By Ian Peggs, Ph.D., P.E
The benefits of performing geoelectric leak location or liner integrity surveys (LIS) as the final stage of geomembrane liner construction quality assurance are well established.
In fact, the New York State Department of Environmental Conservation (NYS DEC) Division of Solid & Hazardous Materials (DSHM) is proposing regulatory changes to its solid waste management regulations that will apply the practice to the floor secondary (lower) liner of new landfill cells as well as to the primary (upper) liner. Robert Phaneuf of DSHM reported that, based on data obtained from voluntary implementation of LIS on landfill projects where these test methods were applied as part of the construction quality assurance (CQA) program, a significant reduction in the number of defects were found compared to projects that did not have LISs done as part of the CQA program.
Phaneuf indicated that the outcome of this result is improved performance of the upper liner system of the double lined landfill, with eased start up of these new cells and significant reductions in leakage flow rates (LFR). The objective of requiring LIS for the lower liner system is to best ensure that all construction related defects in the entire containment system are found and repaired since secondary liner systems will not be easily serviced after operation has begun.
In addition to New York, eight other states require or advise the performance of an LIS as the final stage of landfill liner CQA: Florida, Wisconsin, Minnesota, California, Washington, New Jersey, Texas, and Ohio (currently considering).
Developed primarily for landfill liners, LIS is now routinely applied to all types of geomembrane containment systems: leachate ponds, wastewater treatment plant ponds, anaerobic digesters, farm waste lagoons, potable water reservoirs, fuel oil tank farm secondary containment, canal liners, phosphogypsum stacks, mining solvent extraction settlers/mixers, et al.
The survey technology is simple. An electric potential is applied across the liner. The potential gradients in the soil or water covering the liner are progressively measured looking for the unique signal that occurs at a penetrating hole (leak). The signal is such that a hole can be located exactly.
Four ASTM standards have been developed by Committee D35 on Geosynthetics to guide proper application of the technology:
- D6747 is a guide to the method appropriate for a given installation.
- D7002 is for surveys on exposed liners using the puddle method. A standard for the water lance technique is under way.
- D7007 is for soil- and water-covered liner surveys.
- D7240 is for surveys on geomembranes manufactured with a conductive layer on the underside.
However, not all lining systems can be surveyed, so designs should permit an LIS as part of CQA if required. And as-built drawings should be reviewed to determine whether a survey on a given lining system can be effective in locating a troublesome leak.
The need for experienced LIS technicians is growing, with five standards and (currently) eight states supporting the use of surveys. This has created both a need for new practitioners/companies in the field to offer the service, and education and training to support their successful entrance into the practice.
a few years ago surveys were performed only by a few companies that had developed their own equipment and survey procedures. Equipment was not commercially available and there was an aura of high technology associated with its implementation.
In fact, it is far from rocket science. Correctly interpreting an LIS survey signal takes practice and coaching, but it can be learned relatively quickly and effectively, as the real science involved is simple electrical theory plus gaining familiarity with the equipment. An LIS requires that four boundary conditions are satisfied for successful implementation:
- a conductive medium on top of the geomembrane.
- a conductive medium in the holes.
- conductive medium immediately under the geomembrane.
- no contact between media above and below the liner other than through the holes being sought.
Thus, a single liner with soil on top and over the edges of the liner would not meet Condition 4. A double liner with only a geonet/geotextile composite between primary and secondary geomembranes would not meet Condition 3. Both of these lining systems could not be effectively surveyed without special preparation. Such preparation might require that the soil survey is performed before cover soil is placed at the top of the sideslopes, while backfilling the LDS with water might be required for the double liner.