542F03015
Evapotranspiration Landfill
Cover Systems Fact Sheet
INTRODUCTION
Alternative final cover systems, such as
evapotranspiration (ET) cover systems, are
increasingly being considered for use at waste
disposal sites, including municipal solid waste
(MSW) and hazardous waste landfills when
equivalent performance to conventional final cover
systems can be demonstrated. Unlike
conventional cover system designs that use
materials with low hydraulic permeability (barrier
layers) to minimize the downward migration of
water from the cover to the waste (percolation), ET
cover systems use water balance components to
minimize percolation. These cover systems rely
on the properties of soil to store water until it is
either transpired through vegetation or evaporated
from the soil surface. Compared to conventional
cover systems, ET cover systems are expected to
be less costly to construct. While ET cover
systems are being proposed, tested, or have been
installed at a number of waste disposal sites, field
performance data and design guidance for these
cover systems are limited (Benson and others
2002; Mauser, Weand, and Gill 2001).
This fact sheet provides a brief summary of ET
cover systems, including general considerations in
their design, performance, monitoring, cost,
current status, limitations on their use, and project-
specific examples. It is intended to provide basic
information to site owners and operators,
regulators, consulting engineers, and other
interested parties about these potential design
alternatives. An on-line database has been
developed that provides more information about
specific projects using ET covers, and is available
at http:lfcluin.orgfproducts/altcovers. Additional
sources of information are also provided.
The information contained in this fact sheet was
obtained from currently available technical
literature and from discussions with site managers.
It is not intended to serve as guidance for design
or construction, nor indicate the appropriateness of
using ET final cover systems at a particular site.
The fact sheet does not address alternative
materials (for example, geosyntheticclay liners) for
use in final cover systems, or other alternative
cover system designs, such as asphalt covers.
Online Database:
http:llcluin.orglproductslaltcovers
BACKGROUND
Final cover systems are used at landfills and other
types of waste disposal sites to control moisture
and percolation, promote surface water runoff,
minimize erosion, prevent direct exposure to the
waste, control gas emissions and odors, prevent
occurrence of disease vectors and other
nuisances, and meet aesthetic and other end-use
purposes. Final cover systems are intended to
remain in place and maintain'their functions for an
extended period of time.
In addition, cover systems are also used in the
remediation of hazardous waste sites. For
example, cover systems may be applied to source
areas contaminated at or near the ground surface
or at abandoned dumps. In such cases, the cover
system may be used alone or in conjunction with
other technologies to contain the waste {for
example, slurry walls and groundwater pump and
treat systems).
The design of cover systems is site-specific and
depends on the intended function of the final cover
- components can range from a single-layer
system to a complex multi-layer system. To
77?/s fact sheet is intended solely to provide general information about evapotranspiration covers, it is not intended, nor can it be
relied upon, to create any rights enforceable by any party in litigation with the United States. Use or mention of trade names does
not constitute endorsement or recommendation for use.
United States
Environmental Protection
Agency
Solid Waste and
Emergency Response
(5102G)
EPA542-F-03-015
September 2003
www.epa.gov
http://cluin.org
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minimize percolation, conventional cover systems use
low-permeability barrier layers. These barrier layers
are often constructed of compacted clay,
geomembranes, geosynthetic clay liners, or
combinations of these materials.
Depending on the material type and construction
method, the saturated hydraulic conductivities for these
barrier layers are typically between 1x10'5 and 1x109
centimeters per second (cm/s). In addition,
conventional cover systems generally include
additional layers, such as surface layers to prevent
erosion; protection layers to minimize freeze/thaw
damage; internal drainage layers; and gas collection
layers (Environmental Protection Agency [EPA] 1991;
Mauser, Weand, and Gill 2001).
Regulations under the Resource Conservation and
Recovery Act (RCRA) for the design and construction
of final cover systems are based on using a barrier
layer (conventional cover system). Under RCRA
Subtitle D (40 CFR 258.60), the minimum design
requirements for final cover systems at MSW landfills
depend on the bottom liner system or the natural
subsoils, if no liner system is present. The final cover
system must have a permeability less than that of the
bottom liner system (or natural subsoils) or less than
1x10'5 cm/s, whichever is less. This design
requirement was established to minimize the "bathtub
effect," which occurs when the landfill fills with liquid
because the cover system is more permeable than the
bottom liner system. This "bathtub effect" greatly
increases the potential for generation of leachate.
Figure 1 shows an example of a RCRA D cover at a
MSW landfill with a 6-inch soil erosion layer, a
geomembrane, and an 18-inch barrier layer of soil that
is compacted to yield a hydraulic conductivity equal to
or less than 1x10'5 cm/s (EPA 1992).
Figure 1. Examples of Final Cover Systems
Geomembrane
)• Composite barrier
(a) MSW Landfill
As required £$8M Topsoil layer •£$£?
-.•y* &l&&U.1^2AV:£A.-.U*.>*M^LVv!kl.£jv&.^
» Frost penetration griSjES Cover soil layer
Geomembrane
M C°T^?'^y9r P Composite barrier
:«lsS. --.,.k *,1.9, ,cr!1/B, .. - . .^ I
As required ,:! Gas drainage layer ;.
(b) Hazardous Waste Landfill
For hazardous waste landfills, RCRA Subtitle C (40
CFR 264 and 265) provides certain performance
criteria for final cover systems. While RCRA does not
specify minimum design requirements, EPA has issued
guidance for the minimum design of these final cover
systems. Figure 1 shows an example of a RCRA C
cover at a hazardous waste landfill (EPA 1989).
The design and construction requirements, as defined
in the RCRA regulations, may also be applied under
cleanup programs, such as Superfund or state cleanup
programs, as part of a remedy for hazardous waste
sites such as abandoned dumps. In these instances,
the RCRA regulations for conventional covers usually
are identified as applicable or relevant and appropriate
requirements for the site.
