ARCHIVED REPORTS XR0000669 - Laserfiche WebLink

Home Browse Search

L 31 January 2003 • AGE-NC Project No 96-0232 ' Page 15 of 17 '1 been towards a slight depression in the water table located between wells MW-613 and MW-9A It is not clear at this time why substantial MTBE is present in MW-8A after nine consecutive quarters of being non-detect, however, since the initial high concentration detected in well MW-8A, the concentrations have steadily decreased, to 4 7 µg/l in December 2002 1 ' In general, MTBE concentrations increased in well samples collected in December 2002, subtle to I moderate increases in MTBE concentrations were reported from monitoring wells MW-2, MW-7, MW-9A, MW-11A and MW-1113 and all extraction wells except EW-6 and EW-9 In addition to extraction wells EW-6 and EW-9, subtle MTBE decreases were also detected in monitoring wells ' MW-1, MW-4 and MW-11D Analytical results from the December 2002 quarterly monitoring event indicates the most intense on- ; site MTBE impact is in the First Zone near EW-5 (33 µg/1) and EW-8 (53 µg/1),the most intcnse off site MTBE impact continues to be the First Zone near MW-7 and Three B's Truck Plaza well MW-4 (26 µg/1 and 67 µg/l,respectively), and in the Rocky's Upper Zone wells MW-i 1B (96 µg/1),MW- 15 (46 µg/1) and MW-18 (220 µg/l) It is not clear at this time why the Three B's Truck Plaza well MW-4 has consistently had greater MTBE-impact than that of intermediate well MW-7 Figures 11 and 12 depict the approximate extent of MTBE-impacted ground water beneath the site in cross sectional views C-C' and D-D', Figures 8, 9 and 10 depict the estimated extent dissolved I MTBE-Impact in plan view These figures depict Flag City Chevron wells EW-1 through EW-10, MW-2, MW-7, MW-8, MW-9, MW-11, MW-14 through MW-18 and Three B's Truck Plaza well MW-4 to be located within dissolved MTBE-impacted plumes The vertical extent of-impacted I ground water on-site appears to lie between 13 feet bsg(estimated depth to ground water)and 50 feet bsg (Figures 11 and 12) Low concentrations of MTBE have been detected in the Second Zone Aquifer(MW-9A)and Primary Aquifer NW-90,but MTBE has not been detected at,MW-9C for five consecutive sampling events Going off-site toward the west,the MTBE-impacted plume appears to have extended to as much as 75 feet bsg approaching the area near MW-11, near the former Rocky's Restaurant domestic well (RDW-1), where the underlying fine-grained sedimentary units may thin and pinch out, low concentrations of MTBE have been detected in Primary Aquifer (MW-11D), screened at between I 130 and 150 feet bsg The plume of water impacted by MTBE appears to extend westward beyond Flag City Chevron wells MW-16, 17, 18 and Three B's Truck Plaza well MW-4, with a vertical extent from approximatelyl 3 feet bsg to at least 75 feet bsg (Figure 11) The full westward extent Iof the Flag City Chevron plume is uncertain at this time,previous contaminants encountered in First „ Zone ground water at MW-13 include TPH-d and TPH-mo which suggest that the contaminants may be from a source other than the Flag City plume It is not known if the suspected plumes are co- mingled in the area between Three B's Truck Plaza wells MW-1 through MW-6 and Flag City Chevron wells MW-7, MW-11, MW-12 and MW-13, AGE has modeled them as separate plumes (Figure 8) I Advanced GeoEnveronmental,Inc i