time and appropriate methods for characterizing them are still evolving. Mitigation strategies such as subslab depressurization can prevent vapor intrusion exposure. As a precautionary measure, vapor mitigation could be built into all new construction on or near known VOC groundwater plumes. Vapor mitigation systems require monitoring over the long term to ensure that they are operating properly.

TECHNOLOGY DEVELOPMENT TO SUPPORT LONG-TERM MANAGEMENT

Despite years of characterization and implementation of remedial technologies, many complex federal and private industrial facilities with contaminated groundwater will require long-term management that could extend for decades or longer. Chapter 6 discusses technological developments that can aid in the transition from active remediation to more passive strategies and provide more cost-effective and protective long-term management of complex sites. In particular, transitioning to and improving long-term management can be achieved through (1) better understanding of the spatial distribution of contaminants, exposure pathways, and processes controlling contaminant mass flux and attenuation along exposure pathways; (2) improved spatio-temporal monitoring of groundwater contamination through better application of conventional monitoring techniques, the use of proxy measurements, and development of sensors; and (3) application of emerging diagnostic and modeling tools. The chapter also explores emerging remediation technologies that have yet to receive extensive field testing and evaluation, and it reviews the state of federal funding for relevant research and development. The following conclusions and recommendations are offered.

Long-term management of complex sites requires an appropriately detailed understanding of geologic complexity and the potential distribution of contaminants among the aqueous, vapor, sorbed, and NAPL phases, as well as the unique biogeochemical dynamics associated with both the source area and downgradient plume. Recent improvements to the understanding of subsurface biogeochemical processes have not been accompanied by cost-effective site characterization methods capable of fully distinguishing between different contaminant compartments. Management of residual contamination to reduce the exposure risks via the vapor intrusion pathway is challenged by the highly variable nature of exposure, as well as uncertain interactions between subsurface sources and indoor background contamination.

Existing protocols for assessing monitored natural attenuation and



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