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3.4 Site Hydrogeology The subject site is located near the west-central portion of the Central Valley groundwater basin This basin has furnished a significant portion of San Joaquin County's domestic, agricultural and industrial water supply for many years Regionally, the San Joaquin Delta, west of the site, acts ' to recharge to the basin with groundwater flows eastward toward a large groundwater depression approximately eight miles east of the site (San Joaquin County Flood Control District's, Semi- Annual Groundwater Report, Fall, 1991) Through discussions with the San Joaquin County ' Flood Control District, this groundwater depression is the result of groundwater pumping for agricultural purposes ' The regional groundwater flow direction in the site vicinity is toward the east-northeast at a gradient of 3 16 x 10-3 feet/foot (16 7 feet/mile) A map of regional groundwater elevation ' contours is presented as Figure 7 In 1988, groundwater beneath the site was encountered at a depth of approximately 25 feet bgs, ' in the zone 3 sand Monitoring wells MW-1, MW-2 and MW-3 were completed in this zone with screened intervals from approximately 21 5 feet to 45 5 feet bgs, as shown on cross sections A-A' and B-B' The groundwater flow direction and gradient magnitude were ' calculated based on these three wells in December 1988 and the results indicated a flow direction toward the northeast at a gradient of 2 8 x 1073 ft/ft Monitoring well MW-1 was interpreted to be in the upgradient location and thus was considered a background well Wells MW-2 and MW-3 were interpreted to be located in the center and downgradient portions of a hydrocarbon plume The groundwater was interpreted to be an unconfined water table aquifer within the zone 3 sand Due to the drought conditions which occurred between 1989 and the installation of the ' Phase II wells, the water table dropped approximately 26 feet, as depicted on the cross sections, and is now present in the finer-grained zone 4 This aquifer is interpreted to also be an unconfinLd water table aquifer which is hydraulically interconnected with the zone 3 sand This ' interpretation is supported by the lithologic logs which indicate that there is no clay barrier or aquitard between the two zones The two zones are essentially parts of the same unconfined aquifer in which the water table elevation fluctuates according to the amount of regional ' groundwater recharge The groundwater flow directions and gradient magnitudes in zone 4 do not lend themselves to ' a simple interpretation Figures 8 and 9 show the groundwater elevations plotted on site maps for two separate dates Depending on which three wells are selected for the calculation, several different flow directions may be determined, as shown on the figures These measurements and ' calculations indicate that the water table surface is not planer and that there is no simple upgradient and downgradient flow direction ' To further illustrate the morphology of the water table surface, Figures 10 and 11 present a contour map of the groundwater elevation data and a three dimensional projection of the ' contours, respectively The non-planer water table surface is clearly apparent and hence, based on the presently available data, the predominant flow direction and gradient magnitude for the site is indeterminant Rnsrsrc wr 14 OMIM