ARCHIVED REPORTS_2010 ANNUAL REPORT

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2. Identify the status of all wells, including: a. Identifying wells which are of immediate concern as potential receptors or conduits of contaminants of concern; b. Determining short-term and long-term effect of wells on groundwater remediation efforts; c. Recommending action plans for the wells that may directly or indirectly affect remediation efforts. ECM and AMEC Geomatrix conducted investigation activities jointly in September 2010 in accordance with the M-10A Investigation Work Plan 16 to assess the causes of recent increases in concentrations of TCE and associated byproducts observed in monitoring well M-10A. ECM presented a description of the field activities, a summary and discussion of the analytical results, conclusions, and recommendations in the M-10A Investigation Report". A1.7 HYDROGEOLOGY The Study Area is located in the Eastern San Joaquin County Groundwater Basin (ESJCGB), which is located in the northeastern San Joaquin Valley. The major physical features in the ESJCGB include the Sierra-Nevada foothills, the alluvial plain, and the Stanislaus River,18 which divides San Joaquin County from Stanislaus County. The Stanislaus River, located in the southern portion of the Study Area, flows to the west/southwest into the San Joaquin River. The San Joaquin River then flows northwestward to the inverted river delta of the Sacramento-San Joaquin Delta, which forms the largest estuary on the Pacific Coast of the United States19. The Study Area is underlain by fluvial deposits associated with the Stanislaus River and alluvial fan deposits. Detailed information regarding the physiographic setting and geology of the region is presented in the Updated Site Conceptual Model (SCM) prepared by AMEC-Geomatrix20. Groundwater is first encountered at a depth of 20 to 40 feet below ground surface (bgs) beneath Ripon. Groundwater is locally affected by recharge and discharge areas and fluctuates with precipitation cycles and the stage of the Stanislaus River. Groundwater generally flows away from recharge areas. The recharge areas include the City municipal WWTP lagoons, irrigation fields currently located on property formerly owned by Neenah Paper Company, and the Stanislaus River. Groundwater generally flows toward pumping wells located north and south of the Stanislaus River. The alluvial aquifer system underlying the Study Area consists of three high-producing water- bearing zones designated as the Upper Aquifer, the Intermediate Aquifer, and the Lower Aquifer. Each of these typically contains more than one water-bearing unit with similar water level elevations and water chemistry. Aquitards consisting of fine-grained silts and clays generally separate water-bearing units. A regionally extensive clay aquitard, called the Corcoran Clay, separates the Intermediate Aquifer from the confined Lower Aquifer zone below it. Upper Aquifer, primarily consisting of the B and A units, generally ranging from 25 to 115 feet bgs. The A zone is further subdivided in the Updated SCM into 3 zones: Upper A zone (50-80 ft bgs), middle A zone (80-95 ft bgs) and the lower A zone (100-115 ft bgs). Soils in the 0 to 25 feet bgs range are typically unsaturated (Vadose Zone). Intermediate Aquifer, consisting of the C1 and C2 units, generally ranging from 110 to 190 feet bgs; and Appendix A Page A3 of A7