United States
Environmental Protection
Agency
Hazardous Waste Engineering
Research Laboratory
Cincinnati OH 45268
Research and Development
EPA/540/S2-85/004 Jan. 1986
Project Summary
Leachate Plume Management
Edward Repa and Douglas P. Doerr
A handbook was developed on the
management of leachate plumes, a
problem that has been somewhat ag-
gravated by a lack of understanding of
plume dynamics and the various reme-
dial options available. The handbook
describes factors that affect leachate
•plume movement and key considera-
tions in delineating the current and
future extent of the leachate plume.
Four technologies for controlling the
migration of the plume are also dis-
cussed: (1) groundwater pumping to
extract water from or inject water into
wells to capture a plume or alter the
direction of groundwater movement;
(2) subsurface drains consisting of
permeable barriers designed to inter-
cept groundwater systems; (3) vertical
underground barriers made of low-per-
meability materials to divert ground-
water flow or minimize leachate genera-
tion and plume movement; and (4)
innovative technologies that biological-
ly or chemically remove or attenuate
contaminants in the subsurface.
This Project Summary was developed
by EPA's Hazardous Waste Engineering
Research Laboratory, Cincinnati, OH,
to announce key findings of the research
project that is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Introduction
Cleaning up the thousands of hazard-
ous waste release sites identified across
the Untied States is a serious environ-
mental challenge to the nation. Of partic-
ular importance is protecting and treating
contaminated groundwaters that now
serve or could serve as drinking water
supplies. Successful control of leachate
plumes from uncontrolled hazardous
waste sites requires a thorough under-
standing of plume dynamics and aquifer
restoration technologies. The problem of
leachate plume management has been
intensified to some extent by a lack of
understanding of plume dynamics and
the various site remedies available. Meth-
ods for controlling the migration of a
leachate plume generally fall into one of
four categories—groundwater pumping,
subsurface drains, low-permeability bar-
riers, or innovative technologies.
The purpose of this project was to .
prepare a reference manual on the move-
ment of hazardous leachate plumes and
their management. This summary pre-
sents an overview of the manual, which
consists of eight chapters:
1. Introduction,
2. Plume dynamics,
3. Plume delineation,
4. Plume control technologies,
5. Groundwater pumping,
6. Subsurface drains,
7. Low-permeability barriers, and
8. Innovative technologies
Plume Dynamics
In terms of plume movement, the
simplest type of leachate plume is one
that mixes with groundwater and moves
with it at the same rates and in the same
directions. In this case, plume velocity
(ignoring attenuation and other geochem-
ical factors) can be expressed by Darcy's
Law:
V = K i/n=Ti/bn
where V is the velocity of the plume, K is
the effective permeability or hydraulic
conductivity of the aquifer, i is the hy-
draulic gradient, the slope of the water
table, or the potentiometric (pressure)
surface, n is the porosity of the aquifer, T
is the transmissivity of the aquifer or its
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ability to transmit water to wells, and b is
the saturated thickness of the aquifer
through which groundwater flows.
Thus, the velocity of a plume moving
with groundwater will be directly related
to thetransmissivity and the gradient and
inversely related to the saturated thick-
ness and the aquifer's porosity.
The direction in which simple plumes
move is controlled by the gradient of the
potentiometric surface. In unconfined
aquifers, this gradient commonly follows
the surface topography, but it can be
affected by a variety of hydrologic and
geologic factors, including aquifer geom-
etry and uniformity, geologic discontinu-
ities, and hydraulic barriers.
In addition to hydrogeologic factors,
manmade alterations of surface or sub-
surface conditions can also affect the
direction and rate of leachate plume
migration. Thecumulative effects of these
and other factors can make predicting
plume migration patterns an extremely
complex task, even for water soluble
plumes that move with groundwater flow.
Consequently, it is imperative to delineate
plumes on a site-specific basis.
Plume Delineation
Before the options for leachate plume
management at a given site can be
developed, the dimensions of the plume
must be delineated. A variety of direct and
indirect methods exist for obtaining data
on plume dimensions including the fol-
lowing:
• Aerial image interpretation
* Hydrogeologic investigations
* Geophysical investigations
* Groundwater sampling
* Hydrologic (computer) modeling
Aerial imagery refers to pictorial repre-
sentations produced by electromagnetic
radiation that is emitted or reflected from
the earth and recorded by aircraft-
mounted sensors.
