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Well Monitoring Program -2- 11 September 1992 DDRW, Tracy Screen Interval Screen Interval to Ground Water Well (Feet bgs) Upper Horizon Elevation (Feet msl ) LM-5 20-30 Above 55.50 LM-31 36.5-46.5 In 55.41 LM-32 18-28 Above 55.22 LM-33 49-59 In 55.16 LM-58 20-30 Above 54.70 LM-59 46-56 In 54.67 Because both sets of wells hydraulically appear to be in the same water bearing zone, all of the data should be used to accurately assess the extent of the ground water contamination. This concept should also be extended to the composite wells (screened in the lower part of the upper horizon and the upper part of the middle horizon) and for the wells screened below the lower horizon, provided there are no significant head differences. I am specifically concerned that the extent of the trichloroethylene (TCE) and tetrachloroethylene (PCE) plumes for the upper horizon is more extensive than is depicted in Figures 3-1 and 3-2. Based on the data from wells screened above the upper horizon, the extent of the plume boundaries for the upper horizon should be expanded to include LM-32 and LM-58. As presented in these figures, these wells are outside of the plume boundaries. The TCE and PCE concentrations ranged from 41 to 75.8 micrograms per liter (µg/1 ) and 85 to 215 µg/1 , respectively, in LM-32 and from 11 to 21 µg/l and 10 to 24.8 µg/1 , respectively, in LM-58. Because concentrations of TCE and PCE were not detected in samples from LM-43, this indicates that there may be a source area near LM-32. In addition, high TCE and PCE concentrations (31 to 51.5 µg/l and 295 to 457 µg/1 , respectively) in samples from LM-30 also indicate a source area near this well . Some of the highest concentrations for TCE, PCE and total volatile hydrocarbons from the soil gas survey, were measured in SV137, SV147 and SV149 (Table 2.2-1 of the Comprehensive RI/FS Work Plan) which are in the same area as LM-32 and LM-58, and in SV115 and SV121 which are near LM-30. The soil gas data and ground water data from wells screened above the upper horizon indicate that the majority of the contamination, specifically for PCE, may be in the unsaturated zone and the uppermost ground water. These data also indicate that an extraction well (s) may be needed in the areas of LM-30 and LM-32 (currently depicted outside of the plume boundaries) and that a vapor extraction system may be appropriate for remediation of the unsaturated zone in these two areas. Recommendations for Well Monitoring. Many of the recommendations for on-going monitoring in the Draft Report appear to be excessive. Based on the data collected in the first four consecutive and comprehensive quarters, TCE and PCE still appear to be the primary ground water contaminants. Contamination by pesticides and metals appears to be limited to specific waste disposal areas. On-going monitoring for pesticides and metals in the majority of the monitor wells appears to be unnecessary at this time. Until the site-wide RI is conducted for all waste management unit (WMU) and underground storage tank (UST) potential source areas, the ground water monitoring program should be limited in scope and should only continue in areas