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• I• INTRODUCTION
<br /> Former Exxon Retail Site (RS) 7-3942 is a vacant lot located at 4444 North Pershing Avenue in
<br /> IStockton, California, on the southeast corner of the intersection of North Pershing Avenue and
<br /> Rosemarie Lane (Figure 1) Used oil and three grades of gasoline were stored in four underground
<br /> storage tanks, which were removed in July 1986
<br /> A Shell service station and a 7-11 food and gasoline service store are located on the southwest and
<br />' northwest comers, respectively, of the intersection of North Pershing Avenue and Rosemarie Lane
<br /> Previous laboratory analyses have consistently detected Total Petroleum Hydrocarbons as gasoline
<br /> (TPH-g), benzene, toluene, ethylbenzene, and total xylenes (BTEX), and methyl t-butyl ether
<br />' (MTBE) in groundwater samples collected from MW8 and MW9, winch are located adjacent to the
<br /> Shell site and the 7-11 site, upgradient and crossgradzent of former Exxon RS 7-3942
<br /> 2. GROUNDWATER MONITORING
<br /> On 11 September 2000, groundwater monitoring wells MW1-MW7,MW10, MW13, UVB-MW1,
<br />' and UVB-MW2, groundwater recovery wells RW1, RW2 and RW4, soil vapor extraction wells V3
<br /> and V4, and injection well IW1 were gauged for depth to water and for the presence of liquid-phase
<br /> hydrocarbons (LPH) Groundwater samples were collected from wells MW1-MW7, MW10, UVB-
<br />' MW1, WB-MW2, RW1, RW2, RW4, V3, and IW1 Figure 1 shows the locations of former site
<br /> facilities and the locations of all Exxon_Mobil wells on and off the site The locations of offsite
<br /> . monitoring wells MW8, MW9, MW 11, and MW 13 are shown in Figure 2
<br />' On 11 September, 17 wells were monitored for the presence of LPH, using an optical interface
<br /> probe LPH was detected in well V4, at a thickness of 0 05 feet The depth to groundwater was
<br />' gauged, and groundwater elevations were calculated The approximate groundwater flow direction
<br /> on 11 September was to the northeast, with a gradient of approximately 0 004 to 0 006, which is
<br /> consistent with previous groundwater monitoring data for the site Groundwater elevations on 11
<br />' September are presented in boldface type xn Table 1, along with previous data, and are shown in
<br /> Figure 1 Field documents are included as Appendix A
<br />' After the depths to water were measured, the wells were purged Approximately 3 well casing
<br /> volumes of purge water were removed from each well Wells that purged dry were allowed to
<br /> recover at least 80% prior to sampling After purging, groundwater samples were collected with
<br />' factory-cleaned disposable bailers Groundwater samples were collected in 40-m1 VOA vials The
<br /> vials were labeled and stored in an ice-filled cooler and transported under chain of custody The
<br /> groundwater samples were submitted to SPL Inc , in Houston, Texas, where they were analyzed for
<br />' TPH-g by California EPA-modified EPA Method 8015 and for BTEX and MTBE by EPA Method
<br /> 8021B The analytical results for the samples are presented in boldface type in Table 1, along with
<br /> previous data The laboratory analytical report and chain-of-custody documentation are included
<br />' in Appendix B Groundwater sample analytical results are shown in Figure 1
<br /> 3. GROUNDWATER TREATMENT SYSTEM OPERATION
<br />' A vacuum vaporizer wells stem, known as UVB (Unterdruck Verdam fer Brunnen , began
<br /> operating at the site on 6 February 1997 A description of the UVB system is included in an
<br />' installation report issued in June 1997 The UVB system operated continuously during the third
<br /> F SP1-J a(91739a21NIAS rFR\WMQ9004.doe 1
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