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I 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 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 144°and 0.0008 ft/ft. The groundwater flow direction has been toward the southeast for the past 15 ! 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-BS,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 Site Characterization r Investigation. Additional investigation is necessary to delineate the downgradient and crossgradient extent of the 1,2-DCA plume. 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,in MW-DIS-1 during the March 29,2000 sampling event,and in SVE-North during this sampling event. r Lead scavengers EDB and 1,2-DCA have been detected in monitoring wells MW-DIS-1, NP-1-90',NP-1-80',NP-1-70',and SVE-South. Concentrations are consistent with the previous sampling event. 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 approved by DTSC and the Central Valley Regional Water Quality Control Board for a PG&E Former MGP site,located in downtown Stockton. The decision tree organizes the monitoring wells as upgradient,downgradient, crossgradient, or within the plume as follows: i