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o . CM OZ2 CID San Joaquin County Public Health Services May 26, 1995 (9347-1246) • Replacing the oxidizer with an air diffuser system designed specifically to Increase the dissolved oxygen content of ground water may provide an effective and feasible method to enhance the attenuation of the remaining dissolved and adsorbed petroleum hydrocarbons Following is a discussion of the site background, remediation history and the proposed air diffuser system This correspondence is based, in part, on reports prepared by Delta Environmental Consultants, Inc Background Four former underground steel fuel storage tanks (UST) and product piping were removed from the site in November 1989 The steel UST were replaced with three double-walled fiberglass tanks, each of 12,000-gallon capacity The fiberglass tanks were installed in a new basin (Figure 2) on the north side of the site The upgraded tank system also includes double- walled fiberglass product lines, a fuel containment box at each fill port, and an electronic leak detection/prevention system Monitoring wells were first installed on the site In December 1989 and January 1990, • following the tank upgrade The depth to ground water at the site during remediation has averaged about 30 feet below surface The (vadose) soils above the saturated zone are composed primarily of silts and sands in varying proportions and have allowed the SVE system to operate very effectively The ground water gradient averages about 0 03 ft/ft and is typically to the north, towards well MW-10 off site The downgradient extent of the dissolved hydrocarbon plume has been defined and does not appear to be migrating The dissolved hydrocarbon concentrations in well MW-7 on the north side of the Shell site have been decreasing Well MW-10 was installed approximately 170 feet downgradient from well MW-7 to monitor any potential migration of dissolved hydrocarbons Well MW-10 has been below the respective laboratory detection limits for all petroleum hydrocarbons since July 1992 The SVE system is composed of seven venting/monitoring wells mamfolded to a thermal/catalytic-oxidizer emissions control unit, Model VAC 25, rated at a maximum airflow of 250 standard cubic feet per minute, manufactured by ThermTech Inc Aside from shutdowns for routine maintenance, the SVE system operated continuously from September 17, 1992 to March 5, 1993, and from April 9 to September 1, 1993 The oxidizer has been "pulsed", 1 e , cycled on and off from September 1993 to the present The total actual operating time of the oxidizer is 541 days, or 1 48 years Pulsing has increased the efficiency, or yield, of the SVE system by maximizing the recovery of petroleum hydrocarbons 93471246 LLT 2-