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.10. Hydrogeologic Setting <br />4.11-14.10.1 Regional Hydrogeology <br />Most of groundwater used in the vicinity of eastern San Joaquin County is drawn from <br />the Arroyo Seco, Laguna, and Mehrten Formations. These regional groundwater <br />systems are continuously being recharged by percolation of surface waters. The <br />groundwater gradient direction is generally to the southwest. Shallower groundwater <br />bearing zones can produce significant yield in some areas. These areas are typically <br />located adjacent to streams where Quaternary alluvial systems deposited thin sand and <br />gravel beds. During the 1985 Geologic Investigation groundwater levels were measured <br />in three domestic wells located within a 1 -mile radius of the landfill. Groundwater <br />elevations ranged from 64 to 110 feet MSL (Appendix D). <br />4.11.24.10.2 Site Hydrogeology <br />^.44.10.2.1 Hydraulic Conductivity <br />During the 1985 Geologic Investigation, six borings were advanced to depths <br />ranging from 70 to 100 feet in the central area of the site (Appendix D). <br />Approximately two-thirds of the soils encountered in the six borings consisted of <br />moderately cemented clay/claystone and cemented silt/siltstone with in-place <br />hydraulic conductivities on the order of 10-6 to 10-7 cm/sec. These same soils <br />had remolded hydraulic conductivities as low as 10-9 cm/sec. The remaining <br />soils consisted of cemented silty sand/silty sandstone with.in-place hydraulic <br />conductivities on the order of 10-3 cm/sec and remolded hydraulic conductivities <br />between 10-4 to 10-5 cm/sec (Appendix D). <br />Foothill Sanitary Landfill Page 35 Public Works/Solid Waste <br />Joint Technical Document County of San Joaquin — Revised 7/30/10 <br />