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amec— <br /> (during <br /> solvent recovery) to the sewer lateral likely contained dissolved TCE, and this may <br /> also have leaked. The relatively high concentrations of TCE in soil vapor and shallow <br /> groundwater (VE-4) detected near the western portion of the Decaffeination Building are <br /> consistent with this hypothesis. Elevated TCE in soil vapor samples collected in 1987 and <br /> groundwater samples from well M-813 near the intersection of 4t" Street and Stockton Avenue <br /> (Figure 9) suggest a leak occurred at this location. These potential release areas are shown in <br /> Figure 9. <br /> 3.5.2 Historical Migration of TCE in Groundwater <br /> Between 1957 and 1970, well N-1 was the primary production well in operation, and pumped <br /> at an average rate of 340 gpm (RI, 1994a). Although the well construction records for N-1 <br /> were not available at the time this report was prepared, the hydraulic response of B-zone <br /> through C2-zone water levels to pumping of N-1 (at a lower rate of 140 gpm during 1994 when <br /> it was used for hydraulic containment) has been documented (RUST, 1994a; RUST, 1995). <br /> Therefore, a reasonable assumption is that while N-1 operated, the general direction of <br /> groundwater flow through the upper 200 feet of the saturated zone was northward, from the <br /> decaffeination area towards N-1. Furthermore, because N-1 was pumping at a relatively high <br /> rate, it is likely that the water table was lower beneath the northern portion of the Site, in the <br /> vicinity of N-1, such that the B-zone may have been unsaturated (i.e. dry) beneath the <br /> northern portion of the Site in the 1950's through the 1970's when pumping rates were high. <br /> The relatively high concentrations of TCE in the Upper A-zone (Section 3.4.2) represent a <br /> depth interval that may have been impacted by shallow recharge of water from sewer leaks as <br /> well as from TCE vapor. TCE-affected groundwater (at concentrations above 1,000 pg/L) is <br /> shallower near the former Decaffeination Building (VE-4 contained groundwater with TCE > <br /> 1,000 pg/L) perhaps because the water table has historically been shallower farther from the <br /> production wells, but deeper beneath the northern portion of the Site closer to N-1 and N-3. <br /> Increased water use with the initiation of tea production in 1962, and increased pumping at <br /> wells N-3 and N-4 (210 gpm on average; RUST, 1994a), until at least 1974, may have <br /> promoted a northeasterly component of groundwater flow from the decaffeination area towards <br /> these wells, and would have increased the drawdown beneath the Site, such that much of the <br /> northern portion of the Site may have been dewatered to depths of 60 feet or more. A large <br /> hydraulic stagnation zone (a zone of little to no groundwater flow) likely existed between wells <br /> N-1 and N-3 during this time. This pumping and release history is consistent with the irregular <br /> distribution of elevated TCE concentrations between N-1 and N-3 in the Upper A-zone (Figure <br /> 8). <br /> AMEC Geomatrix, Inc. <br /> I:\Doc_Safe\9000s\9837.005\4000 REGULATORY\SCM_01.30.09\1_text\SCM Report Final.doc 23 <br />