SITE INFORMATION AND CORRESPONDENCE

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Site-Wide Comprehensive ROD -10- 1 May 1997 DDRW Tracy detected in soil gas samples and in downgradient ground water monitoring wells LM 61 and LM 137 at concentrations ranging from 10 to 60 ppb. TCE should be specified as CoC which poses a threat to groundwater in the table describing the fate and transport characteristics of the site. The ROD should clarify that the selected remedy includes completing the characterization of TCE and PCE soil contamination. The site description for Area 3 should state that the site continues to be a source area for TCE and PCE groundwater contamination. Chemographs for monitoring wells LM032A and LM058A indicate that VOC concentrations have been continuously detected in these downgradient monitoring wells. The ROD should clarify that the selected remedy includes completing the characterization of TCE and PCE soil contamination. 2. SWMU 4. The selected remedy presented in the ROD is not adequately justified, in part, because the site characterization summary is incomplete for this site. The ROD should summarize the subsurface soil data collected as part of site characterization activities conducted prior to finalizing the FS process to define the nature and extent of vadose zone contaminants at this site. This data is necessary to adequately assess the potential impacts to water quality and to properly evaluate the potential remedial alternatives. Text should clarify that evaluation of the remedial alternatives included an assessment of data collected to characterize the vertical extent of soil contaminants beneath the sediments in the lagoon. The ROD should further state that ground water has been impacted at this site, and this site is intended for continued use as a storm water holding pond. The ROD should also clarify how a one-time removal of sediments will meet the ARARs (see general comment #3). 3. SWA1U 6 - Building 28 Sump. The site characterization summary table indicates that this site is not a potential source area for VOCs. We do not concur. VOCs have been detected in monitoring well LM017A which is the nearest downgradient monitoring well. In addition, concentrations of PCE detected in LM017A have ranged from 22 ug/L to 10 ug/L. Soil gas investigations conducted at SWMU 6 detected PCE concentrations up to 64 ug/L and TCE concentrations up to 10 ug/L. These findings suggest a VOC source area for groundwater contamination exists at SWMU 6. The ROD should indicate that the selected remedy will further evaluate if SWMU 6 is a source area for VOCs detected in groundwater. The selected remedy should include VOC sampling as part of the confirmation sampling program for the excavation of soils beneath the former sump. Additional text should clarify that the VOC sampling results will be evaluated and the remedial alternatives will be re- evaluated if VOCs are determined to pose a threat to groundwater at this site. The selected remedy should also clarify that the downgradient monitoring well (LM017A)will be replaced when the adjacent Building 28 is expanded and that the replacement well will continue to be monitored for VOCs and pesticides. 4. SWA1U 7: Based on groundwater sampling results presented in the RDFS Report, dioxins/furans have been detected in monitoring well LM095A (5.5 x 10 E-8 ug/kg) which appear to be associated with the dioxins/furans detected (3.3 x 10 E-5 to 6.9 x 10 E-3 ug/kg) at depth in soils in the south area pits (pit F). Dioxins/furans have also been detected at depth (2.8 x 10 E-5 to 5.3 x 10 E-3 ug/kg) in the north area pits (pits A and Q. The site characterization should list dioxins and furans as threats to groundwater ( in addition to those listed in this Section 5.2) which will be evaluated as part of the groundwater monitoring included in selected remedy.