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Historical Background & Site Conceptual Model
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<br />' AGE-NC Project No 01-0827
<br /> Page 7 of 9
<br /> historically detected TPH-g and BTEX compounds in water samples collected from wells MW-1,
<br /> VW-1 and VW-2, the highest dissolved concentrations were detected in wells MW-1 and VW-2
<br /> during their first sampling events on 22 December 1988 and 26 November 1997, respectively The
<br /> highest concentrations of TPH-g was 34,000 micrograms per liter(pg/1)from well VW-2d,followed
<br /> closely in magnitude by 33,700 pg/l in well MW-1 and 32,000 µg/1 or 29,000 pg/l in vapor well
<br /> VW-5s and VW-1,respectively TPH-g and BTEX compound concentrations generally declined at
<br /> well MW-1 and the former UST area during the declining ground water table elevation of the early
<br /> 1990's The TPH-g detections then rebounded to a high concentration in October 1996 during which
<br />' time water rose to 30 feet bsg or within the mayor coarse sand and gravel layers at the site, and/or
<br /> the water table was five feet above the top of the MW-1 well screen BTEX compounds have been
<br /> detected as great as 6,100 µg/1 benzene, 4,900 pg/l toluene, 2,300 pg/l ethyl-benzene and 6,100
<br />' xylene (VW-2d)
<br /> However,all these concentrations decreased after the initial sampling TPH-g and BTEX,along with
<br /> fuel additive concentrations decreased in vapor wells and the momtoring wells,during S VE and IAS
<br /> remediation near the former UST area Monitoring well MW-1,vapor wells VW-1 and VW-5d have
<br /> declined to non-detect levels, since July of 2003 The concentration trends of TPH-g and BTEX
<br /> detections in the samples from well VW-2d, at the former UST area
<br /> Laboratory analysis of samples from well MW-6, representing the deeper ground water, have
<br />' detected TPH-g as high as 1,000 pg/l in the July 2003 sample Concentrations of TPH-g in well
<br /> MW-6 have steadily declined to a non-detect level during six sampling events, over 18 months
<br /> Additionally, BTEX compounds had been observed in the samples from well MW-6 at
<br /> concentrations up to 62 pg/l of benzene, 7 4 pg/l of toluene, 31 pg/l of ethyl-benzene and 19 µg/1
<br /> of xylene The BTEX concentrations have also declined to non-detect levels or at levels below
<br /> concern or maximum contaminant limits, since July 2003 The December 2004 well sample from
<br /> well MW-6 was non-detect for hydrocarbons
<br /> Laboratory analysis has also detected the fuel additives, methyl tert butyl ether (MTBE),
<br /> 1,2-dichloroethane (1,2-DCA) and randomly di-isopropyl ether (DIPE), tert butanol (TBA), ethyl
<br /> tertiary-butyl ether (ETBE) and tertiary-amyl methyl ether (TAME) The following fuel additives
<br /> Ihave been detected constantly
<br /> • MTBE in well VW-2d at concentrations as high as 120 µg/l (31 December 2002) and
<br /> currently is detected near 16 gg/l, while MTBE was previously steadily detected in well
<br /> MW-1 and VW-5d at low concentration, these well samples are currently non detect for
<br /> MTBE,
<br /> • 1,2-DCA in monitoring well MW-1 and vapor wells VW-1, VW-2d and VW-5d, 1,2-DCA
<br /> is currently non-detect in these wells,the last detection of 1,2-DCA was in the August 2004
<br /> Isample of VW-2d, at a concentration of 6 4 pg/l,
<br /> Advanced GeoEnvironmental,Inc
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