), it is possible that some portion of
the footprint of the structure will be in the lateral
inclusion zone, even if the well is clean.
After assessing the need for additional wells, install
those that are needed and sample and analyze
ground water to redefine the space assigned to the
inclusion zone. If a structure is contained within
a lateral inclusion zone, then the structure should
be evaluated to determine if it is within the vertical
inclusion zone as described in Section 3.0
The above discussion presumed that the initial
site characterization was conducted without
consideration of a lateral inclusion zone (or
petroleum vapor intrusion). Thus the lateral
inclusion zone extent is being added to the existing
site conceptual model. If the definition of the
lateral inclusion zone is planned initially as a part
of the site assessment, then some effort may be
minimized. For example, monitoring wells could be
located initially to assess building impacts, as was
done with the additional well placed in Figure 11.
2.3.4 Test the Inclusion Zone Against Simple
Transport Calculations
The contaminant transport equation provides a
means to forecast the distance that a contaminant
might travel with flowing ground water. Because
choices must be made for parameters whose true
values are unknown or uncertain, the forecasts
from the transport equation are rough estimates
rather than definitive guides. However, the rough
estimates provide a second line of evidence that
can be used to evaluate the inclusion zones.
Equations for a one-dimensional, steady-state
transport equation solution are given in Appendix C.
U.S. EPA provides a calculator to forecast plume
length with these equations at http://www.epa.
Qov/athens/learn2model/part-two/onsite/lenath.
html. The calculations are also provided under tab
Plume Lengths in the EXCEL file Inclusion Zone
Calculations.
For three of the monitoring wells in the case
study, an estimate of hydraulic conductivity (Ks)
1 8 Ground We
for Developing Site-Specific Lateral and Vertical Inclusion Zones
-------
was available from rising head slug tests in the
wells. Values were input into Column C of Plume
Lengths. The initial concentration of benzene along
the flow path was assumed to be the concentration
in the well. Values were input in Column K. The
final concentration along the flow path was taken
to be 0.14 mg/L. This is the target ground water
concentration corresponding to the target indoor air
concentration when the indoor air attenuation factor
is 0.001 (U.S. EPA, 2002). This value was input
into Column L. The average hydraulic gradient
was 0.0174 (Cell D11 in tab data Flow Vectors).
This value was input into Column E of tab Plume
Lengths.
The target ground water concentration is
derived from a target indoor air concentra-
tion for benzene of 31 ug/m3 (U.S. EPA, 2002
Table 2a). The air concentration was divided
by the dimensionless Henry's Law constant
(0.22 = mg/L in air divided by mg/L in water)
to get an equivalent concentration in water,
and then multiplied by 1000 to allow for at-
tenuation of concentrations between benzene
in soil gas beneath a building and concentra-
tions within the building.
>*^ _^
Ground water contaminated with petroleum
hydrocarbons is consistently anaerobic. Suarez
and Rifai (1999) reported that the mean rate
constant for anaerobic biodegradation of benzene
at 45 field studies was 0.003 per day, corresponding
to a half life of 230 days. Falta et al. (2012)
recommends a first order rate constant of 1.1
per year (equivalent to a half life of 230 days)
to model anaerobic degradation of benzene at
gasoline release sites. Data from a variety of
field and laboratory studies are collated in the tab
Rates of Benzene Degradation in the EXCEL file
Inclusion Zone Calculations. Most of the rates
were published in Aronson and Howard (1997). The
median half life was 248 days.
A half life of 230 days was used to make the first
estimate of plume length, and as a sensitivity
analysis, a half life of 693 days was also used to
estimate plume length. A value for the degradation
half life of 693 days includes 75% of the half lives
collated under tab Rates of Benzene Degradation.
Values for half life are input in Column G of tab
Plume Lengths. A sensitivity analysis was also
performed with reasonable values of the effective
porosity. Values of 0.20 and 0.25 were input into
Column D of tab Plume Lengths.
In Column O of tab Plume Lengths, the
spreadsheet calculates the lengths of the plumes
that are forecast for these specified conditions.
The calculations use a value for the longitudinal
dispersivity (a) that is input in Column I of tab
Plume Lengths. The spreadsheet uses the
formula of Xu and Eckstein (1995) to estimate an
appropriate value of a from the calculated length.
Manually input different values for a into cells in
Column I until the input value in Column I matches
the calculated value in Column J. When values in
Columns I and J agree within a foot, the value for
the plume length in Column O can be taken as the
forecast of plume length.
Table 3 provides the plume lengths from the
sensitivity analysis. As a worked example, the
forecast of plume length for the plume originating
from MW-11was calculated as follows. Where
the hydraulic gradient (H) is 0.0174 foot per foot,
the hydraulic conductivity (K5) is 1.66 feet per day,
and the effective porosity (6) is 0.25 ft3 per ft3; the
seepage velocity (v) is:
v =
H*Ks _0.0176* 1.66
6 0.25
= 0.1156feet per day
Where the half life of natural biodegradation
is 230 days, the first order rate constant (A,) is
0.003013 per day. For well MW-11, the initial
concentration of benzene (c0) is 4.5 mg/L (see
Figure 4). As mentioned above, the acceptable
concentration of benzene (c) is taken to be
0.14 mg/L. Input of trial values for the longitudinal
dispersivity (a) into Column I of tab Plume Lengths
predicts a value of a of 10 feet, based on the plume
length equation from Appendix C.
