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2.3 Site Geology The delta deposits at RRI are underlain by the Mehrten Formation, comprised of andesitic conglomerate, silt, clay, andesitic agglomerate, and interbedded black sands (Tetra Tech EM Inc. 1998). The Mehrten Formation is late Miocene in age, approximately 700 feet thick, and is encountered at a depth of approximately 1,500 feet beneath RRI. The Mehrten Formation is an important regional aquifer in the Sacramento and San Joaquin Valleys. Groundwater from the Mehrten Formation underlying the Stockton area is saline and the formation contains natural gas (Tetra Tech EM Inc. 1998). The Mehrten Formation is underlain by the Valley Springs Formation which is comprised of consolidated ryholtic tuffs, conglomerate, "clay shales", and sandstone. Beneath RRI, the Valley Springs Formation is encountered at a depth of approximately 2,200 feet and is approximately 500 feet thick. It is reported to be saline and contain natural gas. In addition, it overlies Eocene and older rocks considered to be non-water bearing. 2.4 Regional Groundwater Flow Groundwater in the Central Valley deposits naturally flow away from recharge areas at valley margins and toward discharge areas in centrally located streams such as the San Joaquin River or lakes such as the ancestral Tulare Lake (Tetra Tech EM Inc. 1998). Significant recharge of the Central Valley aquifer system occurs through stream beds where streams enter the Valley. Prior to large-scale aquifer pumping, groundwater most likely discharges from the Central Valley aquifers to the delta. Recent water level measurements reported by San Joaquin County (1995) confirm that groundwater recharge occurs through seepage beneath stream channels (Tetra Tech EM Inc. 1998). However pumping of wells east of Stockton currently causes groundwater levels Rough&Ready Island 6 Tetra Tech,Inc. Revised Draft Work Plan November 18, 1998