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Stockton Avenue GroundwatePExtraction System Shut-down Request • <br /> November 5, 2009 <br /> pumping in well E-2 shown on the figure beginning in 1999. The concentration of TCE in <br /> monitoring well M-88 has decreased steadily since the start of extraction from well E-2, <br /> suggesting that sources related to historical sewer leaks have been contained and <br /> mitigated. Additionally, the concentrations observed in well E-2 from the Upper Aquifer <br /> suggest that no source-like mass currently remains in the Upper Aquifer in this area. <br /> Therefore, it can be concluded that any residual source material was remediated over <br /> the eight year of operation of the Stockton Avenue System. <br /> The Intermediate Aquifer in the vicinity of the Stockton Avenue System is monitored with <br /> wells M-8C1 and M-1801. Data for these wells during active groundwater remediation is <br /> depicted in Table 1. Figure 4 shows concentrations of TCE in wells M-8C1, and M- <br /> 18C1, over time, with the influence of pumping in E-6 beginning in 2004. As indicated by <br /> the figures, a decrease in VOC concentrations is observed in groundwater samples from <br /> well E-6 since pumping operations began at both high and low pumping rates. This <br /> suggests that no source like mass exists at this location. <br /> Recent numerical groundwater flow modeling and fate and transport simulations (using <br /> the ECM MODFLOW/RT3D model) also indicate that current low levels of dissolved <br /> VOC concentrations in groundwater beneath the area of the Stockton Avenue System <br /> are unlikely to migrate away from this area at concentrations which would impact any off- <br /> site receptors. Figures 6 through 9 illustrate these modeling results for future scenarios <br /> assessing TCE fate and transport in the Stockton Avenue area within the Upper Aquifer <br /> (Figures 6 and 7), and within the Intermediate Aquifer (Figure 8 and 9). A network of <br /> existing and proposed monitoring wells in both the Upper and Intermediate Aquifers is <br /> available to monitor COC concentrations after the shut down of the groundwater <br /> extraction and treatment system. <br /> PROPOSED SYSTEM SHUT-DOWN AND POST-REMEDIAL MONITORING <br /> This proposal for shut-down of the Stockton Avenue System supported by the following <br /> observations: <br /> 1) The low COC concentrations in groundwater samples from the Upper and <br /> Intermediate Aquifer monitoring wells and extraction wells in the Stockton <br /> Avenue System area, <br /> 2) COC concentrations in groundwater samples from extraction wells E-2 and E-6 <br /> have decreased while pumping has decreased, indicating that no source exists <br /> near these wells (COC concentrations should increase with decreased pumping <br /> near a source zone). <br /> 3) Numerical simulations using the existing MODFLOW/RT3D model suggest that <br /> dissolved VOC migration is unlikely to impact any downgradient receptors <br /> following the proposed shut-down of the Stockton Avenue System. <br /> To monitor post-remedial groundwater conditions and to confirm the absence of <br /> significant VOC concentrations beneath the Stockton Avenue area, ECM has proposed <br /> installing two new wells for monitoring the Stockton Avenue System area. These wells <br /> were proposed in the work plan titled Monitoring Well Installation Work Plan for Industrial <br /> and Stockton Avenue Extraction and Treatment and dated June 19, 2009. <br /> Discussions between James Brownell and ECM regarding this proposal for additional <br /> monitoring wells have resulted in installation of three new monitoring wells. Two of the <br /> 3 <br />