2oc In c/
2*12*ln
x =
5 _
0.14
4.5
1-
4*0.003013*10
0.1156
= 162feet
Figure 12 plots the plurne lengths in Table 3 against
two configurations of the inclusion zone. The
An Approach for Developing Site-Specific Lateral and Vertical I
' Water Issue 19
-------
Table 3. Maximum plume length forecast from the maximum concentration of benzene in a monitoring well, the
hydraulic conductivity at that location, and an estimate of effective porosity and degradation half life.
Concentration
(mg/L)
Hydraulic
Conductivity
(feet per day)
Effective Porosity
ft3/ft3
Half Life
(days)
Maximum
One-dimensional Plume
Length
(feet)
MW-9
6.3
6.3
6.3
6.3
0.77
0.77
0.77
0.77
0.20
0.20
0.25
0.25
230
693
230
693
106
300
88
242
MW-11
4.5
4.5
4.5
4.5
1.66
1.66
1.66
1.66
0.20
0.20
0.25
0.25
230
693
230
693
198
560
162
453
MW-6
1.89
1.89
1.89
1.89
2.21
2.21
2.21
2.21
0.20
0.20
0.25
0.25
230
693
230
693
191
546
153
440
MW-13
MWM4
MW-16
Figure 12. Comparison of forecasts of plume lengths to two configurations of the inclusion zone.
20 Ground We
for Developing Site-Specific Lateral and Vertical Inclusion Zones
-------
red arrows originating at wells MW-9, MW-6 and
MW-11 in Figure 12 correspond to the forecasts
associated with a porosity of 0.25 and a half life of
230 days. The blue arrows correspond to a half
life of 693 days. The arrows extend in the average
direction of ground water flow.
In the red-colored inclusion zone, the ratio of the
distance that the inclusion zone extends past the
clean wells to the distance between the clean
wells is set at 1.0. The lengths of the plumes that
are predicted from the average rate of benzene
biodegradation in ground water (red arrows) are
contained within the red inclusion zone. For
average conditions, there is no evidence from the
forecast that the inclusion zone is not protective.
However, this is not the case for plume lengths
that are based on a rate of degradation that would
include 75% of rates in the literature (the blue
arrows). The forecast plume lengths from wells
MW-6 and MW-11 extend past the red inclusion
zone. To make the inclusion zone conform to the
forecast for well MW-6, it was necessary to set the
ratio at 2.0 (blue-colored inclusion zone. It is not
possible to adjust the inclusion zone to include the
forecast from well MW-11 with any reasonable ratio
of distances.
This process should be repeated for every well
within the area enclosed by clean monitoring wells
using well-specific input parameters. The forecasts
have the most value to understand the expected
locations of the plume where no monitoring data
are available (such as the forecast from well
MW-6). The forecasts have less value for regions
that are represented by real monitoring data (such
as the forecast from well MW-11 compared to the
measurement at well MW-17).
Although the inclusion zone seems to be greatly
expanded by the forecast, it must be recalled
that there are no monitoring data to support the
assumed location of the toe of the plume (Figure 5).
Adding a monitoring well in the primary direction
of ground water flow would greatly increase the
credibility of the site assessment, and very likely
reduce the size of the inclusion zone.
If information is available about the flow of ground
water at the site, this information can be used to
adjust the configuration of the inclusion zone. If
information about the flow of ground water is not
available, then the configuration of the inclusion
zone must be determined by professional judgment
or by local policy. Over time as information is
collected on actual impacts to residences and
the impact that was predicted by a particular
configuration of the inclusion zone, it will be
possible to optimize this screening process.
3.0 THE VERTICAL INCLUSION ZONE
After characterizing the extent of contamination and
defining a lateral inclusion zone, there still may be
a number of residences potentially at risk for vapor
intrusion. At this point, the vertical separation
criteria should be applied. Table 4 provides example
vertical separation-distances based on Davis (2009)
and Cal EPA (2012). The separation distance for
ground water contamination is the distance between
the lowest part of the structure of concern and the
highest historical elevation of the water table. The
separation distance for LNAPL is the minimum
extent of clean soil that is required between the
contaminated sample and the receptor. It is not
the separation distance between the contaminated
sample and the receptor. There may be additional
contamination in soil between the sample and the
receptor. In addition, data on the stratigraphy at the
site, which should be incorporated into the CSM,
should be considered in determining whether there
is sufficient oxygen in the subsurface to promote
aerobic biodegradation or whether relatively
impermeable layers may prevent the intrusion of
vapors into overlying buildings.
The limits on the vertical separation distance
that would cause a structure to be included in a
vertical inclusion zone are based on experience
with biodegradation of vapors of petroleum
hydrocarbons in the unsaturated zone (Lahvis et al.,
1999; API, 2000; DeVaull, 2007; Davis, 2009; Cal
EPA, 2012). There are two important assumptions
in applying the vertical separation distance: that
the soil is "clean" and that there is adequate
moisture in the soil to support biodegradation of the
hydrocarbon vapors.
For a PVI investigation, "clean" soil does not
necessarily mean that it is contaminant-free, but
rather that the level of any contamination present is
low enough so that the biological activity of the soil
is not diminished and the subsurface environment
will support sufficient populations of microorganisms
An Approach for Developing Site-Specific Lateral and Vertical Incli
Ground Water Issue 21
-------
Table 4. Example conditions for a structure to be included in the Vertical Inclusion Zone. If any condition applies,
the structure is in the Vertical Inclusion Zone.
These conditions are provisional and are for illustration purposes only. They are based on Davis (2009)
and Cal EPA (2012). At such time as U.S. EPA Office of Underground Storage Tanks (OUST) issues the
Guidance for Addressing Petroleum Vapor Intrusion at Leaking Underground Storage Tank Sites, the con-
ditions for vertical separation in the Guidance will supersede the conditions in this table.
