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4. ENVIRONMENTAL ANALYSIS 4.8 HYDROLOGY AND WATER QUALrrY ranges in thickness from 200 to 500 feet. Figure 4.8-2 presents a conceptual cross-section of the geologic formations in the proposed project vicinity. All three types of groundwater aquifers (confined aquifer,unconfined aquifer, and perched _ aquifer)are present in the vicinity of the proposed project site(JMM, 1993). Groundwater is generally present beneath the site in a shallow unconfined zone within about 25 feet of the land surface;in the confined, semi-confined, and unconfined zones in the upper Tulare Formation; and in the confined fresh water zone in the lower Tulare Formation(Davis 1959 in DWR, 2003b). Most of the larger agricultural, municipal,and industrial groundwater production wells pump from below the Corcoran Clay. Groundwater above the Corcoran Clay is often of poorer — quality,although small domestic wells often obtain their supply from this water-bearing zone. Groundwater Flow and Water Levels Regional Groundwater flow direction in both the upper and lower aquifers was historically northwest and — parallel to the San Joaquin River(Hotchkiss and Balding, 1971). Recent data provided by the DWR indicates groundwater flow to the north and east,toward the San Joaquin River. The _ annual groundwater maps developed by the DWR indicate that the general trend in groundwater flow direction in the unconfined aquifer zone is influenced by topography and flows eastward from the Coast Range towards the San Joaquin River. A review of the historic groundwater flow — patterns indicate that the general trend in groundwater flow directions has not significantly changed over the past 20 years (Stanislaus County, 1993). Groundwater maps prepared by the USGS indicate that the regional flow directions in the lower, confined aquifer are influenced by groundwater pumping patterns(USGS, 2003). Currently,the general flow direction is toward the north,where groundwater pumping in the vicinity of Tracy and Stockton has created groundwater depressions(i.e., zones of unsaturated geologic materials which formerly were saturated). Before the agricultural development of the Central Valley,the regional groundwater flow direction in the Vernalis area was north and west towards the Sacramento-San Joaquin Delta(Williamson,et. al., 1989). However,increased agricultural and municipal pumping in the Stockton-Tracy area has modified the general flow direction to the north as groundwater moves to fill the depressions. Review of hydrographs for the Tracy Sub-basin indicate that, except for seasonal changes in _ recharge and pumping, the majority of water levels in wells have remained relatively stable over at least the last 10 years (DWR, 2003a). On average,the Delta-Mendota Sub-basin water levels increased from 1970 through 1985 about 7.5 feet then decreased from 1985 to 1994. The overall fluctuation was 2.2 feet from 1970 to 2000. RMC Pacific Vemalis Quany Muting and Reclamation Project 4.8-6 ESA/203015 A,tmmistradw Dmft EIR Much 2004