A hydrogeologic site investigation typ-
ically consists of installing a number of
wells and conducting tests which char-
acterize the ability of the aquifer to store
and transmit water. Information on site
geology and pedology, leachate seeps,
geologic discontinuities, and discharge
and recharge barriers are required.
A geophysical survey can provide a cost
effective way of collecting data on sub-
surface geology, especially when the
geology of the site is highly variable. The
geophysical methods most commonly
used in site investigations are resistivity,
seismic, metal detection, and radar. Prop-
erly applied geophysical survey methods
can determine the lateral and vertical
extent of the plume, approximate changes
in contamination with depth, establish
depth of bedrock, and locate metal drums
or debris.
The most direct means of delineating a
leachate plume is by groundwater sam-
pling. Typically, the wells installed during
the hydrogeologic investigation are also
used for obtaining samples of the contam-
inated groundwater. However, obtaining
valid groundwater data is extremely dif-
ficult because of the large number of
variables involved. The results of the
chemical analyses of these samples can
be plotted on a site map and contoured in
much the same way as water tables and
land surface elevations. These maps can
then be used to determine the direction of
plume movement.
Computer models of groundwater have
been used to evaluate the extent and
expected movement of plumes. Two
categories exist for groundwater models:
analytical models, which use simplified,
explicit expressions generated from par-
tial differential equations, and numerical
models, which reduce partial differential
equations to a set of algebraic equations
that are solved through linear algebra.
Both types of models can evaluate plume
movement in three dimensions.
Plume Control Technologies
Selection and Evaluation
Identifying the most appropriate tech-
nique for managing a leachate plume can
be extremely complex. The selection
process involves the acquisition, evalua-
tion, and application of data that vary in
reliability, applicability, and depth of
detail. Though data development is rel-
atively straightforward, the evaluation
and use of the data can be difficult
because of the many interrelated ele-
ments. Furthermore, developing and eval-
uating plans for site-specific remedies
requires a great deal of technical judg-
ment. Basically, four plume management
technologies are available—well systems,
subsurface drainage systems, subsurface
barriers, and in situ treatment techniques.
Variations and combinations of these
technologies can result in many possible
plume management alternatives, how-
ever.
Once the technologies and their re-
quired auxiliary measures have been
identified and developed into a site re-
storation alternative, a preliminary eval-
uation can be initiated for each alter-
native. This screening step involves as-
sessing the suitability of each alternative
relative to specific site conditions.
After the problems with a site have
been assessed and priorities have been
set for mitigating them, specific response
goals can be established. These can be
stated in specific or general terms. Estab-
lishing goals before the technology
screening and detailed analysis steps is
extremely important to focus control
efforts on the most critical problems of
the site.
The cost evaluation involves comparing
the costs of alternatives that produce
similar environmental, public health, and
public welfare benefits.
Groundwater Pumping
Pumping technologies have been
shown to be most effective for plume
management at sites where underlying
aquifers have high intrinsic permeabil-
ities (e.g., coarse-grained sands) and
where the contaminants move readily
with groundwater (e.g. benzene). Pump-
ing methods have also been used with
some effectiveness at sites where pol-
lutant movement is occurring along frac-
tured or jointed bedrock. Note, however,
that the fracture patterns must be traced
in detail to ensure proper well placement.
Well systems can be designed to per-
form several functions with or without
the assistance of other technologies (e.g.,
barrier walls). The main applications in
plume management are groundwater
level adjustment, plume containment,
and plume removal.
Groundwater levels can be adjusted by
using extraction wells to lower water
levels or injection wells to create ground-
water mounds or barriers. By adjusting
groundwater levels, plume development
can be stopped at the source, or the speed
and direction of the plume can be altered.
In either case, contaminated water is not
extracted from the groundwater system
as it is with containment and removal
techniques.
A well system used to contain a plume
may incorporate extraction wells or a
combination of extraction and injection
wells. Containment differs from removal
in that the source of contamination is not
generally stopped, so contamination is an
ongoing process. Because containment
requires removing contaminated ground-
water, a treatment or disposal method
must be developed to handle the dis-
charge from the system.
