ARCHIVED REPORTS_SITE CONCEPTUAL MODEL 2009

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amec�9 sewer line discharges or other sources, or onsite impacts. Nonetheless, the B-aquifer TCE plume(s) appears to be limited in extent and has attenuated over time. The transient increase in cDCE and vinyl chloride concentrations in April 2006 post-dates the carbohydrate injection at this well in October 2005 and is therefore a localized phenomenon related to the pilot test performed at this location (Geomatrix, 2006). 4.6.2 Upper Aquifer—A-Zone Wells The spatial distribution of the maximum TCE, cDCE, and vinyl chloride concentrations reported from groundwater samples collected from Nestle A-zone monitoring wells from first quarter 2006 to second quarter 2008 is shown in Figure 31. TCE is the primary VOC in groundwater north of 4t" Street and south of HWY 99, near the former facility at slightly higher concentrations (<11 Opg/L; Figure 31). The interpretation of the difference in chemical signature suggests that TCE reported in samples from wells M-18A, M-6A, M-10A, M-12A, and M-22A may be related to a different source (other than the WWTF lagoons). An alternate explanation for the higher TCE/cDCE ratios in these samples, if related to the WWTF lagoons, is that the higher concentrations of TCE relative to cDCE reflect the earlier discharges of untreated industrial wastewater at the WWTF lagoons, before the aforementioned capacity problems arose in the 1960s, when co-disposal of domestic sewage at the industrial ponds was temporarily implemented. Earlier discharges of industrial wastewater that was disposed in separate ponds, without mixing with domestic water, would have had lower organic matter content, and the discharged TCE would be less likely to undergo biotransformation to cDCE. VOC concentrations in groundwater samples from the Upper Aquifer generally have been decreasing, although an increase in TCE concentration has been observed in samples collected from monitoring well M-1 OA recently (Appendix C). 4.6.3 Intermediate Aquifer— C-Zone Wells The spatial distribution of the maximum TCE, cDCE, and vinyl chloride concentrations reported from groundwater samples collected from the Intermediate Aquifer (C, Cl-, and C2- zone monitoring wells; first quarter 2006 —second quarter 2008) is shown in Figure 32. Groundwater in the Intermediate Aquifer is affected by TCE, cDCE, and vinyl chloride, where cDCE is generally the dominant COC in samples from the Intermediate Aquifer. The highest COC concentrations in 2007/2008 were reported in samples from M-31 C1 (TCE, cDCE and vinyl chloride were 97.1, 240, and 47 lag/L, respectively); M-31 C1 is located approximately 750 feet east of the southeast WWTP lagoon, and approximately 4,000 feet south (upgradient) of the Site. Higher VOC concentrations were also reported for samples from wells M-1 7C1 and M-20C1, north of the WWTP lagoons and south of the Site, from which vinyl chloride has been reported in recent samples at concentrations one to two orders of magnitude above cDCE and TCE, AMEC Geomatrix, Inc. I:\Doc_Safe\9000s\9837.005\4000 REGULATORY\SCM_01.30.09\1_text\SCM Report Final.doc 45