Under RCRA, an alternative design, such as an ET
cover, can be proposed in lieu of a RCRA design if it
can be demonstrated that the alternative provides
equivalent performance with respect to reduction in
percolation and other criteria, such as erosion
resistance and gas control.
DESCRIPTION
ET cover systems use one or more vegetated soil
layers to retain water until it is either transpired through
vegetation or evaporated from the soil surface. These
cover systems rely on the water storage capacity of the
soil layer, rather than low hydraulic conductivity
materials, to minimize percolation. ET cover system
designs are based on using the hydrological processes
(water balance components) at a site, which include
the water storage capacity of the soil, precipitation,
surface runoff, evapotranspiration, and infiltration. The
greater the storage capacity and evapotranspirative
properties, the lower the potential for percolation
through the cover system. ET cover system designs
tend to emphasize the following (Dwyer 2003;
Hakonson 1997; Mauser, Weand and Gill 2001):
• Fine-grained soils, such as silts and clayey silts,
that have a relatively high water storage capacity
• Native vegetation to increase evapotranspiration
• Locally available soils to streamline construction
and provide for cost savings
In addition to being called ET cover systems, these
types of covers have also been referred to in the
literature as water balance covers, alternative earthen
final covers, vegetative landfill covers, soil-plant
covers, and store-and-release covers.
Two general types of ET cover systems are monolithic
barriers and capillary barriers. Monolithic covers, also
referred to as monofill covers, use a single vegetated
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soil layer to retain water until it is either transpired
through vegetation or evaporated from the soil surface.
A conceptual design of a monolithic cover system is
shown in Figure 2. Exhibit 1 provides an example of a
full-scale monolithic cover at a MSW landfill.
Capillary barrier cover systems consist of a finer-
grained soil layer (like that of a monolithic cover
system) overlying a coarser-grained material layer,
usually sand or gravel, as shown conceptually in
Figure 3. The differences in the unsaturated hydraulic
properties between the two layers minimize percolation
into the coarser-grained (lower) layer under
unsaturated conditions. The finer-grained layer of a
capillary barrier cover system has the same function as
the monolithic soil layer; that is, it stores water until it
is removed from the soil by evaporation or transpiration
mechanisms. The coarser-grained layer forms a
capillary break at the interface of the two layers, which
allows the finer-grained layer to retain more water than
a monolithic cover system of equal thickness.
Capillary forces hold the water in the finer-grained
Figure 2. Conceptual Design of a Monolithic ET
Final Cover
Vegetation
Fine-grained Layer
Interim Cover
Waste
layer until the soil near the interface approaches
saturation. If saturation of the finer-grained layer
occurs, the water will move relatively quickly into and
through the coarser-grained layer and to the waste
below. Exhibit 2 provides an example of a capillary
barrier field demonstration at a MSW landfill (Dwyer
2003, Stormont 1997).
Exhibit 1. Monolithic ET Cover at Lopez Canyon Sanitary Landfill, Los Angeles, CA
Site type: Municipal solid waste landfill
Scale: Full-scale
Cover design: The ET cover was installed in 1999 and consists of a 3-foot silty sand/clayey sand layer, which
overlies a 2-foot foundation layer. The cover soil was placed in 18-inch lifts and compacted to 95 percent with
a permeability of less than 3x10"5 cm/s. Native vegetation was planted, including artemesia, salvia, lupines,
sugar bush, poppy, and grasses.
Regulatory status: In 1998, Lopez Canyon Sanitary Landfill received conditional approval for an ET cover,
which required a minimum of two years of field performance data to validate the model used for the design. An
analysis was conducted and provided the basis for final regulatory approval of the ET cover. The cover was
fully approved in October 2002 by the California Regional Water Quality Control Board - Los Angeles Region.
Performance data: Two moisture monitoring systems were installed, one at Disposal Area A and one at
Disposal Area ABplus in May and November 1999, respectively. Each monitoring system has two stacks of
time domain reflectometry probes that measure soil moisture at 24-inch intervals to a maximum depth of 78
inches, and a station for collecting weather data. Based on nearly 3 years of data, there is generally less than
a 5 percent change in the relative volumetric moisture content at the bottom of the cover compared to nearly 90
percent change near the surface. This implies that most of the water infiltrating the cover is being removed via
evapotranspiration and is not reaching the bottom of the cover.
Modeling: The numerical model UNSAT-H was used to predict the annual and cumulative percolation through
the cover. The model was calibrated with 12 months of soil moisture content and weather data. Following
calibration, UNSAT-H predicted a cumulative percolation of 50 cm for the ET cover and 95 cm for a
conventional cover over a 10-year period. The model predicted an annual percolation of approximately 0 cm
for both covers during the first year. During years 3 through 10 of the simulation, the model predicted less
annual percolation for the ET cover than for the conventional cover.
Maintenance activities: During the first 18 months, irrigation was conducted to help establish the vegetation.
Once or twice a year, brush is cleared to comply with Fire Department regulations. Prior to the rainy season,
an inspection is conducted to check and clear debris basins and deck inlets. No mowing activities or fertilizer
applications have been conducted or are planned.
Cosf: Costs were estimated at $4.5 million, which includes soil importation, revegetation, quality control and
assurance, construction management, and installation and operation of moisture monitoring systems.
Sources: City of Los Angeles 2003, Hadj-Hamou and Kavazanjian 2003.
More information available at http:llcluin.orglproductslaltcovers
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Figure 3. Conceptual Design of a Capillary
Barrier ET Final Cover
•Vegetation
Fine-grained Layer
Coarse-grained Layer
Interim Cover
• Waste
In addition to being potentially less costly to construct,
ET covers have the potential to provide equal or
superior performance compared to conventional cover
systems, especially in arid and semi-arid
environments. In these environments, they may be
less prone to deterioration from dessication, cracking,
and freezing/thawing cycles. ET covers also may be
able to minimize side slope instability, because they do
not contain geomembrane layers, which can cause
slippage (Weand and others 1999; Benson and others
2002; Dwyer, Stormont, and Anderson 1999).