Plume removal implies a complete
purging of the groundwater system to
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remove contaminants. Removal tech-
niques are suitable when contaminant
sources have been stopped (e.g., by waste
removal or site capping) or contained
(e.g., by barrier walls) and aquifer restora-
tion is desired. Extraction wells or extrac-
tion and injection well systems can be
used in plume removal. Numerous arrays
and patterns are available for injection
and extraction wells, and the choice
usually depends on suitability for a spe-
cific job. Extraction and injection tech-
niques can also be used with flushing
compounds to accelerate contaminant
removal. As with containment designs,
treatment of pumped water is necessary.
Subsurface Drains
Subsurface drains include any type of
buried conduit used to collect liquid
discharges (i.e., contaminated ground-
water) by gravity flow. The major com-
ponents of a subsurface drainage system
include drain pipes, envelope or filter or
both, backfill, manholes or wetwells, and
pumping stations.
Subsurface drains function like an
infinite line of extraction wells. That is,
they form a continuous zone of depression
that runs the length of the drainage
trench.
Functionally, two basic types of drains
exist—relief drains and interceptor
drains. Relief drains are installed in areas
where the hydraulic gradient is relatively
flat. They are generally used to lower the
water table beneath a site or to prevent
contamination from reaching a deeper
underlying aquifer. Relief drains are
installed in parallel on either side of the
site so that their areas of influence
overlap and contaminated groundwater
does not flow between the drain lines.
They can also be installed completely
around the perimeter of the site.
Interceptor drains, on the other hand,
are used to collect groundwater from an
upgradient source to prevent leachate
from reaching wells or surface water
located hydraulically downgradient from
the site. They are installed perpendicular
to groundwater flow. A single interceptor
at the toe of a landfill or two or more
parallel interceptors may be needed,
depending on the circumstances.
Whether a drain functions like an
interceptor or a relief drain is determined
by the hydraulic gradient. The design of
these drain types differs, but construction
and installation are the same.
Though subsurface drains perform
many of the same functions as pumping
systems, drains may be more cost effec-
tive under some circumstances. For ex-
ample, they may be particularly well
suited to sites with relatively low perme-
ability where the cost of pumping may be
prohibitively high because wells need to
be spaced very closely.
A number of limitations exist on the use
of subsurface drains as a remedial tech-
nique. They are not well suited to areas of
high permeability and high flow rate.
Also, contamination at great depth may
cause prohibitive construction costs, par-
ticularly if a substantial amount of hard
rock must be excavated. Subsurface
drains are also unsuitable for viscous or
reactive plumes, since such leachate may
clog the drian systems.
Low-Permeability Barriers
Low-permeability barriers can be used
to divert groundwater flow from a waste
disposal site or to contain contaminated
groundwater leaking from a waste site.
Three major types of low-permeability
barriers are applicable to leachate plume
management—slurry walls, diaphragm
walls, and grout curtains.
Slurry Walls
A slurry wall is a subsurface barrier
consisting of an excavated trench that
uses a bentonite and water slurry to
support the sides. The trench is then
backfilled with materials of far lower
permeability than the surrounding soils.
The slurry backfill trench or slurry wall
reduces or redirects the flow of ground-
water.
Slurry walls include two major types of
barrier walls—soil-bentonite and soil-
cement walls. Soil-bentonite walls are
composed of soil (often trench spoils)
mixed with small amounts of the benton-
ite slurry from the trench. Cement-ben-
tonite walls are made of a slurry of
Portland cement and bentonite.
Soil-bentonite walls generally have the
lowest permeability, the widest range of
waste compatibilities, and the lowest
cost. They also offer the least structural
strength (highest elasticity), usually re-
quire the largest work area, and are
restricted to a relatively flat topography
unless the site can be terraced.
Cement-bentonite walls can be in-
stalled at sites where there is insufficient
work area to mix and place soil-bentonite
backfill. These walls can be installed in a
more extreme topography if wall sections
are allowed to harden and the wall is
continued at a higher or lower elevation.
