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Underground Storage Tank Leak Detection System (ELDS)
A Study Prepared By: WESTEC
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Figure 1 |
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 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.
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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
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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.
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Figure 4
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Figure 5
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