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amec�9 <br /> reducing groundwater velocities (and chemical migration rates), and therefore mass recovery <br /> rates. Thus, the rapid decline in mass removal rates indicates that groundwater from outside of <br /> the zone of high TCE occurrence is being captured, suggesting that hydraulic containment has <br /> been rapidly established, however, at the expense of slower rates of mass removal. <br /> Calculated mass recovery rates for offsite groundwater extraction well E-2 ranged from <br /> approximately 0.005 to 0.008 lbs/day in 2008, while pumping at a rate of approximately <br /> 70 gpm (Table 3). The TCE concentration reported for the July 2008 sample from E-2 was <br /> 5.3 pg/L (Table 3). This indicates that the migration of groundwater from the vicinity of <br /> 519 Stockton Avenue is not a likely threat to downgradient groundwater production wells and <br /> that pumping at well E-2 for hydraulic containment and/or source treatment is unnecessary. In <br /> other words, the mass flux of COCs in shallow groundwater in the vicinity of Stockton Avenue <br /> is too small to impact City of Ripon production wells. <br /> Calculated mass recovery rates for the deeper Intermediate Aquifer well E-6 ranged from <br /> approximately 0.2 to 0.26 lbs/day in 2008. As discussed in later sections of this report, this well <br /> captures a portion of a large area of relatively dilute COC concentrations (predominantly cDCE <br /> with lower concentrations of TCE) likely associated with the discharge of VOCs from the City <br /> of Ripon wastewater treatment plant lagoons, and potentially other sources. Groundwater <br /> extraction at well E-6 is simply providing containment for a portion of COC-affected <br /> groundwater in the Intermediate Aquifer. Groundwater beneath the City of Ripon may be more <br /> effectively managed by optimizing pumping of production wells as an alternative to ongoing <br /> pumping at well E-6. <br /> 3.5 CONCEPTUAL MODEL FOR TCE RELEASE, FATE, AND TRANSPORT <br /> A conceptual model for the release of TCE, and its subsequent migration in the vadose zone <br /> and Site groundwater, has been formulated based on available information regarding Site <br /> operations, hydrogeologic considerations, and the subsurface TCE distribution based on soil <br /> vapor and groundwater samples discussed in earlier parts of Section 3. This conceptual model <br /> is based on the information discussed in this report, and is subject to revision as new <br /> information is available. <br /> 3.5.1 Timing and Location of TCE Release <br /> Decaffeination with TCE occurred between 1957 and 1970. It is reasonable to assume that <br /> inadvertent TCE surface spills may have occurred during this time near the western end of the <br /> Decaffeination Building. There is also evidence indicating that leaks from the onsite sewer <br /> lateral, and its connection to the industrial sewer that ran south along Industrial Avenue, may <br /> have occurred (loose joints between sections of 14-inch concrete pipe were documented in a <br /> 1974 industrial sewer inspection; RUST, 1994a). Water discharged from the gravity separator <br /> AMEC Geomatrix, Inc. <br /> I:\Doc_Safe\9000s\9837.005\4000 REGULATORY\SCM_01.30.09\1_text\SCM Report Final.doc 22 <br />