Media
Soil
(mg/kg)
Groundwater
(M9/L)
Benzene Concentration
<10
>10 (LNAPL)
<5,000
>5,000 (LNAPL)
TPH Concentration
<250
>250 (LNAPL)
<30,000
>30,000 (LNAPL)
Vertical Separation Distance*
(feet)
<6
<15
<6
<15
The thresholds for LNAPL indicated in this table are indirect evidence of the presence of LNAPL. These thresholds
may vary depending on site-specific conditions (e.g., soil type, LNAPL source). Investigators may have different
experiences with LNAPL indicators and may use them as appropriate. Direct indicators of LNAPL also apply; these
include measurable accumulations of free product, oily sheens, and saturated bulk soil samples. For more informa-
tion, see API (2000).
The vertical separation distance represents the thickness of clean (TPH < 250 mg/kg), biologically active soil
between the source of PHC vapors (LNAPL, residual LNAPL, or dissolved PHCs) and the lowest (deepest) point
of a receptor (building foundation, basement, or slab). EPA recommends that sub-slab sampling be conducted to
evaluate the risk of PVI whenever contamination above the specified threshold is present in any sample and the
distance between the contamination and an overlying building is less than these vertical distances.
to aerobically biodegrade PHC vapors. As a point
of departure, soil with less than 250 mg/kg TPH can
be considered "clean." State agencies may choose
a different definition based on their local conditions
and circumstances. The California Environmental
Protection Agency uses a value of 100 mg/kg (Cal
EPA, 2012).
Establishing that there is adequate moisture to
support growth of bacteria is a substantial challenge
in desert climates. U.S. EPA (2013a) notes that
soil moisture content greater than 2% is adequate
to support biodegradation activity (Leeson and
Hinchee, 1996). However, biodegradation is limited
when the moisture content is at or below the
permanent wilting point (Zwicketal., 1995; Holden,
Halverson, and Firestone, 1997). Adequate soil
moisture is indicated if the landscape supports the
growth of indigenous vegetation (Riser-Roberts,
1992). Agencies in states with desert landscapes
may wish to take advantage of their local knowledge
and apply local criteria.
3.1 Steps to Apply a Vertical Separation
Distance to Core Samples
There are five steps to defining a vertical inclusion
distance:
1. Acquire core samples or a series of core samples
that represent the entire interval from the
receptor to the lowest potential location of the
water table.
2. Screen the core samples in the field with an
Organic Vapor Meter (OVM) to determine if
samples should be acquired for laboratory
analysis. If contamination is detected by the
OVM screening, analyze the sediment samples
for the concentrations of Total Petroleum
Hydrocarbons and for Benzene.
3. Compare depths and concentrations of
contaminants in core samples to the Vertical
Separation Distance Criteria.
4. Acquire a sample of ground water and analyze
for concentrations of TPH and Benzene.
2 2 Ground Water Issue
Developing Site-Specific Lateral and Vertical Inclusion Zones
-------
5. Compare depths to ground water and
concentrations of contaminants in ground water
to the Vertical Separation Distance Criteria.
3.1.1 Acquire Core Samples for Screening
Determining the vertical separation distance for
contamination in the unsaturated zone can be
challenging. To apply the criteria in Table 4, it is
necessary to document that the clean soil is in fact
clean. Exterior bulk soil samples should be collected
from near the perimeter of the building in the
direction of the source of contamination. To avoid
missing a depth interval that might be contaminated,
it is necessary to recover a complete profile of core
samples from the land surface to the water table.
If possible, it is better to recover core samples to
a depth equal to the lowest elevation of the water
table over time.
To assure that the core profile is complete, compare
the length of the core that is recovered (including
material in the core retainer and the cutting shoe)
to the depth interval that the core barrel was driven
into the earth. In some subsurface materials, core
samplers driven two or three feet will recover an
equivalent length of core sample, but core samplers
driven four or five feet will not. Adjust the depth
interval driven in each core if necessary to recover
a complete core sample.
TPHGRO(mg/kg)
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Figure 13. Distribution of TPH (panel a) and benzene (panel b) and hydraulic conductivity with depth below land
surface at a gasoline release site in Golden, OK.
An Approach for Developing Site-Specific Lateral and Vertical lnclusi<
round Water Issue 23
-------
On occasion, material with a high concentration of
TPH will literally be well lubricated, and will fall out
of the core sampler as it is being recovered. Do
not ignore the missing sample. Attempt to collect
core samples in an adjacent bore hole, starting just
above the elevation that would correspond to the
missing sample, and drive the core sampler the
maximum interval that will acquire a complete core
sample.
Figure 13 compares the vertical distribution of
Total Petroleum Hydrocarbons (TPH) at a site in
Oklahoma. Point estimates of hydraulic conductivity
at the site were made with a pneumatic slug test
(Butler et al., 2002). Notice at the site that the
greater mass of TPH was confined to material
that has low hydraulic conductivity. Petroleum
hydrocarbons tend to be held by capillary attraction
to fine textured materials. At many gasoline
service station sites, the first aquifer to produce
enough water to allow sampling by a monitoring
well is effectively a confined aquifer. Much of the
time, the free water surface will be up in the fine
textured material containing the TPH, and much of
the TPH will be covered in water and not in contact
with soil gas. In times of drought, the free water
surface often will drop to the contact between the
fine textured material containing the TPH and the
transmissive material that comprises the aquifer
proper. During times of drought, more of the TPH in
the fine textured material may be in contact with soil
gas.
