by Michael E. Henderson

Welsh Engineering Science & Technology (WESTEC) has developed an economical remote sensing system which is capable of locating and quantifying changes in soil moisture conditions, based on proven geophysical technology. For waste containment facilities, this system is installed below any liner system for which information on the location and quantity of leakage is required.

This system, know as the "Electronic Leak Detection System" or ELDST, consists of hardware and computer software to perform this remote sensing. The hardware consists of electrical nodes which are installed during construction of the liner system. These nodes are connected to an exterior panel which is used to systematically energize the nodes and to monitor the subgrade response, automatically performing a resistivity survey of the entire area. The resistivity meter used for the data collection is the Syscal R1 Plus system produced by IRIS Instruments. This unit features automatic SP correction, digital stacking and averaging, and memory storage.

The software for the system performs both polling and data analysis functions. The polling subroutines allow the system to electrically energize the nodes and perform the corresponding field measurements. The interpretation routines change the input data from raw digital form to values which correspond to soil moisture conditions. Numerical analysis subroutines use advanced statistical techniques such as kriging to reduce the data and to compare each data set to a baseline condition, in order to determine if a leak has occurred. The kriging techniques allow the system to accurately locate leaks.

After the initial installation of each system, a series of calibration tests are conducted. These test introduce simulated leachate in carefully controlled amounts to the subgrade being monitored. The response of the system is then compared to the input leachate. The system calibration provides for verification of the system operation and allows for a response correlation which is used to calculate leakage quantities.

A typical response to an introduced leak is shown on in the figure. This calibration demonstrated that the sensitivity of the particular system was approximately 20 gallons in total introduced leachate, and the location ability of the software was within 10 to 20 feet. This 20-gallon detection level is equivalent to a 4-foot-square zone of saturated soil, assuming a 1-foot thickness.

The first system, which was installed in 1987, is still in operation without showing any signs of degradation or failure. Since this time, six additional systems have been installed totalling approximately 20 million square feet of containment facilities.

The installed systems have been used to locate and repair liners on several occasions. In WESTEC's experience, most damage to liners found to result in leakage has been caused by either improper installation or by damage during placement of the initial layer of material directly over the liner. The ELDST concept has worked amazingly well under these conditions, as the liner has been easily accessible and repairs have been relatively easy to effect.