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Ir � ERI 205503 R02 Tosco 76 Service Station 5886,Stockton,California March 2,1999 • Submit groundwater samples for analysis for total purgeable petroleum hydrocarbons as gasoline (TPPHg), total extractable petroleum hydrocarbons as diesel (TEPHd), benzene, toluene, ethylbenzene, and total xylenes (BTEX), MTBE, ethanol, tertiary butanol, di-Isopropyl ether (DIPS), ethyl tertiary butyl ether (ETBE), and ternary amyl methyl ether {TAME} • Prepare and submit a report presenting the conclusions The County approved the amended Work Plan and the proposed CPT locations (provided In ERI's January 28, 1998, Quarterly Summary Report) in a letter dated January 14, 1999 (Attachment A) BACKGROUND The site is Immediately west of Interstate I-5 on the northwestern corner of March Lane and the I-5 offramp in Stockton, California, as shown on the Site Vicinity Map (Plate 1) The location of the existing underground storage tank (UST) field, dispenser pump islands, other selected site features, and surrounding properties are shown on the Generalized Site Pian (Plate 2) Properties in the vicinity of the site are generally occupied by commercial developments It is ERI'S understanding that Union Oil Company of California (Union Oil) purchased the land in 1967, at which time the property was vacant In 1982, Unocal constructed a service station at the site and installed. three 12,000-gallon UST's to store gasoline and diesel fuels (Delta Environmental Consultants, Inc [Delta], February 6, 1996) To date, environmental work at the site has included the following . • Removal and replacement of USTs, product lines, and dispensers • Removal and disposal of approximately 1,200 cubic yards of soil at Forward Landfill in Manteca, California • Completion of a well survey within a 1/a mile radius of the site • Drilling 32 soil borings (21 Geoprobe®borings, eight soil borings, and three hand-auger borings), and installing three single-casing groundwater monitoring wells and four dual-casing groundwater monitoring wells • Performing a tank integrity test • Performing quarterly groundwater monitoring and sampling INTERIM INVESTIGATION Prior to the January 14, 1999, County approval of the proposed work scope, Tosca directed ERI to perform the following Items proposed in the Work Plan and Addendum to the Work Plan This work was completed between July 1997 and December 1998 Evaluation of MTBE Data from Previous InvestigationN ERI requested Sequoia Analytical Laboratory (Sequoia) provide estimates of MTBE concentrations in water samples collected from Geoprobe3 borings (Delta, February 6, 1996) Sequoia personnel indicated the chromatograms of the water sample analyses were unavailable since the data had been • 2