Heritage Geophysics - Underground Storage Tank Leak Detection System (ELDS)

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Underground Storage Tank
Leak Detection System (ELDS)

A Study Prepared By: WESTEC

This report describes a demonstration of the patented WESTEC, Inc. (WESTEC) Electronic Leak Detection System (ELDS) for the Underground Storage Tank Integrated Demonstration (USTID), at the Westinghouse Hanford facility near Richland, Washington.

ELDS is an integrated system of hardware and software which evaluates soil electrical properties. By quantifying changes in soil electrical properties, and relating those changes to soil moisture content, ELDS can determine the position, relative volume, and movement of liquids within the subsurface.

ELDS utilizes a Wenner electrode configuration to measure soil electrical properties. The Wenner configuration employs four equally spaced electrodes for each measurement. Figure 2 is a schematic diagram of the ELDS used at the USTID, showing the locations of current electrodes (A and B), potential electrodes
(M and N), and the virtual position where the apparent resistivity is calculated (X). To collect an ELDS measurement, a current is

Figure 1

induced in the soil by maintaining voltage across the outer current electrodes (A and B in figure 2). Simultaneously the voltage difference is measured across the inner potential electrodes (M and N in Figure 2). Polarity of the voltage maintained across the current electrodes is switched at intervals to minimize polarization of the electrodes.

Because of the geometry of the Wenner Array, the resistance value is calculated at a point between the two inner or potential electrodes. This value is called apparent resistivity because the point where it was calculated is not directly measured by the electrodes. The point where the apparent resistivity is calculated is referred to as a virtual position (labelled X in figure 2).

The ELDS software provides data reduction and baseline comparison functions used to correlate the changes in apparent resistivity to changes in the moisture content of the soil. The apparent resistivity for the subsurface is calculated using an equation from geophysics that relates the measured voltage and induced current to the apparent resistivity of the soil through a geometric factor. The geometric factor is a function of the electrode configuration and spatial separation.

Figure 2

ELDS was initially developed for monitoring heap leaching containment facilities used by the gold mining industry. At these facilities the potential for loss of valuable gold-bearing solutions, the environmental consequences of a release, and overall size of the lines impoundments (80 go 640 acres) provided commercial incentive for the development of the ELDS technology. ELDS is used in these and other applications to provide the owner/operators with the ability to locate and quantify possible sources of leakage. Once the leak is located and subsequently repaired, ELDS is used to quantify the effectiveness of repairs to the liner.

The first commercial application of the ELDS was in 1987, at a gold mining operation in Nevada. Since then, 12 systems have been installed in Nevada, California, Utah, and Italy with more proposed for the Western United State, South America, and Europe. These existing and proposed installations include heap leach and tailings facilities for the mining industry, underground petroleum storage tanks, hazardous waste impoundments, and municipal sanitary landfills.

Baseline ELDS Data

Prior to the onset of solution release, ELDS data sets were collected which represent baseline conditions for the electrode grid. Figure 3 is a plot of the baseline ELDS resistivity data contoured in units of 0.5 ohm-meters. Color on the plot are arranged so that the low resistivity areas (1.0 to 2.5 ohm-meters) are warm colors, and higher resistivity areas are cold colors. Figure 3 shows the location of the steel tank and its relation to the area of decreased resistivity.

The elongate pattern of low resistivity colored orange in the central area of figure 3 results from geometric effects within the ELDS measurement method.

Figure 3

These effects are a function of the orientation of current and potential electrodes relative to the metallic tank. With a larger number of electrodes distributed symmetrically about the tank, ELDS data would tend to show a more rounded anomaly in the area of the steel tank.

Test Procedures

A leak was introduced on the north-northeast side of the simulated waste tank by WHC personnel. The leak was supplied from a set of 1,000 gallon tanks containing a 0.08 molar NaCl solution. The leak was initiated on July 25 between 9:00 and 10:00 A.M., and ceased on July 31. A total leakage volume of 1,000 gallons was introduces over 151 hours at an average flow rate of 6.9 gallons per hour.

ELDS readings were taken from the grid twice daily from July 25, to July 30, 1995. The readings were taken at approximately 07:00 and 22:00 hours on measurement dates.

Results

Using the Wenner electrode array, ELDS located a resistivity anomaly in the subsurface originating at the north margin of the demo tank, and subsequent migrating in a north-northeast direction. The released liquid was detected in the first ELDS data set collected after initiation of the release. Figure 4 is a plot of the ELDS data collected after 69 gallons of solution was released, with the area of greatest percent difference from baseline colored green. Another release is indicated at the east margin of the tank, and extends to the central area under the tank. This area is also colored green on figure 4.

Figure 4 is contoured in units of 0.5 percent change, which is a measure of the drop in apparent resistivity from the baseline data set to the data collected after 69 gallons of solution was released. The largest observed percent change in the data was 0.2%, concentrated near the northeast margin of the tank, and beneath the center of the tank.

After a total of 897 gallons of solution had been introduced to the soil, the change of apparent resistivity values increase to a maximum of 2.5%. The release was concentrated in the same locations as observed in Figure 4, but the magnitude of change has increased to almost 2.5 percent difference. Figure 5 is a plot of percent changes vs resistivity after 897 gallons of solution introduction, contoured in units of 0.5 percent change. The color scale is the same as used in Figure 4. An increase in the size of the affected area, and the magnitude of the change in resistivity is observed in Figure 5.

Collectively, these three figures show the ability of ELDS to observe the movement of a plume of liquid in the subsurface, and to quantify the volumetric response of the system to introduces solution.