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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Environmental Protection
Agency
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Information
Cincinnati OH 45268
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