A hydrogeologic investigation, groundwater monitoring, Remedial Action Plan (RAP) preparation, and remedial implementation activities were completed by Cameron-Cole at this St. Louis, MO, site from 1995 to 2005. Two former gasoline stations operated on leased properties at the site. Three groundwater plumes (Plumes 1 through 3) containing benzene, toluene, ethylbenzene and xylenes (BTEX) resulted from leaking underground storage tanks. Following the installation of a groundwater monitoring well network and extensive soil and groundwater sampling, a RAP for a dual-phase extraction system, the most cost-effective and technically effective remedial option for the site based on numerical fate and transport modeling (MODFLOW/MT3D), was prepared and accepted by the lead regulatory agency, the Missouri Department of Natural Resources (MDNR). The detailed chemical transport modeling and risk characterization in the RAP was used to determine health-based groundwater cleanup levels and the time necessary to attain groundwater cleanup standards. The fate and transport modeling of the current system performance indicated that benzene concentrations would reach concentrations below the MDNR LUST cleanup guideline of 5 ug/L within a reasonable time frame. The dual-phase extraction system was installed, consisting of a Soil Vapor Extraction (SVE) component and a Free-Product Recovery (FPR) component to address the on-site BTEX plumes. BTEX concentrations and lateral extent were reduced significantly during the eight-year operation of the treatment system. The selection of the most cost-effective remedial option (dual-phase extraction) combined with the regulatory approval resulted in estimated cost savings of $250,000 compared to conventional pump-and-treat, In Situ Chemical Oxidation (ISCO), and In Situ Bioremediation (ISBR), and at the same time addressed the separate phase hydrocarbons (free product) present on site.