ARCHIVED REPORTS_SITE CONCEPTUAL MODEL 2009

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amec_"_" ; • The WWTF lagoons are situated at a higher water elevation than the adjacent Stanislaus River, and therefore, some water from the lagoons likely discharges to the Stanislaus River. 4.3 RECHARGE AND PUMPING Groundwater beneath the Study Area is and has been strongly controlled by recharge in the vicinity of the WWTF and Stanislaus River, and pumping to the north for industrial and municipal supply, and to the south, for irrigation. Additional information on groundwater recharge and pumping is provided below. 4.3.1 Influence of River Stage on Water Levels Depending on river stage, the reach of the Stanislaus River that crosses the Study Area can either recharge groundwater or remove groundwater from the aquifer system. When the river is strongly gaining or losing, it may function as a hydraulic barrier for shallow groundwater flow, but less so during transitional periods, when the river is not gaining or losing substantial amounts of groundwater. The Stanislaus River has been characterized as a gaining stream in the spring of 1964 along the reach encompassing Ripon (DWR, 1965, cited in ECO:LOGIC, 2005a and Lawrence and Associates, 1991). In general, the Stanislaus River was a gaining stream prior to 1975, until increased pumping to the north, towards Stockton, lowered the water table and caused the Stanislaus River to discharge to groundwater, although near the City of Ripon WWTF lagoons, the Stanislaus River may have been gaining due to local recharge at the lagoons causing higher groundwater levels. On September 11, 2006, the water level in the Stanislaus River was surveyed at approximately 1:30 PM at the location show in Figure 12 (river survey station), and was reported as 40.06 feet above North American Vertical Datum 1988 (NAVD 88; http://cdec.water.ca.gov/queryTools.html; Appendix E). This location is approximately 500 feet east-northeast of Intermediate Aquifer monitoring wells M-30C1 and M-30C2, and approximately 750 feet south southeast of Neenah Paper Upper Aquifer monitoring well TH-9. Hourly river stage data is recorded by the Department of Water Resources at a gauging station located on the Stanislaus River where it is crossed by State Highway 99. The water levels measured at 1 PM and 2 PM on September 11, 2006 were 40.47 and 40.65 feet above the DWR datum, respectively. Assuming the river stage at 1:30 PM was the average of these two values (40.56 feet), an offset of-0.50 feet, when applied to the DWR data, can be used to estimate historical river stage at the survey location relative to NAVD 88, assuming that an incremental rise in river stage at the DWR station translates to the same rise at the river survey station. Figure 12 includes a plot of estimated river stage at the survey station and water levels in the nearest monitoring wells (M-30C1, M-30C2, and TH-9). In general, groundwater elevations in AMEC Geomatrix, Inc. I:\Doc_Safe\9000s\9837.005\4000 REGULATORY\SCM_01.30.09\1_text\SCM Report Final.doc 28