United States
Environmental Protection
Agency
Hazardous Waste Engineering
Research Laboratory
Cincinnati OH 45268
Research and Development
EPA-600/S2-84-180 Jan. 1985
Project Summary
Electrical Resistivity
Technique to Assess the
Integrity of Geomembrane Liners
David W. Shultz, Bob M. Duff, and Wendell R. Peters
An electrical resistivity survey tech-
nique has been developed and tested for
assessing the integrity of geomembrane
liner systems installed in fluid impound-
ments. Development of the technique
included two-dimensional computer
modeling and three-dimensional phys-
ical model testing. A 0.4-ha (1-acre)
geomembrane-lined surface impound-
ment was used for full-scale testing of
the technique. Tests were conducted to
detect and locate single and multiple
leaks of different sizes. Results indicate
that the technique can be used to detect
and locate single and multiple leaks as
small as 2.5 cm (1 in.) in diameter
within 1.5 m (5 ft) of the leak.
Though no full-scale tests were con-
ducted with soil fill over the liner, scale
model tests on a simulated landfill
indicated strongly that the technique
could be used to survey geomembranes
installed at landfills.
Thit 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
Detecting and locating leaks in geo-
membrane liner systems at hazardous
waste disposal and storage facilities is
necessary to the performance of the liner
system. Southwest Research Institute has
developed an electrical resistivity tech-
nique to detect and locate leaks in these
liner systems. The technique takes ad-
vantage of the high electrical insulating
properties of a liner compared with the
fluid contained above the liner and the
soil under the liner.
Geomembrane liners made from im-
pervious plastics and rubbers exhibit high
electrical resistance. When a liner is
installed in a landfill or surface impound-
ment, it effectively acts as an electrical
insulator between the materials con-
tained in the facility and the surrounding
earth. If the liner is physically punctured
or separated, conductive fluid flows
through the leak and establishes an
electrical shunt through the liner. The
low-resistance shunt forms an electrically
detectable region that is the basis by
which a leak may be detected and located.
The basic electrical resistivity technique
for detecting and locating leaks in a
geomembrane liner (Figure 1) uses a
current source to inject current across
the boundary of the liner. The liner has an
electrical leak path, as shown. When a
voltage is applied between the source and
remote current return electrodes, current
flows through the leak as shown in the
figure. If a soil cover were present over
the edges of the liner, current would also
flow through this soil cover to the remote
current return electrode bypassing the
liner. Potentials measured on the surface
are affected by the current distributions
near the leak and can be used to locate
the leak.
Computer and physical modeling stud-
ies were performed to validate the electri-
cal resistivity liner survey technique.
These studies provided an opportunity to
analyze the current distribution and sur-
face potentials resulting from various
leak configurations. This information was
needed to develop field survey equipment.
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Remote Current
Return Electrode
Current Source
Electrode
Figure 1.
Conceptual electrical resistivity testing technique applied to detect and locate leaks
in a geomembrane liner system.
Computer Modeling Studies
Model Design
Computer modeling studies with a two-
dimensional resistive network were per-
formed to predict the influence of an
electrial current penetration through a
geomembrane liner and associated sur-
face potential voltages inside a landfill or
fluid impoundment. The influences
sought were the magnitude of the surface
potentials at given locations for cases
where fluid leaks or electrical current
paths existed through the liner. The two-
dimensional resistor network designed to
simulate a liner was modeled using a
general purpose curcuit simulation com-
puter program called SPICE. This software
allows simulation of circuits containing
resistors, capacitors, inductors, and vol-
tage and current sources. The resistivity
of the fill inside the liner (either soil or
fluid) and the earth surrounding the liner
was modeled using a normalized resis-
tance value of 1 ohm. The liner was
characterized in the model by using
parallel 1000-ohm resistors along the
path of the liner (the solid line marked
"liner" in Figure 2). To simulate a leak or
conductive path through the line, one of
the 1000-ohm resistors representing the
liner was replaced by a 1 -ohm resistor. As
a result, the leak path has the same
resistance as the earth. In effect, the liner
is removed at that location.