If a nearby monitoring well is available, determine
the depth to the free water surface. If an
established monitoring well is not available,
determine the depth to water in the borehole used
to acquire the core samples. Examine the texture
of the core samples taken in the depth interval
across the free water surface. If the material has
a fine texture, and particularly if the borehole stays
open, continue to acquire core samples until more
transmissive material is reached.
Apply the Soil Media Criteria in Table 4 to the TPH
values, even if the material is below the water table
at the time the cores were acquired.
3.1.2 Screen Core Samples for Subsequent
Laboratory Analysis
In the past, core samples for analysis of TPH were
often acquired at an arbitrary depth below grade
or an arbitrary depth above the location of the
water table at the time of sampling. This sort of
conventional sampling is illustrated in Figure 14.
At the site in South Carolina, the depth from land
surface to the water table varied from six to eight
feet. Over this vertical interval up to five samples
were taken for organic vapor monitoring.
The OUST guidance applies criteria based on the
thickness of clean, biologically active soil between
the top of the contamination and the receptor (U.S.
EPA. 2013a). To apply the criteria, it is necessary
to document that the soil is clean across the entire
separation distance between the contamination and
the receptor. To minimize the chance of missing
a contaminated depth interval, it is good practice
to screen the core samples with an Organic Vapor
Monitor (OVM) every 0.5 foot starting at 1.0 foot
below land surface or 1.0 foot below the bottom
of the structure of concern. Continue screening
until the depth of the core samples exceeds the
lowest possible position of the water table. If the
OVM meter reading exceeds 100 ppm, a sample
should be analyzed in the laboratory for benzene
and total petroleum hydrocarbons (TPH). Detailed
recommendations for extracting and analyzing core
samples are provided in Appendix A.
Figure 13 presents the vertical profile of TPH
resulting from a gasoline release in Golden,
Oklahoma. The concentration of TPH in the interval
from 7 feet to 9 feet below grade was < 29 mg/Kg.
Notice the sharp increase in concentrations of
TPH and benzene in core material at a depth that
is just less than 10 feet below land surface. The
concentration of TPH at a depth of 9.75 feet was
21,000 mg/kg and the concentration of benzene
was 197 mg/Kg.
3.1.3 Compare the Distribution of
Contamination in Sediment to the Vertical
Separation Criteria
A recent study by EPA (2013b) indicates that for
an oxygen shadow to form beneath a building,
and thus appreciably reduce the effectiveness of
biodegradation to prevent PVI, three conditions
must be met: the building must be very large
(including the surrounding impermeable cover),
the source of vapors must be highly concentrated,
and the vapor source must be in relatively close
proximity to the bottom of the building. For a
2 4 Ground Water Issue
Developing Site-Specific Lateral and Vertical Inclusion Zones
-------
typical single family dwelling, it will generally be
sufficient to collect exterior soil vapor and bulk
soil samples from only one location immediately
adjacent to the structure on the side facing the
source of contamination. The screening criteria
applied will be based on the sample analyses from
this one location. For larger structures, it may be
necessary to collect samples and apply the criteria
at several locations along the building perimeter and
potentially from locations on all sides of the building.
The criteria for the vertical separation distances
are provided in Table 4. If either of these criteria
for vertical separation is satisfied, this site is in
the vertical inclusion zone, and requires further
assessment. As indicated in Figure 13, the site in
Oklahoma is in the vertical inclusion zone because
there was less than 15 feet of clean soil between
the receptor and the first bulk soil sample with
>250 mg/L of TPH. The separation distance to the
receptor was the land surface because the receptor
had a pier-and-beam foundation.
3.2 Steps to Apply a Vertical Separation
Distance to Ground Water Samples
Applying the vertical criteria for ground water is
less challenging. Install a monitoring well in the
borehole used to acquire the core samples, and
sample ground water for analysis of benzene and
TPH. Measure the elevation of the water table in
the new well. If a nearby monitoring well has an
extensive monitoring record, use the variation in
water table elevations in the older well to estimate
the variation in elevation of the water table at
the new location. Compare the elevation of the
bottom of the structure of concern to the highest
elevation of ground water under the structure. The
vertical separation for ground water does not make
allowance for the capillary fringe. Compare the
vertical separation to the free water surface.
At the site in South Carolina as depicted in
Figure 14, the depth to water at the structure of
concern is near 8 feet. If there is no residual TPH
in the unsaturated zone, the inclusion zone is
based solely on the depth to contaminated ground
water. A depth of 8 feet is greater than a separation
distance of 6 feet as described in Table 4. The
structure of concern would not require any further
investigation if the concentration of benzene in
ground water is < 5 mg/L and TPH is < 30 mg/L.
At the site in Oklahoma as depicted in Figure 13,
the depth to the free-water surface was 13.2 feet.
However, the aquifer did not yield significant water
until a depth of 17 feet, which is considerably
below the major mass of residual gasoline. The
concentration of benzene in the ground water
was 823 ug/L and the concentration of TPH was
12,300 ug/L. Based on the concentration of
benzene or TPH in ground water and the separation
distance, this site would not be in the vertical
inclusion zone, and would not require further action.
However, as the site failed the soil screening (i.e.,
TPH at a depth of 9.75 feet was 21,000 mg/kg),
additional investigation for PVI is recommended.
This example illustrates the importance of acquiring
bulk soil samples for analysis, and not relying on
ground water samples alone.
s~~ ^N.
The data used in the case study are provided
as an illustration. They do not necessarily
reflect current conditions at the site, and have no
bearing on past or current regulatory action taken
by the Oklahoma Corporation Commission.
