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