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S E C 0 R
<br /> concentrations have historically not been detected at or above laboratory MRLs in wells
<br /> MW-5, and MW-8 through MW-13 Evaluation of dissolved concentrations through the
<br /> second quarter 2005 indicates that the highest concentrations of petroleum
<br /> hydrocarbons continue to be detected off-site in the vicinity of Shell wells S-1 and S-2
<br /> During the second quarter 2005, maximum concentrations of GRO and benzene were
<br /> detected at 12,000 pg/L (S-2) and 330 pg/L (S-1) With the exception of a minor
<br /> concentration of benzene (0 65 pg/L) In well MW-12, petroleum hydrocarbons were not
<br /> detected in the site wells this quarter
<br /> The constituents of concern at the site consist of dissolved MtBE and the byproduct of
<br /> MtBE metabolism, TBA MtBE has historically been detected in the majority of the site
<br /> wells, with the highest concentrations reported in wells MW-3 and MW-7 at site x
<br /> maximum concentrations of 198,000 pg/L (MW-3) and 233,000 pg/L (MW-7) in 2001
<br /> During the second quarter 2005, relatively minor concentrations of MtBE up to 22 pg/L
<br /> (Shell well S-2) were detected in on-site wells MW-1, MW-4, MW-5, MW-11, and MW-
<br /> 12, and in off-site Shell wells S-2 and S-4 Well MW-11 is screened in a deeper water-
<br /> bearing zone (approximately 68 to 79 feet bgs) in comparison to the rest of the site
<br /> wells, which were completed with 10 to 15-foot screens at a depth between 28 feet and
<br /> 45 feet bgs Elevated concentrations of TBA were detected in on-site wells MW-3
<br /> (24,000 pg/L), MW-7 (30,000 pg/L), deeper well MWA 1 (10,000 pg/L), and in off-site F°
<br /> Shell well S-2 (1,400 pg/L, while relatively minor concentrations of TBA were detected in
<br /> on-site well MW-5 (5 4 pg/L) and off-site Shell well S-1 (32 pg/L) Downgradient Shell ,
<br /> wells S-5 and S-6 (located northeast of the site) have historically contained elevated
<br /> concentrations of MtBE and TBA, split samples collected by TRC during the last two
<br /> quarters contained MtBE and TBA up to 3,100 pg/L (S-6, February 25, 2005) and 1,400
<br /> pg/L (S-5, November 24, 2004), respectively
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<br /> MtBE has significantly declined in the site wells over time since 2001, which is likely due
<br /> to the metabolism of MtBE into TBA by natural attenuation, and as a result of active x:
<br /> remediation through the operation of the OS system Since the initiation of consistent
<br /> TBA sampling during the first quarter 2002, TBA peaked In wells MW-3 (240,000 pg/L),
<br /> and MW-7 (12,000 pg/L) in 2002, and although elevated concentrations continue to be -w
<br /> 14
<br /> detected In these wells, TBA levels have since declined in these wells over time TBA 14
<br /> levels in wells MW-1, MW-4 through MW-6, MW-8 through MW-10, MW-12, and MW-13
<br /> have either decreased to relatively minor to non-detectable concentrations, or have
<br /> generally been non-detectable Deeper well MW-11, which has contained relatively
<br /> minor levels of MtBE, contained elevated concentrations of TBA with the highest levels -x
<br /> reported in 2004 (12,000 pg/L) TBA in the well has since fluctuated in concentration, r
<br /> with elevated concentrations reported in the well over subsequent quarters zw
<br /> CHARACTERIZATION STATUS
<br /> The extent of the dissolved GRO and benzene plumes has been delineated, except westq
<br /> of Shell well S-1 The dissolved MtBE plume has laterally been delineated, except
<br /> northeast of Shell wells S-5 and S-6 The presence of dissolved TBA has laterally been Ile
<br /> delineated In the vicinity of the site, except northeast of Shell wells S-5 and S-6, and
<br /> west of Shell well S-1 The vertical extent of dissolved MtBE and TBA beneath the site
<br /> Ana
<br /> has not been delineated, based on the presence of MtBE and TBA in CPT borings drilled ,w
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<br /> IConocoPhilhps\Retail Sites1111921QSSR1200512Q2005 QSSR and QRPS doc a
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