SERA NRMRL
EPA/600/F-10/017
July2010
www.epa.gov/nrmrl
NATIONAL RISK MANAGEMENT RESEARCH LABORATORY
GROUND WATER AND ECOSYSTEMS RESTORATION RESEARCH
Control of Subsurface Contaminant Migration by Vertical Engineered Barriers
This Fact Sheet provides a synopsis on the use of vertical
engineered barriers (VEBs) to control the migration of
contamination in the subsurface. This Fact Sheet is intended
to provide remedial project managers (RPMs), on-scene
coordinators (OSCs), contractors, and other remediation
stakeholders with a basic overview of hazardous waste
containment systems constructed to prevent or limit the migration
of contamination in ground water as well as their limitations.
Physical containment systems are constructed to isolate
contaminated soil, ground water and aquifer materials by creating
engineered barriers to ground water flow and recharge. By
isolating the source(s), such systems can prevent or reduce the
degradation of ground water and potential threats to human health
and the environment outside the contained area. Conceptually, a
containment system can be visualized as a box, whose sides, top
and bottom are represented by VEBs, a cap, and an underlying
low permeability unit or aquitard, respectively. Containment
systems also typically include a ground water extraction system
and a monitoring system (Figure I).1-2
Figure 1. Major conceptual components of a
containment system "box" include the cap (top),
vertical engineered barrier (walls), aquitard (bottom),
and monitoring wells.
Vertical engineered barriers (VEBs), or cut-off walls,
are most commonly slurry walls composed of native soils
enriched with bentonite or another type of clay. Other
materials such as cement, geomembranes, and steel sheet
piling can also be used separately or in combination. Testing
will generally be required to ensure that the VEB materials
of construction are compatible with the wastes to be
contained.1'3'4'5'6'7
A low permeability cap is normally constructed to prohibit
or reduce infiltration into the containment system. The
cap may be constructed of various layers of natural and/or
geosynthetic materials. If consistent with future land use,
concrete or asphalt may also be used.1'2'8
• The bottom or 'floor' of a containment system is typically
a low hydraulic conductivity (K) unit, into which the
wall is constructed or 'keyed'.9 The presence of a lower
confining unit and an adequate key significantly reduce the
horizontal and vertical advective flow of contaminants from
a containment system.10'11 Technologies for the emplacement
of low K 'floors' in situ have been demonstrated for
relatively shallow contaminant sources.12 Hanging wall
containment systems lacking 'floors' have been used to
prevent the lateral movement of light nonaqueous phase
hydrocarbons.1
• A ground water extraction system is typically required
to maintain inward and upward hydraulic gradients, so
that the flux of water through the walls or floor is into the
containment facility rather than out of it. Vertical extraction
wells or horizontal drain systems can be used to remove
fluids at a rate sufficient to maintain the desired inward
hydraulic gradients. The extracted ground water will
generally be contaminated and will require treatment.1
• A good monitoring system will incorporate a variety
of monitoring techniques, rather than relying on a single
method.13 A monitoring system may include piezometers
inside and outside the VEB to demonstrate that inward
gradients are maintained, and in the underlying aquifer
beneath the 'floor' to demonstrate an upward gradient.
Ground water quality monitoring surrounding the facility
may be used to demonstrate that contaminants are not
leaving the system at unacceptable rates.14'15 Monitoring
will generally be needed for as long as contaminants
within the containment system are potentially able to cause
unacceptable exposures outside of the system.16
While the concept of a containment system is relatively simple,
successful implementation may be difficult.17 Available
information on the performance of VEBs for hazardous waste
containment suggests that the primary short term factor affecting
their performance is poor construction.10'16'18 Successful
construction of a wall that meets design specification for K
(typically 10'8 to 10~9 meters/second1'10'14) requires deployment
of an experienced construction crew,18'19'20 strict adherence to
construction quality control/construction quality assurance
(CQC/CQA),1'14'16'18 and the selection of appropriate construction
materials for the contaminants of concern and hydrogeologic
setting.10'21 Construction difficulties could create "windows"
of higher hydraulic conductivity in some places in the wall,
allowing for an outward advective flux of contaminants, or
U.S. Environmental Protection Agency
Office of Research and Development, National Risk Management Research Laboratory
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requiring a greater ground water extraction rate to maintain
an inward gradient.5'16'22 "Windows" of higher permeability
or discontinuities can also occur naturally in the underlying
aquitard.9'23 Figure 2 illustrates some of the problems that can
occur when containment systems are improperly designed.
constructed, and/or operated. The higher water level inside the
containment system relative to the upper and lower aquifers may
result in leakage out of the system.
