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NP-1-80',NP-1-90',SVE-South and SVE-North are consistent with previous sampling events (see Table 4). Concentrations detected in MW-DIS-1 during this monitoring event were higher than those in February 1999. TPH as gasoline was detected in MW-DIS-1 at a concentration of 6,800 µg/L and benzene,toluene, ethylbenzene,and total xylenes were detected concentrations of 440,450,280,and 1,200�tg/L, respectively. i . Oxygenates Methyl-tert-Butyl Ether (MTBE), tert-Butanol(TBA),Di-isopropyl ether (RIPE), Ethyl-tert- butyl ether(ETBE),and tert-Amyl Methyl ether(TAME)were not detected in any of the wells during this monitoring event. All oxygenates were analyzed by EPA Method 8260 during this sampling event. There have been no confirmed detections of the MTBE,DIPE,ETBE, and TAME at this site. TBA has been detected once,in five different wells,but has not been detected in any well during the past four to five consecutive sampling events. Conclusions Since 1991,groundwater levels have risen approximately 30 feet(from 77 ft bgs to 47 ft bgs), and the direction of groundwater flow has changed directions by almost 180 degrees. Between 1991 and July 1994,the groundwater flow direction was predominantly towards the northwest at an average groundwater flow direction and magnitude of 312° and 0.0006 ft/ft. From October 1995, through today, the groundwater flow direction has been consistently towards the southeast,at an average direction and magnitude of 143°and 0.0008 ft/ft. The groundwater flow direction has been toward the southeast for the past 12 sampling events,indicating a stable flow regime. Due to the switch in flow directions, previous downgradient wells are now upgradient wells. Prior to the switch in groundwater flow directions, the extent of the plume was defined horizontally by monitoring wells MW-DIS-3,MW-DIS-8S,MW-DIS-9,and MW-DIS-10.The vertical extent downgradient of the source area was defined by monitoring well MW-DIS-8D. Currently,the extent of the plume in the southeast direction is unknown. Analytical results from all wells at the site have been extremely consistent,with the exception of monitoring wells MW-DIS-1 and-2,which have had significant reductions in concentrations of petroleum hydrocarbons. There have been no confirmed detections of gasoline additives or oxygenates at this site. Tert-butanol(TBA)has been detected a single time in five different wells.TBA was detected in monitoring wells NP-1-70',NP-1-90',and SVE-North on December 23, 1997. TBA has not detected in any of these wells during the past five consecutive sampling events.TBA was detected in monitoring wells MW-DIS-1,and SVE-South on May 21, 1998. TBA has not been detected in these wells during the past four consecutive sampling events.The Disco tanks were in operation between 1966 and 1976. MTBE was first used as an octane booster in the mid-1980's,and widespread use began in the 1990-1992 period in order to meet air quality requirements.Therefore,it is highly unlikely that the gasoline released from the Disco site 1 contained any additives or oxygenates. r 3