ARCHIVED REPORTS_SITE CONCEPTUAL MODEL 2009

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amec— northerly groundwater flow direction from the WWTF area toward the MW-1 and the Site production wells. 4.6 SPATIAL AND TEMPORAL DISTRIBUTION OF COCS Groundwater samples collected from monitoring wells installed in the Upper, Intermediate, and Lower Aquifer have been reported to contain COCs, primarily TCE, cDCE, and vinyl chloride. The distribution of COC detections throughout the aquifer system beneath Ripon reflects multiple COC release areas, preferential flow paths (conduit wells) and the history of variable pumping and recharge that has affected groundwater movement over time, as discussed in Section 4.5.3. The distribution and temporal concentration trends have been reviewed and considered in the context of the SCM, in an attempt to explain the evolution of multiple COC plumes within a consistent hydrogeologic framework, as summarized below, for each major aquifer zone monitored. 4.6.1 Upper Aquifer— B-Zone Wells As discussed in Section 3.6, the most likely mechanism for TCE to migrate offsite is by discharge to the industrial sewer, which is documented to have had loose joints in 1974 (RUST, 1994a). Elevated TCE in soil vapor samples collected in 1987 and groundwater samples from well M-813 near the intersection of 4th Street and Stockton Avenue (Figure 9) suggest a leak occurred at this location. The spatial distribution of the maximum TCE, cDCE, and vinyl chloride concentrations reported from groundwater samples collected from Nestle 13- zone monitoring wells from first quarter 2006 to second quarter 2008 is shown in Figure 30. The distribution of CDCs in samples from B-zone monitoring wells (water table aquifer) suggests that the primary source for COCs, primarily TCE, recently at concentrations ranging from 10 — 600 pg/L, is likely from an industrial sewer leak at 4th Street and Stockton Avenue (Figure 9), and potentially from other locations. The distribution of TCE in B-zone groundwater samples follows a narrow, northeasterly direction towards the Site from monitoring well M-813, consistent with the 1994 B-zone groundwater contour map which indicated a groundwater flow direction towards well N-1, from the general direction of well M-813 (RUST, 1995). Historically, the highest TCE concentrations (greater than 1,000 pg/L) have been reported in samples from offsite wells M-613 and M-813, with lower concentrations reported in samples from wells M-22B and onsite well M-1 B. Recent TCE concentrations in B-Aquifer monitoring wells have been below 100 pg/L with the exception of groundwater samples from monitoring wells M-613 and M-1 B. TCE concentrations have been decreasing since 1987 at well M-813, and increasing between 1987 and 1993 at well M-613, followed by a decreasing trend (Appendix C). At well M-1 B, TCE concentrations have increased from 0.5 pg/L in 1987 to 590 lag/L in 2005 (Appendix C). It is not clear if these trends and spatial distribution are the result of a historical discharge from the industrial sewer near well M-813, or if they represent multiple historical AMEC Geomatrix, Inc. I:\Doc_Safe\9000s\9837.005\4000 REGULATORY\SCM_01.30.09\1_text\SCM Report Final.doc 44