Computer Results
Figure 2 illustrates the results of one of
the computer-generated, two-dimension-
al analyses of a liner having a leak in the
bottom. This figure represents a two-
dimensional cross-section through the
liner and surrounding earth. The outline
of the liner has been sketched in the
figure and is represented by the three-
sided trapezoidal figure in the center of
the plots. The equipotential lines in the
figure are generated and plotted auto-
matically by the computer program
described earlier. The equipotential lines
showing the voltage distribution patterns
were computer generated. The current
flow paths are at right angles to the
equipotential lines and were sketched in
by hand.
This figure shows that, as predicted,
when a leak has penetrated the liner,
current flow between electrodes located
inside and outside the liner will follow
two paths, namely, through the leak and
over the buried edges of the liner. Note
the nonsymmetry of the equipotential
lines terminating on the surface of the
facility above the liner. The voltage gra-
dient is clearly steeper along the surface
on the left side of the current injection
electrode above the leak. This voltage
gradient is due to more current flow in the
direction of the leak.
Physical Scale Modeling Studies
After completion of the computer
modeling studies, a three-dimensional
physical scale model was designed and
constructed. The objectives of using this
model were to: (1) observe the surface
voltage distribution patterns created by a
fixed liner geometry, injection current
location, fill material inside the liner, and
leak configuration; (2) test the system
instrumentation and assess its suitability
for full-scale testing; and (3) determine
the accuracy of the technique in locating
a leak.
Model Design
The model is designed to study how
accurately the technique could locate a
leak in the liner. Outside dimensions
were established at 3.05 m (10 ft) on each
side, for a total lined floor area of 9.3 m2
(100ft2). Maximum depth was established
at 1 ft, allowing variation of water depth
during the experiments. A black poly-
ethylene sheet 6 mils thick was selected
as the geomembrane. A sand bed was
placed belowthe liner. The moisture level
of the sand was typical of that of the
surrounding soil in the area.
Results
Surface potential measurements were
made with 0.1 m (0.33 ft) of water in the
model. Figure 3 presents a polar coordi-
Figure 2. Two-dimensional computer model of a liner with a leak. Equipotential lines at the top
are nonsymmetrical, indicating the presence of a leak.
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200°
750°
760°
220
240
260
2SO'
300
320
340° 0° 20°
Figure 3. Equipotential plot of voltages on the surface of the water with a single leak.
40°
nate equipotential plot of the measured
data from one experimental configuration.
The current injection electrode is shown
in the center of the plot. A leak is shown
on the 340° radial. The distortion of the
equipotential lines indicates increased
flow of current around the leak.
Testing on the water-filled model
(simulating a fluid impoundments) con-
firmed that leaks can be detected and
located by this technique. To determine
whether the system would achieve the
same performance on a simulated landfill,
the water in the model was replaced with
soil. Figure4 presents the results of a test
with 0.06 m (0.22 ft) of soil in the model
and one leak. The current injection elec-
trode is shown in the center of the plot.
The equipotential contours are distorted
in the area of the leak as with the water-
filled condition.
These studies further demonstrated the
fundamental concepts of the approach,
which were first defined by the computer
modeling efforts. Based on these results,
the measurement equipment, data pro-
cessing software, and electrodes were
specified for full-scale testing at a 0.4-ha
(1 -acre) lined facility located on Institute
grounds. Testing at this scale was per-
formed to measure the performance of
the technique at full-scale conditions that
more accurately simulate actual field
conditions.
Full-Scale Studies
The electrical resistivity leak detection
technique was tested at a 0.4-ha (1 -acre)
lined, water-filled impoundment. The
overall goal of the testing program was to
determine how well the technique worked
in a field environment under full-scale
conditions. To accomplish this goal, the
instrumentation required to apply the
electrical resistivity technique was as-
sembled. A 100-mil geomembrane liner
made of high-density polyethylene was
installed in a 1 -acre impoundment. Con-
trollable leaks were installed in the liner
for detection and location studies. Experi-
ments were then performed to evaluate
the technique and the instrumentation.
Impoundment Design
A 0.4-ha (1-acre) impoundment was
used to test the liner assessment tech-
nique. Thefacility was designed to accom-
modate up to 2 m (6.5 ft) of water. Overall
dimensions were approximately 65.8 m2
(216 ft2) from the top of the berms. Side
slopes were approximately 3 to 1. An
access road around the facility allowed
for vehicular traffic during testing. A100-
mil high-density polyethylene liner was
installed in the facility to serve as the test
liner. The liner was anchored at the top of
the berm in a 0.6-m (2-ft) deep trench.
