Modern waste containment systems rely on surface and subsurface engineered barriers to contain hazardous and toxic waste, to prevent the offsite flow of contaminants, and/ or to render waste less harmful to humans and ecosystems for tens to hundreds or thousands of years, depending on the type of waste, local conditions (e.g., geological setting, climate, land use), and regulations. The barriers may be at the bottom, top (cover), and/or sides (lateral barriers or walls) of the waste containment system, and they usually employ a variety of materials and mechanisms (e.g., liquid extraction) to control contaminant transport. Barriers are made of natural (e.g., soil, clay) and/or synthetic materials, such as polymeric materials (e.g., geomembranes, geosynthetic clay liners), usually arranged in layers.
Engineered barrier systems are monitored for effectiveness and proper functioning. Specified parameters that are observed or measured at the time of construction (e.g., hydraulic conductivity) provide an indication of whether the components of the systems will function as designed. Common measures of the effectiveness of a barrier system include the rate of release of contaminants from the barrier system and/or the detection of concentrations of contaminants beyond the boundaries of the barrier that exceed specified allowable maximum values. Design, initial performance, and monitoring criteria for these waste containment systems are governed by federal and state environmental regulations initially put in place beginning in the mid-1970s.
At the request of the Environmental Protection Agency (EPA), Department of Energy (DOE), National Science Foundation (NSF), and Nuclear Regulatory Commission (USNRC), the National Academies Committee to Assess the Performance of Engineered Barriers was established to provide a technical assessment of the available information on engineered barrier performance over time. The committee was charged with the following tasks:
Identify engineered barrier systems used for surface and subsurface waste containment.
Describe how performance is defined, predicted, measured, and monitored.
Present information on field performance of engineered barrier systems.
Evaluate the information on field performance.
Assess methodologies and capabilities for predicting and monitoring performance and for assessing risk.
Identify information needed to fill the knowledge gaps.
This report focuses on engineered barriers designed to contain municipal solid waste, other nonhazardous solid and liquid waste, hazardous and toxic wastes, and low-level radioactive wastes. The primary questions addressed are: How well are these engineered barrier systems working? How long are they likely to work effectively? Because engineered barrier systems constructed in compliance with current regulatory requirements have been operating for only a few decades at most, the assessment necessarily focuses on short- and medium-term performance. Predictions of long-term performance must be based on models and extrapolation of data and observations obtained over shorter periods of time. In this report, performance periods are defined as follows:
short term: the period until completion of construction of the barrier component,
medium term: the operating period of the waste unit, and
long term: the postclosure period.
Based on as much as 20 years of observations, the committee concluded that most engineered waste containment barrier systems that have been designed, constructed, operated, and maintained in accordance with current statutory regulations and requirements have thus far provided environmental protection at or above specified levels. Extrapolations of long-term performance can be made from existing data
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.
Granular drainage layers are important barrier components for reducing leachate head on liners and covers because they enhance stability and cut off advective and diffusive transport. Their short-term performance may be degraded by inadequate discharge capacity and clogging. Over the medium and long terms, granular drainage layers can become clogged as a result of soil infiltration, biological activity, and chemical precipitation. Geosynthetic drainage layers are susceptible to similar problems. Installation damage and inadequate capacity degrade their short-term performance, and clogging caused by soil infiltration, soil and geosynthetics penetration, creep of the geonet core, biological activity, and mineral precipitation degrade their medium- and long-term performance.
Evapotranspirative barriers are now beginning to be used in capacitive cover systems. Only a few years of data are available, but they suggest that evapotranspirative barriers can be an effective alternative to compacted clay or composite covers in arid and semiarid climates. Significantly more data over much longer time frames and/or studies of natural analogs that have functioned for hundreds or thousands of years are required to make a reliable prediction of the long-term performance of evapotranspirative barriers.
The short-term performance of vertical cutoff walls is primarily affected by the quality of construction. Construction defects that can compromise wall performance include gaps in the wall caused by poor mixing or defective material and high-permeability zones caused by caving or trapping of low-quality material at joints between panels. Chemical incompatibility, desiccation above the water table, and cracking caused by various mechanisms all adversely affect the medium- and long-term performance of vertical cutoff walls. Defective materials, cracking, and degradation are also performance concerns for asphalt cement barriers.
CONTAINMENT SYSTEM PERFORMANCE
Although existing data suggest that modern containment systems are performing well thus far, they have not been in existence long enough to allow a direct assessment of long-term performance. Likewise, models appear to be capable of predicting long-term performance, although relatively few field data exist to verify the models. Models that predict the long-term performance of containment systems depend on predictions of the long-term integrity of containment system elements. Thus, maintaining the integrity of containment system elements over the active life of the wastes they contain appears to be required to assure satisfactory long-term performance of engineered barrier systems. Moreover, redundant design appears to enable the waste containment system to serve as an effective barrier to contaminant transport, even if the performance of an individual component degrades with time.
