United States
Environmental Protection
Agency
Risk Reduction
Engineering Laboratory
Cincinnati OH 45268
Research and Development
EPA/530/SW-91/054a Aug. 1991
EPA Project Summary
Design, Construction, and
Operation of Hazardous and
Nonhazardous Waste Surface
Impoundments
Robert P. Hartley
The document outlined here sum-
marizes the state of knowledge regard-
Ing the design, construction, and
operation of hazardous and nonhazard-
ous waste surface impoundments con-
talning liquids and sludges. The
document draws upon research, mainly
sponsored by the U.S. Environmental
Protection Agency (EPA), and the prac-
tical experience of Impoundment de-
sign engineers. It also draws upon the
experience of the manufacturers and
fabricators of containment materials
who continually strive to Improve the
material's resistance to failure when
exposed to various waste chemicals.
Rather than providing details, the docu-
ment summarizes and directs the reader
to appropriate references.
The document first outlines the fed-
eral regulations under the Resource
Conservation and Recovery Act (RCRA)
that apply to waste-containing surface
Impoundments. It then describes
predeslgn considerations, design char-
acteristics, construction, and operation
and maintenance of a completed facil-
ity. Contingency planning and response
action plans to be Implemented In the
event of system failure or excessive
leaks are discussed In some detail. Fi-
nally, a chapter Is devoted to the clo-
sure of surface Impoundments, with
particular attention directed at hazard-
ous waste surface Impoundments. The
chapter Includes a description of final
covers and post-closure maintenance.
This Project Summary was devel-
oped by EPA's Risk Reduction Engi-
neering Laboratory, Cincinnati, OH, to
announce key findings of the research
project that Is fully documented In a
Technical Resource Document of the
same title (see Project Report ordering
Information at back).
Introduction
There are approximately 180,000
waste-containing surface impoundments in
the United States. The majority are used
for the storage of nonhazardous waste,
but many are used for the treatment, stor-
age, and disposal of hazardous waste.
Hazardous waste surface impoundments
are subject to more restrictive regulation
than are nonhazardous waste impound-
ments. Much of the waste impoundment
technology development has been directed
at hazardous waste containment, but it
appears that most of that technology is
also applicable to nonhazardous waste
surface impoundments.
Regulatory Requirements
In general, RCRA regulations require
that hazardous waste surface impound-
ments have double liners with a leak col-
lection layer between the liners (Figure 1).
EPA guidance recommends that the bot-
tom liner be a composite of a geomembrane
in direct contact with low-permeability soil.
They must operate effectively through ac-
tive impoundment use and a 30-yr post-
closure period. All parts of the system
must meet performance requirements. For
example, chemical constituents of the
waste must not migrate into the top liner
nor migrate through the bottom liner.
Printed on Recycled Paper
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protective soil layer
geomembrane liners with
protective geotextiles
composite liner
^r^-j*^-compacted soil ^sr^r^?
^^ leak detection/collection layer
Figure 1. Cross section of double liner with composite bottom liner for a hazardous or nonhazardous
surface impoundment.
Monitoring and inspection is required dur-
ing construction and during the im-
poundment's operation for leaks, damage,
and imperfections. Repairs must be made
when necessary, or the unit must be re-
moved from service. Contingency plans and
response action plans are required as part
of the permit process.
Closure must be either "clean closure," in
which all contaminated residues are re-
moved, or "in-place closure," in which the
wastes are permanently stabilized and cov-
ered in place. Post-closure care is required
for a 30-yr period for those facilities closed
in place. It is not required for clean closure.
Pre-Design Considerations
A surface impoundment design engineer
must consider the environmental surround-
ings and his design must include features
that will protect against any potential failures
that could be induced or promoted by the
characteristics of those surroundings. Fac-
tors that must be considered are topography,
surface and subsurface hydrology, geology,
soil conditions, existing and future land uses,
climate, and air quality. These factors
should first be used in determining whether
a surface impoundment is appropriate at the
proposed site. If appropriate, then they will
influence the magnitude of engineering
safety factors used for structural component
design.
geomembrane liners
with protective .
geotextiles
protective soil layer
Design
The first step in the impoundment design
process is selecting the number, size, posi-
tion, area, and depth of the desired struc-
tures. Generally these are most dependent
on the type and amount of waste liquid to be
handled and the rate of precipitation and
evaporation.
An analysis must be made of the bearing
capacity of the native soil in addition to a
stability analysis of proposed dikes. A criti-
cal part of the design is the selection of liner
materials for the required double liner sys-
tem. Qeomembrane liners are an essential
part of the system to achieve impermeability,
but they first must be proven capable of
long-term containment of the waste that is to
be handled. EPA guidance recommends a
composite geomembrane-soil bottom liner.
The hydraulic conductivity of the compacted
soil component must be no greater than 1 x
10"7 cm/sec. The geomembrane and soil
must be in direct contact to minimize lateral
flow between them should a leak develop in
the geomembrane.
The top and bottom liners of a double-liner
system must be separated by a drainage
layer that will allow liquid flow sufficient to
remove any potential leakage through the
top liner (Figure 2). The drainage layer may
be of granular soil or geosynthetic material,
so longas it can maintain a transmissivity of
3x10 m2/sec over the long term.
leak collection
layer
cut-slope
low-permeability
soil layer
Figure 2. Surface impoundment dike and liner interfaces and layers.
