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Y Commingled Plume#35 Chevron 9-4275/ARCO#2133 Site Conceptual Model September 1,2006 Page 10 5.0 GEOLOGIC AND HYDROGEOLOGIC SETTINGS 5.1 Geology Regional geologic cross-sections for the area are ,presented in Figures 7 through 9. These cross sections depict the following conditions: • A predominantly clay and silt layer encountered from the ground surface to approximately 10 feet bgs. • A silty sand and poorly graded and well. graded sand layer encountered from 10 feet bgs to approximately 1.9 to 28 feet bgs. • A clay layer described as both a lean clay ,and fat clay in various borings encountered from approximately 25 to 30 feet bgs to 45 to 50 feet bgs. • A poorly graded sand to silty sand layer encountered between 45 to 50 feet bgs to 52 to 57 feet bgs. • The deepest layer encountered to date is a clay layer below 57 feet bgs. • Groundwater is generally encountered at approximately.8 to 15 feet bgs and appears to be an unconfined aquifer. Depth to groundwater.has varied from approximately six feet to 20 feet bgs since the installation of monitoring wells began in, 1989. Soil samples collected from first encountered groundwater to the total depth explored of 70 feet bgs have been classified at times from saturated within the coarser-grained strata to moist, damp, and dry within the fine=grained strata. Historical boring logs are provided in Appendix H. 5.2 Hydrogeology The CPT investigation conducted by SECOR in 2001 determined the presence of shallow groundwater which is consistent with routine groundwater monitoring data. In general, shallow groundwater flows from the Chevron site to the northeast. Historically, groundwater flow has been dominantly to the northeast at a gradient of 0.0001 to 0.0009 ft/ft. Historical groundwater gradient data is included in Table 5. The second quarter 2006 groundwater elevation contour map is included as Figure 5. 94275-ARCO 2133 SCM 9-2006 10