An optimized “Three-Dimensional Compound Specific Isotope Analysis (3D-CSIA)'' investigation was conducted at a chlorinated hydrocarbon–contaminated site in order to (1) determine if multiple onsite sources of groundwater contamination existed and (2) demonstrate the cost-effectiveness of applying isotope fingerprinting at such a complex contaminated site. Previous groundwater investigations identified chlorinated hydrocarbons at levels that significantly exceed drinking-water standards but failed to determine the source(s) of contamination due to the lack of vadose-zone contamination and the absence of groundwater contaminants in shallow portions of the surficial aquifer. To better understand the contaminant source(s), groundwater samples were taken and tested for both the presence of chlorinated hydrocarbons and their isotopic signatures of 13C/12C, 37Cl/35Cl, and 2H/1H. A site investigation with an optimized 3D-CSIA approach revealed multiple chlorinated hydrocarbon releases from different sources, which was also cost-effective considering the new lines of evidence of target contaminants obtained with the 3D-CSIA approach instead of any traditional fingerprinting approaches. In addition, the 3D-CSIA results inferred in situ bioremediation of chlorinated hydrocarbons would be feasible at the site. © 2013 Wiley Periodicals, Inc.