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I• INTRODUCTION
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<br /> Former Exxon Retail Site (RS) 7-3942 is a vacant lot located at 4444 North Pershing Avenue in
<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), benzene, toluene, ethylbenzene, and total xylenes (BTEX), and methyl t-butyl ether
<br /> (MTBE) in groundwater samples collected from MW8 and MW9, which are located adjacent to the
<br /> Shell site and the 7-11 site, upgradient and crossgradient of former Exxon RS 7-3942
<br /> A risk assessment work plan for the site was submitted to San Joaquin County Public Health
<br /> Services in October 2000 The proposed work was performed in December 2000
<br /> 2. GROUNDWATER MONITORING
<br /> On 21 June 2001, groundwater monitoring wells MW1-MW10, MW12, MW13, UVB-MW1, and
<br />' UVB-MW2, groundwater recovery wells RWI, RW2, and RW4, soil vapor extraction wells V1-V4,
<br /> and injection well IW1 were gauged Groundwater samples were collected from wells MWI-
<br />' MW 10,MW 12, MW13, WB-MW I, UVB-MW2, RWI, RW2, RW4, V1, V3, and IW 1 Figure 1
<br /> shows the locations of former site facilities and the Iocations of all ExxonMobil wells on and off the
<br /> site The locations of offsite monitoring wells MW8, MW9,MW 11, and MW13 are shown in Figure
<br /> 2
<br /> On 21 June 2001, wells were monitored for the presence of liquid-phase hydrocarbons (LPH) using
<br /> I an optical interface probe A thickness of 0 02 feet of LPH was measured in wells V2, V4, and
<br /> UVB-MW1 LPH was not detected in any other well The depth to groundwater was gauged, and
<br /> groundwater elevations were calculated The approximate groundwater flow direction on 21 June
<br /> was to the northeast, with a gradient of approximately 0 003 to 0 006, which is consistent with
<br /> previous groundwater monitoring data for the site Current groundwater elevations are presented
<br /> in boldface type in Table 1, along with previous data, and are shown in Figure I Copies of the field
<br /> 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 dewatered during purging were
<br /> allowed to recover prior to sampling After purging, groundwater samples were collected with
<br /> factory-cleaned disposable bailers Groundwater samples were collected in 40-m1 glass VOA vials
<br /> The 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 Samples in which MTBE was detected were also analyzed for MTBE by EPA Method
<br /> 8260B for confirmation The analytical results for the samples are presented in boldface type in
<br /> Table 1, along with previous data The laboratory analytical report and chain-of-custody
<br /> I documentation are included in Appendix B Groundwater sample analytical results are shown in
<br /> Figure I
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