4.0. NEXT STEPS
Approaches to screen for PVI are not limited to
the approach presented in this Issue Paper. The
inclusion zones discussed in this Issue Paper are
defined by proximity to contaminated ground water
or to LNAPL hydrocarbons in the unsaturated zone.
If a structure is in the inclusion zone as defined by
benzene or TPH in ground water or TPH in core
samples, one possible next step is to evaluate the
concentrations of hydrocarbons in the soil gas.
Samples of soil gas can be acquired from sub-slab
monitoring points, or vapor probes, and analyzed
for contaminants of concern such as benzene. The
measured concentrations can then be compared to
concentration limits in the OSWER draft guidance
for evaluating vapor intrusion (U.S. EPA 2002).
The possibility of vapor intrusion of petroleum
hydrocarbons is inversely related to the possibility
of aerobic biodegradation of the petroleum vapors
in the unsaturated zone (DeVaull, 2007). In turn,
the possibility of biodegradation is related to the
separation distance, the oxygen demand of the all
the hydrocarbons in soil gas at the source of the
An Approach for Developing Site-Specific Lateral and Vertical Incli
Ground Water Issue 25
-------
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26 Ground We
for Developing Site-Specific Lateral and Vertical Inclusion Zones
-------
vapors, and the concentration of benzene at the
source.
Another approach is to use these parameters to
screen sites for PVI. The ratio of the concentration
of benzene in indoor air to the concentration of
benzene in vapors at the source is called the
attenuation factor (U.S. EPA 2002). Abreu et
al., 2009) performed computer simulations that
predicted the effect of biodegradation on the
attenuation factor. The results of a large number
of complex simulations are summarized in a simple
figure that plots the attenuation factor against the
total oxygen demand for a variety of separation
distances. In their approach, the figure is used
to predict an attenuation factor that is specific for
conditions at a particular site. To complete the
evaluation, the attenuation factor is multiplied by the
measured concentration of benzene in soil gas at
the source of the vapors.
The approach of Abreu et al. (2009) may have
application at many sites. However, it is important
to attain a robust estimate of the total oxygen
demand. Jewell and Wilson (2011) applied the
approach to several gasoline release sites in
Oklahoma. They took precautions to measure
methane in the soil gas as well as concentrations
of petroleum hydrocarbons. At three of eleven
sites, including the contribution of methane to the
total oxygen demand caused the predicted indoor
air concentration of benzene to exceed the U.S.
EPA Generic Screening Level for indoor air (9.8E-
03 ppm v/v). The sites would not have exceeded
the Generic Screening Level if the oxygen demand
was calculated from the concentration of petroleum
hydrocarbons alone.
Conventional ground water monitoring wells at
gasoline service stations are usually screened
across the water table. This means that monitoring
wells can often be used to collect soil gas. Jewell
and Wilson (2011) used conventional wells to
acquire their soil gas samples. At many sites, it
may be possible to use the same wells that were
previously used to screen ground water to screen
soil gas.
5.0 SUMMARY
U.S. EPA's Guidance for Addressing Petroleum
Vapor Intrusion at Leaking Underground Storage
Tank Sites (U.S. EPA, 2013a) is intended to provide
general criteria to identify structures that are at risk
from petroleum vapor intrusion (PVI). This issue
paper provides one approach to apply criteria set
forth in U.S. EPA (2013a), but does not represent
U.S. EPA guidance.
An inclusion zone is used to recognize structures
that may be at risk from PVI. The inclusion zone
generally consists of a lateral zone based on the
delineation of a clean perimeter and a vertical zone
based on the vertical separation distance between
the structure and contamination in the subsurface.
The delineation of the lateral inclusion zone in
this approach recognizes the fact that the lateral
separation distance between a residence and
contaminated ground water is dependent on the
identification of the edge of a contaminant mass,
whether it is mobile LNAPL, residual LNAPL, or a
dissolved plume. Many sets of site characterization
data do not explicitly define this boundary, but rely
on drawn contours that may be arbitrary. In this
approach, the lateral inclusion zone depends on
the delineation of a clean perimeter. If monitoring
points at a site are scarce or are widely separated,
there will be uncertainty about the location of
contamination in the areas between the monitoring
points. A building may be at risk even though it
is marginally outside the clean perimeter. The
approach provides a reasonable procedure to
extend the lateral inclusion zone based on the
location and spacing of monitoring points. As site
monitoring data are collected over time, the lateral
inclusion zone may be reduced in its extent.
Once a lateral inclusion zone is identified, it can be
further refined to optimize the screening process
and avoid unnecessary risk characterization within
buildings. As is shown in the examples in this Issue
Paper, it may be necessary to acquire more data
before the approach can be used with confidence to
screen structures for PVI. The lateral inclusion zone
may present a clear picture of the best locations
for new wells. Ground water flow directions vary
at most sites, so data collected over time on the
direction of ground water flow can be used to
refine the inclusion zone, very possibly shrinking
An Approach for Developing Site-Specific Lateral and Vertical Incli
Ground Water Issue 2 7
-------
it. If information is available on the hydrological
characteristics of the site, a simple transport and
fate model can be used to forecast the lateral extent
of contaminated ground water from particular wells.
These forecasts can provide an additional line of
evidence to evaluate or further refine the lateral
inclusion zone.
