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It has been documented that two major groundwater bearing zones occur under the Tracy locale. These are referred to as the upper and lower sections of the Tulare formation, and are separated by a prominent clay confining stratum referred to as the Corcoran Clay Layer. This clay layer extends from the Diablo Mountain Range to the west, eastward to the San Joaquin River. The bottom of the Corcoran Clay is found approximately 200 feet below ground surface(bgs)near the Diablo Range and about 550 feet bgs near the San Joaquin River. Correlating these depths with mean sea level(MSL),the clay layer is approximately 50 feet MSL near the Diablo Range and-300 feet MSL near the San Joaquin River. Examination of the "E"-Log for the CSA-44A well (Eley. 125 feet MSL) indicates the Corcoran Clay layer to be between 280 feet and 380 feet bgs, corresponding to -155 ft MSL and-255 ft MSL. From studies conducted by the USGS and DWR, it has been determined that the lower Tracy area groundwater basin is actually a sub-basin of a regional aquifer system and not hydraulically isolated. This aquifer system extends from Redding in the north to Kern County in the south. Additionally, these studies have focused on a larger groundwater study area than just the Tracy area exclusively. FINDINGS WATER QUANTITY There are primarily three issues that pertain to groundwater quantity exclusive to the CSA-44A well: 1.) Water supply from the shallow domestic wells that serve individual homes in and around the Valpico Section, 2.) Affect on water supply provided by the CSA-44 wells (the Feliz well and the Fairoaks well), and 3.) Production from the lower aquifer zones. The groundwater volume in the upper zone which provides water for shallow domestic wells is substantial. Table 1 of the 1971 USGS Open File Report,illustrates that in the Tracy area,the upper portion of the Tulare formation has a maximum thickness of about 200 feet-of which the majority is saturated,and the lower zone has a maximum thickness of approximately 500 feet. There is little influence to the upper zone when pumping from the lower zone, unless a well is cross-zoned in which the well is screened in both the upper and lower zones. The main influence from lower zone pumping may be the potential increase in leakage from the upper zone to the lower zone. Both the Bookman-Edmonston and Kennedy/Jenks Studies estimate the leakage to be between 1,000 and 4,000 acre-feet of water over the study area of 69,000 acres. Water quantity regarding the upper zone within the Valpico Section will be further addressed in the Overdraft Section of this report. Lower confined zone water supply originates from the following areas: 1.)Groundwater inflow from the upper zone by leakage,2.)Inflow from the lower zone forebay area near the Diablo Range which is in lateral hydraulic continuity with the lower confined zone,3.)Inflow from the south of the study area with an assumed transmissivity of 50,000 gpd/ft,an underflow perimeter width of-8 miles and Page -2- Vaffey Ag Research