Although cement-bentonite walls are
stronger than soil-bentonite walls, they
are at least an order of magnitude more
permeable, resistant to fewer chemicals,
and more costly.
Diaphragm Walls
Diaphragm walls are one of the three
major types of barrier walls used (along
with soil-bentonite and cement-bentonite
walls). A diaphragm wall is a subsurface
barrier designed for structural strength
and integrity in addition to low perme-
ability. Diaphragm walls can be made of
cast-in-place concrete or precast panels
with cast-in-place joints.
Diaphragm walls are the strongest of
the three types of barrier walls as well as
the most costly. If the joints between
panels are installed correctly, diaphragm
walls have approximately the same per-
meability as cement-bentonite walls, and
because the materials are similar, about
the same chemical compatibilities. Dia-
phragm walls are most typically used in
situations requiring structural strength
and relatively low permeability.
Grout Curtains
A grout curtain is a subsurface barrier
formed through the pressure injection of
one of several special grouts into a rock or
soil body to seal and strengthen it. Once
in place, these grouts set or gel into the
rock or soil voids, greatly reducing soil
permeability and imparting increased
mechanical strength to the grouted mass.
This process results in a grout wall or
curtain. Because a grout curtain can be
three times as costly as a slurry wall, it is
rarely used when groundwater has to be
controlled in soil or loose overburden.
Grout is used primarily to seal voids in
porous or fractured rock when other
methods of controlling groundwater are
impractical.
Four basic techniques exist for install-
ing a grout curtairi:
• Stage-up method
• Stage-down method
• Grout-port method
• Vibrating-beam method
As with slurry walls, placing a grout
curtain upgradient from a waste site can
redirect the flow so that groundwater no
longer contacts the wastes that are
creating the leachate plume. However,
placement of a grout curtain downgrad-
ient from a hazardous waste site may not
be successful because of grout/leachate
interactions. For example, the grout
setting time is often hard to control, thus
making it difficult to emplace a curtain of
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reliable integrity. Additional problems can
occur when attempting to grout a hori-
zontal curtain or layer beneath a waste
site. In such cases, injection holes must
be drilled either directionally from the site
perimeter or directly through the wastes.
Innovative Technologies
Leachate plumes may be treated in
place by certain biological and chemical
treatments that are specifically designed
to remove plume contaminants. Contam-
inated plumes consisting chiefly of bio-
degradable organics may be treated by
in-place bioreclamation (biodegradation).
Chemicals may also be injected into the
leachate plume to neutralize, stabilize, or
mobilize contaminants. For example, soil
flushing Is used as an in situ technique to
flush residual contaminants from soil
particles to mobilize them for collection
and treatment. Usually water is the
flushing medium, but dilute solutions of
surfactants and organic solvents have
also been proposed.
Bioreclamation
Bioreclamation is an in situ ground-
water treatment that uses a combination
of microorganisms, aeration, and the
addition of nutrients to accelerate the
biodegradation rate of groundwater con-
taminants.
Many species of bacteria, actinomy-
cetes, and fungi have been found to
degrade hydrocarbons associated with
petroleum. Bacteria are the prime micro-
organisms involved with the biodegrada-
tion of petroleum and other organics in
groundwater. Naturally occurring species
of the genera Pseudomonas, Arthro-
bacter. NocardiaAchromobacterium, and
Flavobacterfum have been found to attack
petroleum hydrocarbons and other organ-
ic chemicals. With the addition of nutri-
ents and oxygen, these bacteria can be
stimulated to develop a population that is
adaptedto readily degrade organic chem-
icals present in groundwater.
An alternative to developing adapted
populations from naturally occurring bac-
teria is to inoculate the subsurface with
mutant microorganisms developed in the
laboratory to degrade specific organic
chemicals or chemical groups. This alter-
native is advantageous because it may
increase overall biodegradation rates of
specific organics and it eliminates the
time required for adaptation of a naturally
occurring population.
The selection of bioreclamation as a
plume management technique depends
on the biodegradability of the components
in the contaminant plume. Biodegradabil-
ities of various organic substances are
based on the ratio of biochemical oxygen
demand (BOD) to chemical oxygen de-
mand (COD). Compounds are considered
relatively undegradable if their BOD/COD
ratio is less than 0.01, moderately de-
gradable if their ratio is 0.01 to 0.1, and
degradable if their ratio is 0.1 or greater.