After identifying a lateral inclusion zone, there still
may be a large number of residences potentially
at risk for vapor intrusion. At some sites it may not
be possible to define a lateral inclusion zone. At
this point, the vertical inclusion criteria should be
applied. This Issue Paper recommends five simple
steps to determination of the vertical extent of clean
soil between the building and the contamination
below the building, and to compare that extent
of clean soil to the criteria for vertical separation
distance in U.S. EPA (2013a).
In combination, definition of lateral and vertical
inclusion zones make the best use of site
characterization data for assessing the risk of
PVI to structures at a LUST site. Ultimately, a
useful prediction of the possibility of petroleum
vapor intrusion in a particular building depends
on knowledge of contaminant transport and
transformation, and the site-specific distribution of
contaminants. The procedures outlined in this Issue
Paper provide a realistic data-driven approach to
screen buildings for vulnerability to PVI.
NOTICE
The U.S. Environmental Protection Agency through
its Office of Research and Development conducted
the research described here as an in-house effort.
This Report has been subjected to the Agency's
peer and administrative review and has been
approved for publication as an EPA document.
6.0 REFERENCES
Abreu, L. D. V., R. Ettinger, and T McAlary. 2009.
Simulated Soil v\Vapor Intrusion Attenuation
Factors including Biodegradation for Petroleum
Hydrocarbons. Ground Water Monitoring &
Remediation 29(1): 105-117.
Aronson D. and PH. Howard. 1997. Anaerobic
biodegradation of organic chemicals in ground
water: A summary of field and laboratory studies.
PreparedfortheAmerican Petroleum Institute, SRC
TR-97-0223F.
API. 2000. Non-Aqueous Phase Liquid (NAPL) Mobility
Limitsin Soil. American Petroleum Institute. Soil and
Ground water Research Bulletin No. 9.
API. 2005. Collecting and Interpreting Soil Gas
Samples from the Vadose Zone: A Practical Strategy
for Assessing the Subsurface Vapor-to-indoor Air
Migration Pathway at Petroleum Hydrocarbon Sites.
American Petroleum Institute. API Publication 4741.
Butler, J.J, J.M. Healey, G.W. McCall, E.J. Garnett,
and S.P Loheide II. 2002. Hydraulic tests with
direct-push equipment. Ground water40 (1): 25-36.
Cal EPA. 2012. Low-Threat Underground Storage
Tank Case Closure Policy. California Environmental
Protection Agency, State Water Resources Control
Board, http://www.swrcb.ca.gov/ust/lt els plcv.
shtml.
Davis, R.V 2009. Update on recent studies and
proposed screening criteria for the vapor-intrusion
pathway. LU.S.T.LINE Bulletin 61, pages 1-14.
DeVaull, G. E. 2007. Indoorvaporintrusion with oxygen-
limited biodegradation for a subsurface gasoline
source. Environmental Science and Technology
41(9): 3241-3248.
EPA. 2004a. Performance Monitoring of MNARemedies
for VOCs in Ground Water. EPA/600/R-04/027.
Office of Research and Development, National Risk
Management Research Laboratory, Ada, OK.
EPA. 2004b. Monitored Natural Attenuation. Chapter IX
inHowToEvaluateAlternativeCleanupTechnologies
For Underground Storage Tank Sites: A Guide For
Corrective Action Plan Reviewers (EPA 510-B-94-
003; EPA510-B-95-007; and EPA 510-R-04-002).
Office of Underground Storage Tanks, Washington,
DC.
Falta, R.W, A.N.M. Ahsanuzzaman, M.B. Stacy and
R.C. Earle. 2012. REMFuel: Remediation Evaluation
Model for Fuel Hydrocarbons, User's Manual Version
1.0. EPA/600/R-12/028. Available: http://www.eoa.
gov/nrmrl/awerd/csmos/models/remfuel.html
Gelhar, L.W, C. Welty, and K.R. Rehfeldt. 1992. A
critical review of data on field-scale dispersion in
aquifers, Water Resources Research 28(7): 1955-
1974.
Goode, D.J. and L.F. Konikow. 1990. Apparent
2 8 Ground Water Issue
Developing Site-Specific Lateral and Vertical Inclusion Zones
-------
dispersion in transient ground water flow. Water
Resources Research. 26(10): 2339-2351.
Holden, P. A., LJ. Halverson, andM.K. Firestone. 1997.
Water stress effects on toluene biodegradation by
Pseudomonas putida. Biodegradation 8(3): 143-151.
Jewell, K. P., J. T. Wilson. 2011. A New Screening
Method for Methane in Soil Gas Using Existing
Groundwater Monitoring Wells. Ground Water
Monitoring & Remediation. 31 (3): 22-94 (2011).
Lahvis, M., A. Baehr, and R. Baker. 1999. Quantification
of aerobic biodegradation and volatilization rates of
gasoline hydrocarbons near the water table under
natural attenuation conditions. Water Resources
Research. 35(3):753-765.
Leeson, A., and R.E. Hinchee. 1996. Principles and
Practices of Bioventing. Volume 1: Bioventing
Principles and Volume 2: Bioventing Design. Battelle
Memorial Institute. September.
Mace, R.E., R.S. Fisher, D.M. Welch, and S.P Parra.
1997. Extent, mass, and duration of hydrocarbon
plumes from leaking petroleum storage tank sites in
Texas. Geological Circular97-1, Bureau of Economic
Geology, The University of Texas, Austin, Texas.
Minnesota Pollution Control Agency. 2008. Soil Sample
Collection and Analysis Procedures. Guidance
Document 4-04. Petroleum Remediation Program.
Available at: http://www.pca.state.mn.us/index.php/
view-document. html?gid=3025
Riser-Roberts, E. 1992. Bioremediation of Petroleum
Contaminated Sites. Florida: CRC Press, Inc.
