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3. SOIL VAPOR EXTRACTION AND TREATMENT SYSTEM ' A vapor extraction and treatment (VET) system has been operating at the site since May 1994 The system extracts of soil vapor from the vadose zone through seven wells (V 1, V2, V4, MW through MW6) and treats the extracted hydrocarbons with an oxidizer The system is augmented ' with air sparging equipment that infects air below the water table in infection well IW 1 The oxidizer was converted from thermal to catalytic operation in May 1995, following a steady decline in extracted soil vapor concentrations, as shown in Figure 4 and in Table 2 The decline ' in extracted soil vapor concentrations has continued since the conversion to catalytic operation ' During the current quarter, oxidizer influent samples did not contain TPH-g above the method detection limit (mdl) of 10 mg/L, but benzene was detected at 0 052 µg/L (mdl = 0 05 ,ug1L) The consistently low levels of TPH-g and benzene concentrations observed during the past year in the ' oxidizer influent indicate significant progress in the cleanup of the vadose zone ' 4. WORK PROPOSED FOR NEXT QUARTER ' Groundwater elevations will be measured and groundwater samples will be collected in August 1996 The samples will be analyzed for TPH-g, BTEX, and MTBE Because of the proximity of wells IW1 and MW4, well P1 is sampled only twice a year Similarly, because of the proximity of wells IW 1 and RW2, well MW2 is sampled only twice a ' year On the same basis, well MW 1 is sampled only twice a year, because of its proximity to well P2 Because concentrations of petroleum hydrocarbons in wells MW 12 and MW 13 have been consistently below method detection limits for the last five quarterly sampling events, those ' wells will only be sampled once a year Next quarter, in August 1996, the following wells will be sampled MW2 through MW 10, ' recovery wells RW 1 through RW4, piezometers P1 and P2, and infection well IW i Submerged vapor wells V2 through V4 will be sampled if there is no sheen present 1 ' W%1 73942C,196 TX 5 3