SITE INFORMATION AND CORRESPONDENCE

Home Browse Search

Mr. Marshall Cloud March 13, 2008 Page 3 11. Figures 4.3-1 a, 4.3-1b, 4.3-2a, 4.3-2b, Interpreted Contaminant Plumes and Capture Zones; TCE isopleth drawn around extraction well (EW) EW020 and EW019 is not defined by close analytical data to the northeast (NE), east (E), or southeast (SE), and should redrawn with dashed or inferred lines. The closest monitoring point in the A-zone aquifer is Long-Term Monitoring (LM) well LM150A, located approximately 400+ feet down/cross-gradient, with a 3Q07 concentration of 3.4 pg/L. Additionally, LM151B, located adjacent to LM150A, detected TCE at 9.3 pg/L in 3Q07 and is not defined by any monitoring well data for over 1000+ feet down/cross-gradient, currently defined by LM 155B. The lack of close proximity monitoring wells for the NE TCE plumes results in lack of definition of the plumes' lateral extents. The figure's TCE plumes' isopleths to the NE, E, and SE should be redrawn to a dashed or inferred line since no monitoring data is available to define the actual location of the 5 pg/L iso-concentration line. 12. Attachment 1, Groundwater Flow Model 3Q07 Conditions, Figure 1; The COC plume depicted surrounding EW020 and EW019 does not accurately depict the lateral extents of the TCE plume due to lack of monitoring points to define actual toe-of-plume. 3Q07 groundwater data shows TCE concentrations of 9.3 pg/L in LM151 B, which is almost twice the Federal Maximum Contaminant Level (MCL). LM150, and LM151 are outside the hydraulic influence of EW019A and EW020A and allowing the plume to migrate further to the N/NE. DTSC recommends that the DDJC-Tracy Groundwater Model should place particles at LM150 and LM151 and further down-gradient to assess the performance and adequacy of the current groundwater remedy. 13. Attachment 1, Groundwater Flow Model 3Q07 Conditions, Figure 3; As stated in comment #11, the Groundwater Model shows incomplete capture of contaminants exceeding the MCL in the middle hydrologic zone associated with B-zoned wells, including EW0246, EW02513, and LM151B. Optimization efforts for the ground water remedy should include evaluating the lack of hydraulic influence for the NE TCE plumes and options to install additional extraction wells if necessary to maintain adequate hydraulic influence of contaminant plumes. 14. Page 5-3, Section 5.3, SVE Performance Summary —2007 Monitoring Period; 2007 SVE monitoring data for Area 1, Area 2, and Area 3 suggests that additional SVE system operations or an alternative remedy is necessary to achieve the cleanup goals for TCE and PCE. If the selected remedy for Area 2 or Area 3 changes from SVE to a Soil Removal Action, the DDJC-Tracy Record of Decision will need to be amended to document the remedy change.