ARCHIVED REPORTS_SITE CONCEPTUAL MODEL 2009

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amec— and at Neenah Paper monitoring well TH-10, east of the southeast aeration pond. The presence of TCE breakdown products, the spatial distribution of TCE breakdown products in groundwater, and historical groundwater flow patterns that include mounding in the vicinity of the WWTF lagoons (discussed in Section 4.5.3), suggest that the major source for VOCs detected in C-zone monitoring well samples is the WWTP lagoons. Organics present in domestic and industrial sewage held at the WWTP have likely promoted the breakdown of TCE. This dilute VOC plume underlies the largest portion of the Study Area, perhaps because of higher groundwater velocities in the Intermediate Aquifer due to higher permeability and pumping, or perhaps because the shallower flow systems were more strongly influenced by Stanislaus River stage variations causing localized reversals of hydraulic gradient near the WWTF. A third possibility is that unidentified conduit wells (e.g., former and undestroyed irrigation wells) near the lagoons, or idle Simpson Paper well S-2, approximately 1,000 feet east of the WWTF, may have facilitated recharge of contaminated groundwater from the WWTF to the Intermediate Aquifer. The COC distribution in the vicinity of the Stanislaus River and to the south of the Stanislaus River is discussed in greater detail in Section 4.6.5. Trends in data showing the occurrence of reductive dechlorination indicate that C-zone chlorinated VOCs are undergoing intrinsic remediation (vinyl chloride has increased relative to cDCE and TCE over time, as shown in the concentration versus time plot for M-1 7C1 in Figure 34). Also, changes in hydraulic gradient can have an influence on COC trends, also shown in Figure 34. Between 1994 and 1998, water levels at well M-17C1 were often much higher than at well M-23C1, approximately 700 feet south near the WWTF. A corresponding decrease in TCE, cDCE and vinyl chloride concentrations occurred in groundwater samples from well M- 17C1 during this timeframe, perhaps due to mounding caused by recharge nearby. After the water levels decreased in well M-1 7C1 relative to well M-23C1, COC concentrations increased, where vinyl chloride has since been the major COC present in groundwater samples. It is unclear if this increase in vinyl chloride concentration is related to the additional input of organics associated with the recharge of organic-rich water to this area, or simply related to a change in groundwater flow direction. As of the date of this report, additional characterization of non-chlorinated breakdown products (ethene) and geochemical conditions is being conducted to evaluate the intrinsic remediation processes in curtailing the migration of VOCs in the Intermediate Aquifer. However, it is important to note that ethene is also subject to biodegradation. Therefore, it's the absence of ethene is not necessarily an indication that the reductive dechlorination process stops at vinyl chloride. Vinyl chloride can also be degraded to carbon dioxide and chloride under anaerobic conditions. AMEC Geomatrix, Inc. I:\Doc_Safe\9000s\9837.005\4000 REGULATORY\SCM_01.30.09\1_text\SCM Report Final.doc 46