of states. Confidence in the long-term proper performance of waste containment systems can be gained only if the proper monitoring protocols are implemented.
Indirect monitoring of engineered barrier performance by monitoring for contaminant migration downstream of the waste containment system is commonplace, as it is mandated by regulations. Direct monitoring of barrier system component integrity is generally limited to an end-of-construction assessment of the component. Modern construction quality assurance procedures generally provide a high level of reliability for barrier component integrity in the short term. However, there has been little long-term direct monitoring of the integrity of individual barrier system components.
The primary liner in a double-liner system is perhaps the only type of engineered barrier system in which postconstruction integrity is routinely monitored directly. Liquids collected in the leak detection layer between the primary and secondary liners provide a direct assessment of the performance of the primary liner system. The absence of direct postconstruction monitoring of barrier integrity for other types of systems may be attributed to a variety of factors, including
difficulty in directly monitoring barrier integrity, particularly for barrier systems overlain by tens to hundreds of meters of waste or soil;
a philosophy that it is the overall performance of a waste containment system, not the integrity of individual elements of the system, that is important; and
the reluctance of designers, owners, operators, and regulators to monitor something they may not be able to remedy or that would be exceedingly costly to address.
Of these factors, the technological limitation is perhaps the easiest to overcome, particularly for caps and many near-surface vertical barriers. A variety of techniques can be used to monitor the postconstruction integrity of caps. Since a cap generally has relatively shallow soil cover, exhumation and recovery of samples of cap material and tests for degradation of their properties are feasible in most cases. While this has been done for short-term or early medium-term monitoring of geosynthetic clay liners, no long-term evaluations of buried cover system elements of this type have been conducted to the committee’s knowledge. In situ moisture content monitoring of soil layers in caps above, in, and below the barrier system can also provide an indication of cap performance. Furthermore, electrical surveys and leak detection surveys could be employed with geosynthetic (geomembrane and perhaps geosynthetic clay liner) caps if a wire grid is placed below the barrier layer during construction, and other geophysical monitoring techniques (e.g., electromagnetic surveys) could be used to assess changes in the physical properties of the cap over time. Temperatures can be monitored in caps and bottom liner systems to determine the service environments for soil and geosynthetic barrier components.
Geophysical techniques also offer promise for cost-effective long-term monitoring of vertical barriers. Electrical resistivity surveys and electromagnetic surveys offer the potential to detect gross defects that concentrate flow in vertical barriers. Tomographic imaging and seismic velocity surveys have the potential to detect changes in physical properties over time that may suggest barrier degradation. Inferred changes in barrier properties could be evaluated by in situ testing of the barrier or by physical sampling and laboratory testing when warranted.
Airborne and satellite-based remote monitoring techniques may offer the potential for cost-effective indirect monitoring of cap and vertical barrier effectiveness. Multi-spectral imaging can indicate water content and temperature changes in near-surface soils, as well as distress and other changes in vegetation, each of which may indicate barrier performance problems. Interferometric synthetic aperture radar, light detection and ranging, and other airborne/satellite techniques can resolve centimeter-scale deformations caused by local or global instability or barrier performance problems. Autonomous monitoring systems could detect moisture fluxes from cracks in caps. However, these techniques have not yet been demonstrated as useful tools for evaluating containment barrier effectiveness.