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collected-in the immediate vicinity,of the underground fuel storage tanks, used oil storage, tank, and dispenser islands did not detect total,petroleum hydrocarbons as gasoline (TPH-G) above the reported detection limit. Benzene was detected in, two soil samples at . 'concentrations.of up to 0.12 parts per' million (ppm) (Resna, 1993). Analysis of groundwater samples collected during subsequent monitoring events in 1.993 and-1994 detected concentrations of up to 8,700 parts per billion (ppb) TPH-G and 3200 ppb benzene 1 (Alisto, 1993/1994). , 1.4 Regional Geolod and Hydrogeolou- ► ' The site is situated in the Central Valley, which comprises the lowlands between the Sierra Nevada to the east and the coastal range to the west.' As a structural trough, the Central Valley received sediment from the adjacent mountains under both marine and continental conditions. The Sierra Mountains are the source area of silt, sand, and gravel in alluvial fans along the valley margins. The topography of the site vicinity slopes gently to the southwest (Page, Ben-M., 1966). 2.0 FIELD METHODS The procedures and methods used,during field activities are described in the following sections: 2.1 Drilling and Sampling Before drilling, a well installation permit was acquired from the SJCEHD, and an encroadiment permit,was acquired from the City of Stockton. Copies'of the permits are presented in Appendix A. On March 30, 1994, Borings' MW-5, MW-6, and MW-7 were' drilled on and offsite to a depth of 32 feet below grade. Drilling was performed by Soils Exploration Services, Vacaville, California, using a CME 75 drilling rig,equipped with 8-inch- diameter, hollow-stem augers. Drilling and soil sampling procedures are presented in , 1 Appendix B. Boring logs were prepared using the Unified Soils Classification System, which includes a description of soil characteristics.such.as color, moisture, and consistency. The boring logs are presented in Appendix C. 2.2 Monitoring Well Installation.and Construction On March 30, 1994, Soil Borings MW-5, MW-6, and MW-7 were converted into Monitoring Wells MW-5, MW-6, and MW-7 in accordance with the field procedures for groundwater.. monitoring well installation presented in'Appendix B. The monitoring-wells were constructed using 2-inch-diameter, flush-threaded, Schedule 40, polyvinyl chloride blank casing and 0.010-inch slotted casing. .The slotted casing was installed from approximately ' .12-1/2-and 32 feet below grade. Well construction details are included on the boring logs presented in Appendix C. , \o\iaise\ss[.xroexx 2 Nel