Even when a containment system is designed, constructed, and
operated to design specifications, diffusive flux of dissolved
volatile organic compounds (VOCs) through slurry walls.
geomembranes, and the containment system floor can still
occur.24'25'26'27'28'29'30 Site specific ground water flow conditions
will determine whether this steady state flux rate will result in
ground water concentrations exceeding ground water cleanup
criteria.
Site conditions may have an adverse impact on construction and
performance of containment systems. It is not uncommon for
some contamination to remain outside the perimeter of a VEB.
and this can cause confusion about the integrity of the system.1
At many sites, it may be difficult to determine the continuity and
integrity of the underlying aquitard in the containment area.9'23
Fractures in the aquitard that are hydraulically active are difficult
to characterize and may allow short-circuiting of contaminants
out of the containment system and into a lower aquifer.31
Monitoring and maintenance of the containment system are
crucial to ensure that the system remains effective for as long
as contamination poses a risk to areas beyond the containment
system.16 The rate at which the effectiveness of a containment
system will diminish over time depends on the conservativeness
of the original design, the effectiveness of the CQC/CQA
program, and the adequacy of system maintenance. Currently
there is insufficient data documenting the long-term performance
of containment systems to predict their useful life with any
degree of reliability.15'22'32 Wells, pumps, the treatment system
and its related infrastructure, and the cap will all require regular
maintenance. Due to the long term (decades or centuries) nature
of most of the containment systems, it is likely that some or all of
the components will require repair and/or replacement during the
lifetime of the system.1'16'33
The presence of non-aqueous phase liquids (NAPLs) in
containment systems creates additional concerns. Many NAPLs
may impact the integrity of the wall. For example, some NAPLs
may cause shrinkage of bentonite slurry walls that may increase
the K of the wall.5'18'34 Dense NAPLs (DNAPLs) may accumulate
behind a wall and eventually penetrate the lower confining unit
and contaminate a deeper aquifer. Fractures in the aquitard
may allow rapid downward migration of DNAPLs.23'35'36 High
dissolved phase concentrations adjacent to the wall caused
by the proximity of NAPLs may allow diffusive transport of
contaminants from the containment system at rates that cause
unacceptable groundwater concentrations outside of the system.30
Figure 2. Potential leakage pathways and causes
for contaminants to leak out of a containment system
include high K windows, discontinuities in the
aquitard and inadequate keying of wall into aquitard,
and higher hydraulic heads inside then out.
Making the determination whether or not a containment system
has significant leakage can be accomplished using hydraulic
head data and is relatively straight forward at some sites.37 For
example, significant temporal changes in water levels indicate
that water is either entering or exiting the containment system.
The installation of transducer-type water level recorders inside
and outside the wall may be helpful in understanding the water
balance of the system.1'16'37 However, determining the magnitude
and specific location(s) of leakage will generally not be feasible
with the monitoring well networks commonly found at most
sites. If leakage from a containment system is deemed significant
or unacceptable, additional site characterization will likely be
required to determine the specific locations of leakage.1'3'15'33'37'38
Tracers added to the contained waste may be helpful in
determining the location of leakage.
At some sites, containment systems can be implemented
relatively quickly to reduce the spread of contamination
in the subsurface. However, the long term performance of
containment systems has not been verified. Frequent maintenance
and monitoring is required to maintain the desired level of
effectiveness of the system, 1,3,15,16,33,37,38 Source remediation
may be required within the containment system to improve the
system's effectiveness.
For more information, contact Eva Davis at (580) 436-8548 or
davis.eva(@,epa. gov or Randall Ross at (580) 436-8611 or ross.
randall(@,epa.gov.
See Also: http://www.epa.gov/nrmrl/gwerd/
References are available at: http://www.epa. gov/nrmrl/
pubs/600f 10017/600f 10017ref.pdf
U.S. Environmental Protection Agency
Office of Research and Development, National Risk Management Research Laboratory
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SERA NRMRL
EPA/600/F-10/017
July2010
www.epa.gov/nrmrl
NATIONAL RISK MANAGEMENT RESEARCH LABORATORY
GROUND WATER AND ECOSYSTEMS RESTORATION RESEARCH
References for Control of Subsurface Contaminant Migration by
Vertical Engineered Barriers
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U.S. Environmental Protection Agency
Office of Research and Development, National Risk Management Research Laboratory
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U.S. Environmental Protection Agency
Office of Research and Development, National Risk Management Research Laboratory
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