The reduction in the trichloroethylene (TCE) vapor phase screening level by the United States Environmental Protection Agency (USEPA) in 2004 prompted a re-evaluation of groundwater contaminant source areas, transport mechanisms, and commingling of multiple CVOC plumes within a complex river basin at a confidential site in the Midwest. A USEPA Administrative Order on Consent (AOC) dictated that the former owner of the facility investigate and perform residential and commercial vapor phase removal action to achieve compliance with revised indoor air and subslab action levels. The AOC did not differentiate contaminant source areas, transport, or commingled contaminants. In response, a comprehensive re-evaluation of the river basin hydrogeology and groundwater chlorinated volatile organic compound (CVOC) distribution was completed to facilitate demarcation of the AOC vapor phase removal action boundary and to minimize cleanup of contaminants not attributable to the facility. CVOCs, including TCE, are hydrophobic compounds that are heavier than water which tend to persist as residual contamination in aquifer matrix and bleed off slowly over time into the groundwater. In 2007, an integrated investigation and review of remediation reports filed with state regulators, USGS hydrogeologic reports, and historical groundwater elevation data was conducted. The data were evaluated to identify additional CVOC source areas, map known CVOC plumes, establish groundwater flow transport pathways, and determine the potential for commingled CVOC plumes. Understanding the complex groundwater flow regime, strongly influenced by river stages, flood control structures, municipal well field production, and engineered recharge basins, was critical to resolving the migration pathway of multiple CVOC plumes. All data collected was compiled into a series of CVOC overlay maps to provide a working river basin model of CVOC distribution and migration based on groundwater flow. The distribution of CVOC source areas results in numerous instances of CVOCs plumes becoming commingled due to the groundwater flow patterns. As a result, the former owner recommended the reduction of the AOC vapor phase removal action boundary area by over 60%, thus limiting the action area to immediately downgradient of the facility based on groundwater flow while identifying additional potential responsible parties (PRPs) for future cost recovery actions.