Capillary barrier ET cover systems may also eliminate
the need for a separate biointrusion and/or gas
collection layer. The coarser-grained layer can act as
a biointrusion layer to resist root penetration and
animal intrusion, due to its particle size and low water
content. The coarser-grained layer also can act as a
gas collection layer, because the soil properties and
location within the cover system are comparable to a
typical gas collection layer in a conventional cover
system (Dwyer 2003, Stormont 1997).
LIMITATIONS
ET cover systems are generally considered potentially
applicable only in areas that have arid or semi-arid
climates; their application is generally considered
limited to the western United States. In addition, site-
specific conditions, such as site location and landfill
characteristics, may limit the use or effectiveness of ET
cover systems. Local climatic conditions, such as
amount, distribution, and form of precipitation,
including amount of snow pack, can limit the
effectiveness of an ET cover at a given site. For
example, if a large amount of snow melted when
vegetation was dormant, the cover may not have
sufficient water storage capacity, and percolation might
occur (EPA 2000a; Mauser, Weand, and Gill 2001).
Further, landfill characteristics, such as production of
landfill gases, may limit the use of ET covers. The
cover system may not adequately control gas
emissions since typical ET cover designs do not have
impermeable layers to restrict gas movement. If gas
collection is required at the site, it may be necessary to
modify the design of the cover to capture and vent the
gas generated in the landfill. In addition, landfill gas
may limit the effectiveness of an ET cover, because
the gases may be toxic to the vegetation (Weand and
others 1999; EPA 2000a).
Limited data are available to describe the performance
of ET cover systems in terms of minimizing percolation,
as well as the covers' ability to minimize erosion, resist
biointrusion, and remain effective for an extended
period of time. While the principles of ET covers and
Exhibit 2. Capillary Barrier ET Cover at Lake County Landfill, Poison, Montana
Site type: Municipal solid waste landfill
Scale: Field demonstration under Alternative Cover Assessment Program (ACAP)
Cover designs: The capillary barrier test section was installed in November 1999. From the surface
downward, it is composed of 6 inches of topsoil, 18 inches of moderately compacted silt, and 24 inches of
sandy gravel. The cover was seeded in March 2000 with a mixture of grasses, forbs, and shrubs, including
bluegrass, wheatgrass, alfalfa, and prickly rose shrubs. A conventional composite cover test section was also
constructed at the site.
Performance data: Percolation is being measured with a lysimeter connected to flow monitoring systems, soil
moisture is being measured with water content reflectometers, and soil matric potential and soil temperature
are being monitored with heat dissipation units. From November 1999 through July 2002, the capillary barrier
cover system had a cumulative percolation of 0.5 mm. Total precipitation was 837 mm over the 32-month
period. Additional field data are expected to be collected through 2005.
Modeling: Numerical modeling was conducted using HYDRUS 2-D, which simulated the wettest year on
record over the simulation period of 10 years. The model predicted approximately 0.6 mm of percolation during
the first year, and 0.1 mm per year for the remaining 9 years.
Sources: Bolen and others 2001, Benson and others 2002.
More information available at http:llcluin.orglproductslaltcovers
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their corresponding soil properties have been
understood for many years, their application as final
cover systems for landfills has emerged only within the
past 10 years. Limited performance data are available
on which to base applicability or equivalency decisions
(Dwyer 2003; Dwyer, Stormont, and Anderson 1999;
Mauser and Weand 1998).
Numerical models are used to predict the performance
and assist in the design of final cover systems. The
availability of models used to conduct water balance
analyses of ET cover systems is currently limited, and
the results can be inconsistent. For example, models
such as Hydrologic Evaluation of Landfill Performance
(HELP) and Unsaturated Water and Heat Flow
(UNSAT-H) do not address all of the factors related to
ET cover system performance. These models, for
instance, do not consider percolation through
preferential pathways; may underestimate or
overestimate percolation; and have different levels of
detail regarding weather, soil, and vegetation. In
addition, HELP does not account for physical
processes, such as matric potential, that generally
govern unsaturated flow in ET covers. Further
information about numerical models is provided under
the Performance and Monitoring section of this fact
sheet (Dwyer 2003; Weand and others 1999; Khire,
Benson, and Bosscher 1997).
GENERAL CONSIDERATIONS
The design of ET cover systems is based on providing
sufficient water storage capacity and
evapotranspiration to control moisture and water
percolation into the underlying waste. The following
considerations generally are involved in the design of
ET covers.
Climate - The total amount of precipitation over a
year, as well as its form and distribution, determines
the total amount of water storage capacity needed for
the cover system. The cover may need to
accommodate a spring snowmelt event that causes the
amount of water at the cover to be relatively high for a
short period of time or conditions during cool winter
weather with persistent, light precipitation. Storage
capacity is particularly important if the event occurs
when local vegetation is dormant, yielding less
evapotranspiration. Other factors related to climate
that are important to cover design are temperature,
atmospheric pressure, and relative humidity (Benson
2001; EPA 2000a; Hauser, Weand, and Gill 2001).
Soil type - Finer-grained materials, such as silts and
clayey silts, are typically used for monolithic ET cover
systems and the top layer of a capillary barrier ET
cover system because they contain finer particles and
provide a greater storage capacity than sandy soils.
Sandy soils are typically used for the bottom layer of
the capillary barrier cover system to provide a contrast
in unsaturated hydraulic properties between the two
layers. Many ET covers are constructed of soils that
include clay loam, silty loam, silty sand, clays, and
sandy loam.
