Chlorinated solvents have been used extensively in industry and are common groundwater contaminants. Their limited aqueous solubility often leads to the presence of dense non-aqueous phase liquid (DNAPL) source zones, which may persist due to mass transfer limitations from the DNAPL to groundwater. While recent research effort has been directed to the development of effective source zone remediation methods, considerably less attention has been given to establishing reliable methods to quantify the progress toward, and achievement of, remediation targets. At many sites, particularly early in the release history, a significant proportion of the mass is present as a DNAPL. Evaluating remediation against cleanup objectives therefore requires quantification of DNAPL mass depletion. Current methods typically fail to provide a complete description of the total DNAPL mass. Furthermore, uncertainty in DNAPL mass estimates are rarely quantified, despite having significant bearing on whether the remediation has met the clean up criteria. Here we describe the development of an uncertainty-based performance metric for quantifying the treatment of a trichloroethene DNAPL source zone. Conclusions and recommendations are presented by reference to lessons learned at the Source Area BioREmediation (SABRE) site and from current literature and best practice.