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i 160, 12,and 620 gg/L respectively. 1,2-DCA was not detected in SVE-North during the previous sampling event. { Conclusions Since 1991,groundwater levels have risen approximately 33 feet(from 77 ft bgs to 44 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 3121 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 14 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-BD.Recently, the downgradient extent of the TPH-gas and BTEX plume-was delineated with CPT grab groundwater samples during the Additional Characterization Investigation. 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 oxygenates at this site with the exception of TBA. TBA is a former gasoline additive,and was used as an octane booster prior to the advent of oxygenated gas. TBA was detected in monitoring wells NP-1-70',NP-1-90',and SVE-North on December 23, 1997,in MW-DIS-1,and SVE-South on May 21,1998,and in MW-DIS-1 during this event. Lead scavengers 1,2-DCA and EDB have been detected in monitoring wells MW-DIS-1, NP-1-90',NP-1-80',NP-1-70',and SVE-South. Recommendations Extent of Plume Additional groundwater monitoring wells are needed to monitor the cross-gradient and downgradient extent of the hydrocarbon/1,2-DCA plume present at the Disco site. Frequency of sampling The Disco site has accumulated a large data set of groundwater sample results since monitoring began in 1991. Monitoring wells containing petroleum hydrocarbons have either stable or reducing concentrations.Monitoring wells outside of the dissolved hydrocarbon plume have been consistently non-detect. A Groundwater Monitoring Frequency Decision Tree,Figure 4,is presented to evaluate the frequency of sampling for each monitoring well at the site,based upon the location of the well with respect to the plume and gradient direction. A similar decision tree has been 3