ARCHIVED REPORTS XR0006666 - Laserfiche WebLink

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SEDC ® R MW-1, MW-4, and MW-6 have since declined over time Petroleum hydrocarbon concentrations have generally not been detected at or above laboratory method reporting limits (MRLs) in wells MW-5, and MW-8 through MW-10, MW-12, and MW-13 Well MW-11, which is screened in a deeper water-bearing zone beneath the site, has occasionally contained concentrations of GRO since November 2002, with a maximum concentration detected at 370 pglL during the third quarter 2005 The laboratory, however, indicated that this value was an estimated value Evaluation of dissolved concentrations through the third quarter 2005 indicates that the highest concentrations of petroleum hydrocarbons continue to be detected off-site in the vicinity of Shell wells S-1 and S-2 During the third quarter 2005, maximum concentrations of GRO and benzene were detected at 15,000 pg/L (S-2) and 260 NgIL (S-1) With the exception of a minor concentration of ethylbenzene (0 17 pg/L) in well MW-13, which was reported by the laboratory as an estimate, concentrations of BTEX constituents were not detected in the site wells this quarter Split samples collected from the Shell wells indicated the presence of MtBE in wells S-1, S-2, S-3, S-5, and S-6, with the highest detections reported in off-site wells S-5 (400 pg/L) and S-6 (140 lag/L) The constituents of concern at the site include dissolved MtBE and the presumed byproduct of MtBE metabolism, TBA, although TBA can sometimes be found in gasoline MtBE has historically been detected in the majority of the site wells, with the highest concentrations reported in wells MW-3 and MW-7 at site maximum concentrations of 198,000 NgIL (MW-3) and 233,000 lag/L (MW-7) in 2001 MtBE has significantly declined in the site wells over time since 2001, which is likely due to the metabolism of MtBE into TBA by natural attenuation and/or as a result of active remediation through the operation of the OS system During the third quarter 2005, MtBE was detected in wells MW-1, MW-3 through MW-7, MW-11, and MW-12 up to a maximum concentration of 29 NgIL Since the initiation of consistent TBA sampling during the first quarter 2002, TBA peaked in wells MW-3 (240,000 pg/L), and MW-7 (220,000 pg/L) in 2002, and although elevated concentrations continue to be detected in these wells, TBA levels have since declined in these wells over time During the third quarter 2005, wells MW-3 and MW-7 contained TBA at respective concentrations of 25,000 NgIL and 29,000 pg/L, which is a decline of over 80 percent from 2002 levels TBA levels in wells MW-1, MW-4 through MW-6, MW- 8 through MW-10, MW-12, and MW-13 have either decreased to relatively minor to non- detectable concentrations, or have generally been non-detectable Deeper well MW-11, which has contained relatively minor levels of MtBE, contained elevated concentrations of TBA with the highest levels reported during third quarter 2005 (16,000 pg/L) A generally increasing TBA trend has been observed in the well since November 2002 Historical analysis of TBA in the off-site Shell wells indicated that the highest concentrations have historically been detected in on-site well S-2 (1,700 tag/L, split samples collected by TRC in November 2004 and February 2005), and in off-site well S- 5 (1,400 pg/L, November 2004) TBA was present in wells S-1 (21 pg/L), S-2 (1,300 pg/L), and S-5 (36 pg/L) during the third quarter 2005 Other fuel oxygenates and the analytes acetone, tertiary butyl formate, and tertiary-butyl acetate were not detected in the site wells at or above laboratory MRLs during the third quarter 2005, with the exception of the presence of acetone and EtBE in well MW-3 at estimated concentrations of 6 8 NgIL and 0 43 pg/L, respectively The samples collected from the Shell wells did not contain other fuel oxygenates, 1,2-DCA, and EDB at or rY