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ug/L MTBE MW I I is offsite and cross gradient of the R603 hydrocarbon plume and may represent <br /> the leading edge of other MTBE plume(s) in the area <br /> LABORATORY LOWER DETECTION LIMITS FOR OXYGENANTS <br /> t-Butanol (TBA) 100 ug/L Methyl-t-butyl ether (MTBE) 5 ug/L <br /> Di-isopropyl ether (DIPE) 5 ug/L Ethyl-t-butyl ether (ETBE) 5 ug/L <br /> t-Amylmethyl ether (TAME) 5 ug/L <br /> MW-2 had elevated lower detection limits due to high levels of non-target compounds (TBA 10000 <br /> ug/L, MTBE 500 ug/L, DIPE 500 ug/L, ETBE 500 ug/L and TAME 500 ug/L) <br /> Groundwater samples obtained for the December 1997 quarterly sampling were cross contaminated <br /> by the submersible pump used to collect representative Dissolved Oxygen samples These samples <br /> 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 Since that time two quarterly sampling events <br /> have occurred and demonstrate that the levels indicated during the December, 1997 sampling are not <br /> representative A water sample of the final rinse of the pump was obtained and indicates that all or <br /> some of the elevated concentrations of TPHg and BTEX in all wells could have resulted from the <br /> submersible pump decon practices The wells were sampled in the following order MW9, MW7, <br /> MW6, MW4, MW5, and MW11 These wells in the past have all been below laboratory lower <br /> detection limits The following wells which have shown petroleum hydrocarbons in past samplings <br /> were then sampled in the following order MW12, MWS, MW1, MW2, MW13, MW3, and MW10 <br /> The pump (Grundfos Redi-Flo2) was washed and rinsed after each well The final rinse was <br /> sampled and contained the following concentrations TPHg 0 12 mg/L, Benzene 1 6 ug/L, Toluene <br /> 12 ug/L, Ethylbenzene 5 5 ug/L, Xylenes 39 ug/L and MTBE <5ug/L This pump was used instead <br /> of LTT vacuum lift pump in order to obtain representative 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 /> 2 R603 Update#54/CLOSURE 3 12/98 <br />