WO1987003987A1

WO1987003987A1 - Method and apparatus for detecting and locating leaks in geomembrane liners

Info

Publication number: WO1987003987A1
Application number: PCT/US1986/002749
Authority: WO
Inventors: Merle Edward Converse; Wendell Richard Peters; Lewis Spencer Fountain
Original assignee: Southwest Research Institute
Priority date: 1985-12-17
Filing date: 1986-12-15
Publication date: 1987-07-02
Also published as: AU6836587A; EP0250561A1

Abstract

A method and apparatus for detecting and locating leaks in geomembrane liners (20) used to contain liquids (17) within surface impoundment facilities. A nonintrusive electrical measurement technique is used to survey a liquid impoundment for the existence of a leak (11) and to obtain a precise location of the leak after such leak has been detected. A voltage source (12) is used to establish a voltage gradient in the liquid contained by the liner. The voltage gradient in the liquid has a characteristic distribution which can be correlated with an unfaulted liner or a faulted liner. Similarly, the current flow in the liquid can be correlated with a faulted liner or unfaulted liner. By measuring the current flow through the liner, the integrity of the geomembrane can be continuously monitored. A current flow above a certain predetermined level can be used to trigger an alarm (38) indicating the existence of a leak in the liner. Once the existence of a leak (11) has been detected, a voltage gradient detection probe (22) is moved through the contained liquid in a systematic survey pattern to detect distortions in the voltage gradient distribution which can be correlated with the location of a leak in the geomembrane liner.

Description

METHOD AND APPARATUS FOR DETECTING AND LO ATI LEAKS IN GEOMEMBRANE LINERS FIELD OF THE INVENTION

The present invention relates to a method and apparatus for detecting and locating leaks in geomembrane liners used to contain liquids within surface impoundment facilities. More specifically, the present invention uses a nonin- trusive electrical measurement technique to survey a surface impoundment for the existence of a leak and to obtain a precise location of the leak after such a leak has been detected.

BACKGROUND Geomembrane liners (often called flexible liners) are large sheets of plastic or rubber material used as a barrier to contain liquids in a surface structure. Facilities where these liners are commonly used include hazardous waste landfills and- liquid impoundments, water reservoirs and other surface impoundments.

At certain types of facilities, such as hazardous liquid impoundments, it is extremely important to know 'at all times whether the liner is intact and is performing its intended containment function. Failure to detect and repair a leak can lead to serious groundwater and surface water contamination.

Geomembrane liners are generally inspected for physical integrity during installation. Such inspection usually consists of a visual inspection of the surface of the individual sheets which are bonded together to form the liner combined with testing of the seams which join the various sheets.

Methods of monitoring the performance of liners after installation and use have typically been based on ground water sampling using a plurality of monitoring wells at spaced intervals along the perimeter of the impoundment. The ground water sampling method, however, provides only an indirect and delayed indication of leakage and, therefore, is not adequate for monitoring liner performance since ground water contamination may take years to occur. Furthermore, by the time a leak has been detected by this method, substantial ground water contamination may have already occurred. Another source of inadequacy in the ground water sampling method stems from the need to have the monitoring well in the particular aquifer which is transporting the contaminants. An adequate ground water monitoring program, therefore, requires a large number of monitoring wells along the perimeter of the impoundment with a sufficient number of wells sampling water from different levels within the various aquifers under the impoundment. Even the most elaborate ground water monitoring system cannot provide monitoring as accurate and timely as the invention system described herein because of the inherent limitations discussed above.

One method for detecting and locating leaks in geomembrane liners uses an electrical measurement technique which takes advantage of the high electrical insulating properties of the liner in comparison with the liquid contained above the liner and the soil under the liner. In general, geomembrane liners made from an impervious plastic material or rubber have a very high, electrical resis¬ tance. A liner installed in a landfill or surface impoundment, therefore, effec¬ tively acts as an electrical insulator between the materials contained within the liner and the surrounding environment. If the integrity of the liner is lost because of a puncture or seam separation, however, conductive liquid may then flow through the leak, thus establishing an electrical path through the liner between the contained liquid and the conductive earth in surrounding contact with the liner. The leak is a low resistance path to electric current flow and as such forms an electrically detectable region corresponding to the position of a leak which may be detected and located.

