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submitted to Sequoia Analytical, where they were analyzed for TPH-g by California EPA-modified EPA Method 8015 and for BTEX by EPA Method 8020 The samples were also analyzed for methyl t-butyl ether (MTBE) by EPA Method 8020 and confirmed by using EPA Method 8260, this was done at the request of Harlin Knoll of the San Joaquin County Environmental Health Services The analytical results for the samples are presented in boldface type in Table 1, along with previous data The laboratory analytical report and chain-of-custody documentation are included in Appendix B TPH-g, BTEX, and MTBE were not detected in the samples collected from wells MW3, MWS, MW6 and MWI I—MW13 at concentrations above the method detection limits Detectable concentrations of TPH-g in the remaining groundwater samples ranged from 79 ,ug/L (MW7) to 150,000�zg/L (V2) The concentration of TPH-g in the groundwater sample collected from MW8, located upgradient of the site and adjacent to the Shell service station, was 26,000 kig/L Detectable benzene concentrations ranged from 1 4 jzg/L (MW2) to 13,000 mg/L (UVB---MWI) MTBE, by EPA Method 8020, was detected in groundwater samples at concentrations ranging from 3 3 ,ug/L (MW2)to 3,200 EZg/L. (UVB—MWl) The ' laboratory results for MTBE using EPA Method 8260 ranged from 3 7 ,ug/L (P1) to 68 yg/L (MW9) Groundwater sample analytical results are shown in Figures 1 and 2 3 GROUNDWATER TREATMENT SYSTEM A vacuum vaporizer well system, known as UVB (Unterdruck Verdampfer Brunnen) has been operating at the site since 6 February 1997 A description of the UVB system is included in an installation report issued in June 1997 During the current quarter the UVB system operated continuously Since the start-up of the UVB system, approximately 612 pounds of TPH-g have been stripped from the groundwater Table 2 provides a summary of field data and laboratory analytical results for the UVB system A graphic representation of the UVB system performance to date is shown in Figure 4 The analytical results of the current quarterly groundwater sampling event indicate a resurgence in petroleum hydrocarbon concentrations in downgradzent wells MW10, V3, and RW4 From the start-up of the UVB system in February 1997 until this quarter, these three wells had successive reductions in TPH-g concentrations (more than one order of magnitude at MW10 and RW4) which clearly demonstrated that the WB system successfully controlled further offsite migration The resurgence in downgradient TPH-g concentrations during the latest event is a result of an approximate 5-feet increase 2 n water Ievels at the site The seasonal water level increase resulted in a decreased air flow through the UVB air stripper, and consequently less efficient hydrocarbon removal The system will be closely monitored for an increase in air flow and stripped hydrocarbon concentrations, concurrent with an anticipated seasonal decrease in water levels ' WP61 73942 Q398 TX 2 2