Implementation of the bioreclamation
process involves placing extraction wells
to control migration of the contaminant
plume by pumping. Groundwater pumped
to the surface is mixed with nutrients and
reinjected upgradient of the extraction
wells. Specialized mutant bacteria may
also be added along with the nutrients.
The groundwater may be oxygenated with
air, oxygen, or hydrogen peroxide.
Bioreclamation has been used success-
fully in many cases to treat contaminated
groundwater plumes from underground
gasoline and hydrocarbon leaks. The
technique has not yet been demonstrated
for groundwater treatment at uncon-
trolled hazardous waste disposal sites,
but its potential fortreating hydrocarbon-
contaminated groundwater establishes it
as a viable technique.
In Situ Chemical Treatment
In situ chemical treatment techniques
involve the injection of a chemical into a
leachate plume to neutralize, detoxify,
precipitate, or otherwise affect the con-
taminant materials. These techniques are
highly dependent on the contaminant and
have in the past been used only for spills
of specific chemicals. Dilute solutions of
acids or bases such as nitric acid or
sodium hydroxide could theoretically be
used to neutralize acidic or basic ground-
water contaminants. A system of extrac-
tion and injection wells could be used to
disperse the neutralizing agent and to
contain and cycle groundwater until the
appropriate pH was attained. Other chem-
ical agents could also be used in this
manner to detoxify plume contaminants.
Sodium hypochlorite, for instance, has
been used to oxidize cyanide-contam-
inated groundwaters. Other oxidizing
chemicals such as hydrogen peroxide and
ozone may be applicable to this type of
remedial approach. Solutions of sodium
sulf ide have also been proposed to precip-
itate toxic metals from groundwater and
thereby immobilize them. Recently, an
underground spill of acrylate monomer
was attenuated by the injection of a
catalyst that caused the contaminant
plume to polymerize and solidify.
Block Displacement
Block displacement is a technique
developed for completely isolating a large
mass of contaminated soil. The concept
rejies on producing a fixed underground
barrier around and beneath the contam-
inated zone to encapsulate a block of
earth. The method involves construction
of a perimeter and a bottom barrier that
are interconnected to allow block dis-
placement.
The block displacement method was
developed for application where an un-
weathered bedrock or impermeable stra-
tum below a contaminated zone is not
shallow enough for a perimeter (e.g.,
slurry wall) alone to provide cost-effective
isolation of the contaminant plume. In
situ isolation using a slurry wall or other
conventional vertical barriers requires
keying the barriers into a naturally oc-
curring impermeable stratum. Block dis-
placement is also designed to minimize
the volume of soil or earth to be isolated.
Thus block displacement would theoret-
ically isolate a contaminated area through
the creation of a man-made confining
layer immediately surrounding the con-
taminated zone.
Problems associated with block dis-
placement include (1) difficulties in en-
suring bottom barrier continuity, (2)
health and safety, and (3) environmental
and construction risks of drilling injection
holes through a contaminated zone and
potentially through hazardous materials.
Even if injection holes could be construc-
ted safely, they could potentially serve as
conduits for increased rates of vertical
contaminant migration before slurry in-
jection.
The full report was submitted in fulfill-
ment of Contract No. 68-03-3113 by JRB
Associates under the sponsorship of the
U.S. Environmental Protection Agency.
•&U. S. GOVERNMENT PRINTING OFFICE:1986/646~116/20745
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Edward Repa is with JRB Associates, McLean, VA 22102; and the EPA author
Douglas P. Doerr is with the Hazardous Waste Engineering Research
Laboratory, Cincinnati, OH 45268.
Naomi P. Barkley is the EPA Project Officer (see below).
The complete report, entitled "Leachate Plume Management," (Order No. PB
86-122 330/AS; Cost: $46.95, 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:
Hazardous Waste Engineering Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
Unnod States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
BULK RATE
POSTAGE & FEES PAID
EPA
PERMIT No. G-35
Official Business
Panolty for Private Use $300
EPA/540/S2-85/004
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