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are not representative of the site and should be ignored when evaluating the site in reference to the
<br /> dissolved TPHg and BTEX in water compounds A water sample of the final rinse of the pump was
<br /> obtained and indicates that all or some of the elevated concentrations of TPHg and BTEX in all
<br /> wells could have resulted from the submersible pump decon practices The wells were sampled in
<br /> the following order MW9, MW7, MW6, MW4, MW5, and MW 11 These wells in the past have all
<br /> been below laboratory lower detection limits The following wells which have shown petroleum
<br /> hydrocarbons in past samplings were then sampled in the following order MW12, MW8, MW1,
<br /> MW2, MW13, MW3, and MW10 The pump (Grundfos Redi-Flog) was washed and rinsed after
<br /> each well The final rinse was sampled and contained the following concentrations TPHg 0 12
<br /> mg/L, Benzene 16 ug/L, Toluene 12 ug/L, Ethylbenzene 5 5 ug/L, Xylenes 39 ug/L and MTBE
<br /> <5ug/L This pump was used instead of LTT vacuum lift pump in order to obtain representative
<br /> Dissolved Oxygen samples
<br /> SITE LOCATION
<br /> Former Regal Station 4603, is an operating EXXON service station located at 6425 Pacific Avenue
<br /> at the northwest corner of Pacific Avenue and Douglas Road, Stockton, in San Joaquin County,
<br /> California The site lies in section 21, T2N, R6E, MDB&M at approximately 12 feet above mean
<br /> sea level (MSL), see Figures 1, 2 and 3 The parcel is #31 and is recorded in Assessor's Book 97
<br /> GROUNDWATER MONITORING
<br /> Depths to groundwater measurements were obtained from all monitor wells associated with this site
<br /> (MW1, MW2, MW3, MW4, MW5, MW6, MW7, MW8, MW9, MW10, MW11, MW12 and MW13)
<br /> on March 11, 1998 No vacuum influence was measured, the vapor extraction/sparging system had
<br /> been non-operative for eight days, see Appendix A - Field Notes and Methods and Procedures
<br /> Figure 4 represents the groundwater gradient obtained from measurements obtained on March 11,
<br /> 1998 and shows a groundwater low (trough) with depressions at MW i and MW3 flowing inward in
<br /> all directions
<br /> The most recent groundwater quarterly sampled only the wells that historically have contained TPHg
<br /> (MWi, MW2, MW3, MW8, MW10 and MW12) These samples were analyzed for Total Petroleum
<br /> Hydrocarbons as gasoline (TPHg), Benzene, Toluene, Ethylbenzene, Xylenes and Methyl tertiary
<br /> Butyl Ether (MBTEX) using EPA method 8020, see Table 1 and Figure 8
<br /> The groundwater plume of dissolved gasoline range hydrocarbons has been reduced in size and
<br /> concentrations, the following figures historically display the plume reduction stability
<br /> Figure 5 represents TPHg and Benzene measurements obtained in March 1991
<br /> Figure 6 represents TPHg and Benzene measurements obtained in March 1994
<br /> 2 R603 Update#51 5198
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