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5.0 EVALUATION OF LOCAL HYDROGEOLOGY 5.1 Hydrogeologic Profile Soils encountered during this investigation varied from sand to silty sand and sandy silt to clay Idealized hydrogeologic cross sections are presented in Figure 5 and Figure 6. The boring logs are presented in Appendix D and document the field logs prepared durnng the drilling activities The presence of a clay layer at a depth of 17 feet forms the lower boundary of this hydrologic zone Naturally occurring clean sand in the saturated zone makes this upper zone potentially highly permeable particularly if the sand layers are continuous laterally The fast recovery rate in the well, as found during well development confirms the locally high permeability. The water levels were found to be higher than initially anticipated and are at a depth of approximately 8 feet 5.2 Hydraulic Gradient The depth to groundwater at the site was between 7 80 and 8 02 feet as indicated in Table 3. The groundwater gradient determined from on-site was north 14' east with a magrutude of 0 37ft/100 ft. The calculated gradient direction differs by 35° from the gradient determined from off-site sources at the nearby Mobil Distribution Center site 6.0 CONCLUSIONS The following conclusions can be made on the basis of the information collected during this investigation - Detectable levels of hydrocarbons are present in the vadose zone of the undisturbed soil in the vicinity of the tank - Non-detectable hydrocarbon concentrations are found in the water in a down gradient direction of the potential source area • The upper hydrogeologrcal zone is highly permeable and consists of sands and silty sands. - Soils used to backfill the UST excavation contain detectable concentrations of hydrocarbons - The hydraulic gradient as determined on site is directed north 14' east 10 A BRUNSING ASSOCIATES, INC 40