The storage capacity of the soil varies among different
types of soil, and depends on the quantity of fine
particles and the bulk density of the soil. Compaction
impacts bulk density, which in turn affects the storage
capacity of the soil and the growth of roots. One key
aspect of construction is minimizing the amount of
compaction during placement. Higher bulk densities
may reduce the storage capacity of the soil and inhibit
growth of roots (Chadwick and others 1999; Hauser,
Weand, and Gill 2001).
Soil thickness - The thickness of the soil layer(s)
depends on the required storage capacity, which is
determined by the water balance at the site. The soil
layers need to accommodate extreme water
conditions, such as snowmelts and summer
thunderstorms, or periods of time during which ET
rates are low and plants are dormant. Monolithic ET
covers have been constructed with soil layers ranging
from 2 feet to 10 feet. Capillary barrier ET covers have
been constructed with finer-grained layers ranging from
1.5 feet to 5 feet, and coarser-grained layers ranging
from 0.5 foot to 2 feet.
Vegetation types - Vegetation for the cover system is
used to promote transpiration and minimize erosion by
stabilizing the surface of the cover. Grasses
(wheatgrass and clover), shrubs (rabbitbrush and
sagebrush), and trees (willow and hybrid poplar) have
been used on ET covers. A mixture of native plants
consisting of warm- and cool-season species usually
is planted, because native vegetation is more tolerant
than imported vegetation to regional conditions, such
as extreme weather and disease. The combination of
warm- and cool-season species provides water uptake
throughout the entire growing season, which enhances
transpiration. In addition, native vegetation is usually
planted, because these species are less likely to
disturb the natural ecosystem (Dwyer, Stormont, and
Anderson 1999; EPA 2000a).
Soil and organic properties - Nutrient and salinity
levels affect the ability of the soil to support vegetation.
The soil layers need to be capable of providing
nutrients to promote vegetation growth and maintain
the vegetation system. Low nutrient or high salinity
levels can be detrimental to vegetation growth, and if
present, supplemental nutrients may need to be added
to promote vegetation growth. For example, at Fort
Carson, Colorado, biosolids were added to a
monolithic ET cover to increase organic matter and
provide a slow release of nitrogen to enhance
vegetation growth. In addition, topsoil promotes
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growth of vegetation and reduces erosion. For ET
covers, the topsoil layer is generally a minimum of six
inches thick (McGuire, England, and Andraski 2001).
Control layer types - Control layers, such as those
used to minimize animal intrusion, promote drainage,
and control and collect landfill gas, are often included
for conventional cover systems and may also be
incorporated in ET cover system designs. For
example, a proposed monolithic ET cover at Sandia
National Laboratories in New Mexico will have a
biointrusion fence with 1/4-inch squares between the
topsoil layer and the native soil layer to prevent
animals from creating preferential pathways, potentially
resulting in percolation. The biointrusion layer,
however, will not inhibit root growth to allow for
transpiration. At another site, Monticello Uranium Mill
Tailings Site in Utah, a capillary barrier ET design has
a 12-inch soil/rock admixture as an animal intrusion
layer located 44 inches below the surface, directly
above the capillary barrier layer.
In addition, a capillary barrier cover demonstration at
Sandia National Laboratories has a drainage layer
located above the capillary break. A drainage layer
consisting of an upper layer of sand and a lower layer
of gravel is located directly below the topsoil layer.
The sand serves as a filter to prevent topsoil from
clogging the drainage layer, while the gravel allows for
lateral drainage of water that has infiltrated through the
topsoil (Bolen and others 2001, Dwyer 2003).
In more recent applications, several types of ET cover
designs also have incorporated synthetic materials,
such as geomembranes, which are used to enhance
the function of minimizing water into the waste. For
example, the Operating Industries Inc. Landfill in
California has incorporated a soil layer with a
geosynthetic clay liner in the design. The cover
system for this site will reduce surface gas emissions,
prevent oxygen intrusion and percolation, and provide
for erosion control (EPA 2000b).
PERFORMANCE AND MONITORING
Protection of groundwater quality is a primary
performance goal for ail waste containment systems,
including final cover systems. The potential adverse
impact to groundwater quality results from the release
of leachate generated in landfills or other waste
disposal units such as surface impoundments. The
rate of leachate generation (and potential impact on
groundwater) can be minimized by keeping liquids out
of a landfill or contaminated source area of a
remediation site. As a result, the function of minimizing
percolation becomes a key performance criterion for a
final cover system (EPA 1991).
Monitoring the performance of ET cover systems has
generally focused on evaluating the ability of these
designs to minimize water drainage into the waste.
Percolation performance typically is reported as a flux
rate (inches or millimeters of water that have migrated
downward through the base of the cover in a period of
time, generally considered as 1 year). Percolation
monitoring for ET cover systems is measured directly
using monitoring systems such as lysimeters or
estimated indirectly using soil moisture measurements
and calculating a flux rate. A more detailed summary
on the advantages and disadvantages of both
approaches can be found in Benson and others 2001
(EPA 1991, Benson and others 2001).
Percolation monitoring can also be evaluated indirectly
by using leachate collection and removal systems. For
landfills underlain with these systems, the amount and
composition of leachate generated can be used as an
indicator of the performance of a cover system (the
higher the percolation, the more leachate that will be
generated) (EPA 1991).
Although the ability to minimize percolation is a
performance criterion for final cover systems, limited
data are available about percolation performance for
final cover systems for both conventional and
alternative designs. Most of the recent data on flux
rates have been generated by two federal research
programs, the Alternative Landfill Cover Demonstration
(ALCD) and the Alternative Cover Assessment
Program (ACAP); see Exhibits 3 and 4, respectively,
for further information on these programs. From these
programs, flux rate performance data are available for
14 sites with demonstration-scale ET cover systems
(Dwyer 2003, Benson and others 2002).