The electrical measurement technique described above is discussed in greater detail in the publication "Electrical Resistivity Technique to Assess the Integrity of Geomembrane Liners," Final Technical Report, Southwest Research Institute, Project No. 14-6289, EPA Contract No. 68-03-3033 (1984), which by this reference is incorporated for all purposes.

SUMMARY OF THE INVENTION

The invention apparatus and method for detecting and locating leaks operates by, first, injecting an electrical current from a source into an essen¬ tially insulating enclosure and returning this current through a path defined by the conducting earth in external contact with the enclosure. The current source is comprised of a voltage source (either AC or DC) having one electrode in the conducting liquid and another electrode in the conducting earth. Current flows from the voltage source through the resistance provided by a series resistor in combination with the resistance of the conducting liquid, the resis- tance of conducting earch and the contact resistance of the electrodes of the current source. A characteristic voltage gradient associated with the current flow is thus established within the contained liquid and the surrounding earth.

In the preferred embodiment, the insulating enclosure is comprised of a geomembrane liner having a very high resistivity. For a perfectly insulating liner with no leaks, the voltage gradients in the conducting liquid will have a characteristic distribution and a small or negligible amount of current will flow through the liner. If the liner has a leak, however, a localized low resistance path is established between the contained liquid and the conducting earh sur¬ rounding the liner. A current proportional to the size of the leak flows through the liner via the low resistance path. Thus, by measuring the current flow through the liner, the integrity of the geomembrane can be continuously moni¬ tored. A current flow above a certain predetermined level can be used to trigger an alarm indicating the existence of a leak in the liner.

The existence of a leak in the liner and the associated increase in current flow through the liner at the point of the leak causes a distortion in the voltage gradient in the contained liquid. This distortion in the voltage gradient can be used to determine the exact location of the leak. A probe having differential electrodes is moved through the liquid in a systematic pattern to measure changes in the potential gradient with the impoundment. As the probe is moved closer to the leak, an increased potential is developed between the potential measuring electrodes. A signal corresponding to the potential between the electrodes is amplified by electronic circuits and observed as an audio frequency signal from a voltage-controlled oscillator and/or on a meter indicator. An operator wearing a pair of headphones or observing the meter is able to correlate the change in frequency output of the oscillator or the varying meter reading with the location of the leak in the liner. BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional side view of a liquid impoundment showing a schematic representation of the detection system of the present invention.

FIG. la is a schematic representation of the equivalent resistance of the leak monitor system components shown in FIG. 1.

FIG. 2 is the sectional side view of FIG. 1 showing the test probe used in the invention detection system to locate a leak in a geomembrane liner.

FIG. 3 is a top plan view of a hypothetical surface impoundment showing a pattern for moving a probe in an impoundment in a systematic manner to ensure detection of all possible leaks in a geomembrane liner.

FIG. 4 is a schematic block diagram of the system components of the monitor system for detecting the existence of a leak in a geomembrane liner.

FIG. 5 is a schematic block diagram of the signal processing circuitry contained in the differential dipole probe of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The apparatus for utilizing the electrical measurement technique to monitor leaks in a geomembrane liner 20 is shown generally in FIG. 1. In the preferred embodiment, a voltage source 12 having a source electrode 16 and a sink elec- trode 18 is connected in series with a resistor 14. The voltage source 12 has a typical output of 50 volts DC and the series resistor 14 has a typical resis¬ tance of 100 ohms. An AC voltage source can be used in place of the DC source shown in the preferred embodiment.

As can be seen in FIGS. 1 and 2, the source electrode 16 is immersed in the conducting liquid 17 contained within the impoundment 10 while the sink electrode 18 is emplaced in the conducting earth 13 at some point along the perimeter of the impoundment 10.

