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4.0 SUBSURFACE CONDITIONS Stratus has developed geologic cross sections based on available historical soil boring and CPT logs, and soil types encountered during this phase of investigation. Interpreted geologic relationships are illustrated in geologic cross sections A-A' (Figure 3) and B-B' (Figure 4). The surface trace of the cross sections is included on Figure 2. Historical soil boring logs, CPT logs, well detail diagrams, and groundwater elevation contour maps are not attached in order to make this document more concise. 4.1 Geologic Conditions Fine grained soils (silt and clay intervals) have predominately been observed between surface grade and approximately 95 to 110 feet bgs. In the western portion of the site, and across Fremont Street to the north, sandy strata are predominately observed from approximately 15 to 35 feet bgs and 42 to 65 feet bgs. These sandy strata appear to decrease in thickness in the eastern portion of the site. Mid-depth monitoring wells MW-16 and MW-18 at the Apache site were situated within sandy strata that have been observed in the eastern portion of the site between approximately 85 and 90 feet bgs. Sandy soils at this depth do not appear to extend beneath the former UST area at the Apache site, but appear to be continuous northward beneath the former Sunland Gasoline/Stockton City Cab Company site. Laterally continuous sandy soils, with thicknesses ranging from approximately 10 to 35 feet . on-site and greater than 40 feet thick in the vicinity of boring CPT —7, are observed beneath the upper fine grained soils. The deeply screened monitoring well network wells are generally screened within these sandy soils. Fine grained soils underlie these laterally continuous sandy stratum to approximately 160 feet bgs, the maximum depth explored beneath the site. 4.2 Hydrogeology Depth to groundwater has ranged from approximately 42 feet bgs (2001) to 69 feet bgs (1990) since monitoring was initiated in 1989. Based on discussions with SJ'CFHD personnel, depth to groundwater in the vicinity of the site was measured as low as approximately 110 feet bgs in 1979. Depth to groundwater was generally measured at 45 to 47 feet bgs at the time of the second quarter 2006 monitoring event. Groundwater elevations are generally similar within the shallow and deeply screened monitoring wells. Groundwater flow beneath the site has generally been calculated to be towards the southeast and east at low gradients. The southeast and east groundwater flow directions have been calculated using data from the shallow and more deeply screened wells. Figure 5 and Figure 7 illustrate groundwater elevation contours using second quarter 2006 monitoring data for the shallow and deep monitoring well networks, respectively. Groundwater elevations at the mid-depth monitoring wells are illustrated on Figure 6. K;\Apache PlasticslReports\apache-sem-assessment report2006.doc Page 10 STKAW5