In addition, previous studies have been conducted that
monitored the performance of ET covers. Selected
studies include the following: integrated test plot
experiment in Los Alamos, NM, which monitored both
types of ET covers from 1984 to 1987 (Nyhan,
Hakonson, and Drennon 1990); Hill Air Force Base
alternative cover study in Utah, which evaluated three
different covers (RCRA Subtitle D, monolithic ET, and
capillary barrier ET) over a 4-year period (Hakonson
and others 1994); and Hanford field lysimeter test
facility in Richland, WA, which monitored ET covers for
6 years (Gee and others 1993).
Additional demonstration projects of ET covers
conducted in the 1980's and early 1990's are
discussed in the ACAP Phase I Report, which is
available at http:llwww.acap.dri.edu.
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Exhibit 3. Alternative Landfill Cover Demonstration (ALCD)
The U.S. Department of Energy (DOE) has sponsored the ALCD, which is a large-scale field test of two conventional
designs (RCRA Subtitle C and Subtitle D) and four alternative landfill covers {monolithic ET cover, capillary barrier ET
cover, geosynthetic clay liner cover, and anisotropic [layered capillary barrier] ET cover). The test was conducted at
Sandia National Laboratories, located on Kirtland Air Force Base in Albuquerque, New Mexico, with cover design
information available at http://www.sandia.goviSubsurfacelfactshtslert/alcd.pdf. The ALCD has collected information on
construction, cost, and performance that is needed to compare alternative cover designs with conventional covers. The
RCRA covers were constructed in 1 995, and the ET covers were constructed in 1 996. All of the covers are 43 feet wide by
328 feet long and were seeded with native vegetation. The purpose of the project is to use the performance data to help
demonstrate equivalency and refine numerical models to more accurately predict cover system performance {Dwyer 2003).
The ALCD has collected data on percolation using a lysimeter and soil moisture to monitor cover performance. Total
precipitation {precip.) and percolation (perc.) volumes based on 5 years of data are provided below. The ET covers
generally have less percolation than the Subtitle D cover for each year shown below. More information on the ALCD cover
performance can be found in Dwyer 2003.
Monolithic
ET
Capillary
barrier ET
Anisotropic
{layered
capillary
barrier) ET
Geosynthetic
clay liner
Subtitle C
Subtitle D
1997
(May 1 -Dec 31)
Precip.
(mm)
267.00
267.00
267.00
267.00
267.00
267.00
Perc.
(mm)
0.08
0.54
0.05
0.51
0.04
3.56
1998
Precip.
(mm)
291.98
291.98
291.98
291.98
291.98
291.98
Perc.
(mm)
0.22
0.41
0.07
0.19
0.15
2.48
1999
Precip.
(mm)
225.23
225.23
225.23
225.23
225.23
225.23
Perc.
(mm)
0.01
0.00
0.14
2.15
0.02
1.56
2000
Precip.
(mm)
299.92
299.92
299.92
299.92
299.92
299.92
Perc.
(mm)
0.00
0.00
o.oo
0.00
0.00
0.00
2001
Precip.
(mm)
254.01
254.01
254.01
254.01
254.01
254.01
Perc.
(mm)
0.00
0.00
0.00
0.02
0.00
0.00
2002
(Jan 1 - Jun 25)
Precip.
(mm)
144.32
144.32
144.32
144.32
144.32
144.32
Perc.
(mm)
0.00
0.00
0.00
0.00
0.00
0.74
Monitoring systems - Lysimeters are installed
underneath a cover system, typically as geomembrane
liners backfilled with a drainage layer and shaped to
collect water percolation. Water collected in the
lysimeter is directed toward a monitoring point and
measured using a variety of devices (for example,
tipping bucket, pressure tranducers). Lysimeters have
been used in the ALCD and ACAP programs for
collecting performance data for ET cover systems.
Soil moisture monitoring can be used to determine
moisture content at discrete locations in cover systems
and to evaluate changes over time in horizontal or
vertical gradients. Soil moisture is measured using
methods to determine relative humidity, soil matrix
potential, and resistance. Table 1 presents examples
of non-destructive techniques that have been used to
assess soil moisture cpntent of ET cover systems. A
high soil moisture value indicates that the water
content of the cover system is approaching its storage
capacity, thereby increasing the potential for
percolation. Soil moisture is especially important for
capillary barrier ET cover systems; when the finer-
grained layer becomes saturated, the capillary barrier
can fail resulting in water percolating through the highly
permeable layer to the waste below (Hakonson 1997).
Maintaining the effectiveness of the cover system for
an extended period of time is another important
performance criterion for ET covers as well as
conventional covers. Short-term and long-term
performance monitoring of a final cover system
includes settlement effects, gas emissions, erosion or
slope failure, and other factors.
Numerical models - While there are limitations to
numerical models, as previously described, they have
been used to predict cover performance and assist in
the design of ET cover systems. Numerical models
have been used to compare the expected performance
of ET cover systems to conventional cover systems.
By entering multiple parameters and evaluating the
design of cover systems, designs can be modified until
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Exhibit 4. Alternative Cover Assessment Program (ACAP)
EPA is conducting the ACAP to evaluate the performance of alternative landfill covers. ACAP began in 1998, and cover
performance is currently being evaluated at 13 sites. The sites are located in eight states from California to Ohio, and
include a variety of landfill types, such as MSW, construction and demolition waste, and hazardous waste landfills. At eight
sites, conventional and ET covers are being tested side by side. At the remaining five sites, only ET covers are being
tested.
