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I• INTRODUCTION ' Former Exxon Retail Site (RS) 7-3942 is a vacant lot located at 4444 North Pershing Avenue, Stockton, California, on the southeast corner of the intersection of North Pershing Avenue and Rosemarie Lane (Figure 1) Used oil and three grades of gasoline were stored in four underground ' storage tanks, which were removed in July 1986 A Shell service station and a 7-11 food and gasoline service store are located on the southwest and ' northwest corners, respectively, of the intersection of North Pershing Avenue and Rosemarie Lane Previous laboratory analyses have consistently detected Total Petroleum Hydrocarbons as gasoline (TPH-g), benzene, toluene, ethylbenzene, and total xylenes (BTEX), and methyl t-butyl ether ' (MTBE) in groundwater samples collected from MW8 and MW9,which are located adjacent to the Shell site and the 7-11 site, upgradient and crossgradient of former Exxon RS 7-3942 ' 2 GROUNDWATER MONITORING ' On 16 May 2000, groundwater monitoring wells MWI-MWI3, UVB-MWI, and UVB-MW2, groundwater recovery wells RW1, RW2 and RW4, soil vapor extraction well V4, and injection well IW1 were gauged for depth to water and for the presence of liquid-phase hydrocarbons (LPH) ' Groundwater samples were collected from wells MW2, MW4-MW7, UVB-MWI, UVB-MW2, RW1, RW2, RW4, and IW1 Figure 1 shows the locations of former site facilities and the locations of all ExxonMobil wells on and off the site The locations of offsite monitoring wells MW8, MW9, MWI1, and MW13 are shown in Figure 2 On 16 May, 20 wells were monitored for the presence of LPH, using an optical interface probe LPH was detected in well V4, at a thickness of 0 04 feet The depth to groundwater was gauged, and groundwater elevations were calculated The approximate groundwater flow direction on 16 May was to the northeast, with a gradient of approximately 0 002 to 0 005 (Figure 3), which is consistent with previous groundwater monitoring data for the site Groundwater elevations on 16 May are presented in boldface type in Table 1, along with previous data, and are shown in Figure 3 Field documents are included as Appendix A IAfter the depths to water were measured, the wells were purged Approximately 3 well casing volumes of purge water were removed from each well After purging, groundwater samples were collected with factory-cleaned disposable bailers Groundwater samples were collected in 40-m1 VOA vials The vials were labeled and stored in an ice-filled cooler and transported under chain of custody The groundwater samples and a laboratory-prepared trip blank were submitted to SPL Inc , in Houston, Texas, where they were analyzed for TPH-g by California EPA-modified EPA Method 8015 and for BTEX and MTBE by EPA Method 8021B The analytical results for the samples are presented in boldface type in Table 1, along with previous data The labotatory analytical report and chain-of-custody documentation are included in Appendix B Groundwatei sample analytical results are shown in Figure 1 3. GROUNDWATER TREATMENT SYSTEM OPERATION A vacuum vaporizer well system, known as UVB (Unterdruck Verdampfet Brunnen), has been operating at the site since 6 February 1997 A desciiption of the UVB system is mcluded in an installation report issued in lune 1997 Fhe UVB systenn operated continuously duttng the second I TI"J«Is-t).1_%]11]I X\kVI'V100 t'do 1