ARCHIVED REPORTS_2013 ANNUAL REPORT

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Nestl6 USA, Inc., Ripon, California March 26, 2014 2013 Annual Report located north of the City of Ripon WWTP (Figure 11) may indicate that the western extent of the plume in the M-17C1 area has receded. Currently, it is not known whether this is due to nondestructive processes such as dilution, dispersion, or sorption or to COC destruction; additional data collection and evaluation are planned (see Section 3.3). • VC was detected in four wells located in the vicinity of the Stockton Avenue treatment system (E-6, M-6C2, M-8C1, and M-54C) and in eight wells in the vicinity of the City of Ripon WWTP (M-11C1, M-17C1, M-20C1, M-30C1, M-31 C1, TH-8, TH-10, and TH-11). Concentrations of VC and cis-1,2-DCE are greater than concentrations of TCE in each of these wells, which provides evidence of intrinsic remediation via reductive dechlorinations in these areas. • VC concentrations above 100 pg/L in the vicinity of wells TH-10 and M-30C1 have been observed for over 5 years, indicating sustained reductive dechlorination is occurring in this area. Lower Aquifer • There is no evidence in the 2013 data of significant changes in the extent of the COC plume in the Lower Aquifer (Appendix B). • Well M-20D, located near the City of Ripon WWTP, is still the only Lower Aquifer well to contain VC. 2.3.2 COC TREND ANALYSIS ECM evaluated COC concentration data over time for each Nestl6 monitoring well and City of Ripon municipal wells (Table 6, Appendix B). COC concentrations at each well location respond to spatial and temporal variations in the flow directions within the aquifers. As discussed in Section 2.2.2, changes in the level of the Stanislaus River control the groundwater flow in the Upper and Intermediate aquifers. Most of the wells with COC impacts have decreasing concentration trends over the last five years, indicating overall plume attenuation. Since the plume boundaries have not increased, this suggests that destruction of COCs is occurring through natural attenuation processes. ECM has provided a qualitative COC trend analysis for each of the three monitored aquifers, including observations at select wells, in the following subsections. Refer to Appendix C to review all historical well data presented in graphical format. Upper Aquifer • Ninety percent of the Upper Aquifer wells with historical COC detections have had decreasing or relatively constant TCE concentration trends over the last five years of monitoring. • TCE concentrations in wells M-413, M-5A, M-7A, M-8A, M-11A, M-1413, M-15A, M-17A, and M-32A have decreased significantly to levels below the RLs since initial monitoring. TCE concentrations in well M-32A most recently dropped below RLs in January 2012. • TCE concentrations in wells located in the vicinity of the Site, such as wells M-1A, M-2A, M-613, M-46A, M-47A, and M-48A, show significant decreases over time (Appendix C). 10