extraction (including thermal enhancements), air sparging, and in situ oxidation. In addition, several technologies have potential applicability primarily for aqueous-phase contamination dissolved from DNAPLs. These technologies include various bioremediation and reactive barrier wall systems, in addition to conventional pump-and-treat systems.

Recommendation: Although the potential of the DNAPL remediation technologies listed above has been demonstrated, SCFA should fund additional, carefully controlled tests in conjunction with external technology developers (where appropriate) to provide cost and performance data to facilitate application of these technologies. There is much uncertainty in technology evaluations due to the limited amount of high-quality data, and SCFA can play a key role in generating the needed data.

Finding. Technologies for treating contaminant mixtures are in short supply, and the efficacy of many technologies for treating mixtures has not been established. The permeable reactive barrier is the most promising method for preventing the migration of mixtures of dissolved metals, radionuclides, and DNAPL components under appropriate conditions, but the longevity of barrier materials needs to be established.

Recommendation: SCFA should fund tests designed to develop and determine performance limits for technologies capable of treating the types of contaminant mixtures that occur at DOE sites. In particular, SCFA should continue to fund studies of the longevity of reactive barriers in terms of reactivity, permeability, and integrity.

Finding. Removing all sources of groundwater contamination, particularly DNAPLs, will be technically impracticable at a number of DOE sites, and long-term containment systems will be necessary for these sites. Methods will be needed to monitor the performance of containment barriers, because the longevity of barrier materials is uncertain. Electrical resistance tomography methods have received a significant amount of SCFA funding for studies of the integrity of subsurface barriers (including reactive barriers and conventional containment systems), but these methods have considerable limitations for this application.

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