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amec�9 <br /> 2) pumping groundwater near the Stanislaus River and WWTF lagoons would not <br /> be a feasible containment option because of the rapid infiltration of surface water <br /> from the WWTF and Stanislaus River (increased pumping will increase recharge) <br /> would encumber capture zone width, <br /> 3) treated groundwater would likely be disposed of at the WWTF or used for <br /> irrigation, which would in turn enhance infiltration, <br /> 4) other groundwater quality problems (unrelated to Nestle) such as elevated <br /> TDS and arsenic would greatly increase the cost of groundwater treatment, and <br /> 5) the downward gradients caused by pumping would enhance the vertical <br /> migration of high TDS water. <br /> 5.0 MAJOR CONCLUSIONS AND IMPLICATIONS OF THIS SCM UPDATE <br /> The major conclusions and implications of this SCM update are: <br /> 1. The hydrostratigraphy beneath the Site has been revised. Four depth intervals <br /> have been characterized between the water table and base of the Upper Aquifer at <br /> approximately 115 feet bgs as follows: the B-zone (water table to approximately <br /> 40 feet bgs), the Upper A-zone (approximately 50 —80 feet bgs), the Middle A-zone <br /> (approximately 80 — 95 feet bgs), the Lower A-zone (approximately 100 — <br /> 115 feet bgs). <br /> 2. Onsite releases would have been contained by onsite pumping for facility supply. <br /> Therefore, COC impacts discovered in 1986 (wells MW-1, MW-6, S-3, and <br /> domestic wells on Acacia Avenue) were likely not the result of COC migration from <br /> onsite releases, but were likely related to other discharges, including discharges <br /> from the WWTF. <br /> 3. The Corcoran Clay is a major regional aquitard beneath the Study Area. It has an <br /> average thickness of approximately 15 feet and slopes to the west. Increasing <br /> vertical gradients across the Corcoran Clay have been observed since <br /> approximately 2000. These increasing vertical gradients will increase the rate of <br /> COC migration to the Lower Aquifer via conduit wells. <br /> 4. The B-aquifer is primarily impacted by offsite historical leaks from the industrial <br /> sewer. <br /> 5. The Upper Aquifer (A-zone) and Intermediate Aquifer are primarily affected by <br /> historical discharges from the WWTF. <br /> 6. Future groundwater remediation should focus on the B-zone and Upper A-zone <br /> beneath the Site (approximately 40 — 60 feet bgs)where the highest TCE <br /> concentrations have been delineated. <br /> 7. Vertical conduit wells to the Lower Aquifer should be identified and properly <br /> decommissioned. <br /> AMEC Geomatrix, Inc. <br /> hDoc_Safe\9000s\9837.005\4000 REGULATORY\SCM_01.30.09\1_text\SCM Report Final.doc 53 <br />