The alternative covers typically were constructed with local soils and native vegetation. At two facilities, however, hybrid
poplar trees were used as vegetation, At 1 1 sites, percolation performance is being evaluated by lysimeters. At the other
two sites, performance is being evaluated indirectly by monitoring leachate production. Soil moisture is also being
evaluated at all 13 sites. Below is an example of the field data for precipitation (precip.) and percolation (perc.) volumes at
3 of the sites. A summary of field cover performance for all 1 3 sites through July 2002 is provided in Albright and Benson
2002. More information about ACAP is available on the Desert Research Institute website at http://www.acap.dri.edul.
Site
Altamont, CA
(semi-arid)
Poison, MT
(semi-arid)
Omaha, NE
(humid)
Cover Design
Monolithic ET
Composite/
compacted clay
Capillary barrier ET
Composite/
compacted clay
Capillary barrier ET
(thick)
Capillary barrier ET
(thin)
Composite/
compacted clay
Start
Date
11/00
11/00
11/99
11/99
10/00
10/00
10/00
Yearl
Precip.
(mm)
225
225
300
300
600
600
600
Perc.
(mm)
negligible
negligible
0.05
0.5
55
100
5
Year 2
Precip.
(mm)
300
300
300
300
200
200
200
Perc.
(mm)
1.5
negligible
0.05
0.5
negligible
negligible
negligible
Year 3
Precip.
(mm)
250
250
Perc.
(mm)
0.45
0.5
Table 1. Examples of Non-Destructive Soil Moisture Monitoring Methods
Method
Tensiometer
Psych rometer
Electrical resistance blocks
Neutron attenuation
Time domain reflect rometry
Description
Measures the matric potential of a given soil,
which is converted to soil moisture content
Measures relative humidity (soil moisture)
within a soil
Measures resistance resulting from a gradient
between the sensor and the soil; higher
resistance indicates lower soil moisture
Emits high-energy neutrons into the soil that
collide with hydrogen atoms associated with
soil water and counts the number of pulses,
which is correlated to moisture content
Sends pulses through a cable and observes
the reflected waveform, which is correlated to
soil moisture
Instrumentation
Commonly consists of a porous ceramic cup
connected to a pressure measuring device
through a rigid plastic tube
Generally consists of a thermocouple, a
reference electrode, a heat sink, a porous
ceramic bulb or wire mesh screen, and a
recorder
Consists of electrodes embedded in a
gypsum, nylon, or fiberglass porous material
Consists of a probe inserted into access
boreholes with aluminum or polyvinyl chloride
casing
Consists of a cable tester (or specifically
designed commercial time domain
reflectrometry unit), coaxial cable, and a
stainless steel probe
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• specific performance results are achieved. The
numerical model HELP is the most widely used water
balance model for landfill cover design. UNSAT-H and
HYDRUS-2D are two other numerical models that have
been used frequently for the design of ET covers.
HELP and UNSAT-H are in the public domain, while
HYDRUS-2D is available from the International Ground
Water Modeling Center in Golden, CO
http:lltyphoon. mines.edu (Dwyer2003; Khire, Benson,
and Bosscher 1997).
Recent studies have compared available numerical
models and found that cover design depends on site-
specific factors, such as climate and cover type, and
that no single mode! is adequate to accurately predict
the performance of all ET covers. Several of the
studies identified are: intercede comparisons for
simulating water balance of surficial sediments in semi-
arid regions, which compared results of seven
numerical models for nonvegetated, engineered
covers in semiarid regions; water balance
measurements and computer simulations of landfill
covers, which evaluated ALCD cover performance and
predicted results from HELP and UNSAT-H; and field
hydrology and model predictions for final covers in the
ACAP, which compared performance results with those
predicted by HELP and UNSAT-H (Scanlon and others
2002; Dwyer 2003; Roesler, Benson, and Albright
2002).
COST
Limited cost data are available for the construction and
operation and maintenance (O&M) of ET cover
systems. The available construction cost data indicate
that these cover systems have the potential to be less
expensive to construct than conventional cover
systems. Factors affecting the cost of construction
include availability of materiats, ease of installation,
and project scale. Locally available soils, which are
usually less costly than imported clay soils, are
typically used for ET cover systems. In addition, the
use of local materials generally minimizes
transportation costs {Dwyer 2003, EPA 2000a).
While the construction cost for an ET cover is expected
to be less than that for a conventional cover,
uncertainty exists about the costs for O&M after
construction. Several factors affecting the O&M cost
include frequency and level of maintenance (for
example, irrigation and nutrient addition), and activities
needed to address erosion and biointrusion. In
addition, when comparing the costs for ET and
conventional covers, it is important to consider the
types of components for each cover and their intended
function. For example, it would generally not be
appropriate to compare the costs for a conventional
cover with a gas collection layer to an ET cover with no
such layer. Additional information about the costs for
specific ET cover systems is provided in project
profiles, discussed below under Technology Status.
TECHNOLOGY STATUS
A searchable on-line database has been developed
with information about ET cover systems and is
available at http:flcluin.org/productslaltcovers. As of
September 2003, the database contained 56 projects
with monolithic ET cover systems and 21 projects with
capillary barrier ET cover systems; these systems have
been proposed, tested, or installed at 64 sites located
throughout the United States, generally from Georgia
to Oregon. Some sites have multiple projects, and
some projects have multiple covers and/or cover types.
The database provides project profiles that include site
background information (for example, site type,
climate, precipitation), project information (for example,
purpose, scale, status), cover information (for example,
design, vegetation, installation), performance and cost
information, points of contact, and references. Table
2 provides a summary of key information from the
database for 34 recent projects with monolithic ET or
capillary barrier ET covers.
In addition to this on-line database, several ongoing
federal and state initiated programs are demonstrating
and assessing the performance of ET cover systems.
The following programs provide performance data,
reports, and other useful information to help evaluate
the applicability of ET designs for final cover systems.
