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4.8 Hydrology and Water Quality 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. r 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 r 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. Local and Project Site Specific Developing groundwater setting information for this EIR included identifying groundwater wells within one mile of the proposed project site. The DWR collects groundwater data on a select number of agricultural and domestic wells in the proposed project area and provides that data to the public through their internet website. The DWR groundwater monitoring well locations listed and discussed in this EIR do not include all wells on the proposed project site or in the vicinity. There are actually three agricultural wells and two domestic wells on the proposed project site but _ the DWR compiles data on only one, identified as 4S/6E-04N01, which is located in the central portion of the proposed RMC mine site. The review of DWR groundwater monitoring well data identified a total of 8 other groundwater wells within a mile of the proposed project. These DWR r, groundwater monitoring well locations are listed below on Table 4.8-2 and shown on Figure 4.8-3.This EIR uses data from these DWR groundwater monitoring wells to describe groundwater level trends in the project vicinity. Offsite groundwater well 4S/6E-05A01 (Costa r RMC Pacific Vernalis Quarry Mining and Reclamation Project 4.8-7 ESA/203015 Draft Environmental Impact Report May 2006 r