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1.3 Project Background A retail gasoline outlet has been located at this site since at least 1983. BP Oil Company acquired the site from Mobil Oil Corporation in 1989 and subsequently sold the site to Tosco in 1994. Tosco operated the site under the BP brand before the station was re-branded as a Union 76 outlet, and is the current owner of the facility. In preparation for the sale of the facility, BP Oil installed five monitoring wells in 1992 to obtain baseline chemical data. The investigation was not prompted by the suspicion that a release had occurred because tightness testing results and inventory reconciliation data were known to be within acceptable tolerances. Soil samples collected in the immediate vicinity of the underground fuel storage tanks, a used oil storage tank, and dispenser islands were reported to contain petroleum hydrocarbons at concentrations of up to 120 milligrams per kilogram (mg/kg)total petroleum hydrocarbons as gasoline (TPH-G) and 16 mg/kg benzene. The initial groundwater samples collected from Monitoring Wells MW-3 and MWA had reported concentrations of up to 10,000 micrograms per liter (ug/1) TPH-G and 6.7 ug/1 xylenes (Resna, 1993). In September 1993, two groundwater monitoring wells,MW-6 and MW-7,were installed. Soil samples collected at a depth of 6 feet were not reported to contain petroleum hydrocarbons (Alisto, 1994a). Since 1993, the groundwater monitoring wells have been sampled on a periodic basis. Depth to groundwater has ranged from 6 to 12 feet below grade, and groundwater elevation has ranged from 3 feet below mean sea level (MSL) to 3 feet above MSL. The hydraulic gradient is typically flat; on the order of 0.001 feet per feet or less, and the direction of groundwater flow has been variable. Appendix A presents the groundwater flow maps included with the quarterly groundwater monitoring reports (Alisto, 1993, 1994b, 1995, 1996, 1997, and 1998b). The results of groundwater and soil analysis performed to date are summarized in Tables 1, 2, and 3. 1.4 Other Chemical Release Sites Review of regulatory agency files revealed the following two nearby properties with reported chemical releases: • In 1987, three wells were installed after removal of a used oil tank at an Arco service station,located approximately 100 feet north of the BP Oil site. Petroleum hydrocarbons were detected at concentrations of up to 29 parts per million (ppm)in two soil samples, and total lead at concentrations of up to 12 parts per billion (ppb) in groundwater samples collected at the site. The groundwater gradient direction at the Arco service station has varied from southwesterly to southeasterly (Brown and Caldwell, 1992). Mr. Michael Infurna of SJCDEH indicated on April 10, 1998, the regulatory agency granted the site case closure and all associated monitoring wells have been destroyed. • The J. R. Simplot Company (formerly Occidental Chemical Agricultural Products, Inc.) operates a fertilizer manufacturing and formulation plant approximately 1 mile east- southeast of the BP Oil site. The groundwater remediation system,which currently includes more than 20 groundwater monitoring,recovery, and reinjection wells, was 3 R/*\