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r • benzene, toluene, eth Ibenzene, and xylenes (BTEX) EPA Method 8020 The samples were also analyzed for methyl t-butyl ether (MTBE) by EPA Method 8020 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 Detectable concentrations of TPH-g in the groundwater samples ranged from 63 µg/L (MW 10) to 270,000µg/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 35,000,ug/L Detectable benzene concentrations ranged from 2 1 kig/L (MW 10) to 12,000µg/L (UVB-MW2) MTBE was detected in groundwater samples at concentrations ranging from 11 µglL(P2) to 1,300µg/L (UVB-MW2) Groundwater sample analytical results are shown in Figure 1 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 The UVB system was not in operation from 9 July 1997 to 11 September 1997 due to a quarterly maintenance of the air stripper components for the well The vapor extraction system (VES) was operated while the UVB system was shut down The VES and UVB cannot be run concurrently Since the start-up of the UVB system, approximately 342 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 3 Twice since the start-up of the UVB system, on 28 February 1997 and on 8 May 1997, the influent (UVB-MWI) and effluent (UVB-MW2) monitoring wells were sampled for TPH-g, BTEX, and MTBE analysis, along with the other wells on the site, to determine the stripping efficiency of the UVB system On 8 May 1997, the stripping efficiency for TPH-g was 99 9 percent, for BTEX the stripping efficiency was 100 percent As can be seen in Table 1, effluent (UVB-MW2) samples typically have TPH-g and BTEX concentrations at least one order of magnitude lower than the corresponding concentrations in influent (UVB-MW 1) samples The analytical results for influent (UVB-MW 1) and effluent (UVB-MW2) samples collected on 26 August 1997 are surprising, because they indicate that TPH-g and BTEX concentrations in the effluent are higher than in the influent It is possible that the current results for UVB-MW 1 and UVB-MW2 are a result of sample mislabeling The UVB system was not in operation when the samples were collected In order to verify the analytical results in question for UVB-MW I and UVB-MW2, grab samples were collected from the two wells on 2 October 1997 and submitted for TPH-g, BTEX, and MTBE analysis by EPA Methods 8015/8020 The grab sample analytical results indicate that influent and effluent concentrations were 17,000µg/L and below the reporting limit (50,ug/L) for TPH-g, correspondingly The Iaboratory analytical report and chain- of-custody documentation for the grab samples are presented in Appendix B WP61 73942 Q597 TX 4 2