Assessment of the Performance of Engineered Waste Containment Barriers (2007)

“Modern” engineered waste containment systems have been in existence for only a few decades. Thus, the committee’s assessment of these systems is necessarily limited to their short- and medium-term performance. The principal findings and overarching recommendations for actions and studies needed to both reduce the uncertainties surrounding the evaluation of barrier performance and better ensure that contained wastes do not provide a risk to health, safety, or the environment in the future are given below.

As much as 20 years of field observations suggest that engineered waste containment barrier systems that have been designed, constructed, operated, and maintained in accordance with current statutory regulations and requirements have so far provided environmental protection at or above specified levels. Extrapolations of long-term performance can be made from existing data using both empirical and physical and chemical models, but they will have high uncertainties until field data are accumulated for longer periods, perhaps 100 years or more for some systems.

Our ability to predict the long-term performance of engineered waste containment systems depends strongly on accurate prediction of the service life of the individual components (i.e., compacted clay layers [CCLs], geomembranes, geosynthetic clay liners [GCLs], vertical walls of various types). Based on the available data, the committee draws the following conclusions regarding the performance of barrier systems:

• Modern composite liners generally appear to be working well, up to the assessed period of about 20 years, with double composite liners constructed according to rigorous construction quality assurance guidelines providing the best protection against advective contaminant migration.

• Cover systems can be effective at isolating waste and limiting infiltration. Most cover systems require periodic maintenance to maintain their integrity.

• Direct monitoring of vertical barriers (e.g., soil-bentonite and cement-bentonite walls) has been insufficient to draw conclusions about their field performance. Monitoring of contaminants down gradient of the barrier (indirect monitoring) suggests that most vertical barriers are functioning as intended. However, more extensive monitoring and detailed analyses are needed before definite conclusions can be reached.

Individual components of barrier systems can degrade as a result of chemical interactions, environmental effects (freeze/thaw and desiccation of CCLs and GCLs), elevated temperature (geomembranes), deformation-induced cracking (covers and vertical barriers), and clogging caused by biological action or soil intrusion (drainage layers). However, even if the performance of an individual component degrades with time, redundant design appears to enable the overall waste containment system to still serve as an effective barrier to contaminant transport. Incorporation of specific provisions for repair or for recovery and replacement would further strengthen designs and likely enhance the performance of new waste containment systems.

Although engineered waste containment barrier systems have been working well in the short term, it is not known how long they will continue to work. A number of observations about performance (e.g., liners can get hot, leachate collection systems and drains can clog, long-term exposure of geosynthetics to high temperatures and chemicals can degrade their properties, incompatibility between CCL, GCL, and leachate may require many years to develop) suggest that considerable care in design is needed to avoid problems in the 20- to 100-year time frame. Consequently, it is important to continue monitoring performance well beyond the few decades for which performance data are now available.

Much information on barrier components and systems is collected in accordance with regulations. This data collec-

tion is necessarily targeted toward regulatory compliance, which tends to focus on chemical concentrations in gas and groundwater at defined points of compliance. The overall result of the focus on compliance is that key data on barrier performance are either not collected or are not collected long enough to enable reliable predictions of performance. A systematic approach to data collection and reporting that targets the most important data on barrier performance and makes the data readily accessible would greatly facilitate periodic assessments of the long-term performance of engineered barriers.

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.

Key types of data that should be collected are listed in Table 6.1.

Noninvasive geophysical monitoring techniques (e.g., electrical surveys, radar, seismic tomography) have the potential to reduce the number of monitoring and observation wells needed and thus reduce costs. Geophysical techniques may also enable continuous, rather than episodic, assessments of barrier integrity. Additional evaluation is needed to determine the extent to which these methods are capable of providing the information listed in Table 6.1.

Most field monitoring of waste containment systems is performed either immediately at the end of construction and before the placement of waste or indirectly afterward through measurements such as cover settlement or concentrations of chemical constituents in gas and groundwater. Although this practice satisfies regulatory requirements, the lack of direct monitoring data introduces uncertainties about how well the individual parts of the overall containment system are working. Such information could help operators avoid an unacceptable release of contaminants and is also essential to designing better systems and materials for future waste containment systems. New techniques are needed to directly monitor the integrity and performance of other barrier configurations and individual barrier system components.

