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5.4. Hydrogeologic Setting <br /> 5.4.1. Regional Hydrogeology <br /> Most of groundwater used in the vicinity of eastern San Joaquin County is drawn from the <br /> Arroyo Seco,Laguna, and Mehrten Formations. These regional groundwater systems are <br /> continuously being recharged by percolation of surface waters. The groundwater gradient <br /> direction is generally to the southwest. Shallower groundwater bearing zones can produce <br /> significant yield in some areas. These areas are typically located adjacent to streams where <br /> Quaternary alluvial systems deposited thin sand and gravel beds. During the 1985 Geologic <br /> Investigation groundwater levels were measured in three domestic wells located within a 1-mile <br /> radius of the landfill. Groundwater elevations ranged from 64 to 110 feet MSL (Appendix D). <br /> 5.4.2. Site Hydrogeology <br /> 5.4.2.1. Hydraulic Conductivity <br /> During the 1985 Geologic Investigation, six borings were advanced to depths ranging from 70 to <br /> 100 feet in the central area of the site (Appendix D). Approximately two-thirds of the soils <br /> encountered in the six borings consisted of moderately cemented clay/claystone and cemented <br /> silt/siltstone with in-place hydraulic conductivities on the order of 10-6 to 10-7 cm/sec. These <br /> same soils had remolded hydraulic conductivities as low as 10-9 cm/sec. The remaining soils <br /> consisted of cemented silty sand/silty sandstone with in-place hydraulic conductivities on the <br /> order of 10-3 cm/sec and remolded hydraulic conductivities between 10-4 to 10-5 cm/sec <br /> (Appendix D). <br /> During a Geologic Investigation (Kleinfelder 1990), seven soil borings (B-1 through B-7) were <br /> advanced to depths ranging from 30 to 65 feet in the east, southeast, and southern areas of the <br /> site. The soils encountered in the seven borings consisted of cemented clay/claystone, cemented <br /> silt/siltstone, and cemented sand/sandstone with in-place hydraulic conductivities ranging from <br /> 10-3 to 10-5 cm/sec. Remolded soils had hydraulic conductivities ranging from 10-4 to 10-6 <br /> cm/sec. The results of the laboratory testing are tabulated in a report dated January 2, 1991 <br /> (Plate 9 and Appendix E). <br /> It <br /> Foothill Sanitary Landfill Page 25 of 62 Public Works/Solid Waste <br /> JTD County of San Joaquin—Revised 10/6/05 <br />