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_ 4.0 HYDROGEOLOGY <br /> The soils in the vicinity of the Chevron site have been mapped by the U.S.D.A. Soil <br /> Conservation Service as Stockton clay. Stockton clay is a fine-textured, dark grey to black clay <br /> deposited by quiescent or slow moving water. Soils encountered during drilling consist <br /> primarily of lean clay to 20 feet, sands and silts to 35 feet, and lean clay to total depth. <br /> The site is located in the Stockton Fast Water District According to the Semi-Annual <br /> Groundwater Report by the San Joaquin County Flood Control and Water Conservation District <br /> for Spring 1990, the regional groundwater gradient is to the east - northeast (Figure 4). The <br /> closest surface water feature is Mormon Slough approximately one mile to the south of the site. <br /> 5.0 DRILLING SUMMARY <br /> Chevron U S.A. Inc. completed the following worksteps to provide additional 'information <br /> regarding subsurface conditions at the facility. To assess current groundwater quality beneath <br /> the site, three groundwater monitoring wells were drilled on April 29 through May 1, 1991 <br /> • (Figure 5). The wells were situated to provide information regarding groundwater quality and <br /> flow direction beneath the site. One monitoring well was located southeast of the former fuel <br /> storage area to monitor upgradient groundwater quality Two monitoring wells were located in <br /> the assumed downgradient direction of the former tank pit Drilling was conducted by Spectrum <br /> I <br /> Exploration of Stockton, California, under a C57 license, using a Hollow-stem auger drill ng. <br /> r <br /> l 5.1 Sampling Procedures <br /> All borings were sampled at 5-foot depth intervals, beginning at a depth of 5 feet, using a <br /> E modified California split-spoon sampler with brass liners A liner from each split-spoon sampler <br /> was sealed at each end with a Teflon sheet and plastic cap. The cap was secured with <br /> I polyethylene tape and the sample was preserved on ice Drill cuttings were logged continuously <br /> by a qualified geologist using the Unified Soil Classification System. Descriptions of soils <br /> encountered and monitoring well construction details were recorded on bonnglwell logs which <br /> are included in Appendix H. <br /> i <br /> f <br /> chr"M U.S.&me.-761 <br /> LCSA079[ <br />