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well purge logs are included in Appendix A and copies of laboratory reports and chain-of-custody documentation are included in Appendix B ' 3.0 HYDROGEOLOGIC SETTING ' The site is located in the Northern San Joaquin Valley, part of the Great Valley Province, California, a large north-westward trending asymmetric structural trough The trough is filled with as much as six miles of sediment ranging from Jurassic to Holocene The immediate ' subsurface geology are Pleistocene and Recent alluvial deposits These alluvial deposits consist of heterogeneous sequences of sands and gavels originating from active stream channels, and silts and clays originating from overbank and marsh depositional environments The regional groundwater flow direction is expected to be north-northwest First groundwater was present in the tank pit during excavation at approximately 16 feet below surface grade The San Joaquin River lies approximately 3 6 miles west of the site and Walthal Slough lies 3 6 nules to the southwest According to the San Joaquin County Flood Control and Water Conservation District (FC&WCD), groundwater lies at approximately 16 feet below grade at an elevation of approximately 10 feet above sea level 4.0 SITE BACKGROUND 4.1 Tank Removals ' The subject property has been occupied by Manteca Unified School District since 1972 The site encompasses the District Office, Transportation Depot, Food Storage and Maintenance Buildings for Manteca Unified School District (Figure 2) ' According to WHY Consultants' August 31, 1993, Remedial Excavation Report, in June 1993, two underground storage tanks were removed from the site by SEMCO The tanks, one 12,000- gallon gasoline and one 12,000-gallon diesel, were removed on June 15, 1993 A 1,000-gallon tank had been previously removed Analytical results for soil samples collected during the removal recorded elevated concentrations of petroleum hydrocarbons 1 4.2 Overexcavation of Tank Locations ' On July 7-9, 1993, the former tank location was over-excavated to a depth of 18 feet below grade Confirmation soil and grab groundwater samples were collected from the excavation on ' July 8-9, 1993 at the locations shown on Figure 4 Samples were placed on ice and transported under chain-of-custody documentation to Sparger Technology, in Sacramento, California The samples were analyzed for TPHg and BTEX by appropriate EPA methodologies The samples contained TPHg and BTEX above the method detection limits Analytical data are summarized in Table 3 w 194337Tclwure doe 2 • SMTH 1