The purpose of this summary is to report on some of the oral discussions. The papers presented (included in Appendix D) provide more detailed information on barrier technology development and implementation. Significant information on barrier technology has been published in reports of two recent, DOE-cosponsored meetings (Gee and Wing, 1994; Rumer and Mitchell, 1996) and a DuPont Company workshop (Rumer and Ryan, 1995).
The text that follows is a synthesis of the oral discussions at the workshop. It does not represent the opinions of the committee.
Workshop Overview
The use of surface barrier research within the DOE nuclear weapons complex and the installation and use of vertical subsurface barriers at sites primarily outside of the DOE complex were the focus of two introductory presentations at the workshop (see Appendix B for program of the workshop). It was noted that about 70 million cubic meters of radioactively contaminated soil within the DOE complex require remediation. Regardless of the remedial methods pursued for individual sites, engineered containment barriers will be needed to ensure short-term (tens to hundreds or years) to long-term (hundreds to thousands of years) isolation of residual materials. Representatives from DOE mentioned that estimates of costs for development, construction, and maintenance of barriers at DOE sites are on the order of tens of billions of dollars.
Following the introductory speakers, there were six presentations on surface barriers and five on subsurface barriers, both horizontal and vertical. In addition, the findings of a study of ground water cleanup alternatives by the National Research Council (1994) were summarized. The presentations addressed such topics as types of barrier construction materials, biointrusion, application of freezing to achieve temporary subsurface confinement, and barrier installation techniques. The subject of barriers and regulatory compliance was not presented formally, but it was raised several times during the workshop.
To be effective, surface barriers must control infiltration of precipitation and surface runoff, erosion, and biointrusion, with minimal maintenance. However, it was noted that it is impractical to eliminate entirely the potential for degradation of surface barriers over long time periods. It was suggested that surface barrier sites should be visited at least once a year for maintenance and monitoring to ensure long-term performance as designed.
Workshop participants discussed the importance of being able to demonstrate that a surface barrier will remain effective for periods of 200 to 1,000 years of isolation. They also expressed concern that surface barriers may require a large volume of construction materials. If a thick surface cover is to be constructed from natural materials, sufficient amounts of the materials with proper characteristics may not be readily available at an affordable cost to the sites in question.
Technologies for constructing surface barriers under a variety of site-specific conditions, including climate, are still being developed and demonstrated. Although there have been instances of failures of surface barriers, there are locations where barriers have been effective generally in their application to a range of waste site remediation conditions. Some of the failures are associated with applying a technology to a site where the design parameters are inconsistent with site conditions; others may be the result of a lack of well-defined performance standards having good quality assurance and quality control metrics, or of poor construction practices. Undoubtedly, surface barriers will continue to play an important role in the future, but even the best design can fail if the barrier was installed and maintained incorrectly.
Subsurface barriers are likely to be effective as a temporary measure to prevent migration of contaminants of concern while more effective removal or neutralization technologies are developed and demonstrated. In addition, some subsurface barriers appear to offer the potential for long-term containment of contaminants. Subsurface barriers have been used in the private sector for nearly 30 years, and vertical cutoff walls have been constructed to depths of several hundreds of feet. The installation of subsurface horizontal barriers beneath large structures or contaminated areas (such as under a tank farm) is likely to challenge current installation technology.
Work is still- needed in the design of surface and subsurface barriers that would lead to more effective construction and testing, as well as minimizing costs without jeopardizing protection to the public and the environment. Some workshop participants suggested that research and development efforts in barriers need to be continued by DOE in areas such as (1) collection and use of both laboratory and field data to advance the development and application of mathematical models and to bring about greater confidence in model predictions regarding barrier system performance, and (2) techniques for monitoring migration of contaminants contained by barriers and for detecting defects in barriers.
Participants discussed the challenge of pursuing innovative containment technology within the DOE waste complex, addressing both regulator and stakeholder skepticism associated with unproved approaches, plus the need for selecting experienced contractors within the DOE procurement system. A participant noted that the industry is not sufficiently mature to enable companies to take legal responsibility for emplacement of barriers requiring long-term integrity. It was suggested that DOE and regulators might consider the approach taken in Europe, where the contractor accepts liability related to substandard performance of the containment system for a period of 10 years. Over this period, it is anticipated that the technology may improve such that further modifications to the system, if necessary, may act to ensure satisfactory performance for an extended time.
It was noted that data on the effective performance lifetime as a function of climate, hydrology, and geology should be compiled for selected barriers constructed of both natural and synthetic materials. Convincing scientific and engineering evidence that barriers retain their effectiveness over sufficiently long time periods is needed. A representative from the U.S. Nuclear Regulatory Commission reported that the agency is examining how much credit for isolation, as defined for regulatory purposes, can be given to various engineered barrier systems. Of concern to regulators and the public is the lack of available supporting technical bases and scientific proof of isolation.
The greatest chance of success for barrier deployment will result from use of proper installation techniques by contractors with demonstrated experience and skill, along with quality control and quality assurance measures. Successful installers may be able to provide some useful information to researchers, and vice versa, so that the technical engineering concept may be married to the construction process. Participants encouraged the collection and publication of case studies of valuable information on the performance of barrier systems that could be acquired by instrumenting existing barriers.
The summary of the National Research Council (1994) report on ground water cleanup noted the difficulty of cleaning up contaminated aquifers using pump-and-treat methods (pumping contaminated ground water to the surface for treatment). This presentation prompted a discussion of the causes of this difficulty, including inadequate technology, misapplication of existing technology, and lack of sufficient knowledge regarding the behavior of contaminants in the subsurface environment. The use of barriers to isolate materials in-place might be a reasonable
