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� f�; + ' i+,'ski � �" -. M`�' �� rw�,T ". sa.; �"` �, � �w'T�� s's-'�•. I N= rl.I all J amil _ r I CHAPTER 3 HYDROGEOLOGICAL SETTING I � The hydrogeologic setting for the site is based on past geologic reports and macs for the area. Regional H dro eology San Joaquin County lies at the confluence of the Sacramento and n. San Joaquin River systems. It is bordered on the north and south by broad valleys, on the east by the Surra Nevada Mountains, and to the west by the Delta. F Subsurface geologic formations in San Joaquin County range in age from Pre-Cretaceous to Recent. Nonwater-bearing deposits include the Pre-Tertiary Basement complex, the Eocene Ione Formation, and the Miocene Valley Springs Formation. The main water--bearing deposits include the Late Miocene and/or early Pliocene Mehrten Formation, tine Plio-Pleisroceene Turlock Lake and Laguna, and Tulare Formations. -- Lesser quantities of groundwater are obtained from the Pleistocene Riverbank Formation and the Recent :alluvium. Groundwater occurs in both confined and unconfined aquifers in, the water--bearing deposits. Recharge to the groundwater in the valley is by subsurface inflcw from the Sierra Nevada Mountains to the east, and by percolation of water from surface stream„ b; rainfall irifiltration, and by perco- lation of unconsumed water in irrigated land. Usable grounuwater_ in the central and eastern portion of the Stockton area is stored to a depth of approximately 1,100 feet in the unconsolidated Recent Alluvium, :Riverbank, and Laguna Formations. These formations are not homogeneous sediments, but are composed of discontinuous sand and - gravel layers separated by vaLuble thicknesses of sandy silt, silt, and clay. According to the California Department of Water Resources, Bulletin 146, groundwater historically f",wed from the base. of the Sierra Nevada Mountains westward towar6s the Sacramento-San Joaquin River Delta. However, increased municipal use of groundwater in the Stockton area has created . a ,pumping depression beneath and to the east of �tockLon. This depression has caused an easterly migration. of poor quality saline groundwater from the Delta. `. The geologic formations underlying the central and eastern portions of the Stockton area are a continuation of the Recent Alluvium and the Riverbank, Turlock Lake, Laguna, Mehrten, and Valley Springs Formations. Groundwater in the western portion of the Stockton area is stored, for the most part, in fine-grained deltaic deposits. J'