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Levine-Fricke-Recon • -, DRAft c 3.3 Groundwater Elevation Data and Analytical Results 3.3.1 Groundwater Elevations As described in Sections 2.2.5 and 2.2.6 of the RI/FS Work Plan, Stockton is located west of the axis of a structural trough that controls the regional hydrology. The regional direction of groundwater flow in the Study Area is generally toward the east (Figure 8). 3.3.1.1 Domestic Wells Groundwater elevation data were not available for domestic wells identified in the Study Area. 3.3.1.2 Municipal Supply Wells Groundwater elevation data were not obtained for municipal supply wells located in the Study Area. As discussed in Section 3.2.2, the screened intervals for these wells vary from approximately 150 to 600 feet bgs. Given the variable screen intervals for these wells, and the potential for water levels measured in these wells to be affected by pumping, groundwater elevation data were not evaluated for these wells. 3.3.1.3 Monitoring Wells Groundwater elevation data collected from Chevron and Exxon monitoring wells were evaluated to determine if they would provide useful data to supplement the Site data. The top-of-casing elevations and locations of all existing Chevron monitoring wells have been surveyed to the same datum (mean sea level) as existing Site monitoring wells. Groundwater elevations reported by Chevron's consultants indicate a groundwater flow direction generally to the east. Review of concurrent water level measurements collected from both Chevron and existing Site monitoring wells indicates that both sets of wells reflect very similar patterns of groundwater flow (Gettler-Ryan, Inc. 1995; LFR 1996b). This similarity remains consistent when comparing seasonal groundwater level fluctuations observed in the Chevron monitoring wells (Appendix C) to those observed in existing Site monitoring wells. The top-of-casing elevations of all existing Exxon monitoring wells were surveyed to the same datum as existing Site monitoring wells in 1992. Groundwater elevations reported for the Exxon site show a generally eastward flow direction (WEGE 1995). However, comparison of the recorded pattern of seasonal water level fluctuations observed in the Exxon monitoring wells to those observed in both Chevron and existing Site monitoring wells indicates that groundwater levels fluctuate substantially in the Exxon wells. Groundwater levels in several monitoring wells at Exxon have varied up to 5 feet within a month. The groundwater levels in the Exxon monitoring Page 12 411IWELL.AMPAM1