Available data show that liners constructed following rigorous construction quality assurance guidelines provide protection against offsite contaminant leakage. Composite liners composed of either compacted clay and geomembranes or geosynthetic clay layers and geomembranes provide better protection than any single component acting alone. Reliable predictions of leakage rates through composite liners should take into account holes in geosynthetic wrinkles and elevated leachate head. Cover systems are effective at isolating waste, as long as periodic maintenance is performed. Vertical waste containment barriers have not been monitored sufficiently to draw conclusions about their field performance or the accuracy of predictions of the transport of contaminants through them.
Data Collection and Distribution
A systematic approach to data collection and reporting that targets the most important data and makes those data readily accessible would greatly facilitate periodic assessments of long-term performance. Key types of data that should be collected are listed in Table 6.1. Key parameters that should be monitored include groundwater quality in the saturated zone at the down-gradient edge of the containment facility, gas emissions in the vadose zone around the site if the waste has potential for generating harmful gases (e.g., methane), leachate head acting on the liner inside the containment facility, temperature on geomembrane liners, and the quality and quantity of leachate being generated by the facility.
Recommendation 1: Monitoring programs for new facilities should include provisions for collecting data needed to assess the long-term performance of engineered barriers, and operators of existing facilities should collect these data to the extent practical using in-place monitoring systems.
The performance of many engineered barriers is monitored indirectly, usually through evidence of contaminant migration outside the waste containment system. The absence of direct monitoring data introduces uncertainties about how well the individual elements of the overall containment system are working.
Recommendation 2: Regulatory agencies should develop guidelines to increase direct monitoring of barrier systems and their components, and NSF should sponsor research for the development of new cost-effective monitoring techniques, especially for assessing the effectiveness of vertical barriers, for this purpose.
Assessing or predicting the performance of engineered barriers is made more difficult because the necessary data and observational information do not exist, are hard to find,
are incomplete, or have not been analyzed. The effort to compile and evaluate these data is considerable, but there is enough new information on field performance, material behavior, and monitoring and modeling capabilities to make an assessment of performance worthwhile about every 5 to 10 years. More frequent assessments may be required based on previous monitoring data and performance assessment models.
Recommendation 3: Federal agencies responsible for engineered barrier systems should commission and fund assessments of performance on a regular basis. Given the rate at which performance data and knowledge of waste behavior, contaminant transport, and monitoring accumulate, the interval at which these assessments should take place is probably on the order of once every 5 to 10 years. The results of the assessment should be placed in the public domain in a form that is readily accessible.
Much data used to predict performance come from laboratory experiments, models, and field-constructed prototype barrier systems (e.g., test pads). Although useful for understanding material properties and behavior, these data are no substitute for performance data collected in the field from operating containment systems. An overall comprehensive assessment of performance requires long-term monitoring and analysis of data from different types of waste containment systems constructed from a variety of components and located in different climate regimes.
Recommendation 4: EPA, USNRC, NSF, and DOE should establish a set of observatories at operational containment facilities to assess the long-term performance of waste containment systems at field scale. The program would involve building one or more field facilities, monitoring the site, and analyzing and archiving the data. New sites could be created or adjustments could be made to existing observatories when promising new and innovative concepts and materials become available.
Analytical and numerical models are relied on to predict contaminant transport, containment effectiveness, degradation of materials, and changes in behavior over time, even though some models have shortcomings (e.g., they do not account for advection-dispersion processes; they are used in applications for which they were not designed).
Recommendation 5: Regulatory agencies (e.g., EPA, DOE, USNRC) and research sponsors (e.g., NSF) should support the validation, calibration, and improvement of models to predict the behavior of containment system components and the composite system over long periods of time. These models should be validated and calibrated using the results of field observations and measurements.
The optimum time for monitoring varies with the facility, type of waste, climate, and the observed performance. Yet funding is often not available to continue monitoring until the site no longer poses risk to human health and the environment, and no national policy exists to assure that such funding will be available.
Recommendation 6: EPA should develop financial assurance mechanisms to ensure that funding is available for monitoring and care for as long as the waste poses a threat to human health and the environment.
Performance criteria are needed that account for both barrier performance and impacts to public health and safety that extend beyond the barrier system.
Recommendation 7: EPA and USNRC should develop guidance for the practical implementation of performance-based criteria for assessment of containment system performance as an alternative to prescriptive designs.