The trench was backfilled with soil.
Leaks had to be constructed in the liner
to facilitate testing of the leak detection
technique. Five pipes, each 0.3 m (1 ft) in
diameter and 0.1 m (4 in.) long, were
installed through the floor of the liner.
The pipes were installed with 0.05 m (2
in.) above the liner and 0.05 m (2 in.)
below the liner. Each pipe was made of
high-density polyethylene (HOPE) to allow
the liner to be welded to the pipe, creating
a water-tight seal. A cap was constructed
to fit over this pipe. The cap was made
from two polyvinyl chloride (PVC) rings
that were bolted together. To create
various leak sizes, disc-shaped HOPE
inserts with holes of various sizes were
placed between the rings. The rings were
bolted and placed over the top of the pipe.
These leak points allowed control of leak
location during the performance tests.
Full-Scale Measurements
Figure 5 shows a conceptual drawing
of the full-scale test impoundment and
operation of the equipment. All measure-
ments were taken along radial lines 5° or
10° apart, beginning at the current source
electrode and continuing out to markings
on the berm. The logging cable was
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A.'
Figure 4. Equipotential plot of voltages on the surface of the soil with a single leak.
Current Return
Electrode 60% A way
Mobile Laboratory
Floating Measurement
Electrode
Electric Wench
Support Platform
Technician Maintains Tension
X °" Survey Cable
Figure 5. Conceptual drawing of the test impoundment and operation of the test equipment.
4
positioned on these marks during data
acquisition runs. The location of any
anomaly such as a leak could be calcu-
lated from the radial and the odometer
data. The odometer, located on the electric
winch, established the exact location of
the measurement electrode with respect
to the current electrode. To establish a
baseline condition, the entire surface of
the pond was surveyed with all leaks
closed.
The various leaks were opened by
removing the caps previously described.
Potential measurements were then taken
over the surface of the water. Soil mois-
ture conditions below the liner were not
measured. A contour plotting computer
program was used to produce equipoten-
tial contour plots of the no-leak and leak
data.
No-L eak and Single-L oak Results
A contour plotting computer program
was used to produce an equipotential
contour plot of the voltages measured
over the surface of the water in the no-
leak case (Figure 6). The X and Y axes
define the approximate north and east
water level boundaries of the impound-
ment. The angles of the radials are
indicated. Voltage measurements were
taken at 0.3-m (1-ft) increments along
each radial. The current source electrode
is identified by a dot above the X axis at
the 28-m(92-ft) point. The water was approxi-
mately 1.5 m (5 ft) deep.
The equipotential contour lines in
Figure 6 show insignificant distortions of
the surface potentials across the water
surface. The results are similar to the no-
leak contour plots of data taken with the
physical scale model. The contour lines
tend to be concentric semicircles close to
the current injection point. Moving away
from this region, the contour lines begin
to straighten out due to the effect of small
amounts of current flow across the liner.
Part of this current flow is due to the
capacitive effects of the liner. Overall,
this plot indicates no sudden, unexpected
changes or perturbations in the current
flow on the surface of the water.
To determine whether the survey tech-
nique could detect and locate a single
leak, a 0.3-m (1-ft) diameter leak was
opened. The surface potentials were
measured and the results plotted as
before. An examination of the equipoten-
tial contour plot in Figure 7 shows that
the contour lines close around the loca-
tion of the leak. This indicates that the
flow of current converges in the area of
the leak, shown by the "bull's-eye"
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160
160
140
120
100
110
120
100 -
1
20 40 60 80 100 120 140 160 180
— , '40
Figun 6. An equipotential contour plot of no-leak measurements.
pattern centered on the leak. The bull's-
eye potential pattern located on the
surface of the water serves to graphically
reveal the location of the leak. The plots of
the data for each radial along with the
equipotential contour plots serve as ex-
cellent analytical tools to detect and locate
a leak.