• Alternative Landfill Cover Demonstration - See
Exhibit 3 for more information or
http://www.sandia.gov/Subsurface/factshtslertl
alcd.pdf
* Alternative Cover Assessment Program - See
Exhibit 4 for more information or
http://www. acap.dri. edu
• Interstate Technology and Regulatory Council -
Published a report called Technology Overview
Using Case Studies of Alternative Landfill
Technologies and Associated Regulatory
Topics; March 2003. For further information,
see http://www.iircweb.org
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Table 2. Selected Sites Using or Recently Demonstrating Evapotranspiration (ET) Covers
Site Name and Location ' Site Type
Monolithic ET Covers - Full Scale Projects
Barton County Landfill, Great Bend, KS MSW landfill
Coyote Canyon Landfill, Somis, CA MSW landfill
Duvall Custodial Landfill, Duvall, WA MSW landfill
Fort Carson, Colorado Springs, CO MSW landfill
Hastings Groundwater Contamination Superfund Site, MSW landfill
Hastings, NE
Horseshoe Bend Landfill, Lawrenceburg, TN Industrial waste landfill
Idaho National Engineering and Environmental Laboratory Radioactive waste site
Superfund Site, Idaho Falls, ID
Industrial Excess Landfill Superfund Site, OH Industrial waste landfill
Johnson County Landfill, Shawnee, KS MSW landfill
Lakeside Reclamation Landfill, Beaverton, OR Construction debris
Lopez Canyon Sanitary Landfill, Los Angeles, CA MSW landfill
Marine Corps Logistics Base Superfund Site, GA MSW and hazardous waste landfill
Municipal Waste Landfill at Kirtland Air Force Base, NM MSW landfill
Operating Industries Inc. Landfill Superfund Site, CA MSW landfill
Pantex Plant, Amarillo, TX Construction debris
Site Name and Location Site Type
Capillary Barrier ET Covers - Full Scale Projects
Gaffey Street Sanitary Landfill, Wilmington, CA MSW landfill
Hanford Superfund Site, Richland, WA* Radioactive waste site
McPherson County Landfill, McPherson, KS MSW landfill
Site Name and Location Site Type
Monolithic ET Covers - Demonstration Projects
Altamont Landfill, Livermore, CA (ACAP project) Non-hazardous waste site
Bluestem Landfill #2, Marion, IA (ACAP project) MSW landfill
Finley Buttes Regional Landfill, OR (ACAP project) MSW landfill
Green II Landfill, Logan, OH (ACAP project) MSW and hazardous waste landfill
Kiefer Landfill, Sloughhouse, CA (ACAP project) Non-hazardous waste site
Marine Corps Logistics Base, Albany, GA (ACAP project) MSW and hazardous waste landfill
Milliken Landfill, San Bernadino County, CA (ACAP project) MSW landfill
Monterey Peninsula Landfill, Marina, CA (ACAP project) Non-hazardous waste site
Rocky Mountain Arsenal Superfund Site, Denver, CO Hazardous waste site
Sandia National Laboratories, NM (ALCD project) Non-hazardous waste site
Site Name and Location Site Type
Capillary Barrier ET Covers - Demonstration Projects
Douglas County Landfill, Bennington, NE (ACAP project) MSW landfill
Hill Air Force Base, Ogden, UT Hazardous waste landfill
Lake County Landfill, Poison, MT (ACAP project) MSW landfill
Lewis and Clark County Landfill, MT (ACAP project) Non-hazardous waste site
Sandia National Laboratories, NM (ALCD project) Non-hazardous waste site
Uranium Mill Tailings Repository, UT (ACAP project) Hazardous waste landfill
Status of Project
Installation
Operational
Operational
Operational
Design
Operational
Proposed
Proposed
Installation
Operational
Operational
Proposed
Operational
Operational
Operational
Status of Project
Installation
Operational
Operational
Status of Project
Operational
Operational
Operational
Operational
Operational
Operational
Operational
Operational
Complete
Operational
Status of Project
Operational
Operational
Operational
Operational
Operational
Operational
Date Installed
NA
April 1994
1999
October 2000
NA
1998
NA
NA
NA
1990
1999
NA
2002
May 2000
2000
Date installed
NA
1994
2002
Date Installed
November 2000
October 2000
November 2000
2000
July 1999
March 2000
1997
May 2000
April 1998
1996
Date Installed
August 2000
1994
November 1999
November 1999
1996
July 2000
Notes:
* Project conducted as Superfund treatability test study with cover constructed over an existing waste site
NA Not Applicable
ALCD Alternative Landfill Cover Demonstration; program supported by DOE
ACAP Alternative Cover Assessment Program; program supported by EPA
10
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^REFERENCES
Albright, W.H. and C.H. Benson. 2002. "Alternative
Cover Assessment Program 2002 Annual Report."
Desert Research Institute. Publication No. 41182.
October.
Benson, C.H. 2001. "Alternative Earthen Final Covers
(AEFCs) or 'ET Caps." GEO Institute.
Proceedings, Liners and Covers for Waste
Containment Facilities. Atlanta, Georgia.
November 14 through 16.
Benson, C.H. and others. 2001. "Field Evaluation of
Alternative Earthen Final Covers." International
Journal of Phytoremediation. Volume 3, Number
1. Pages 105 through 127.
Benson, C.H. and others. 2002. "Evaluation of Final
Cover Performance: Field Data from the
Alternative Landfill Cover Assessment Program
(ACAP)." Proceedings, WM 2002 Conference.
Tucson, Arizona. February 24 through 28.
Bolen, M.M. and others. 2001. "Alternative Cover
Assessment Program: Phase II Report."
University of Wisconsin-Madison. Madison,
Wisconsin. September. Geo Engineering Report
01-10.
Chadwick, Jr., D. and others. 1999. "Field Test of
Potential RCRA-Equivalent Covers at the Rocky
Mountain Arsenal, Colorado." Solid Waste
Association. Proceedings, North America's 4th
Annual Landfill Symposium. Denver, Colorado.
June 28 through 30. GR-LM 0004. Pages 21
through 33.
