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alternative to pump-and-treat systems, or it might enhance the effectiveness of pump-and-treat technology for waste site cleanup. One cannot install a barrier and leave it unmonitored after only a few years. It is very difficult to develop engineering plans for unexpected, uncertain, or unpredictable long-term events such as climatic changes because such events may affect local weather patterns and the attendant physical, chemical, and biological factors acting on a barrier. Participants noted that development and refinement of non-invasive monitoring techniques, such as shallow exploration geophysics, may be useful in ensuring that barriers are functioning as designed, as well as for detection of defects in the barriers. More sensitive and robust instruments may be needed to monitor subtle changes that may be forerunners of contaminant migration in the case of long-term isolation; such instruments would require periodic calibration. Tracers that follow migrating contaminants may increase the effectiveness of monitoring instruments. Participants suggested that some case studies should explain the absence of data collection and performance monitoring data for barriers (factors may include cost, absence of short- and long-term in-place monitoring instruments and methods, ambiguity of regulatory requirements, and level of interest in performance evaluation). There was agreement by the workshop participants that mathematical simulation modeling is also important to predict the performance of barrier design and installation. However, participants noted that extended monitoring can be used to verify models of barrier performance. Several presenters noted the importance of both surface and subsurface barriers to prevent vapor transport of contaminants of concern. Vapor transport and infiltration of precipitation into and through a barrier may cause migration of contaminants to the air above the barrier or to the ground water, respectively. It was suggested that, where appropriate, barriers in which such conditions may exist should provide for controlled vapor venting. Several participants noted a need for improved communication among operators at the various DOE sites that may need to use barriers to meet their site-specific remediation objectives and regulatory statutes and agreements. For example, barriers constructed under the UMTRA Project appear to be functioning successfully, but information on these barriers needs to be communicated effectively to other DOE sites where barriers are needed. However, significant potential exists for selecting the wrong barrier for a specific site. A design that works in one situation may not work at all in another. This observation supports the importance of documenting and publicizing both successes and failures of barrier development and operation case studies. Themes Identified at the Workshop Several themes emerged during the panel discussions. The importance of employing proper installation techniques and quality control measures, especially during construction, including using contractors with demonstrated experience and skill. The need for knowledge concerning effective lifetimes for selected barrier materials and resultant barrier systems. The importance of periodic inspection, maintenance, and monitoring, both short- and long-term, of containment barriers. The current dearth of barrier performance monitoring data. The advantages of using barriers in combination with pump-and-treat approaches.
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