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IV. HYDROGEOLOGY A. REGIONAL HYDROGEOLOGY Most of the ground water used in eastern San Joaquin County is pumped from the Arroyo Seco and underlying Laguna Formations. Although Quaternary units, such as the Riverbank and Turlock Lake Formations, generally contain less ground water, some wells in the region draw large yields from discontinuous sand and gravel layers within these units. Ground water is primarily recharged by percolation from the surface that originate in the foothills of the adjacent Sierra Nevada Mountains. EMCON conducted a survey of wells within 1 mile of the site perimeter (see Figure 3 and Appendix D) . Through area reconnaissance and review of Department of Water Resources records, 38 water-supply wells were located (see Figure 3) ; other undocumented wells may exist in the area. Data was available for only 10 water wells. These data show that the cased depths range from 75 to 368 feet and average approxi- mately 164 feet. Recent measurements found depths to ground water range from 121 to 164 feet. Most of the water from the wells is used for irrigation purposes. Extensive irrigation (and resultant ground- water drawdown) began around 1907 and has increased steadily since (Piper et al . , 1939) . Piper et al . (1939) documented an average local ground-water elevation recession of 11 feet from 1907 to 1927 due to intensive pumping; this trend has continued. The measured ground-water elevation in the Thomas well (Well 19 on Figure 3) , located approximately 0.95 mile west of the north end of the site, has declined from 48 feet MSL (depth approximately 50 feet) in 1929 to -27 feet MSL in 1987. The San Joaquin n Count Flood q y F od Control and Water Conservation District (referred to as the District) has monitored ground-water levels and ground-water quality in the county since 1971 . Ground-water elevation PJ9 9390206.00D