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Section 1 INTRODUCTION In April 1997, Pacific Gas and Electric Company (PG&E) initiated a program to investigate subsurface soil and groundwater at approximately 75 gas gathering facilities. The purpose of the investigation was to evaluate the presence or absence of petroleum hydrocarbons (Typically lubrication oil or gas condensate) in the subsurface at each facility. The Central Valley Regional Water Quality Control Board (CVRWQCB) was the lead oversight agency for this effort. The results of the investigation indicated that some sites exhibit petroleum hydrocarbons in subsurface soil or groundwater that appear to be related to releases or leaks from historic operations. Furthermore, where hydrocarbon concentrations exceed the cleanup levels established by the CVRWQCB, PG&E may need to implement remedial actions to reduce hydrocarbon concentrations. PG&E is therefore interested in identifying effective in-situ remedial strategies for active sites where excavation or other intrusive options are considered too disruptive for on-going site operations. PG&E is especially interested in identifying strategies with minimal operation and maintenance requirements for remote location sites that may not have electricity or water supplies. One of the facilities investigated during this program is PG&E's former McMullin Gas Dehydrator Station, located on Airport Road near Manteca, California (Figure 1). The McMullin Gas Dehydrator Station has also been the site of two additional site investigations; a soil investigation performed earlier in 1992 and a comprehensive soil/groundwater investigation performed in November and December 1997. Results of the site characterization activities show that soil and groundwater beneath the site are impacted by petroleum hydrocarbons. PG&E wishes to utilize the McMullin site to demonstrate the viability and effectiveness of an in-situ remedial strategy to accelerate the biodegradation of subsurface petroleum hydrocarbons that, if effective, may be useful at other similar facilities. Several studies have demonstrated that most petroleum hydrocarbons residing in the subsurface will degrade under aerobic conditions. The lack of oxygen has been observed to be the most common factor limiting biodegradation rates in the subsurface. Studies using oxygen releasing compound (ORC) in the saturated zone have shown that its application can result in elevated dissolved oxygen concentrations on the order of 20 ppm and TPH biodecay rates of about 1% per day (Chapman and others, 1997; Data on 402 331-98 63 1-1