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<br />' 1. INTRODUCTION
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<br />' Former Exxon Retail Site (RS) 7-3942 is a vacant lot located at 4444 North Pershing Avenue,
<br /> Stockton, 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 corners, 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), and benzene, toluene, ethylbenzene, and total xylenes (BTEX) in groundwater samples
<br />' collected from MW8 and MW9, which are located adjacent to the Shell site and the 7-11 site,
<br /> upgradient and crossgradlent of former Exxon RS 7-3942
<br />' On 20 August 1999, 12 groundwater monitoring wells (MWI through MW9, MW13, UVB-MW1,
<br /> UVB-MW2), two groundwater recovery wells (RW 1, RW2), three soil vapor extraction wells (V 1,
<br />' V2, and V4), one piezometer(P2), and one infection well (IW1)were monitored for the presence of
<br /> liquid-phase hydrocarbons (LPH) Groundwater samples were collected from wells MW2, MW4,
<br /> MW5, UVB-MW 1, UVB-MW2, RW I, RW2, V2, V4, and P2 Figure I shows the locations of
<br />' former site facilities and the locations of all Exxon wells on and off the site, except MWI I The
<br /> location of MWI 1 is shown in Figure 2
<br /> The following is the proposed annual groundwater monitoring schedule for the site Wells MWl-
<br /> MW10, MW12, MW13, IW1, UVB-MWI, UVB-MW2, and RW4 will be gauged quarterly Well
<br /> MW11 will be gauged once per year during the fourth quarter Wells MW4-MW7, IW1, RWI,
<br />' RW2, RW4, UVB-MWI, UVB-MW2, and V4 will be sampled quarterly Wells MW8-MW13 will
<br /> be sampled once per year during the fourth quarter The chemicals analyzed for will remain the
<br /> same (BTEX, TPH-g, methyl t-butyl ether [MTBE])
<br /> 2 SUMMARY OF RESULTS
<br />' On 20 August 1999, 19 wells were monitored for the presence of LPH, using an optical interface
<br /> probe The depth to groundwater was gauged, and groundwater elevations were calculated The
<br /> approximate groundwater flow direction on 20 August was to the northeast, with a gradient of
<br />' approximately 0 003 to 0 009 (Figure 3), which is consistent with previous groundwater monitoring
<br /> data for the site Groundwater elevations on 20 August are presented in boldface type in Table 1,
<br /> along with previous data, and are shown in Figure 3 Field documents are included as Appendix A
<br />' After the depths to water were measured, wells MW2, MW4, MW5, RW 1, RW2, V2, V4, P2, UVB-
<br /> MWI, and UVB-MW2 were purged Approximately 3 well casing volumes of purge water were
<br />' removed from each well After purging, groundwater samples were collected with factory-cleaned
<br /> disposable polyethylene 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 and a laboratory-prepared trip blank were submitted to Sequoia Analytical,
<br /> where they were analyzed for TPH-g by California EPA-modified EPA Method 8015 and for BTEX
<br /> and MTBE by EPA Method 8020 The analytical results for the samples are presented in boldface
<br />' type in Table 1, along with previous data The laboratory analytical report and chain-of-custody
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