ARCHIVED REPORTS_SITE CONCEPTUAL MODEL 2009

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amec- 3.5.3 Potential Mechanism for Historical Elevated TCE in Lower A-Zone Groundwater Groundwater samples collected early in the project history from monitoring well M-1 A, approximately 80 feet south of well N-1, were reported to contain relatively high concentrations of TCE (as high as 10,000 pg/L). Other A-zone wells installed in this area (ED-1, ED-2, EE-1, EU-1 and EW-1) reportedly had TCE concentrations above 1,000 pg/L. TCE concentrations rapidly decreased by one to two orders of magnitude in these wells after well N-1 was decommissioned and groundwater extraction was implemented at EI-1 (Figure 10). This scenario is consistent with well N-1 acting as a conduit well when idle, with wells N-3 and N-4 pumping, causing a transient southerly flow away from well N-1 towards wells N-3 and N-4. However, there is currently no historical information to indicate that this occurred. 3.6 IMPLICATIONS The findings and interpretations presented in this Section have the following implications: • Historical pumping by onsite production wells at annual average rates as high as 600 gpm would have prevented groundwater movement offsite. Detections of COCs in samples of offsite groundwater since 1986 are most likely related to offsite releases because there is no feasible mechanism for groundwater affected by onsite TCE releases to flow offsite (south, west, or east) during plant operations because of onsite pumping. The primary conceivable mechanism for dissolved TCE to leave the Site is by discharge to the industrial sewer. • Evidence collected to date suggests that releases of TCE (as dissolved solvent but potentially as pure solvent) in the vicinity of the Decaffeination Building caused elevated TCE vapors in the vadose zone in this area, and in shallow groundwater beneath the Decaffeination Building, which was then captured by onsite production well N-1, and potentially by wells N-3 and N-4. Therefore, future groundwater remediation efforts should be focused in the area of the former Decaffeination Building and northward towards the locations of former supply wells N-1 and N-3. Remediation options for this area are addressed in the Revised Feasibility Study, submitted to the Water Board on January 30, 2009. This area is currently addressed with IRAP groundwater extraction wells EU-3 and EU-4, and TCE concentrations may have declined substantially since the 2005 and 2007 characterization programs implemented by Geomatrix and ECM. • The hydrostratigraphy beneath the Site has been revised, where four depth intervals have been characterized between the water table and base of the Upper Aquifer at approximately 115 feet bgs as follows: the B-zone (water table to approximately 40 feet bgs), the Upper A-zone (approximately 50 —80 feet bgs), the Middle A-zone (approximately 80 — 95 feet bgs), the Lower A-zone (approximately 100 — 115 feet bgs). • The area of high TCE concentrations in groundwater is limited to a relatively small area and shallow depth range (the B-zone and upper portion of the Upper A-zone, from 40 — 60 feet bgs). This area is addressed in the Revised Feasibility Study, submitted to the Water Board on January 30, 2009. Concentrations of COCs AMEC Geomatrix, Inc. hDoc_Safe\9000s\9837.005\4000 REGULATORY\SCM_01.30.09\1_text\SCM Report Final.doc 24