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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 East 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 /> M 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 /> "J 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 /> Exploration of Stockton, California, under a C57 license, using a hollow-stem auger drill ng <br /> 5 1 Sampling Procedures <br /> All bonngs were sampled at 5-foot depth intervals, beginning at a depth of 5 feet, using a <br /> 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 /> 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 bonng/well logs which <br /> are included in Appendix Il <br /> cb—u 3&In. seg <br /> CUM 3 <br /> r <br />