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Samples were analyzed by the District's laboratory for BTEX by United States Environmental Protection Agency (USEPA) SW-846 Method 8020, and TPH-g by USEPA SW-846 modified Method 8015 The groundwater samples from MW-8 and MW-1 were also analyzed for total lead by USEPA Method 6010 Quality Control samples were collected to provide a check on the sampling, ' analysis, and shipping procedures A split sample was collected from MW-8 and analyzed for BTEX and TPH-g by an outside non-District laboratory to provide a check on the analytical procedures A duplicate sample was collected from MW-1 and submitted to the District's laboratory for BTEX, TPH-g, and lead analyses The ' quality control samples were collected and handled In the same manner as the other samples One laboratory prepared trip blank, consisting of one 40 ml vial, was shipped with the collected samples to provide a check on the shipping procedures The trip blank was submitted to the District's laboratory for BTEX analyses Analytical results from fourth quarter 1993 sampling are summarized In ' Table 2 The District's laboratory data sheets are provided in the Appendix The quality control sample results confirmed that the samples had been collected, shipped, ' and analyzed appropriately These results are not included in Table 2, but are included in the Appendix Based on the data collected to date, the approximate petroleum hydrocarbon plume delineation is shown on Figure 4-3 The fourth quarter 1993 sampling results are consistent with the third quarter 1993 and previous sampling results However, a trace concentration (0 5 µg/L) of benzene was detected in the groundwater sample from MW-9 No TPH-g or other BTEX constituents were detected In this sample Typically, to samples in which ' contaminants have been detected, the TPH-g concentration is much greater than the BTEX concentrations If gasoline has )Impacted the groundwater at a well, detection of TPH-g in the sample might be expected prior to, or in addition to, detection of ' BTEX constituents Because this is not the case, the relatively low level reported benzene concentration in the MW-9 groundwater sample is suspect First quarter 1994 groundwater monitoring should confirm or invalidate the results from MW-9 If the reported benzene concentration is confirmed, it is an indication that the ' contaminant plume is migrating The plume delineation map (Figure 4-3) will not be modified to incorporate MW-9 until another sampling event confirms the presence of contaminants in this well ' The analyses of the quality control samples provided good correlation with the matrix samples and did not indicate any problems with sampling, analysis, or shipping procedures 931122 East Bay Municipal Utility District 404296004thgtrgw 5 Stockton Service Center -Fourth Quarter 1993