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I. INTRODUCTION
<br /> Former Exxon Retail Site (RS) 7-3942 is a vacant lot located at 4444 North Pershing Avenue,
<br /> Stockton, California, on the southeast comer of the intersection of North Pershing Avenue and
<br /> Rosemarie Lane (Figure I). Used oil and three grades of gasoline were stored in four
<br /> underground 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
<br /> and northwest comers, respectively, of the intersection of North Pershing Avenue and Rosemarie
<br /> Lane. Previous laboratory analyses have consistently detected Total Petroleum Hydrocarbons as
<br /> gasoline (TPH-g) and benzene, toluene, ethylbenzene, and total xylenes (BTEX)in groundwater
<br /> samples collected from MW8 and MW9, which are located adjacent to the Shell site and the 7-11
<br /> site, upgradient and crossgradient of former Exxon RS 7-3942.
<br /> On 11 November 1997, groundwater was purged and sampled from 10 groundwater monitoring
<br /> wells (MW2, MW4, MW8-MW13, UVB-MW1, UVB-MW2), three groundwater recovery
<br /> wells (RWI, RW2, and RW4), two soil vapor extraction wells (V2, V3), two piezometers (P1,
<br /> P2), and one injection well (lWI). Monitoring wells MW3, MW5, MW6, and MW7 and vapor
<br /> extraction well V 1 were not sampled on 11 November 1997, because they did not contain
<br /> IQ measurable quantities of water. Figure 1 shows the locations of former site facilities and the
<br /> locations of all Exxon wells on and off the site, except MW 11. The location of MW 11 is shown
<br /> in Figure 2.
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<br /> 2. SUMMARY OF RESULTS
<br /> 4On 11 November 1997, 25 wells were monitored for the presence of liquid-phase hydrocarbons,
<br /> 14 using an optical interface probe. A sheen was noted in well V4. The depths to water ranged
<br /> from 18.98 (MW8) to 24.83 (MW 11) feet below ground surface. Groundwater elevations ranged
<br /> from-15.12 (MW 13) to -12.53 (MW8) feet relative to mean sea level. The approximate
<br /> groundwater flow direction on 11 November was to the north, with a gradient of
<br /> approximately 0.005 to 0.01 (Figure 3), which is consistent with previous groundwater
<br /> monitoring data for the site. Groundwater elevations on 11 November are presented in boldface
<br /> type in Table 1, along with previous data, and are shown in Figure 3. Field documents are
<br /> 14 included as Appendix A.
<br /> After the depths to water were measured, wells MW2, MW4, MW8-MW 13, RW 1, RW2, RW4,
<br /> V2, V3,Pl, P2, IW 1, UVB-MW 1, and UVB-MW2 were purged by vacuum truck and sampled.
<br /> At least 3 well casing volumes of purge water were removed from each well. After purging,
<br /> 74 groundwater samples were collected with factory-cleaned disposable polyethylene bailers.
<br /> Groundwater samples were collected in 40-m1 VOA vials. The vials were labeled and stored in an
<br /> ice-filled cooler and transported under chain of custody. The groundwater samples, a
<br /> field-prepared rinse blank, and a laboratory-prepared trip blank were submitted to Sequoia
<br /> Analytical, where they were analyzed for TPH-g by California DHS-modified EPA Method 8015
<br /> and for BTEX by EPA Method 8020. The samples were also analyzed for methyl t-butyl
<br /> ether(MTBE) by EPA Method 8020. The MTBE results for the samples collected from wells
<br /> WP61.73942.Q1197.TX.2 I
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