Source Depletion at Contaminated Groundwater Sites: Is it Worth it?

In the late 1980s and early 1990s the research community and EPA began to focus on the constraints on aquifer remediation, particularly constraints related to Dense Non-Aqueous Phase Liquids (DNAPLs) as a secondary source of contamination at chlorinated solvent sites. By the late 1990s, a new paradigm was in general acceptance, where most chlorinated solvent sites were assumed to contain a difficult-to-remove, continuing DNAPL source, even at sites where DNAPL was no directly observed. The presence of this DNAPL presents a tremendous challenge "unprecedented in the field of groundwater engineering." One of the responses to this new paradigm has been to develop better technologies that can remove or destroy residual DNAPL and therefore deplete the source. A variety of methods have been tested, including thermal, chemical, oxidation, and biodegradation-based technologies. Despite the heavy financial investment in these approaches, there is an intense debate on whether these intensive source zone remediation technologies should be applied at many field sites. A U.S. EPA Expert Panel presented a qualitative decision process where the potential benefits from source depletion are evaluated using a "weight of evidence" approach. A list of potential benefits was presented, including: i) reduction of DNAPL mobility; ii) reduction of source longevity; iii) reduction of dissolved plume loading; iv) reduction of loading to receptors; iv) need to achieve cleanup quickly; and iv) intangible benefits. Quantitative results from a Department of Defense Strategic Environmental Research and Development Program (SERDP) project are presented to illustrate the performance of partial source depletion using thermal, chemical, oxidation, and biodegradation-based technologies compared to long-term containment. Planning-level models of source depletion at chlorinated solvent sites are used along with two new databases (a cost and performance database of source depletion case studies from the literature and a source response database from actual field sites) to compare the environmental and economic benefits and costs of partial source depletion vs. long-term containment.