The total current I passing through the system is a function of the voltage produced by the voltage source 12 and the total resistance provided by the ^' series resistor 14 in combination with the resistance of the liquid 17 contained within the liner, the resistance of the geomembrane liner 20, the resistance of the earth 13 in surrounding contact with the liner 20, and the contact resistance of the source and sink electrodes 16 and 18,' respectively.

The source electrode 16 of the preferred embodiment typically consists of a 3 foot diameter brass disk having a 1/16 inch thickness. Brass is particularly well suited as an electrode material for the invention apparatus because it has excellent conductive properties and is resistant to corrosion. Furthermore, the circular shape of the electrode 16 helps to reduce the voltage anomalies in the immediate vicinity of the electrode. The sink electrode 18 completes the current path to the voltage source

12. The sink electrode 18 consists typically of a copper-clad steel rod which is driven into the ground to a depth of approximately 36 inches. Increased surface area for the sink electrode 18 can be achieved by connecting three rods of the type described above with a common conductor. The increased surface area of the multiple electrode arrangement reduces the voltage drop between the electrodes and the earth.

The geomembrane liner 20 is constructed of an impervious plastic or rubber, material having a very high electrical resistance. Typical materials used to form the liner include high density polyethylene or polyvinyl chloride. The resistivity of the liner materials used in the preferred embodiment range from approximately 2 X 10 ohm-cm to 2 X 10¹⁶ ohm-cm. The liner 20 of the preferred embodiment is formed from a plurality of elongated sheets of the resistive plastic material, with complimentary edges of the sheets bonded together to form an integral liner 20. An unfaulted liner 20 has a very high electrical resistance for the reasons discussed above and, therefore, only a very small magnitude of current will pass therethrough. With the voltage source 12 connected to the impoundment 10 having an unfaulted liner 20, the voltage produced by the voltage source 12 is divided approximately according to the resistance of the series resistor 14, R₀, the resistance of the liquid, R_L* the resistance of the geomembrane liner, R_M, the resistance of the earth, R_E, and the contact resistances of the source electrode 16 and sink electrode 18, R_s and R_G, respectively. The current flow produced by the voltage source 12 can be calculated from the relationship:

I = V

(R₀ + R_L + R_M + R_E + R_s + R_G) (1) and, as an example, the voltage drop, V_a, across the series resistor 14 is:

(2)

The liquid 17 contained in the impoundment 10 forms a large distributed resistance in which the geometric distribution of current flow is dependent upon the size, shape, and depth of the body of the liquid 17 and the position of the source electrode 16 in the liquid body. This distribution of current can be characterized in large measure by the magnitude of current passing through each unit area of a closed surface surrounding the source electrode 16. Such a characterization of current is termed the ' current density. The total current, I, is the surface integral of the current density computed over any closed surface surrounding the source electrode 16. Thus, in the case, of the geomem¬ brane liner 20 described above, since all of the source current must flow through the liquid-liner surface interface, the total current can be specifically repre¬ sented by the integral of the current density over the liner surface defined by the liquid level line boundary. Intermediate surfaces within the liquid volume and located between the source electrode 16 and the liquid-liner interface can also be used in such surface integrations to compute the total source current.

By connecting points of equal current density on such successive interme¬ diate surface between the current source electrode 16 and the liner 20, flow lines of constant current density can be established, thereby mapping the distri¬ bution of electric current within the volume of liquid 17 contained by the impoundment 10. By physical principle, each such, current flow line will follow a path of least resistance from the source electrode 16 through the liquid and through the liner 20 to the surrounding earth 13. If the liquid 17 and the liner 20 have uniform resistivities, then the spatial distribution of the current within the liquid will be dependent only upon the size and shape of the liquid- liner interface and the position of the source electrode 16 within the geometry defined by that interface. In the case of an unfaulted geomembrane liner 20, the resistance contrast between the high resistivity liner 20 and the more conductive liquid 17 will be such that the current density through the liquid- liner surface and through the various intermediate surfaces located well away from the source electrode 16 will be relatively uniform.

