2. The Source Zone Remediation Challenge

Remediating groundwater contaminated by chlorinated solvents in the form of dense nonaqueous phase liquids (DNAPLs) poses major technical, economic and institutional challenges. This chapter summarizes those challenges and discusses the developments needed to improve management of DNAPL source zones. There are over 10,000 such sites in the United States, and only a few examples of cleanup to levels allowing unrestricted use and exposure. DNAPLs migrate downward through aquifers in complex patterns. Over time, the solvents can diffuse into low permeability materials, and the later back diffusion can contaminate groundwater for decades or even centuries. Finally, typical cleanup levels are 5 to 6 orders of magnitude below the solubilities, so complete remediation of a source often requires removal or destruction of over 99% of the mass. Such partial mass depletion from DNAPL source zones has been a viable remediation strategy at certain sites, but it has proven difficult to decide when and how to undertake DNAPL depletion, and how best to transition DNAPL sites to passive long term management.