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�E EMCON conducted a well survey of wells within 1 mile of the site perimeter (see Figure 4-8 and Table 4-1). Through area reconnaissance and review of Depart- ment of Water Resources records, 38 water supply wells were located; other un- documented wells may exist in the area. Data was available for only 10 water wells. These data show that the cased depths range form 75 to 368 feet and av- erage approximately 164 feet. Recent measurements found depths to ground water range from 121 to 162 feet. Most of the water from the wells is used for ir- rigation purposes. Extensive irrigation (and resultant ground -water drawdown) began around 1907 and has increased steadily since (Piper, et al., 1939). Piper, et al. 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 No. 19 on Fig- ure 4-8), 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 County Flood Control and Water Conservation District has monitored ground -water levels and ground -water quality in the County since 1971. Ground -water elevation contour maps from the District indicate that ground water is currently first encountered at elevations of -10 to -20 feet MSL, in the site area. The flow gradient is regionally to the southwest. Water level data acquired by EMCON during this investigation indicates that cur- rent water levels in wells in the area range from elevations of -19 to -27 feet MSL (Table 4-2). The local gradient is very flat over most of the area, but appears to be generally to the west-southwest (see Figure 4-9). Actual local flow direction conditions are believed to be influenced by seasonal pumpage of irrigation wells. No springs were identified on site or within 1 mile of the proposed landfill site. The area was inspected by examining aerial photographs and searching for veg- etation reflective of springs. PJ9 9390218A.00W 26 Rev. 0 July 20, 1989