Abstract

A subsurface release of unleaded gasoline was discovered at an operating retail gasoline station in southwestern Pennsylvania during due diligence activities conducted in September 2003. Site characterization activities concluded that dissolved phase unleaded gasoline constituents (specifically benzene, ethylbenzene, naphthalene, and methyl tertiary-butyl ether [MTBE]) were present at concentrations above the Pennsylvania Statewide Health Standards (SHS) Medium-Specific Concentrations (MSC) in the uppermost water-bearing unit underlying the property. Based on pilot testing results and aquifer characterization properties, traditional remediation techniques (i.e., vacuum extraction, pump & treat, etc.) with ex-situ treatment were determined not to be feasible for this low-yielding unconsolidated aquifer. Supporting evidence of biodegradation was present; however, anoxic conditions (i.e., oxygen depleted) were present in the aquifer, and the availability of electron acceptors was limited in areas of constituent impact. Ozone sparging with enhanced bioremediation was determined to be a practical, cost-effective remedial technique for the site conditions. The ozone sparging treatment process consists of a combination of chemical oxidation destruction (primary remedial mechanism), followed by enhanced natural bioremediation (secondary remedial mechanism). A KTI C-Spargerâ unit was installed with ozone distributed through seven spargepoints that began operations in July 2005. Reductions in benzene concentrations from a maximum of 4,400 micrograms per liter (m g/L) to less than the SHS MSC (5 m g/L) have been achieved for groundwater within approximately 19 months. MTBE (maximum 2,600 m g/L), ethylbenzene (maximum 3,000 m g/L), and naphthalene (maximum 840 m g/L) have all been reduced to concentrations below the SHS MSC or laboratory detection limits within approximately 12 months of system operation. The project is approaching closure through a risk-based approach with an active remediation life cycle of only approximately two years with minimal operations and maintenance.