City of Los Angeles. 2003. E-mail Message
Regarding Lopez Canyon Landfill. From Doug
Walters, Sanitary Engineer, Department of Public
Works. To Kelly Madalinski, EPA. September 25.
Dwyer, S. 2003. "Water Balance Measurements and
Computer Simulations of Landfill Covers."
University of New Mexico, Civil Engineering
Department. May.
Dwyer, S.F., J.C. Stormont, and C.E. Anderson. 1999.
"Mixed Waste Landfill Design Report." Sandia
National Laboratories. SAND99-2514. October.
Gee, G. and others. 1993. "Field Lysimeter Test
Facility Status Report IV: FY 1993." Pacific
Laboratory, Richland, Washington. PNL-8911,
UC-902.
Hadj-Hamou, T. and E. Kavazanjian, Jr. 2003.
"Monitoring and Evaluation of Evapotranspirative
Cover Performance." GeoSyntec Consultants.
Hakonson, T.E. 1997. "Capping as an Alternative for
Landfill Closures-Perspectives and Approaches."
Environmental Science and Research Foundation.
Proceedings, Landfill Capping in the Semi-Arid
West: Problems, Perspectives, and Solutions.
Grand Teton National Park, Wyoming. May 21
through 22. ESRF-019. Pages 1 through 18.
Hakonson, T. and others. 1994. "Hydrologic
Evaluation of Four Landfill Cover Designs at Hill
Air Force Base, Utah." Los Alamos National
Laboratory, Los Alamos, New Mexico. LAUR-93-
4469.
Hauser,V.L.,andB.L.Weand. 1998. "Natural Landfill
Covers." Proceedings, Third Tri-Service
Environmental Technology Workshop. San Diego,
California. August 18 through 20.
Hauser, V.L., B.L. Weand, and M.D. Gill. 2001.
"Natural Covers for Landfills and Buried Waste."
Journal of Environmental Engineering.
September. Pages 768 through 775.
Khire, M.V., C.H. Benson, and P.J. Bosscher. 1997.
"Water Balance Modeling of Earthen Final
Covers." Journal of Geotechnical and
Geoenvironmental Engineering. August. Pages
744 through 754.
McGuire, P.E., J.A. England, and B.J. Andraski. 2001.
"An Evapotranspiration Cover for Containment at
a Semiarid Landfill Site." Florida State University.
Proceedings, 2001 International Containment &
Remediation Technology Conference. Orlando,
Florida. June 10 through 13.
Nyhan, J., Hakonson, T., and Drennon, B. 1990. "A
Water Balance Study of Two Landfill Cover
Designs for Semi-arid Regions." Journal of
Environmental Quality. Volume 19. Pages 281
through 288.
Roesler, A.C., C.H. Benson, and W.H. Albright. 2002.
"Field Hydrology and Model Predictions for Final
Covers in the Alternative Cover Assessment
Program-2002." University of Wisconsin-Madison.
Geo Engineering Report No. 02-08. September
20.
Scanlon, B.R., and others. 2002. "Intercede
comparisons for simulating water balance of
surficial sediments in semiarid regions." Water
Resources Research. Volume 38, Number 12.
11
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Stormont, John C. 1997. "Incorporating Capillary
Barriers in Surface Cover Systems."
Environmental Science and Research Foundation.
Proceedings, Landfill Capping in the Semi-Arid
West: Problems, Perspectives, and Solutions.
Grand Teton National Park, Wyoming. May 21
through 22. ESRF-019. Pages 39 through 51.
EPA. 1989. Technical Guidance Document: "Final
Covers on Hazardous Waste Landfills and Surface
Impoundments." EPA/53O-SW-89-047. July.
EPA. 1991. "Seminar Publication, Design and
Construction of RCRA/CERCLA Final Covers."
EPA/625/4-91/025. May.
EPA. 1992. "Subtitle D Clarification." 40 CFR 257 &
258. Federal Register pages 28626
through28632. June.
EPA. 2000a. Introduction to Phytoremediation. Office
of Research and Development. Washington, DC.
EPA/600/R-99/107. February.
EPA. 2000b. "Operating Industries Inc. Final
Construction As Built Report." May.
Weand, B.L., and others. 1999. "Landfill Covers for
Use at Air Force Installations." AFCEE. Brooks
Air Force Base, Texas. February.
NOTICE
Preparation of this fact sheet has been funded wholly
or in part by the U.S. Environmental Protection Agency
under Contract Number 68-W-02-034. For more
information regarding this fact sheet, please contact
Mr. Kelly Madalinski, EPA, at (703) 603-9901 or
madalinski.keily@epa.gov.
This fact sheet is available for viewing or downloading
from EPA's Hazardous Waste Cleanup Information
(CLU-IN) web site at http://cluin.org. Hard copies are
available free of charge from:
U.S. EPA/National Service Center for Environmental
Publications (NSCEP)
P.O. Box42419
Cincinnati, OH 45242-2419
Telephone: (513) 489-8190 or (800) 490-9198
Fax: (513)489-8695
ACKNOWLEDGMENT
Special acknowledgment is given to the following
individuals for their review and thoughtful suggestions
to support the preparation of this fact sheet: David
Carson (EPA), Steve Rock (EPA), Ken Skahn (EPA),
Steve Wall (EPA), Greg Mellama (U.S. Army Corps of
Engineers), Joey Trotsky (U.S. Navy), Bill Albright
(Desert Research Institute), Craig Benson (University
of Wisconsin-Madison), Steve Dwyer (Sandia National
Laboratory), Glendon Gee (Pacific Northwest National
Laboratory), Tanya Goldfield (City of Los Angeles),
Charles Johnson (Colorado Department of Public
Health), Doug Walters (City of Los Angeles), and Tom
Hakonson (Environmental Evaluation Services, LLC).
12
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