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FLrestone Store#3573 400 North El Dorado Street, Stockton, California October 25, 1"S VES Quarterly Report and Request for Closure--4so Page 1 of 14 r 1 .0 INTRODUCTION In 1967, four underground storage tanks (USTs), numbered 1 through 4, were closed-In- place at this site in accordance with applicable regulations. In April 1991, a total of twelve USTs were removed from the site including Tank 4 (previously closed-in-place) which was immediately adjacent to Tank 5 at the facility boundary at North El Dorado Street. Laboratory analyses of samples collected from the UST excavations satisfied regulatory compliance objectives at all locations except for the area surrounding Tanks 4 and 5. The current remediation activity is being performed in this area. This report contains operational information and monitoring results for the second quarter of soil vapor extraction system (SVES) operation at the subject site, and presents system performance data, analytical evidence, and regulatory arguments supporting final shutdown of the extraction system and site closure. The second quarter monitoring period was from February to May of 1995. System startup information and the initial quarterly monitoring results can be reviewed in the letter report submitted by Ryan-Murphy, Inc. (RMI) to the San Joaquin County on May 18, 1995 (Appendix 7.6). 2.0 SECOND QUARTER MONITORING RESULTS System off-vapor sampling has been performed weekly throughout the monitoring period to satisfy air permit requirements (post-carbon treatment effluent samples), and to establish and verify a reduction of volatile organic compound (VOC) concentrations within the extracted soil vapor (pre-carbon treatment influent samples). Vacuum has been applied to the two vapor extraction wells at this site which include the deep well screened from 18- to 28-feet below grade (BG) in native soil (VE-1), and the shallow well screened from 8- to 13-feet BG in non-native gravel backfill material within the former UST pit (VE-2) (Site Diagram, Appendix 7 1) VE-1 was the primary extraction pant throughout SVES operation Use of VE-2 was initiated on February 27, 1995, but was shut down on the same day due to the absence of VOCs within the vapor stream. The system was shutdown during the monitoring period for approximately one month (March 24 through April 21, 1995) to allow subsurface vapor conditions to equilibrate to normal temperatures and pressures Following that equilibration period, the system was re-started to investigate for elevated VOC concentrations as a result of the shutdown period. System shutdown was performed as specified in the approved soil remediation plan sent to San Joaquin County on February 2, 1995. All vapor samples have been analyzed for total petroleum hydrocarbons within the gasoline range (TPH-G) by Modified EPA Method 8015, and benzene-toluene- ethylbenzene-xylene (BTEX) by Modified EPA Method 8020. RYAN M[1RPSY, INCURPORATM Dm"r FAMIRVUMeEtill C,oINtractioo NW01% amen( IiOs