To determine whether the technique
could detect and locate a smaller leak,
2.5-cm (l-in.)diameter leak was installed
on the 90° radial 22 m (72.3 ft) from the
current electrode. Surface potential
measurements were taken at 0.3-m (1 -ft)
increments along the 60° through 120°
radials. The equipotential contour of these
data (Figure 8) indicates a significant
distortion on the 90° radial in the proxim-
ity of the true leak location. The results of
this test indicate that the electrical resis-
tivity technique can be used to detect and
locate a 0.02-m (1-in.) diameter leak in
approximately 3716 m2 (40,000 ft2) of
liner surface area to an accuracy of 1 ft.
This accuracy is obtainable when the
contour plots are used together with the
raw data (not shown). The contour line
shifts along the 100° radial in Figure 8 do
not indicate an anomaly, since the shift is
consistently of the same magnitude.
Multiple Leak Results
Multiple leak configuration experi-
ments were performed to determine the
leak signatures and sensitivity of the
electrical resistivity technique. An experi-
ment was performed in which three 1 -ft-
diameter leaks were detected and located.
These leaks were established at locations
01, 04, and 05. Surface potential meas-
urements were at 1-ft (0.3-m) increments
along the 70° through 125° radial lines.
Voltage peaks occur directly over each of
the three leaks. The equipotential contour
plot (Figure 9) clearly shows the presence
of each leak.
Conclusions
Full-scale tests have demonstrated the
ability of the electrical survey technique
to detect and locate single and multiple
leaks in geomembrane liner systems
installed in water-filled impoundments.
For all tests, the technique detected the
presence of a leak. The accuracy of leak
location ranged from within 0.3 m(1 ft) to
1.5 m (5 ft), depending on the location of
the leak with respect to the current source
electrode and the total number of leaks
present. Although full-scale tests of the
technique with soil fill over the liner were
not conducted as part of this project, the
results of model scale tests strongly
indicate that the technique can be used to
survey geomembranes installed at land-
fills.
The technique will not damage the
geomembrane liner, and instrumentation
and hardware for applying the technique
are portable. Experiments using conduc-
tive metal cans close to a leak path
indicate that the performance of the
technique is not diminished by the pre-
sence of such potentially interfering
objects.
The approach to finding leaks when
their location is unknown would be the
same as that used at the SWRI test
impoundment. Modifications to the sur-
vey approach, such as moving the position
of electrodes, can easily be accomplished
if necessary. Modifications might be
called for if, for example, the leaks are not
along a radial traverse.
The water depth and leak size and
shape will probably influence leak signa-
tures. Though the exact relationships are
not known at this time, increasing depth
appears to reduce the magnitude of the
signature. Depth will become a limiting
factor only if the signature strength falls
below the background noise of the instru-
mentation. Background noise will be a
site-specific characteristic and therefore
cannot be predicted.
The full report was submitted in fulfill-
ment of Contract No. 68-03-3033 by
Southwest Research Institute under the
sponsorship of the U.S. Environmental
Protection Agency.
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180,.
160
140
720 *
110
I
20 40 60 80 700 720 740 750 780
720
730
7 -Foot Diameter
Leak Location
140
Figure 7. An equipotential contour plot showing the distortions from a 1 -foot diameter leak on
the 90° radial.
180 r
160 .
740 -
720 -
700"
7 -Inch Diameter
Leak Location
I
20 40 60 80 700 720 740 760 780
720
Figure 8. An equipotential contour plot showing the distortions from a 1 -in. -diameter leak.
6
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160"
140"
120"
i •
looL
80
60 4.
40 f
20
75 80 85
105
110
70
115
1 -Foot-Diameter
Leak Location
120
125
1 -Foot-Diameter
Leak Location
1 -Foot-Diameter
Leak Location
Current Source Electrode
0 20 40 60 80 100 120 140 160 180
Feet
Figure 9. An equipotential contour plot showing the distortions from three 1 -ft-diameter
leaks on different radials.
David W. Shultz, Bob M. Duff, and Wendell R. Peters are with the Southwest
Research Institute, San Antonio, TX 78284.
Carlton C. Wiles is the EPA Project Officer (see below).
The complete report, entitled "Electrical Resistivity Technique to Assess the
Integrity of Geomembrane Liners," (Order No. PB 85-122 414; Cost: $11.50,
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
It US GOVERNMENT PRINTING OFFICE ins - 559-U1/10778
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United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
BULK RATE
POSTAGE & FEES PA
EPA
PERMIT No. G-35
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Penalty for Private Use $300
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