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1 ' terminated after 120 minutes due to stabilization of vacuum and PID readings Benzene vapor concentrations were detected only in well VW-2 at 0 075 mg/m3 The eluent vapor sample contained low concentrations of toluene (0 18 mg/m3) and xylenes (0 14 mg/m3) This was due to the high flowrate of vapor through the carbon vessels and was not considered to be breakthrough The results of laboratory analyses of collected vapor samples are presented in Table 5 4.4.4 Remediation System Operations and Maintenance ' Clearwater installed a vapor extraction system (VES) at the site in February 1995 to remove petroleum hydrocarbons from soil beneath the site The VES consisted of a moisture separator, particulate filter, two 200-pound carbon units, a vacuum blower connected via 2-inch schedule 80 1 aboveground PVC piping to wells VW-4 and VW-5, and associated electrical and instrumentation The layout of the vapor extraction system is shown on Figure 5 The VES operated continuously from February 9, 1995 until March 16, 1995, when wells VW-1, VW-2, tand VW-3 were incorporated into the system and the carbon units were removed, as requested by PHS/EHD An inspection was performed by San Joaquin Valley Unified Air Pollution Control ' District who verbally approved system operation The system was restarted on April 20, 1995 and operated continuously until January 18, 1996 ' when the system was shut down The system was monitored weekly in accordance with the conditions of the PTO In addition, VES vapor samples were collected for laboratory analysis of TPHg and BTEX The vapor extraction system has removed approximately 190 pounds of petroleum hydrocarbons from soil beneath the site A summary of analytical data and extraction rates is presented in Table 3 ' 4.4.5 Groundwater Remediation Due to the relatively low levels of dissolved petroleum hydrocarbons detected in the groundwater, ' active groundwater remediation was not initiated at the site Natural biodegradation of petroleum constituents was monitored by conducting quarterly groundwater monitoring and sampling activities ' Since groundwater monitoring was initiated, BTEX constituents were nondetectable in all wells except for two events Toluene, ethylbenzene, and xylenes were detected once in well MW 1 and once in well ' MW4 at concentrations well below their respective maximum contaminant level (MCL) or action level (AL) for drinking TPHg was detected once in well MWl in March 1992 but has been nondetectable in all wells since then All four wells have contained TPHd one or two times since morutonng began, ' but at relatively low levels W 195141*art1y%gr-1995 doc 8 • 1 SMTH