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PROBLEM ASSESSMENT REPORT <br /> ' Moreland Court Shell <br /> 7700 Moreland Court,Stockton, California <br /> Delta Project No 40-88-146 <br /> Page 12 <br /> 1 <br /> 2 8 Regional Geology and Rydrogeology <br /> ' Stockton lies within the Great Valley geomorphic province The regional geology consists of unconsolidated <br /> deposits of sand, silt, and clay which were deposited in fluvial and deltaic environments <br /> A regional map showing the ground water elevation contours for the unconfined aquifer for Spring 1989 <br /> suggests ground water is 60 to 70 feet below ground surface and generally flows towards the west <br /> ' 3 0 HYDROGEOLOGIC INVESTIGATIVE RESULTS <br /> On August 1 and 2, 1990,Delta completed three additional soil borings to various depths and converted one <br /> soil boring into a monitoring well (MW-1) Soil boring and monitoring well locations are illustrated in <br /> rFigure 3 Boring logs for monitoring well MW-1 and soil borings SB-9 and SB-10 are provided in <br /> Appendix D <br /> 31 Site Geoloy <br /> Subsurface geology at the site has been inferred from drilling 11 soil borings at the site Soil borings B-1 <br /> through B-6 were drilled during pretank closure assessment, soil borings SB-7 and SB-8 were advanced <br /> during the Phase I investigation, and soil borings SB-9, SB-10, and monitoring well MW-1 were advanced <br /> 1 during the preliminary report implementation <br /> ' Two cross-sections were generated to illustrate the subsurface geology Figure 4 shows the surface trace of <br /> cross-sections A-A' and B-B' which are presented in Figures 5 and 6 Silts and clays predominate from <br /> ground level to approximately 25 feet below grade,whereby a very wet silty sand to sand is encountered and <br /> appears to be continuous across the site This silty sand to sand layer was encountered in soil boring SB-7 <br /> ' at a depth of 29 feet, in soil boring SB-8 at a depth of 24 feet,in soil boring SB-10 at a depth of 35 feet,and <br /> in soil boring SB-9 and the soil boring for monitoring well MW-1 at a depth of 30 feet below grade From <br /> ' 35 to 70 feet, silty clays and sands predominate in soil boring SB-9 Soil boring SB-9 was the only boring <br /> to be drilled past 415 feet <br /> In the vicinity of the former tank excavation, a clay layer is present and extends from ground surface to <br /> approximately 24 feet below grade This clay layer is approximately I1 feet thick beneath the backfill of the <br /> former tank excavation (Figure 5) <br /> 1 <br />