Other layers may be required in the liner
system to prevent intrusion of one layer int'
another or to facilitate liquid drainage or es-
cape of gases. Generally, geotextile filters
or geosynthetic drainage materials are most
appropriate for these uses. They must be
shown to be chemically resistant to any liq-
uid or gaseous waste to be contained in the
impoundment that the material potentially
could contact.
Other components of the design include
active and passive liquid level controls and
warning devices, leak detection systems
(mechanical or electronic) to detect leaks
through the top liner, secondary containment
to contain potential large sudden losses, and
surface water drainage and diversion away
from the impoundment. Controls should be
designed for any potential volatile organic
emissions. Finally, the design should in-
clude a construction quality assurance plan
that addresses all structural components of
the impoundment system during the con-
struction phase. Lack of construction quality
has been found to be the greatest controlling
factor in system failures.
Construction
Foundation soils and any dikes must be
cut and/or compacted to the design bearing
strength. A compacted soil liner is then con-
structed. The desired degree of compac
tion, in all cases, is generally achieved wi
footed compactors. Compacted layers are'
usually finished with smooth rollers. A
smooth surface on the compacted low-per-
meability soil component of the composite
secondary (bottom) liner is critically import-
ant, as it must facilitate uninterrupted direct
contact with the geomembrane on top of it.
Soils are generally placed in several lifts,
individually compacted, to achieve the de-
sired thickness, shape, bearing strength,
and hydraulic conductivity. Test pads are
first required, before actual construction, to
determine if the design soil characteristics
are achievable with the equipment to be
used. Again, the finished soil liner is com-
pacted as smoothly as possible to accom-
modate full contact with the overlying
geomembrane.
Geomembranes and geosynthetic drain-
age materials are installed by experts in the
techniques involved so that seams are
sealed continuously and stress areas, such
as wrinkles and folds, are minimized.
Seams are placed up and down the side-
walls to minimize tensional stresses across
the seams (Rgure 3). Great care should be
taken to make seams as strong as the pan-
els they are connecting.
All construction should be subjected to s
rigid construction quality assurance p'
gram, carried out by a third party who e,.,,
ploys an experienced quality control
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engineer. Inspections and testing of poten-
I points of weakness are done in a statis-
i'ally valid manner. After completion of
construction, the impoundment should be
finally tested, preferably by filling with water
and testing for leaks. Any discovered leaks
must be eliminated.
Operation, Maintenance,
Monitoring, and Contingency
Planning
Operation, maintenance, and monitoring
procedures should be spelled out before
wastes are received, and the operating per-
sonnel should be fully familiar with the pro-
cedures. Maintenance and monitoring
should be routine programs with scheduled
periodic inspection and maintenance activi-
ties.
The procedures described in contingency
plans and leak response plans should be
instilled in operating personnel so that there
is no question about their importance and
the emergency responses that will be
employed. These plans are of utmost im-
portance, because they may prevent wide-
spread damage in case of facility failure.
Closure
All waste-containing surface im-
poundments must eventually be closed. By
yillation, two options are generally avail-
dole, clean closure and In-place closure.
Clean closure requires removal and/or de-
contamination of all contaminated materials.
Removal generally requires the removed
materials to be landfilled. In-place closure
means that the contaminated materials are
decontaminated to the extent possible and
kept on the site permanently. In either case,
the waste requires solidification with no free
liquids remaining. Many treatment options
are available for decontamination and solid-
ification, but their effectiveness will be site-
specific.
perimeter anchor trench
toe of slope
Figure 3. Layout for surface impoundment geomembrane panels.
vegetation/soil
top layer
drainage layer
low-permeability \
geomembrane/soil layer ~\
waste
\\l/
60cm
~Ğ granular or geotextile filter
30cm
-^ 20-mil (0.51 mm) geomembrane
60 cm w/overlying protective geotextile
-Ğ geotextile separation layer
(for low-bearing-strength waste)
Figure 4. EPA-recommended cover system for hazardous and nonhazardous waste surface
Impoundments.
In-place closure requires a permanent
landfill cover that meets the requirements of
RCRA and EPA's minimum technology guid-
ance for landfill covers (Figure 4). The cover
must include a low-permeability hydraulic
barrier, and USEPA recommends a com-
posite geomembrane-soil layer much like
the composite bottom liner recommended
for a double-liner system. The cover re-
quires monitoring and maintenance through-
out a 30-yr post-closure period in accor-
dance with an approved closure plan.
The full report was submitted in fulfill-
ment of EPA Purchase Order No. 1C6081
NATX by Robert P. Hartley under the spon-
sorship of the U.S. Environmental Protec-
tion Agency.
&U.S. GOVERNMENT PRINTING OITICE: lğğl - S4MM/40070
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Robert P. Hartley was with Risk Reduction Engineering Laboratory, Cincinnati, OH
45268.
Robert E. Landreth is the EPA Project Officer (see below).
The complete report, entitled "Design, Construction, and Operations of Hazardous and
Nonhazardous Waste Surface Impoundments," (Order No. PB91-204354/AS; Cost:
$35.00, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Risk Reduction Engineering Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati, OH 45268
BULK RATE
POSTAGE & FEES PAID
EPA PERMIT NO. G-35
Official Business
Penalty for Private Use $300
EPA/530/SW-91/054a
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