Srinivasan, P., Pope, D.F., and Striz, E. 2004. Optimal
Well Locator (OWL): AScreening Tool for Evaluating
Locations of Monitoring Wells, User's Guide Version
1.2. EPA 600/C-04/017 February 2004. Available:
http://www. epa.gov/nrmrl/awerd/csmos/models/
owl, html
Suarez, M. P. and H .S. Rifai. 1999. Biodegradation rates
for fuel hydrocarbons and chlorinated solvents in
ground water. Bioremediation Journal 3 (4): 337-362.
U.S. EPA. 1995. OSWER Directive 9610.17: Use of
Risk-Based Decision Making in UST Corrective
Action Program. Available: http://www.epa.gov/oust/
directiv/od961017.htm.
U.S. EPA. 2002. OSWER DraftGuidanceforEvaluating
the Vapor Intrusion to Indoor Air Pathway from
Ground waterand Soils (Subsurface Vapor Intrusion
Guidance) EPA530-D-02-004. Available: htto://
www. epa. gov/epawaste/hazard/correctiveaction/
eis/vapor/complete.pdf
U.S. EPA. 2013a. [DRAFT] Guidance for Addressing
Petroleum Vapor Intrusion at Leaking Underground
Storage Tank Sites . (EPA-xxx-xx-xx-xxx) Office of
Underground Storage Tanks.
U.S. EPA. 2013b. 3-D Modeling of Aerobic
Biodegradation of Petroleum Vapors: Effect of
Building Area Size on Oxygen Concentration below
the Slab; June 4, 2012 draft report prepared by
ARCADIS U.S., Inc. (EPA-xxx-xx-xx-xxx) Office of
Underground Storage Tanks.
U.S. EPA. 2013. Evaluation of Empirical Data toSupport
Soil Vapor Intrusion Screening Criteria for Petroleum
Hydrocarbon Compounds. (EPA-510-R-13-001)
Office of Underground Storage Tanks.
van Genuchten, M.T andWJ.AIves, 1982. Analytical
Solutions of the One-Dimensional Convective-
Dispersive Transport Equation, U.S. Department
of Agriculture, Agricultural Research Service, U.S.
Salinity Laboratory, Riverside, California, Technical
Bulletin 1661.
Wlson, J.T 2003. Fate and transport of MTBE and
other gasoline components. In: MTBE Remediation
Handbook, Amherst, MA: Amherst Scientific
Publishers, pp. 19-61.
Wlson, J.T, P.M. Kaiser and C.Adair.2005a. Monitored
Natural Attenuation of MTBE as a Risk Management
Option at Leaking Underground Storage Tank Sites.
EPA600/R04/179.
Wlson, J.T, C. Adair, P.M. Kaiser, and R. Kolhatkar.
2005b. Anaerobic biodegradation of MTBE at a
gasoline spill site. Ground Water Monitoring and
Remediation 25(3): 103-115.
Xu, M. and Y. Eckstein. 1995. Use of weighted least-
squares method in evaluation of the relationship
between dispersivity and field scale. Ground water
33(6): 905-908.
Zwick, T.C., A. Leeson, R.E. Hinchee, L. Hoeppel,
and L. Bowling. 1995. Soil Moisture Effects During
Bioventing in Fuel-Contaminated Arid Soils. Third
International In-Situ and On-Site Bioreclamation
Symposium. In-Situ Aeration, v. 3, Batelle Press,
San Diego, CA.
An Approach for Developing Site-Specific Lateral and Vertical Incli
Ground Water Issue 29
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APPENDIX A. RECOMMENDATIONS FOR
SAMPLING AND ANALYSIS
The conventional practice to sample for Total
Petroleum Hydrocarbons in some states is to take
a bulk sample of sediment into a sealed jar, return
the jar to the laboratory on ice, and store the jar in
a refrigerator until a subsample was taken to be
extracted. This practice can result in considerable
loss of VOCs and produce erroneous results. The
authors recommend that sediment samples for
analysis of TPH and Benzene should be preserved
in methanol in the field as soon as possible after the
core samples are acquired. In the absence of other
guidance, the authors recommend the procedures
and requirements as described in Minnesota
Pollution Control Agency (2008).
The authors have had good results using the
following procedure to extract core samples
into methanol. Plug-samplers were constructed
before going to the field by cutting the end from
a 10-ml plastic syringe (Figure A-1 & Figure A-2).
A sediment core was acquired in an acetate
liner. The core was cut through with a saw at the
depth interval to be sampled (Figure A-3). Then
a plug-sampler was driven into the exposed face
of the core sample. The syringe plunger was
used to provide suction to pull the sample into
syringe barrel as the barrel was forced into the
face of the core sample (Figure A-4). Each plug
contained approximately 10 ml of soil and extended
approximately 2.5 inches into the core. After all
the necessary plug samples for a particular depth
interval were acquired (Figure A-5), the core was
measured and cut again to present a fresh face at
the next interval to be sampled.
The authors have found it to be convenient to
take all the samples that might be needed at the
same time. These include one plug sample for field
screening with an OVM, duplicate plug samples into
methanol for analysis of TPH and benzene and a
sample taken into a clean empty vial for analysis
of moisture content. The duplicate plug sample
for TPH and benzene provides a contingency if a
sample is lost, and provided a field duplicate if one
is needed for quality assurance purposes. If the
OVM screening did not reveal contamination, the
other samples were not analyzed. The samples that
were extracted into methanol were returned to the
laboratory and discarded as hazardous waste.
