ARCHIVED REPORTS_SITE CONCEPTUAL MODEL 2009

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amec�9 hydraulic depression caused by pumping at 519 Stockton Avenue (extraction well E-6) and toward well MW-3 pumping, to the northwest. The anthropogenic influence on groundwater flow is apparent from Figure 16. January 2006 (Figure 17): At this time, the Stanislaus River water level was near flood stage, reaching a maximum elevation of approximately 54.29 msl on January 6, 2006. The water level was measured in nearby well TH-9 on January 23, 2006, and was approximately 49.41 feet msl; the river stage had subsided to an elevation of 45.50 feet msl by this time. Groundwater contours shown in Figure 17 indicate that the groundwater flow direction (based on the hydraulic gradient estimated from water levels in the Intermediate Aquifer monitoring wells) was northward, from the Stanislaus River beneath the WWTF lagoons, towards pumping wells (E-6, MW-3 and MW-10). No groundwater mound is evident north of the WWTF. This scenario is consistent with a large input of river water to the groundwater system, causing groundwater to flow northward beneath the lagoons, which is opposite to the flow direction inferred from the July 2006 water levels. Also, there was likely no irrigation pumping south of the Stanislaus River during this time. The groundwater flow system between the WWTF lagoons and the Stanislaus River is dynamic system with systematic gradient reversals due to high river stage that could greatly curtail net groundwater migration over time in this area. July 2004 (Figure 18): There were no monitoring wells near the Stanislaus River at this time, so the flow direction between the WWTF lagoons and the Stanislaus River cannot be evaluated. However, groundwater flow was generally to the northwest beneath the Study Area during this time, based on the interpreted Intermediate Aquifer hydraulic gradient shown in Figure 18. January 2004 (Figure 19): There were no monitoring wells near the Stanislaus River at this time, so the flow direction between the WWTF lagoons and the Stanislaus River cannot be evaluated. However, groundwater flow was generally to the northwest beneath the Study Area during this time, with an apparent hydraulic mound near the WWTF and Neenah Paper discharge areas. The water level in well M-1 7C1 is approximately 0.26 feet higher than that measured for well M-23C1, and similar to well M-20C1, implying a southwesterly flow beneath the WWTF lagoons. The cause for the apparent high water level within the Intermediate Aquifer near M-1 7C1 is unknown. April 1997(Figure 20): Monitoring wells near the Stanislaus River had not yet been installed at this time, so the flow direction between the WWTF lagoons and the Stanislaus River was not evaluated. However, the prevailing direction of the hydraulic gradient in the Intermediate Aquifer was generally to the north and northeast beneath the Study Area during this time, with AMEC Geomatrix, Inc. I:\Doc_Safe\9000s\9837.005\4000 REGULATORY\SCM_01.30.09\1_text\SCM Report Final.doc 41