The finite resistivity of the liquid 17 contained within the liner 20 causes a voltage drop between the surface of the source electrode 16 and the liner 20 when the source power supply 12 is energized. This voltage drop, V_L, is ex¬ pressed by:

(3)

Because the resistance of the liquid body 17, R_L, is distributed resistance, there are also voltage differences within the body of the liquid 17. In particu¬ lar, along each line of constant current density there are incremental voltage drops whose sums can be considered to be approximately equal to the total voltage drop, V_, in the liquid 17. By connecting points of equal potential along each line of constant current density, specific surfaces, termed equipoten- tial surfaces, are identified and the vector directions normal to such equipoten- tial surfaces (parallel to the current density flow direction at each point on the surface) define the direction of the potential gradient within the body of the liquid 17. Such equipotential surfaces and potential gradients are important to this invention since, by their measurement and interpretation, anomalous conditions of current density distribution within the impounded liquid 17 can be measured and interpreted to reveal the presence and location of leaks 11 in the liner 20.

When the current density within the contained fiquid 17 is relatively uniform, as in the case of an unfaulted liner 20, the voltage gradients in the body of liquid 17 are small. When a leak 11 is present in the liner 20, the current flow through the liner 20 tends to concentrate along the lower resis- tance path through the leak 11, thereby causing higher voltage gradients in the vicinity of the leak 11. These anomalous changes in the potential gradients also change the spatial location and shapes of the equipotential surfaces men¬ tioned above.

The magnitude of the current flow through the impoundment 10 can be determined by measuring the voltage across the series resistor 14. Alternatively, an ammeter (not shown) can be connected in place of the series resistor 14 to obtain an indication of the current I flowing through the impoundment. In the absence of a leak 11, a small current will flow through the impoundment 10. If a leak develops, however, the current flow through the impoundment 10 will increase because of the low resistance shunt created across the liner between the contained liquid 17 and the surrounding earth 13, as described above.

The change in voltage across the series resistor 14 associated with the increased current flow detected by the ammeter can be displayed visually or permanently recorded on a strip chart. If the voltage or current detected by the system reaches a predetermined level, an alarm contained in the monitor system 30 will sound indicating the presence of a leak in the impoundment.

Details relating to the monitor system 30 can be seen by referring to the schematic block diagram of FIG. 4. Current flow through the resistor 14 is converted to a scaled voltage by the monitor signal conditioner 32. This circuit also provides all of the signal conditioning required to interface and use the perforation-dependent voltage to operate ancillary monitoring and display devices. Typical displays which can be used include a strip chart recorder 34 which generates a permanent record of the perforation-dependent current vs. time history of the geomembrane liner. An analog or digital voltmeter display 36 can be used to obtain a real-time readout of these same signals. ^"An audible/ visual alarm circuit 38 can be used to compare the leak current against a predetermined threshold value and to activate appropriate alarms to call attention to the existence of a leak in the liner.

Another option for monitoring the leak-dependent voltage is to digitize the signal generated in the monitor signal conditioner 32 and to sample and store the result in a digital processor 40, such as a dedicated microprocessor circuit or a minicomputer. This digital processor can also be used to transit data via telecommunication lines to a monitor station remote from the impound¬ ment site. A modem 44 shown in FIG. 4 connects the computer to a communi¬ cation line . to achieve this mode of operation. Another option is to record the data on a long-term digital storage medium 42, such as random access memory (RAM) with a battery backup to prevent loss of data during power interruptions. Magnetic media, such as disks or tape, can also be used to provide for long- term storage of data pertaining to the geomembrane integrity.

After the existence of a leak has been detected by the monitoring system 30, as described above, the exact position of the leak can be determined by a differential voltage probe 22, shown in FIG. 2. The probe 22 operates by detecting a potential difference between the upper and lower electrodes 22a and 22b, respectively, of the probe 22 as the probe moves through the body of the liquid 17. Higher current density in the liquid volume occurs within the vicinity of perforations, for the reasons discussed above. The probe 22 measures the larger potential difference in the liquid associated with the current flow through the leak 11, thus detecting the leak 11 as an increase in voltage be¬ tween the electrodes 22a and 22b.

