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1 i r Chevron Products Company 2 Chevron SS#9-8264,3775 Tracy Boulevard,Tracy,CA December 29, 1997 Site Background and Previous Investigations The site is an operating Chevron Service Station located in a primarily commercial area on the west side of Tracy Boulevard in Tracy, California. An operating Shell Service Station is located directly south of the site and a former Unocal Service Station was located east (across Tracy Boulevard) of the site. 1 In November 1995, Touchstone Developments oversaw the excavation and removal of the USTs , hydraulic hoists, sump, product dispensers and associated product lines. Soil samples were collected from beneath the USTs, hydraulic hoists, sump, product piping, and dispensers. The highest concentrations of hydrocarbons (830 and 420 ppm TPH-g) were reported near the center and northeast corner of the former UST pit at depths of 9.5 to 12.5 feet bgs. TPH-g concentrations of 1.2 to 66 ppm were reported beneath the product lines. Concentrations of oil and grease in soil beneath the hoist and sump 1 ranged from 91 to 120 ppm, and were reported nondetectable beneath the used oil tank. In February 1996, Touchstone Developments drilled eight onsite borings and installed four monitoring wells in four of the eight borings to an approximate depth of 22 feet below ground surface (bgs). Laboratory analysis of soil samples reported nondetectable levels of total petroleum hydrocarbons calculated as gasoline (TPH-g) in all soil f samples collected from the borings, with the exception of 2.6 ppm and 13 ppm reported in samples from Boring B-1 at 5 and 9.5 feet bgs respectively. MtBE was reported in 5 of the 17 samples analyzed at concentrations ranging from 0.053 to 0.23 ppm. BTEX concentrations were reported from nondetectable to 0.20 ppm. Results of groundwater "grab" sampling from Borings B-1 through B-4 indicated TPH-g concentrations from nondetectable to 890 ppb, and MtBE at concentrations from nondetectable to 1,500 ppb. BTEX concentrations ranged from nondetectable to 150 ppb, and diesel was reported at nondetectable to 580 ppb. Initial groundwater sampling of Wells MW-1 through MW-4 indicated nondetectable concentrations of all analytes, with the exception of 140 and 91 ppb MtBE in Wells MW-3 and MW-4 respectively, and 1.8 ppb toluene in Well MW-4. Depth to groundwater in May 1996 was reported at approximately 9 to 10.4 feet bgs. Wells MW-1 through MW-4 have been sampled quarterly by Blaine Tech Services since May 1996. Concentrations of TPH- and BTEX have remained nondetectable in Y g all four wells. MtBE concentrations have remained nondetectable in Wells MW-1 and MW-2 through August 1997. MtBE has been reported from 32 to 780 ppb in Wells MW-3 and MW-4. M �