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ARCO Station 548S C O R 1612 west Hammer Lane,Stockton77 Remedial Action Plan May 31,2006 f 3.0 . GEOLOGIC AND HYDROGEOLOGIC SETTING 3.1 Regional Geology According to the geologic maps of the Sacramento-San Joaquin Delta, California written by Atwater, B.F., dated 1982, provided by U.S. Geological Survey; the area is generally well established within individual columns but generally speculative between columns. More specifically the area is defined as (Qcr) alluvial-fan deposits derived from unglaciated drainage basins, Calaveras River and vicinity. Subsurface sediments generally consist of silt, clayey silt, and sandy silt, with minor beds and tenses of sand and silty sand from 0 to 65 feet below ground surface (bgs), the maximum depth explored. The sand lenses are not laterally continuous across the site. (URS, 2005). 3.2 Local Geology and Hydrogeology As described in the Site Conceptual Model (URS, 2005a), subsurface sediments generally consist of silt, clayey silt, and sandy silt, with minor beds and lenses of sand and silty sand from 0 to 100 feet bgs, the maximum depth explored. Geologic cross-sections of the site are included in Appendix D. The sand lenses are stream channel deposits and are not laterally continuous across the site. Depth to groundwater in site monitoring wells has ranged from approximately 50 feet bgs, when the wells were first installed in 1991, to approximately 30 feet bgs during fourth quarter 2005. Increasing groundwater elevations have caused the screen intervals of seven of the groundwater monitoring wells to become submerged (well screens range from 43 to 63 feet bgs (Table 1). SVE well screen intervals begin between five and 44 feet bgs and span the saturated-unsaturated soil boundary. Although constructed as extraction wells with large slot sizes (0.05 inch to 0.10 inch) and gravel filter packs, these wells are usable as monitoring points for the groundwater interface. Groundwater has been measured between eight and 15 feet bgs in shallow well VW-5S. This anomalous water level is believed to be a localized perched aquifer in the former UST complex, resulting from porous and permeable backfill in the UST excavation pit underlain by relatively impermeable clayey silts (URS, 2004). Currently, groundwater is being monitored and sampled quarterly from all existing monitoring, SVE, and AS wells. As requested in the letter from the SJEHD dated March 2, 2006, all wells at the site were resurveyed by a California Licensed Land Surveyor on May 4, 2006. This information was used to recalculate the groundwater gradient and flow direction from January 2004 to March 2006 (as requested in the SJEHD March 2, 2006 letter). New groundwater gradient figures are included in Figures 3 through 20. Revised groundwater flow information indicates across the site has ranged from east to northeast, with hydraulic gradients ranging from 0.02 feet/foot (ft/ft) to 0.008 fVft and an average depth to water for the site is approximately 30 feet bgs. Historical groundwater gradient information is included in Table 3. I:ISP-ARC0104 BP valley Portfolio15481Reports\RAP1548 RAP.doc 5