ARCHIVED REPORTS_SITE CONCEPTUAL MODEL 2009

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amec- 2.2 HYDROGEOLOGY The Central Valley Aquifer system is comprised of Post-Eocene continental deposits which overlie deeper marine sediments (Bertoldi et al. 1991). Aquifer vertical thickness averages about 2,400 feet, and is composed mostly of fluvial deposits (from rivers and streams) and interbedded lacustrine deposits (Bertoldi et al. 1991). The average horizontal hydraulic conductivity for the entire Central Valley Aquifer system was estimated to be six feet per day (ft/d), based on a calibrated groundwater flow model; yields greater than 1,000 gpm are common for wells in this aquifer system where specific capacities typically range from 20 — 100 gpm/foot of drawdown (Bertoldi et al., 1991). The relatively low hydraulic conductivity value is not expected for such a high yielding aquifer, however, the 6 ft/d value is a bulk number for the entire aquifer system, and may reflect the lateral discontinuity of sands and gravels (Bertoldi et al., 1991). The recharge of excess irrigation water and withdrawals of substantial quantities of groundwater has altered the natural groundwater movement such that vertical gradients are predominantly downwards in the center of the San Joaquin Valley (Burow et al., 2004; Bertoldi et al., 1991). In other words, this configuration creates localized vertical recirculation of groundwater, where pumping groundwater for irrigation causes concentration of salts by transpiration and evaporation, and where infiltrating irrigation water may dissolve anthropogenic chemicals (pesticides, organic waste, etc.) as it percolates back to the aquifer system. Ripon is located along the north bank of the Stanislaus River, which forms the southern boundary of the Eastern San Joaquin Sub Basin; the Modesto Sub Basin is south of the Stanislaus River. In this area, the Central Valley Aquifer System is typically described in terms of two aquifers, an unconfined or semi-confined aquifer above a confined aquifer. The confining unit separating the two is the Corcoran Clay. The base of the confined aquifer has been mapped as the base of fresh water, where fresh water is defined as having a specific conductance less than 3,000 microsiemens per centimeter, which extends as deep as 700 feet bgs around Ripon and Modesto (Burow et al. 2004). Groundwater pumping from the Eastern San Joaquin Sub Basin exceeds the estimated sustainable yield by approximately 150,000 acre-feet per year (CDM, 2001). The depth to water currently is about 30 feet bgs beneath the Study Area, but is shallower near the Ripon Municipal Lagoons where recharge occurs. Groundwater levels since the 1970s have been fairly constant (between 30 and 45 feet msl) and generally fluctuated with precipitation based water level elevations reported for a San Joaquin County monitoring well located in Ripon, approximately '/2 mile north of the former Nestle facility (Appendix A). The highest water level for this well (45 ft msl)was reported in the spring of 1984, following 1983 which was the wettest year on record, with approximately 27 inches of rain falling in Modesto. Water levels steadily decreased to just above 30 feet msl AMEC Geomatrix, Inc. I:\Doc_Safe\9000s\9837.005\4000 REGULATORY\SCM_01.30.09\1_text\SCM Report Final.doc 6