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1 . 3 HYDROGEOLOGY AND WATER USE The estimated depth to ground water at the site, based on regional information, is approximately 18-20 feet ( Sabiniano 1988, personal communication ) . The regional direction of ground- water flow is northerly ( SJCFCWCD 1985 ) . There is no site- specific information on the hydrogeology of the site. The investigated site is located in the northern part of the San Joaquin Valley, a broad structural trough drained by the San Joaquin River. This structure is underlain by a complex of pre- Cretaceous rock. Basement rocks are overlain by consolidated-to- semiconsolidated Cretaceous and Tertiary deposits and unconsoli- dated continental sediments of Quaternary age. The sediments that are most likely to be directly under the site are alluvial fan deposits consisting of poorly sorted sand, silt, clay, and gravel, with moderate to low permeability. These mate- rials belong to the Victor Formation, of Pleistocene to recent age, and might be 100 to 150 feet thick. Ground water is extensively used for municipal, private domestic, and irrigation in San Joaquin County. In particular, Lathrop has three city wells which supply domestic water for the city' s 1 , 500 households (Reed 1988, personal communication ) . The wells are located more than 0 . 5 mile east of the site, across the Southern Pacific railroad. A number of private wells are used for live- stock watering and irrigation. The locations, pumping rates, and construction details of these wells are considered confidential by the San Joaquin County Environmental Health Department (Wilson 1988 , personal communication ) . 1 . 4 PRINCIPLES OF SOIL VAPOR CONTAMINANT ASSESSMENT The subsurface environment has three basic compartments, soil, air-filled pores, and water-filled pores. Following a subsurface c40/90265 2