The plug sample for field screening was sealed
into a plastic bag containing air. At a later time the
headspace of the bag was analyzed with an organic
vapor meter (OVM) (Figure A-6). Our screening
essentially followed Section A. Headspace Analysis
of Minnesota Pollution Control Agency (2008).
Extraction vials were prepared by delivering 10 ml
of purge-and-trap grade methanol into 40 mL
Volatile Organic Analysis (VOA) vials. In the field,
the plug samples were delivered into the vials
(Figure A-7), the vials were sealed with the screw
cap, and then the vials were shaken to begin the
extraction and preserve the samples (Figure A-8).
In the laboratory, the vials were shaken on a
mechanical shaker for ten minutes. If this was
not adequate to disperse and extract the plug, the
vials were open and the plug was broken up with
a spatula, and the vial was put back on the shaker
for additional extraction. After the sediment was
extracted, the vials were set out on the counter
to allow the solids to settle. Then the vials were
opened and the methanol extracts were taken for
analysis. The methanol extract was diluted into
distilled water, and the water was then analyzed by
EPA Method 8260.
The final plug sample was used to determine the
moisture content of the sediment sample. The plug
was delivered into a clean empty 40 ml VOA vial. In
the laboratory the sample was weighted, then dried
to constant weight and weighed again.
30 Ground Water Issue
Developing Site-Specific Lateral and Vertical Inclusion Zones
-------
FigureA-1. A sampler was constructed by cutting theendoutof FigureA-2. Commercial samplers are also available.
a plastic syringe.
Figure A-3. A core sample acquired in a plastic sleeve is cut to
access the core for sub sampling.
Figure A-4. A sampler is inserted into the cut face of the core
sample to acquire a subsample.
FigureA-5. Additional samples are acquired from the cut face
as needed. One sample is transferred to a plastic bag for
screening of volatile organic hydrocarbons. See Figure A-6.
Figure A-6. After the volatile hydrocarbons in the subsample
equilibrated with the air in the sealed plastic bag, the
concentration of hydrocarbons were measured with an
organic vapor meter.
An Approach for Developing Site-Specific Lateral and Vertical lnclusi<
jnd Water Issue 31
-------
Figure A-7. Each subsample for analysis in the laboratory is
in traduced in to a vial previously prepared with 7 Oml of
methanol
Figure A-8. The vial was sealed and shaken to disperse the
subsample and began the extraction. Note that the empty
weight of the vial and cap and the weight of the vial and
cap plus methanol were recorded in the laboratory when
the vials were made up. When the vial is returned to
laboratory it is weighted again to determine the wet weight
of the subsample.
32 Ground We
for Developing Site-Specific Lateral and Vertical Inclusion Zones
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APPENDIX B. QUALITY ASSURANCE
The following information documents the data
quality of samples collected and analyzed by
U.S. EPA from the site in Oklahoma.
The concentration of benzene in the water sample
from the site in Oklahoma was analyzed using a
modification of EPA Methods 5021A(headspace
sampler) and 8260C (GC/MS). The method
detection limit was 0.18 ug/L and the quantitation
limit was 0.5 |jg/L. Benzene was not detected in
the method blank. The continuing calibration check
was 101 % of nominal.
The concentration of benzene and gasoline range
organics (GRO TPH) in the methanol extracts
were analyzed by EPA Method 5030B, Revision 2
(purge and trap followed by GC/FID). The extract
was diluted 1:50 into water prior to analysis. Some
samples exceeded the calibration range. These
samples were diluted 1:500 and analyzed a second
time.
The method detection limit for GRO in the extracts
was 155 ug/L; the limit of quantitation was
1250 ug/L. The method detection limit for benzene
in the extracts was 6.5 ug/L; the limit of quantitation
was 50 ug/L. Neither GRO nor benzene were
detected in the method blank. The recovery of
GRO in ten continuing calibration checks ranged
from 101% to 118%. The recovery of benzene in
ten continuing calibration checks ranged from 92%
to 114%.
After correcting for dilution of the extract into the
water that was analyzed by purge and trap, and for
the average weight of sample that was extracted,
the limit of quantitation of GRO in the sediment was
9.2 mg/kg and the limit of quantitation of benzene
was 0.37 mg/kg. The detection limit of GRO was
2.1 mg/kg and the detection limit of benzene was
0.091 mg/kg.
The relative percent difference between in
concentrations of benzene in two sets of field
duplicate samples was 2.3% and 1.8%. The relative
percent difference between in concentrations of
GRO in two sets of field duplicate samples was
0.4% and 0.1%.
APPENDIX C. EQUATIONS FOR STEADY STATE
PLUME CALCULATIONS
At steady state,
dlc dc ,
D--v -Ac = 0
dx1 dx
where D is the dispersivity, c is concentration, x is
distance, v is seepage velocity, and X is the decay
constant. Using the boundary conditions
C(0)=
dc_
dx
= 0
The solution for the plume length is
2a In 9
x =
1-J1+
where a is the dispersivity (D=av), and the
dispersivity is presumed independent of plume
length (van Genuchten and Alves, 1982). Since
dispersivity is known to be scale dependent
(Gehlar et al., 1992), an implicit calculation can be
substituted linking dispersivity and plume length:
The Xu and Eckstein (1995) regression can give an
indication of the scale dependence of dispersivity
a= 0.83[logxf414
where a and the plume length, x, are given in
meters.
An Approach for Developing Site-Specific Lateral
Water Issue 33
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34 Ground Water Issue
Developing Site-Specific Lateral and Vertical Inclusion Zones
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