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3. GROUNDWATER ASSESSMENT 3.1 SIITE AND LOCAL HYDROGEOLOGY Chevron SS 9-1918 is located in the northern end of the San Joaquin Valley. This valley is part of the larger geological province known as the Great Valley. The subsurface at Stockton is composed primarily of thick sequences (approximately 1,200 feet thick)of unconsolidated Miocene-Pliocene to Recent sediments deposited by the San Joaquin River. The sediments in the Stockton area may be divided into the Mehrten, Laguna,and Alluvium- Victor formations, from the oldest to youngest. The (Recent) Alluvium and Victor Formation that directly underlies the site generally consists of unconsolidated gravel,sand, silt,and clay deposits with extensive sand and gravel stringers (DWR 1955). In certain areas, however, almost the entire Victor Formation consists of clayey sediments to a depth of 150-200 feet(DWR 1967). A geologic cross-section through the area about 1.5 miles south of Chevron SS 9-1918 (Figure 4)shows clayey and silty sediment(Alluvium and Victor Formation) to a depth of about 150 feet(DWR 1967). Groundwater in the Stockton area occurs in recent sediments,stored in discontinuous sand and gravel layers separated by variable thicknesses of clay,silt,and sandy silt. Groundwater in this deposit is largely unconfined, although local pressure effects may occur where silt and clay layers are extensive. The San Joaquin County semi-annual groundwater report for Fall 1989 indicated a depth to groundwater under the investigated area of about 40 feet(Figure 5)and a groundwater flow direction of east-northeast at a gradient about 20 ft/mile(0.0038). The geology beneath former Chevron SS 9-1918 consists of three different units of unconsoli- dated sediments (Figures 6 and 7). The uppermost unit is a section of silty and sandy clays, along with silts from I0 to 20 feet thick. Groundwater appears to be contained in lenses of sand and silty sands below the shallower clays; this sand occurs at depths below the ground surface from 14 to 43 feet. The sand is immediately underlain by an olive gray clay that is very tough and plastic,and apparently less permeable than either of the overlying layers. Figure 8 shows in perspective view the topography of this lower clay under the site. Details of site geology are in the well logs (Appendix Q. Figure 9 shows isocontours of groundwater elevation on I3 July 1989. The direction of groundwater Plow under the site re- mained generally northeast for as long as it could be determined by well gauging. Between July and October 1989 all but one well went dry. Well gauging data are summarized in Table 3. The wells were_dry_when gauged on 22 March 1991,despite the recent rains. c7g91919,41 5