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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 C DRILLING SUMMARY <br /> el <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 ground Nater monitoring wells were dnlled 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 /> Exploration of Stockton, California, under a C57 license, using a hollow-stem auger drill rig. <br /> 5.1 Sampling Procedures <br /> �a All borings 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 sails <br /> encountered and monitoring well construction details were recorded on bonnglweli logs which <br /> are included in Appendix If <br /> cs..m V"hn xr <br /> MUM 3 <br />