Approximately 100 hectares (250 acres) were investigated as part of the passive soil gas (PSG) investigation that included multiple areas of concern. The objective of the investigation program was to rapidly identify potential source areas and guide future sampling events based on the frequency and magnitude of compounds identified in the survey. PSG sampling was the preferred technology because it does not involve intrusive activities and allowed a comprehensive data set for evaluation before more invasive techniques were implemented. Vadose zone sources were detected using a well-executed and properly conducted non-intrusive PSG survey. A two-year field investigation allowed the development of a refined conceptual site model. The results show that historical disposal operations at this DOE facility resulted in the continuing presence of residual chlorinated contamination in the vadose zone that may present a potential threat for impacting groundwater. Confirmatory site investigation techniques revealed carbon tetrachloride soil gas concentrations at a depth of approximately 35 m (115 ft), near an area that is currently undergoing site remediation. The analysis of discrete soil samples taken in regions where significant soil gas concentration exists resulted in non-detections, indicating that dense non-aqueous phase liquid sources are not present at these locations.