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Mr George Takemon Project OF90-002 04 August 21, 1995 ' Page 2 Two soil samples were collected from the tank excavation for laboratory analysis One of the samples contained detectable concentrations of benzene, toluene, ethylbenzene, and xylenes (BTEX) BTEX was not detected in the other sample, and total petroleum hydrocarbons as gasoline (TPHG) was not detected in either sample In September 1988, the area was overexcavated Confirmation soil samples were collected from the bottom of the excavation Petroleum hydrocarbons were not detected in any of the confirmation soil samples collected ' during this overexcavation activity Between December 1987 and December 1994, following the removal of the tanks, soil borings were drilled and ten groundwater monitoring wells were installed to characterize the lateral and ' vertical extent of soil and groundwater impact (EMCON, Problem Assessment Report and Final Remedianon Plan, December 16, 1992) Dissolved hydrocarbons were detected in the ' vicinity of the USTs Well MW-1 was damaged, and well MW-2 was destroyed during removal of this soil In October 1993, EMCON performed a soil vapor extraction (SVE) test (EMCON, Problem Assessment Report and Final Remediation Plan, December 16, 1992) Results of the test indicate that soil remediation using SVE is feasible In December 1994, a remediation system, consisting of an EI 250-cubic-foot-per-minute (cfm) thermal oxidizer, was started up The system was designed to extract soil vapors from wells YtW-13 and IOW-14 ' SUPPLEMENTAL iNVESTIGATION The following activities were performed to increase removal of dissolved hydrocarbons ' and increase the efficiency of the remediation system On May 8, 1995, a geologist from EMCON observed Woodward Drilling of Rio Vista, California, drill one soil borings (AS-1) The boring was drilled to 81 feet bgs and converted to an air sparge well (AS-1) ' The air sparge well was screened between 77 9 and 79 8 feet bgs During the dulling activities, groundwater was encountered at 66 1 feet bgs, but static groundwater was measured at 67 6 feet bgs EMCON field procedures are described in Appendix A ' The geologist collected soil samples from the borings at approximately 5-foot intervals for possible laboratory analyses and to characterize the geology beneath the site Samples ' were logged using visual and manual methods, and classified according to the Unified Soil Classification System Sediments encountered are shown on the logs of exploratory borings presented in Appendix B Drill cuttings were stockpiled on site and covered with plastic sheeting On May 8, 1995, the geologist collected soil samples from the stockpiled soil (one composite of four samples) for laboratory analyses to evaluate disposal options Because the air sparge well was installed adjacent to well MW-14 and in an area of known ' SAC\N\Pi0k0F9010F900020 4cs-95 PAS 2