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ATTACHMENT A SUMMARY OF PREVIOUS INVESTIGATIONS Chevron 9-3232, 8660 Lower Sacramento Road, Stockton, California According to the data provided by Chevron, the service station was constructed in 1967 Subsurface facilities consisted of one 5,000-gallon and two 10, 000-gallon steel gasoline underground storage tanks (USTs), and one 500-gallon steel waste oil UST In August 1983, the original USTs were replaced with three 10,000-gallon single-walled fiberglass gasoline USTs and one 1,000-gallon single-walled fiberglass waste oil UST Data regarding subsurface conditions at the time of UST replacement are not available In December 1993, the dispensers and product lines were removed The dispensers were replaced and the product lines were upgraded from single-walled fiberglass to double-walled fiberglass The USTs were not removed or upgraded Soil samples were collected from beneath the product lines and the waste oil UST remote fill pipe Total petroleum hydrocarbons quantified as gasoline (TPHg) were detected in one soil sample from beneath the product lines at a concentration of 7 milligrams per kilogram (mg/kg) Benzene was detected in one sample at a concentration of 0 81 mg/kg Lead was detected in three of the five soil samples at concentrations ranging from 5 mg/kg to 6 mg/kg The soil sample collected beneath the waste oil UST remote fill pipe contained TPHg at 3 mg/kg Total petroleum hydrocarbons quantified as diesel (TPHd) were also detected in this sample at a concentration of 350 mg/kg, along with oil and grease (O&G) at a concentration of 1,700 mg/kg and trichloroethane (TCE) at a concentration of 0 037 mg/kg In October 1998, Gettler-Ryan (GR) installed three groundwater-monitoring wells (MW-1 through MW-3) The soil samples analyzed from well bonngs MW-1 and MW-3 did not contain detectable concentrations of TPHg, benzene, toluene, ethylbenzene, and total xylenes (BTEX) or methyl tert-butyl ether (MtBE) with the exception of the sample from boring MW-3 at 49 5 feet below ground surface (bgs) TPHg concentrations ranged from 2 94 to 593 mg/kg, and benzene concentrations ranged from 0 184 to 6 4 mg/kg On November 4, 1998, groundwater was recorded at depths between 31 93 and 36 07 feet below top of well casing The flow direction was to the southwest at a gradient of 0 05 ft/ft Groundwater samples from each of the wells contained concentrations of TPHg and BTEX Well MW-2, located down gradient of the dispenser islands had the highest concentrations of TPHg (120,000 micrograms per liter [pg/L]) and benzene (23,000 µg/L) Fuel oxygenate compounds were not detected in any of the groundwater samples The gasoline and waste oil USTs were removed on March 11, 1999 Petroleum hydrocarbons were not detected in the samples collected beneath the dispenser islands or the product piping TPHg was detected in soil samples collected beneath the middle gasoline UST The highest concentration of TPHg was detected in the sample from beneath the southern end of the middle UST at 19 feet bgs (680 mg/kg) A sample from 21 feet bgs in this area contained 51 mg/kg of TPHg MtBE was detected in eight of the nine soil samples collected beneath the gasoline USTs at beneath the gasoline USTs at concentrations between