Details relating to the detection indicator 23 used to process signals obtained from the probe electrodes can be seen by referring to FIG. 5. Signals from the electrodes 22a and 22b are processed by the differential amplifier 50 to produce an output voltage which is a function of the voltage between the probe electrodes. This signal voltage is amplified by the amplifier 52 and applied t'o the meter M to provide a visual indication of the potential differ¬ ences. As a further degree of signal processing, an audio indication of the poten¬ tial difference between the electrodes 22a and 22b is produced by applying the signal voltage to the input of the voltage-to-pulse repetition rate converter 54. This circuit generates output pulses at an increasing rate as the potential difference increases. Thus, the pulse rate increases as the probe 22 is moved closer to a leak. The output pulse signal is applied to the headphones worn by the operator when performing the search.

The smaller the leak the closer the probe 22 must be located to the leak to detect it. The probe can readily detect a hole having a cross-sectional area as small as 0.0008 square inch at a distance of 18 inches from the leak. There- fore, to assure that all small geomembrane punctures and welded-seam leaks are detected, every square foot of the entire liquid-filled area must be searched in such a manner that no spot on the liner area is missed by more than 18 inches by the probe tip.

The handle of the detection probe 22 is of sufficient length that an opera- tor wading in a liquid impoundment can comfortably move the probe laterally across the bottom of the impoundment. As the operator moves forward in one-foot increments, he will cover an area about seven feet wide in the direc- tion he is walking with the probe tip passing within 12 inches of all of the points on the entire surface area of the liner covered by the search path.

FIG. 3 illustrates the systematic search concept on a hypothetical symmetric impoundment having outer dimensions represented by the reference letter Z. In a typical impoundment, the reference letter Z would represent a distance of 220 feet. However, the systematic search described herein can be used effec¬ tively to locate a leak in an impoundment having virtually any shape or dimen¬ sion.

The first step in implementing the search involves the placing of markers in. the impoundment to define the search path to be followed by the operator. Lines having 1-foot markers are pulled taut across the impoundment, with the first line (1) being placed at a distance of 5 feet, represented by the reference letter X, from the edge of the impoundment. The lines may have floats attached along their length to keep the line near the surface of the liquid for better visibility. A plurality of additional lines, e.g. (2), (3), (4), are then placed in the impoundment on 10-foot centers, represented by the reference letter Y, until the entire impoundment has been marked off into search paths.

To begin the search, the operator centers himself at position (5) between the edge of the impoundment and the first marker line (1). The operator moves forward in 1 foot increments guided by the marker line (1) as he moves the tip of the probe 22 across the impoundment bottom laterally between the edge of the impoundment and the marker line (1), keeping the probe tip in contact with the liner material or at a small distance above it - not more than 12 inches. After searching this first path, the operator returns in the opposite direction centering himself at search position (6) between marker lines (1) and (2), such that a lateral movement of the probe 22 moves the probe tip to scan a lateral path between lines (1) and (2) approximately 7 feet wide beginning at line (1). The probe, therefore, covers more than one-half of the distance between line (1) and line (2). The operator then follows a search path (7) along the next line (2), spanning a 7 foot wide path from line (2) that overlaps the central line (8) between lines (1) and (2).

The search is then continued between lines (2) and (3), and between a plurality of additional marker lines (not shown) until the entire impoundment has been searched. The spacing of 10 feet between guide lines can be adjusted to suit the particular probe handle length and the operator's physical capabilities to achieve adjacent search path spacings such that overlap redundancy is accom- pushed between adjacent search paths to assure that the entire surface area of the liner is covered.

Finally, as leaks are located, an appropriate lead weight is lowered through the liquid to mark the leak location. A marker float attached to the weight by a length of line will then float on the surface of the liquid to indicate the leak location at the surface.

The side slopes 24 of the impoundment are searched for leaks in much the same manner as described above. For this purpose, the probe 22 is equipped with an extended handle so that it can be lowered from the edge of the im- poundment along the side slopes of the liner 20 to the bottom of the impound¬ ment. The probe 22 is then moved laterally a distance of 1 foot and drawn back up to the top of the liquid level along the liner sides 24 where it is again moved laterally a distance of 1 foot and then scanned downward again, with this process continuing until all sides of the impoundment 24 have been surveyed.

