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Excavation and Second Quarter Post Treatment Monitoring and Sampling Report 2 The Southland C ton Store#20304 455 Grant Line Road Tracy,CA June 9 1998 In October 1996,additional recovery wells GWE-2 and GWE-3 were installed and added to the remediation system Benzene and TPH-G were detected in the soil boring for well GWE-3 at 3 7 mg/kg and 2,000 mg/kg,respectively,at 5 feet BGS Quarterly groundwater monitoring and sampling of all site-related wells has been on going since July 1988 Graphs of benzene versus water table elevation (WTE)are presented in Figures 5 through 8 for source wells MW-1,MW-8,MW-9,and MW-10 These graphs indicate an Inconsistent relationship between benzene and WTE,In MW-1 there appears to be a direct relationship,while in MW-9 and MW-10 an inverse relationship is apparent Data indicate that the dissolved hydrocarbon plume beneath the site is stable Benzene and TPH-G concentrations have fluctuated but have generally decreased in the source wells MW-1,MW-8, MW-9, and MW-10 The dissolved hydrocarbon plume beneath the site extends laterally from the former UST pit and dispenser island northeast(downgradient)to well MW-11 and MW-10 Groundwater extraction well GWE-1, located downgradient of MW-10, has not contained detectable benzene and TPH-G concentrations in samples collected since March 23, 1996 Downgradient well MW-5 has not contained detectable benzene or TPH-G concentrations in any of the samples collected since March 20, 1996 Dissolved oxygen readings have been collected from the sate monitoring wells since November 1997 Relatively high dissolved oxygen readings up to 5 8 milligrams per liter(mg/L) have been measured in upgradient monitoring wells MW-4,MW-3,and MW-6,whereas relatively low dissolved oxygen readings(as low as 1 mg/L) have been measured in the source area and downgradient wells MW-1,MW-8, MW-9,MW- 10, W10,GWE-1,MW-16,and MW-5 The low dissolved oxygen readings indicate natural biodegradation of dissolved hydrocarbons in those areas Soil excavation was chosen as the best method of remedial action to remove the remaining residual gasoline hydrocarbons in sod and groundwater beneath the site Several attempts at alternative remedial techniques,including groundwater and sod vapor extraction to remove hydrocarbons at the site were unsuccessful due rapid groundwater recharge and shallow depth to water 1.2 Hydrogeology Tracy is located in the northwestern part of the San Joaquin Valley,Califomia, a broad asymmetric trough filled with Cretaceous and Cenozoic sediments The Quaternary sediments in the Tracy area,were derived from the Coast Ranges to the west The site Is situated at approximately 25 feet above mean sea level on a distal northern part of an alluvial fan The shallow groundwater at the site shows seasonal fluctuation from approximately 3 to 6 feet BGS 20304ex,1101 FLUOR DANIEL GTI