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Site Characterization Report: 7500 West Eleventh Street, Tracy, CA Page 18 R 4.0 HYDROGEOLOGIC REGIME j 4.1 Groundwater Gradient and Flow Direction The depths to groundwater measured in the groundwater-quality monitoring wells on May 11, 2000, together with the surveyed well casing elevations, were used to calculate ' the elevations of the groundwater table beneath the site. The results of those measurements and computations are shown in Table 3. The depth to groundwater as measured on May 11, 2000 varies between 7.0 and 9.5 ft. The elevation of the groundwater table in each well is noted on Figure 9. Figure 9 also shows groundwater table contours constructed using that data. The groundwater contours show that beneath the 7500 West Eleventh Street property itself, the direction of groundwater flow is approximately 3 degrees to the east of north. However, further to the north beneath West Eleventh Street, the direction of flow begins to turn so that, on the other side of that street, the groundwater flow direction is approximately 20 degrees east of north. The mean groundwater gradient beneath the site is approximately 0.004 ft/ft. However, as -� indicated by the groundwater table contours shown on Figure 9, the gradient in the area north of the subject property's frontage onto West Eleventh Street is somewhat steeper than that beneath the subject property itself. <:- The northeasterly trend of the groundwater flow direction derived from the measurement of depths to the water level in Monitoring Well MW-1 through MW-7 is compatible with the general flow of groundwater in the region and follows the general slope of the j topography in the area (U.S. Department of the Interior, Geological Survey 1978, 1981). However, on first inspection, it appears that the groundwater flow direction derived from measurements in those wells might conflict with the direction of groundwater flow computed by the staff of Geological Technics Inc. of Modesto, California (GTI) from their measurements in groundwater-quality monitoring wells located at C&B Equipment at 7474 West Eleventh Street. As shown on Figure 9, that site is located to the east of the { subject property across Chrisman Road. Groundwater-quality monitoring wells were installed at C&B Equipment in 1995 at the direction of the SJCPHS to investigate the extent to which the subsurface beneath that property was affected by a release of fuels hydrocarbons from underground storage tanks previously located there. SJC reviewed a groundwater-quality monitoring report prepared by GTI that, based on _.i measurements of depths to groundwater made by that company on March 18, 1998, indicated groundwater flow to be approximately. 10 degrees west of north (Geological j I Technics Inc. 1998). SJC checked the depth to groundwater and well casing elevation data presented in GTI's report and computed the same apparent direction of groundwater flow beneath the C&B Equipment property. Assuming that the well casing elevation and depth to groundwater data presented by GTI is accurate, this result calls for some explanation. Among the possibilities are seasonal variations in the direction of groundwater flow (these are known to occur at sites along the West Eleventh Street SJC I