Although the invention method has been described in connection with the preferred embodiment, this embodiment is not intended to limit the invention to the particular form set forth, but on the contrary, it is intended to cover such alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims.

Claims

I CLAIMS

1. An apparatus for monitoring and detecting the presence of a leak in a geomembrane liner, said liner having one face in contact with a conductive liquid contained, by said liner and having the opposite face in contact with a

5 conducting material, comprising: means for producing a voltage drop between said contained liquid and said conducting material, thereby creating an electric current through said liquid and said liner, said current having a characteristic flow rate correlatable with an unfaulted liner; 0 means for monitoring said current flow to detect a change in said current flow rate, said monitoring means operable to produce an output signal upon detection of a change in said current flow rate; and alarm means responsive to said output signal to produce an alarm indicating the presence of a leak in said liner. 5

2. An apparatus for monitoring and detecting the presence of a leak in a geomembrane liner according to claim 1, said means for generating a voltage drop comprising a power source connected in series with a source electrode and a sink electrode, said source electrode being immersed in said conducting 0 liquid, said sink electrode being in contact with said conducting material.

3. An apparatus for monitoring and detecting the presence of a leak in a geomembrane liner according to claim 2, said means for generating a voltage drop further comprising a resistor in ^" series with said power source, said source 5 electrode and said sink electrode.

4. An apparatus for monitoring and detecting the presence of a leak in a geomembrane liner according to claim 3, said monitoring means comprising a voltage measuring means operable to measure the voltage drop across said 0 series resistor.

5. An apparatus for monitoring and detecting the presence of a leak in a geomembrane liner according to claim 2, said monitoring means comprising an ammeter in series with said power source, said source electrode and said sink 5 electrode.

6. An apparatus for monitoring and detecting the presence of a leak in a geomembrane liner according to claim 1, said alarm means comprising an audible alarm.

7. An apparatus for monitoring and detecting the presence of a leak in a geomembrane liner according to claim 1, said alarm means comprising a visual indicator means.

8. An apparatus for monitoring and detecting the presence of a leak in a geomembrane liner according to claim 7, said visual indicator means comprising a strip chart recorder.

9. An apparatus for monitoring and detecting the presence of a leak in a geomembrane liner according to claim 1, said monitoring means comprising communications means operable to transmit data from said monitoring means to a remote monitoring station.

10. An apparatus for monitoring and detecting the presence of a leak in a geomembrane liner having a high electrical resistance, said liner having one face in contact with a conductive liquid contained by said liner and having the opposite face in contact with a conducting material, comprising: means for producing voltage drop between said contained liquid and said conducting material, thereby creating a flow of electric current through said liquid and said liner, said current flow having a first magnitude correlatable with an unfaulted liner and a second magnitude correlatable with a faulted liner; means for monitoring said current flow to detect said second magnitude of current flow, said monitoring means operable to produce an output signal upon detection of said second magnitude of current flow; and alarm means responsive to said output signal to produce an alarm indicating the presence of a leak in said liner.

11. An apparatus for monitoring and detecting the presence of a leak in a geomembrane liner according to claim 10, said means for producing a voltage drop comprising a power source connected in series with a source electrode and a sink electrode, said source electrode being immersed in said conducting liquid, said sink electrode being in contact with said conducting material.

12. An apparatus for monitoring and detecting the presence of a leak in a geomembrane liner according to claim 11, said monitoring means comprising a resistor in series with said power source and voltage measuring means operable to measure the voltage drop across said series resistor.

13. An apparatus for monitoring and detecting the presence of a leak in a geomembrane liner according to claim 11, said monitoring means comprising an ammeter in series with said power source, said source electrode and said sink electrode.

14. An apparatus for monitoring and detecting the presence of a leak in a geomembrane liner according to claim 10, said alarm means comprising an audible alarm.

15. An apparatus for monitoring and detecting the presence of a leak in a geomembrane liner according to claim 10, said alarm means comprising a visual indicator means.

16. An apparatus for monitoring and detecting the presence of a leak in a geomembrane liner according to claim 15, said visual indicator means comprising a strip chart recorder.

