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Sal Excavation and Second Quarter Post-Treatment Monitoring and Sampling Report 7 • The Southland Corporation,Store#20304,455 Grant Line Road,Tracy,CA June 9,1998 5.2 Monitoring Well Sampling and Analytical Results Groundwater samples were collected from wells MW-4,MW-5,and newly installed well MW-16 Before sampling, at least four well-casing volumes of groundwater was removed from each monitonng well, unless the well was purged dry Field measurements of temperature,conductivity, and pH were recorded during well purging The groundwater samples were collected using disposable bailers and placed in analysts-specific sample containers with analysis-specific preservatives The samples were then sealed and labeled with the fob identification number,the sample number,date,time, and type of laboratory analysis required The samples were stored on ice in an insulated container and transported under chain-of-custody manifest to Core Laboratories, in Anaheim, California The groundwater samples were analyzed for BTF_X,MTBE and TPH-G using Environmental Protection Agency(EPA) methods 8020/modified 8015,and for oxygenates MTBE,TBA, DIPE, ETBE, and TAME using EPA method 8260 Groundwater purged during the second quarter monitoring and sampling event was placed in Department of Transportation (DOT)-approved drums and left on site pending transportation and disposal at a licensed facility Dissolved benzene and TPH-G were detected in groundwater samples from well MWA 6 at concentrations of 730 microgram per Iter(pg/L) and 13,000 pg/L, respectively(Figure 3) Using EPA method 8260, dissolved MTBE was not reported in any well above detection limits TBA, DIPE, ETBE, and TAME were not detected in any wells Analytical results from the groundwater samples collected on May 15, 1998 are • presented in Table 2,and historical analytical results are summarized in Table 2A The laboratory analytical reports and the chain-of-custody records for this sampling event are included in Appendix D 6 0 CONCLUSION AND CLOSURE ABSTRACT On April 28,29, and 30, f=lour Daniel GTI supervised the excavation of approximately 270 cubic yards of hydrocarbon impacted sod northeast(downgradient) of the former UST pit and beneath the former fuel dispenser island (Figure 2) Groundwater monitoring wells MW-8, MW-9,MW-10,and groundwater extraction well GWE-3 located within the excavation area were properly destroyed prior to the excavation activities On May 8, 1998,groundwater monrtonng well MW-16 was installed in the excavation northeast of the former UST pit to replace monitoring wells MW-8,MW-9, and MWA 0 Soil analytical results from the recent excavation and from previous site investigations indicate that the vertical and lateral extend of the soil hydrocarbon plume beneath the site has been defined Benzene and TPH-G have not been detected below a depth of 8 feet BGS in any of the soil samples collected at the site with the exception of a sample collected in 1990 from groundwater extraction well GWE-1 at 15 feet BGS which contained a benzene concentration of 0 0006 mg/kg (Table 1) Historical data indicate that the sod hydrocarbon plume extends laterally from the former UST pit and dispenser island northeast (downgradient)to the approximate location of well MW-5 iThe current dissolved hydrocarbon plume beneath the site extends laterally from the former UST pit and dispenser island northeast(downgradient)to well MWA and the former location of well MWA 0 FLUOR DANIEL GTI 20301ex.rpf