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most wells average more than 1000 gallons per minute (Hotchkiss and Balding, 1971, Fig. 7). The San Joaquin County Flood Control and Water Conservation District maintains an annual record of elevation of water in wells in San Joaquin County. In their figure "Lines of Equal Elevation of Water in Wells, Fall 1992, contour lines represent ten -foot intervals in ground water elevation. Water at the proposed project site stands at elevation 70 MSL, or at approximate depth 95 feet below the ground surface. Depth to water increases toward the north-northwest. Variation in depth of the water table typically stems from topographic relief, with the water table fairly flat. Limited water level data for selected wells in the project area is on file at the Department of Water Resources, Sacramento District (DWR). Hydrographs of wells in the area indicate that water levels vary from year to year, but tend to return to an equilibrium level that reflects the balance between recharge, pumping, and subsurface inflow and outflow. This balance is affected by long-term changes in irrigation patterns and the relative amounts of ground water and imported surface water used for irrigation. Periods of above- or below-average rainfall and stream flow can cause relatively short-term deviations from the equilibrium water level. Water levels rose in wet years (1969, 1983, and 1986), and declined during droughts (1976-1977, and 1991-1992). Limited ground water quality data for selected wells in the project area is on file at the DWR. Ground water in the vicinity of the project appears to be generally of good quality, with the exception of total dissolved solids, which can exceed California Secondary Drinking Water Standards. (it should be noted that secondary drinking water standards are based on aesthetic consideration including odor and taste, and are not necessarily health related). Water quality in the shallow aquifer system is generally worse than the quality in the deep aquifer system, with higher concentrations of dissolved solids, chloride, boron, and sulfate. These constituents are thought to originate from subsurface inflow of ground water from the marine sedimentary rocks adjacent to the western edge of the basin. The higher quality of the deep water indicates that some of it probably originates from recharge on the eastern side of the valley. Elevated concentrations of nitrate in ground water occur in various parts of San Joaquin County, including the Tracy area. The suspected sources of nitrogen are fertilizer and animal wastes. Water Use and Potential Impact to Nearby Wells Predominant land uses in the project area are agriculture and sand and gravel extraction and processing. Land use near the project site is primarily agricultural, and irrigation is the largest use of water. Significant quantities of water are also utilized in the processing of sand and gravel. To the north, east and south of the site are row crops and orchards. To the west and north of the site are the continuing operations of the RMC LoneStar Industries sand and gravel mining operations. Also in the area are the sand and gravel mining and processing operations of Granite Construction Company and Teichert Aggregates. There are a number of large wells in the project vicinity. The City of Tracy obtains municipal water from a series of wells located at the south end of Section 8 of T3S, R5E, approximately one mile west-northwest of the site, as well as from the Defta-Mendota Canal (by way of a pump -and -treat system). The Tracy Municipal Airport obtains its water from the City of Tracy. Several businesses and schools in the area operate °public water systems', as they serve more than the minimum number of users as required by the Public Health Department. Teichert Aggregates and Granite Construction Company obtain process water from deep wells located in the central portion of Section 9, T3S, R5E, and the northeast portion of Section 17, T3S, R5E, respectively, approximately within a one mile radius of the site. Most municipal and large irrigation wells produce water primarily from the deep aquifer below the Corcoran Clay. As a result of pumping and drawdown of the deep aquifer, water levels in the deep wells are typically lower than water levels in the shallow wells. ER -93-1 -66- (9-27-93)