It is estimated (EPA, 2003) that a total of about 15,000 to 25,000 sites in US are contaminated with significant amounts of dense nonaqueous phase liquids (DNAPLs) that entered the subsurface as a result of either accidental spills or inadequate storage/disposal practices. Extensive contamination of groundwater has been observed at numerous sites, including industrial facilities (e.g., manufacturing sites), federally owned properties (e.g., military bases) and urban sites (e.g., drycleaner operations). DNAPL vapour intrusion into basements is also a major concern in some cases when the DNAPL source is within the vadose zone or the contaminated plume is in unconfined, shallow groundwater. Life-cycle costs for clean up of DNAPL sites may range from $50 to $100 billion dollars (EPA, 2003). Thus, DNAPL sites pose an important and interesting case study for characterization and cleanup of contaminated sites.
Federal and state regulatory statutes in the US drive the remediation targets for cleanup of DNAPL sites. Returning the contaminated ground water to maximum beneficial uses in a reasonable time frame and achieving pollution prevention are the societal goals. Source control is a critical component of contaminated site cleanup. Several field-scale demonstrations have shown that a variety of in-situ remediation technologies can extract or destroy a large fraction (70 to 90+%) of the contaminant mass in source zones. Recent analyses (EPA, 2003) also suggested that at many DNAPL sites, the remedial goal of achieving drinking water standards (i.e., Maximum Contaminant Levels, MCLs) may be impractical, and pointed to the need for consideration of alternative remediation endpoints and the move towards adoption of more rational performance metrics.
Several technical and policy questions are being debated regarding the utility or futility of attempts to aggressively treat DNAPL source zones. Can the spatial extent and contaminant mass distribution present in the source zone be reliably characterized? If so, what fraction of the DNAPL mass should be extracted or destroyed? Which source cleanup technologies perform the best? Which site attributes are useful in optimizing the design and implementation of source remediation? What benefits in risk or liability or cost reduction can be achieved from partial mass depletion of the source? What are the likely negative impacts of aggressive source remediation attempts? What are the appropriate metrics for site evaluation and remedial performance? Are appropriate methods available to measure these metrics? What are the long-term stewardship needs at DNAPL sites where the source is either partially depleted or physically contained?
These questions will be explored, using recent results from theoretical analyses and data from field studies, to assess the benefits of partial source depletion at DNAPL sites. New approaches that are being developed and field tested for establishing the source and plume strength as a robust metric for site assessment and remediation effectiveness will be presented. Model simulations and lab/field data suggest that depletion of DNAPL source mass can lead to short-term benefits that include a reduction in the source strength (i.e., integrated contaminant flux). The magnitude of such reduction depends on the site conditions and the remediation technology used at the site. Long-term, down-gradient impacts of source-strength reduction, including reduction in risks and overall site care costs, are a function of the biogeochemical processes occurring within the dissolved plume. It is necessary to couple the source and plume simulations with both cost analyses and risk analyses to estimate if the reduction in risk levels is justified by the cost involved in source remediation.