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The Bioremediation of DNAPLs (BioDNAPL) Team has published In Situ Bioremediation of Chlorinated Ethene: DNAPL Source Zones (BIODNAPL-3), a guidance document that focuses on in situ bioremediation (ISB) of chlorinated ethene dense, nonaqueous-phase liquid (DNAPL) source zones in the saturated subsurface. A DNAPL source zone can act as a reservoir that sustains a contaminant plume in groundwater over the long term. ISB increases contaminant removal by enhancing dissolution and/or desorption of nonaqueous- and/or sorbed-phase contaminant mass and promoting subsequent biological degradation to nonchlorinated, nontoxic end products. This can help to achieve the goals for DNAPL source treatment, which are to reduce the mass of contaminants within the source area and to minimize contaminant migration above unacceptable levels. While ISB is an established technology for treating dissolved-phase chlorinated ethenes in groundwater, its use for DNAPL source zones is an emerging application. Based on a review of relevant case studies, the BioDNAPL Team concluded that ISB of DNAPL source zones is a viable technology and can be an effective component of a treatment plan. BIODNAPL-3 provides a systematic understanding of the technical and regulatory considerations for the use of ISB at DNAPL source zone sites. The document includes information related to site characterization requirements, application and design criteria, process monitoring, and process optimization and summarizes six case studies of ISB at DNAPL sites.