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. Although the law requires operators of waste containment facilities to make data publicly available, reports, databases, and tables are often not readily accessible. The effort required to collect relevant information from disparate sources can discourage the types of broad-scale analyses needed to evaluate performance. However, accumulation of new information on field performance, as well as advances in understanding of material behavior and in monitoring and modeling capabilities, would make an assessment of performance worthwhile about every 5 years.

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.

Many data used to predict performance come from laboratory experiments, models, and field-constructed prototype barriers, such as 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. A 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. Some of this information could be gathered from existing facilities where sufficient funding is available to expand monitoring or from new facilities where collecting and reporting the types of information listed in Table 6.1 are built into operational plans. But even taking advantage of existing and planned facilities misses opportunities to test innovative concepts and new materials or to control instrument spacing and monitoring periods.

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.

Because published high-quality field data are sparse, facility operators commonly rely on analytical and numerical models to predict contaminant transport, containment effectiveness, degradation of materials, and changes in behavior over time, even though some models have well-known shortcomings. For example, some do not account for known processes (e.g., advection-dispersion processes, leakage caused by holes in geomembrane wrinkles), and others (e.g., the HELP model) are widely 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.

Almost all statutory monitoring programs require an initial 30-year postclosure monitoring period. At the discretion of regulatory authorities, the owners and operators of some sites may have to continue monitoring and maintenance if the waste still poses a threat to human health or the environment. However, financial assurance is frequently required only for the initial postclosure monitoring period. The committee’s analysis of data from engineered barrier systems that contain low-level radioactive waste, hazardous waste, and municipal solid waste suggests that extended monitoring periods (hundreds to thousands of years) will be required in many cases. The necessary duration of monitoring varies with the facility, type of waste, climate, and observed performance. Yet without an appropriate financial assurance mechanism, funding will often not be available to continue monitoring until the site no longer poses risk to human health and the environment. Legislation has been introduced into the Senate (S. 452) that would direct the EPA to develop financial assurance regulations to ensure that liable parties meet their Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) obligations. Whether or not it passes, a national policy that ensures the availability of funding would alleviate concerns that different state financial assurance requirements might create an incentive for shipping waste across state lines.

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.

Common performance criteria used in practice include measures such as acceptable percolation rates through covers and contaminant mass fluxes through low-permeability barriers. Performance-based design is now the norm in many other countries. These performance criteria are generally based on the performance of the prescriptive designs specified in the regulations for many waste containment barrier systems or system components (e.g., prescriptive cover and liner designs described in Chapter 2, prescriptive low-permeability barrier layers described in Chapter 4). However, performance criteria that are based on a prescriptive barrier system design are often narrowly focused on the performance of these components and may miss key aspects of the overall performance of the waste containment system. As a result, flexibility in the design of waste containment systems to provide cost-effective environmental protection is limited.

Regulations often provide for sophisticated risk-based design of containment systems. For instance, Subtitle D regulations for MSW landfills provide a table of risk-based chemical concentrations at the point of compliance that can be used as a basis for regulatory approval of alternative barriers that do not meet the prescriptive requirements. In theory, a risk-based design could result in more effective and economical systems that balance technical performance,

cost, and risk on a project-specific basis. The engineered barrier systems in a risk-based design could be either more or less protective than the prescriptive barrier, depending on the project-specific characteristics of the waste, the geological setting, and the exposure potential for humans and the environment. For example, a design that limits the concentrations in an underlying receptor aquifer provides a measure of the performance of the entire system, but if it fails to take into account the service lives of the system components, the long-term performance of the system could be compromised.

In practice, a risk-based design will rely heavily on validated and calibrated models to minimize uncertainties in predicted performance and is accompanied by field monitoring to confirm performance. Given the current lack of performance data and deficiencies in monitoring technology and validated and calibrated models (Sections 6.2 to 6.4), there is a significant potential for misuse or even abuse of risk-based designs in practice. Thus, until further developed and validated in practice, risk-based designs should be subjected to independent review.

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.

In conclusion, effective long-term containment of wastes is difficult and requires high-level engineering, comprehensive design, use of suitable materials, carefully controlled construction, continual monitoring, and maintenance as required. Evidence to date reveals few failures of engineered waste containment barrier systems that have been designed, constructed, operated, and maintained in accordance with statutory regulations. In those few cases where failures have occurred, repair or limited reconstruction has been possible.

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