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Ld Mr. Harlin Knoll, SJCEHD September 29,2005 Site Investigation Work Plan 103 North E Street, Stockton, CA Page 3 Remedial Activities An estimated 700 cubic yards of impacted soil was excavated in 1987. Soil was bioremediated and aerated on-site, and properly disposed of. Limited additional excavation was performed,in 1988. Another round of excavation.was performed in the vicinity of the former dispensers in 1999. Soil vapor extraction was initiated using well MW-14 in December 1994. Following reconstruction, well MW-13 was added to the extraction system in January 1995. Air sparging was initiated using well AS-1 in May 1995. Bioventing using wells MW-13, MW-14, and AS-1 was initiated in October 2004. Status of Wells Well MW-2 was properly destroyed through excavation activities in 1987. Wells MW-3, MW-4, MW-5, and MW-9 through MW-12 were properly abandoned in 2000. SUBSURFACE CONDITIONS Subsurface relationships, based on soil borings installed to date, are depicted on cross- sections A-A', B-B% and C-C' (Figures 3, 4, and 5, respectively). Cross-section traces i are indicated on the Site Plan(Figure 2). Hydrogeologic Conditions Interbedded sand and fine-grained soils have been observed between surface grade and approximately 50 feet bgs, near the current water table interface. These sandy vadose strata underlie much of the area in the vicinity of the former USTs. Due to the extensive excavation performed in 1987, fill material comprises a significant portion of vadose zone soil strata near the former UST locations. Predominately sandy strata are encountered between approximately 50 and 80 feet bgs, and below approximately 90 feet bgs. Many of the borings and wells installed during the initial phases of the petroleum release investigation were advanced no deeper than 55 feet bgs (currently groundwater is measured at approximately 50 feet bgs). As shown on the cross-sections, the lateral extent of the sandy strata within the saturated zone is not adequately characterized to identify potential pathways for 1,2-DCA migration, and the vertical extent of the sandy strata encountered around 90 feet bgs has not been characterized. ! Depth to groundwater beneath the site has been measured at depths ranging from approximately 37 feet bgs (1987) to approximately 81 feet bgs (1992). Groundwater was measured at approximately 50 feet bgs at the time of the second quarter 2005 monitoring + KATeicherMtockton-1 03%Work PlansVeichert workplan 09-2005.doe STRfiT2J5