and models, but they will have high uncertainties until field data are accumulated for longer periods, perhaps 100 years or more. We will never have all the long-term observations and data that we would like.

Long-term containment is difficult and requires high-quality engineering. Few significant failures have occurred and, in general, repair or limited reconstruction has been possible. Given that development of optimal designs for lifetimes of thousands of years is likely to be both infeasible and prohibitively expensive, designs that allow for recovery, repair, and/or replacement are to be encouraged. Findings and recommendations on specific barrier components, systems, and models are described below.

MONITORING BARRIER PERFORMANCE

Because most waste containment systems are buried, their component systems are usually monitored indirectly. Direct monitoring of the integrity of barrier system components is generally limited to an end-of-construction assessment of the component. Modern construction quality assurance procedures have, in general, been effective in ensuring the integrity of barrier components in the short term.

The primary (top) 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 sandwiched between the primary and secondary (bottom) liners provide a direct assessment of the performance of the primary liner system. The postconstruction integrity of caps (covers) can be monitored by exhumation and testing of cap material. In situ moisture content monitoring of soil layers within and beneath containment system covers (caps) can provide an indirect measure of cap performance.

The performance of engineered barriers and barrier systems should be monitored with a variety of techniques and in a variety of media (surface water, groundwater, air, and soil). Geophysical techniques offer promise for cost-effective, long-term, indirect monitoring of barrier systems. For example, electrical resistivity and electromagnetic surveys may detect gross defects that facilitate concentrated flow through vertical barriers. Tomographic imaging and seismic velocity surveys may detect changes in physical properties caused by vertical barrier degradation. Multispectral imaging can show changes in vegetation and in water content and temperature in near-surface soils caused by problems with caps and vertical barriers. 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. However, to date, these technologies have yielded little data that can be used to quantitatively and reliably monitor barrier systems. Development of these tools for long-term monitoring purposes is an area of ongoing research.

PERFORMANCE OF BARRIER SYSTEM COMPONENTS

Common barrier system components include earthen barriers (e.g., clay liners), geomembranes, geosynthetic clay liners, granular and geosynthetic drainage layers, evapo-transpirative barriers, vertical barriers, and asphalt concrete barriers. Most of the information available is on components used in covers and liners; hence, these are covered in more detail in this report than components used in vertical barriers. Available data indicate that compacted clay layers generally perform effectively as components within barrier systems as long as good construction and/or operational practices are followed. However, secondary permeability may develop in unprotected clay liners and covers as a result of wetting and drying, freezing and thawing, and deformation processes. Diffusion can be a significant contributor to the total migration of chemical contaminants through well-constructed, low-permeability earthen barriers. High temperatures near the barrier and reactions between migrating chemicals and the earthen materials (especially bentonite) used for the barrier have the potential to increase the hydraulic conductivity above the usual target of <1 × 10−9 m/s over the medium and long terms. Additional monitoring will be required to determine whether compacted clay and composite barriers effectively halt volatile organic compound migration in the long term.

Geomembranes installed following strict construction quality assurance protocols exhibit significantly fewer leaks and perform better than those installed without such requirements. Defective materials or seams and physical damage caused during construction can all degrade short-term performance. Over the medium and long terms, geomembrane performance may be reduced by punctures caused by increased overburden pressure, material degradation, and high temperatures. The estimated service lives of geomembranes decrease from 1,000 years at 10­°C to only about 15 years at 60­°C. Geomembranes appear to offer little, if any, resistance to the migration of several types of volatile organic compounds. This lack of resistance can be a short-term problem if a geomembrane is used as the sole barrier, or a medium- or long-term problem if the barrier system is comprised of more than one barrier material or type.

The use of defective materials and/or separation of overlapped panels will decrease the short-term effectiveness of geosynthetic clay liners. Hydraulic conductivity may increase if the liner is exposed to relatively strong liquids (e.g., high ionic strength chemicals) and is a performance concern over all timescales. Medium- and long-term concerns for geosynthetic clay liners include the effects of desiccation and local and global slope instability. Chemical transport through individual geosynthetic clay liners can be a problem when holes are too large to permit self-healing (e.g., through swelling of bentonite) or when the liner is the sole barrier component and is susceptible to diffusion.



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