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Work Plan for Additional Site Assessment and Interim Remediation/Feasibility Testing ARCO Facility No.5469 Stockton,California Page 2 0.187 mg/kg, tert-butyl alcohol'(TBA) at 44 mg/kg, and methyl-tert butyl ether (MTBE) up to 5.16 mg/kg (IT, 2000). Based on the analytical results, limited excavation of hydrocarbon impacted soil was performed in the vicinity of the north end of the two dispenser islands. Approximately 4 cubic yards of impacted soil was excavated from the north end of the western dispenser island. Due to subsurface utilities and native hardpan soils (encountered at approximately 3 feet below ground surface [bgs]), only overlying pea-gravel was hand excavated from the north end of the eastern dispenser island. Approximately 60 cubic yards of soil and pea-gravel were excavated and removed from the site (IT, February 2000). Between June 5, and June 8, 2000, Delta oversaw Woodward Drilling Company of Rio Vista, California, drill and install six groundwater monitoring wells (MW-1 through MW-6). The wells were constructed of 4-inch diameter, 0.020-inch factory-slotted PVC casing from a depth of approximately 33 to 53 feet bgs. The site was subsequently placed on a quarterly monitoring prograrn. In response to the PHS/EHD request for additional assessment to evaluate lateral and vertical impact of petroleum hydrocarbons to soil and groundwater, .Delta oversaw Woodward Drilling Company install four additional monitoring wells (MW-7 through MW-10) on May 30, and May 31, 2001. The wells were constructed using 4-inch diameter, 0.020-inch factory slotted PVC casing installed to a depth of approximately 33 to 53 feet bgs (MW-7 & MW-8), and approximately 59 to 74 feet bgs (MW-9 and MW-10). Delta submitted a Well Installation Report (dated August 15, 2001) documenting the well installation activities. The newly installed wells were incorporated into the quarterly groundwater monitoring program. The groundwater flow direction, based on elevations measured in well MW-1 through MW-8, has been calculated to be to the east and northeast, with an average.gradient of 0.002 ftlft. Figure 2 presents the fourth quarter 2001 groundwater elevation contours. Historical groundwater elevations and analytical data are presented in Appendix A. PROJECT APPROACH Soils encountered at the site from ground surface to 40 feet bgs are predominantly clayey silt, and sandy silt with thin discontinuous lenses of silty sand. Sand with interbedded silt was encountered between 40 and 55 feet bgs. A second sandy unit was encountered between'60 and 73 feet bgs and appears to be separated from the upper sandy unit by silty clay. P:1ARCO\5469%Work P3an15469 wrkpin 03.2002.doc