17. An apparatus for monitoring and detecting the presence of a leak in a geomembrane liner according to claim 10, said monitoring means comprising communications means operable to transmit data from said monitoring means to a remote monitoring station.

18. An apparatus for monitoring and detecting the presence of a leak in a geomembrane liner having a high electrical resistance, said liner having one face in contact with a conductive liquid contained by said liner and having the opposite face in contact with a conducting material, comprising: means for producing a voltage drop between said contained liquid and said conductive material, thereby creating a voltage gradient in said contained liquid, said voltage gradient having a first distribution correlatable with an unfaulted liner and a second distribution correlatable with a faulted liner; means for measuring said voltage gradient in said contained liquid, said measuring means operable to produce an output signal correlatable with said voltage gradient distribution; and indicator means responsive to said output signal and operable to produce an indication of a voltage gradient correlatable with a leak in said liner.

19. An apparatus for monitoring and detecting the presence of a leak in a geomembrane liner according to claim 18, said means for producing a voltage drop comprising a power source connected in series with a source electrode and a sink electrode, said source electrode being immersed in said conducting liquid, sand sink electrode being in contact with said conducting material.

20. An apparatus for monitoring and detecting the presence of a leak in a geomembrane liner according to claim 19, said means for measuring said voltage gradient comprising a probe immersed in said conducting liquid, said probe including first and second electrodes and amplifier means operable to produce an output signal correlatable with a voltage potential difference between said first and said second electrode.

21. An apparatus for monitoring and detecting the presence of a leak in a geomembrane liner according to claim 20, said indicator means comprising means for processing said output signal to produce an audible indication of changes in said potential gradient in said conducting liquid.

22. An apparatus for monitoring and detecting the presence of a leak in a geomembrane liner according to claim 21, said indicator means comprising a meter responsive to said output signal, said meter providing a visual indication of changes in said potential gradient in said conducting liquid.

23. A method for detecting the location of a leak in a geomembrane liner, said liner having one face in contact with conductive liquid contained by said liner and having the opposite face in contact with a conducting material, com¬ prising the steps of: creating a voltage drop between said contained liquid and said conducting material, thereby creating a voltage gradient in said liquid, said voltage gradient having a characteristic distribution correlatable with the location of a leak in said liner; moving a voltage gradient measuring means in said conducting liquid at a plurality of locations, thereby obtaining an indication of the voltage gradient distribution in said liquid at each said location; - correlating said indication of voltage gradient distribution at each said location with the location of a leak in said geomembrane liner.

24. A method according to claim 23, said measuring means comprising a probe having first and second electrodes and means for producing an output signal in response to a potential difference between said first and second electrodes.

25. A method according to claim 24, said step of correlating said indication of voltage gradient distribution further comprising the step of observing an audible signal and correlating said audible signal with the location of a leak in said geomembrane liner.

26. The method according to claim 24, said step of correlating said indication of voltage gradient distribution further comprising the step of observing a visual indicator signal of said measuring means, said visual indicator being responsive to said output and correlating a reading on said indicator with the location of a leak in said geomembrane liner.

27. A method of surveying a liquid impoundment to detect the location of a leak in a geomembrane liner containing a conducting liquid within said impound¬ ment, said geomembrane liner having one face in contact with said conducting liquid and having another face in contact with a conducting material, comprising the steps of: ^* creating a voltage drop between said conducting liquid and said conducting material, thereby creating a voltage gradient in said liquid, said voltage gradient having a distribution correlatable with the location of a leak in said liner; placing a plurality of markers in said impoundment, said markers defining a plurality of linear, parallel search paths, each said search path^' having a longitudinal centerline defined by one-half of the distance between the markers defining said path; placing a voltage gradient detector probe at a plurality of points along each said center line of each said search path; moving said probe along an axis transverse to said longitudinal axis at each said point along said longitudinal axis, thereby obtaining a measurement of said voltage gradient at a plurality of points along each said search path; and correlating said measurements of said voltage gradients at said plurality of points with the location of a leak in a geomembrane liner.