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'1 -- Iwo Ms. Mary Meays Lip December 20, 1995 (9347-3011) ini0ft Findings and Conclusions f Following are the findings and conclusions based on an evaluation of the results of current and 1 previous investigations, regulatory communications, and technical publications: • Of the nine confirmation soil borings drilled in September 1992, TPH-G was only detected in the soil samples collected from CB8 and CB9. Benzene was not detected above reported laboratory detection limits in any of the samples collected and analyzed from the nine additional borings. Borings CB 1 through CB9 were drilled after the VES system had been in operation for less than 1 year. A VES has been operated at the site since January 1992, excluding down periods during treatment system replacement, carbon change-outs, and routine operation and maintenance. • Benzene was detected at up to 0.8 parts per billion (ppb) only in the water samples collected from wells MW-3 and MW-5 in November 1995. TPH-G was not detected above reported detection r limits in the water samples collected from wells MW-1 through MW-5, RW-1, and RW-2. Toluene, ethylbenzene, and total xylenes were detected at 1, 1.2, and 2.3 ppb only in the water sample collected from well MW-3. • TPH G and BTEX constituents were not detected above the California Primary Maximum Contaminant Level (MCL) goal for drinking water, the United States Environmental Protection Agency taste and odor thresholds, or the California State Water Resource Control Board taste and odor thresholds (Marshack, 1993) in the water samples collected from the wells at the site during the November 1995 ground water sampling event. • The fuel USTs were removed from the site in 1981. The residual sources of petroleum hydrocarbons .in the subsurface soil and ground water have been removed from the site by the operation of the VES and ground water recovery system. • The VES has removed all of the petroleum hydrocarbons that can be practically and economically remediated by vapor extraction. The petroleum hydrocarbon extraction rate of the vent system has dropped to approximately one-half of 1 percent of the original startup rate of extraction with a corresponding drop in efficiency. The VES petroleum hydrocarbon extraction rates and cumulative extraction have achieved asymptotic limits. • Since the VES petroleum hydrocarbon extraction rates and cumulative extraction have achieved asymptotic limits, continued operation of the VES will not significantly improve the quality of the ground water at this site. In addition, review of the VES radius of influences and the results of the September 1992 confirmation boring investigation demonstrates that VES was an efficient and effective remedial measure at this site. 93473411.BRP -5-