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,f Quik Stop Markets, Inc. Page 1 Project No. F7132.32 4 APPENDIX A SITE BACKGROUND Quik Stop Markets 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 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 Exceltech, Inc. (Exceltech), installed a vadose monitoring system around the tanks at the site. This system detected hydrocarbon contamination in July 1990. Gasoline--contaminated backfill was removed from the product line trenches and samples of the native soil at the bottoms 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 p ry bonngs 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 po samples from three borings showed little or no petroleum hydrocarbon contamination. Soil samples from one boring, near the northwest portion of the north pump island (13-3) 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, ethyl benzene, 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. 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 1 ppb benzene and detectable levels of ethyl benzene 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, 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, November 1990). New double-walled fiberglass USTs were installed at the site and the remaining excavation was backfilled with clean fill. The canopy, pumps, and concrete surface were replaced and the station put back in operation in March 1991.