ARCHIVED REPORTS_SITE CONCEPTUAL MODEL 2009

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amec�9 • Cessation of pumping at MW-6 in 1992, • Cessation of pumping at N-1 in 1995, • Increased pumping at MW-10 in 1996, and • Cessation of pumping at MW-4 in 2002. An increase in the magnitude of vertical gradients between well M-6C2 and M-61D beginning in 1996 through 2003 is also evident based on the hydrographs shown in Appendix B. These changes to the flow system can be expected to change the migration of COCs comprising a very narrow groundwater plume that may originate from a conduit well across the Corcoran Clay. Therefore, changes in groundwater flow direction may strongly influence the orientation of narrow COC plumes, which may temporarily migrate away from or towards monitoring wells in response to changes in pumping. Increasing or decreasing concentration trends do not necessarily indicate that a plume is expanding or decreasing in length; an alternate explanation is that a narrow COC plume is moving laterally in response to changing in pumping. 4.6.5 COC Impacts to Groundwater near the Stanislaus River Subsurface conditions beneath the Stanislaus River and WWTF lagoons are shown in Figures 35 and 36. The regional dip of the Corcoran Clay is apparent in Figure 35, where it dips towards the west southwest end of the cross section, towards the central axis of the San Joaquin Valley (the Corcoran Clay contacts were based on a review of the boring logs, from which the distinct blue-green color of the clay and higher plasticity were used to determine its depth and vertical thickness). The highest concentrations of CDCs, based on 2008 groundwater data, are associated with groundwater samples from C1-zone monitoring wells located along the north bank of the Stanislaus River. Total COC concentrations (TCE+cDCE+vinyl chloride) were highest in the sample from TH-9, at a total COC concentration of 369 pg/L. These detections and COCs are consistent with discharge of chlorinated VOCs to the WWTF area over time. CDCs have also been detected in two domestic wells south of the Stanislaus River, but at much lower concentrations, at private wells located at 5725 Hall Road (Upper Aquifer well from 100 — 120 feet bgs; Hall Road Well) and 5701 Kiernan Avenue (a domestic well with a total depth of 165 feet bgs, but unknown screen interval, referred to as the Kiernan well), in Salida (Stanislaus County). Groundwater samples from the Hall Road well were reported to contain TCE, cDCE, and vinyl chloride at concentrations ranging from 4.9— 5.5 pg/L, 10— 12 pg/L and 0.75— 0.94 pg/L, respectively (ECM, 2008b). Groundwater samples from the Kiernan AMEC Geomatrix, Inc. I:\Doc_Safe\9000s\9837.005\4000 REGULATORY\SCM_01.30.09\1_text\SCM Report Final.doc 48