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Working to Restore Nature June 1993 Quarterly Groundwater Sampling & Analysis August 16, 1993 Quik Stop Market No. 132, Stockton, California F7132.93 APPENDIX A SITE BACKGROUND Quik Stop Market No. 132 3555 West Hammer Lane Stockton, California Quik Stop Market No. 132 formerly utilized three underground fuel-storage tanks on-site; two 10,000-gallon tanks contained regular unleaded and formerly leaded gasoline and one 8,000-gallon.tank contained super unleaded gasoline. In 1987 at the request of Quik Stop Markets, Inc. (Quik Stop), RESNA Industries Inc. (RESNA), under our former name of Exceltech, Inc. (Exceltech), installed a vadose monitoring system around the tanks at the site. This system detected hydrocarbon contamination in July 1990. Investigation revealed the possibility of a leak in the product delivery lines. Gasoline-contaminated backfill was found and removed from the product line trenches and samples of the native soil at the bottom of the trenches were collected and analyzed for gasoline components. The results of the analysis revealed that hydrocarbons had penetrated the soil adjacent to the east half of the product line trench which serves the north pump island. RESNA drilled four exploratory borings on the site at this time and converted one boring to a groundwater monitoring well on August 23, 1990. The results of laboratory analyses of soil samples from three borings showed little or no petroleum hydrocarbon contamination. Soil samples from one boring, near the northwest portion of the northern pump island (B-3) contained 460 parts per million (ppm) total petroleum hydrocarbons as gasoline (TPHG). The groundwater sample from monitoring well MW-1 contained 470 parts per billion (ppb) TPHG and 21 ppb benzene as well as detectable levels of toluene, ethylbenzene, and total xylenes (Exceltech, September 1990). On September 11, 12, and 13, 1990, Exceltech drilled five additional exploratory borings at the site and converted three of them to groundwater monitoring wells MW-Z, MW-3 and MW-4. The results of the analyses of the soil samples from these borings revealed little or no petroleum hydrocarbon contamination. Analyses of groundwater samples collected from monitoring wells MW-3 and MW-4 did not find any petroleum hydrocarbon related contaminants but samples from MW-2 were found to contain I ppb benzene and detectable ' levels of ethylbenzene and total xylenes (Exceltech, September 1990). During the above investigations, a leak was discovered in the subsurface piping for the fueling system at the site. Excavations to remove the contaminated soils revealed that the contamination had affected the tank backfill and possibly the soil beneath the tank backfill. Upon issuance of the proper permits, the overhead canopy, pump islands, concrete surface, I and three underground fuel storage tanks (USTs) were removed in October and November 1990. Following tank removal, excavation and soil sampling were continued until laboratory analyses of soil samples indicated that petroleum hydrocarbon contaminated soil above the water table in the vicinity of the storage tank excavation had been removed. All contaminated soil was removed from the station and properly disposed (Exceltech, A-1