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LABORATORY LOWER DETECTION LIMITS FOR OXYGENANTS t-Butanol (TBA) 100 ug/L Methyl-t-butyl ether (MTBE) 5 ug/L Di-isopropyl ether (DIPE) 5 ug/L Ethyl-t-butyl ether (ETBE) 5 ug/L t-Amylmethyl ether (TAME) 5 ug/L - MW-2 had elevated lower detection limits due to high levels of non-target compounds (TBA 10000 ug/L, MTBE 500 ug/L, DIPE 500 ug/L, ETBE 500 ug/L and TAME 500 ug/L). Groundwater samples obtained for the December 1997 Quarterly sampling were cross contaminated by the submersible pump used to collect representative Dissolved Oxygen samples. These samples are not representative of the site and should be ignored when evaluating the site in reference to the dissolved TPHg and BTEX in water compounds. Since that time two quarterly sampling events have occurred and demonstrate that the levels indicated during the December, 1997 sampling are not representative. A water sample of the final rinse of the pump was obtained and indicates that all or some of the elevated concentrations of TPHg and BTEX in all wells could have resulted from the submersible pump decon practices. The wells were sampled in the following order: MW9, MW7, MW6, MW4, MW5, and MW 11. These wells in the past have all been below laboratory lower detection limits. The following wells which have shown petroleum hydrocarbons in past samplings were then sampled in the following order: MW12, MW8, MW1, MW2, MW13, MW3, and MW10. The pump (Grundfos Redi-Flog) was washed and rinsed after each well. The final rinse was sampled and contained the following concentrations TPHg 0.12 mg/L, Benzene 1.6 ug/L, Toluene 12 ug/L, Ethylbenzene 5.5 ug/L, Xylenes 39 ug/L and MTBE <5ug/L. This pump was used instead of LTT vacuum lift pump in order to obtain representative Dissolved Oxygen samples. SITE LOCATION Former Regal Station #603, is an operating EXXON service station located at 6425 Pacific Avenue at the northwest comer of Pacific Avenue and Douglas Road, Stockton, in San Joaquin County, California. The site lies in section 21; T2N; R6E; MDB&M at approximately 12 feet above mean sea level (MSL), see Figures 1, 2 and 3. The parcel is #31 and is recorded in Assessor's Book 97. Chronology of remedial actions for Former Regal Station R603 6425 Pacific Avenue, Stockton, California 95207 October 24, 1988 Ust's and dispensing piping removed, first discovery of gasoline tainted soil. December 9, 1988 Workplan to drill and sample six exploratory borings and install 3 groundwater monitoring wells (MW1, MW2 and MW3). 2 R603 Update #52/CLOSURE RV 7198