ARCHIVED REPORTS_SECOND SEMIANNUAL 2018 GROUNDWATER MONITORING REPORT

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J/�COB5 Second Semiannual and 2018 Annual Groundwater Monitoring Report 3.1.2 Nonaqueous-Phase Hydrocarbons NAPH was not detected in any of the groundwater monitoring wells in 2018. NAPH has not been detected • at the site since 2002. 3.1.3 Total Petroleum Hydrocarbons and Benzene, Toluene, Ethylbenzene, and Total Xylenes • TPH and BTEX constituents were not detected in shallow downgradient wells M-8, N-19, and N-20 during this semiannual event, which is consistent with historical monitoring results. TPH and BTEX constituents also were not detected in the deeper well (N-24) during the first and second semiannual events in 2018. TPH and BTEX constituents were not detected in well N-18, except for the periodic detections of benzene • in the past several years at low concentrations and a detection of total xylenes in the 2017 second • semiannual monitoring event. The previous detections of benzene have occurred since 2010, which is consistent with the construction and use of farm buildings in the immediate vicinity of this well. A shallow seep has been observed near N-18 during this time, which could have been from the building septic system. TPH and BTEX constituents continue to be detected in the remaining monitoring wells, which are located in the former release area (N-7 and N-15), on the plume fringe (0-1, N-8, and N-16), and in the former extraction wells (RW-1 and N-17). As shown in the time series concentration charts in Appendix D, since the start of the bioremediation pilot test, the long-term TPH and BTEX concentration trends have continued to decrease for the wells with lower concentrations located on the plume fringe (0-1, M-10, and N-8) and for the wells with higher concentrations located in the former release (source) area (N-7 and N-15). • All of these wells historically had NAPH, which is no longer detectable. The long-term concentrations for former extraction well RW-1 have exhibited a decreasing trend since monitoring of this well began • in 2005. The decreases in wells M-10 and 0-1 have occurred to such an extent that they are almost no longer in the hydrocarbon plume. Before the start of the bioremediation pilot test, no long-term TPH and BTEX concentration trends for extraction well N-17 and nearby monitoring well N-16 were apparent. This • potentially was due to extraction at well N-17 pulling hydrocarbons downgradient from the areas of higher contamination near the source area, which sustained the hydrocarbon concentrations. Since the start of the bioremediation pilot test in February 2004, the concentrations in wells N-16 and N-17 have shown similar behaviors, as described in the following paragraph. Overall, the concentrations of TPH and BTEX constituents began to increase after extraction stopped from well N-17 in February 2004. This likely occurred because clean, downgradient groundwater was no longer being pulled into the capture zone (and into wells N-16 and N-17) to dilute the TPH and BTEX constituents that were being pulled downgradient from the source area. Since the initial concentration increases after the extraction was shut down, a new equilibrium appears to have been established wherein the naturally occurring processes are resulting in decreasing concentrations for the TPH and BTEX constituents. 3.2 Effectiveness of the Current Remediation System • The current remediation system consists of MNA and the phytoremediation system, as summarized in Section 1. The combination of these two strategies is effective, resulting in a sitewide decline in TPH and • BTEX concentrations in groundwater, and a stable-to-shrinking extent of these constituents in groundwater. As discussed with the RWQCB during meetings on July 12, 2007, and September 21, 2007, and reflected in the current MRP issued by the RWQCB, the steady reduction in hydrocarbon mass, as well as , reductions in TPH and BTEX concentrations, at the site are the result of fate-and-transport processes occurring naturally in the subsurface. These naturally occurring processes are documented in the Final Site Characterization Report(CH2M, 2002) and in the 3-Year Evaluation of the Bioremediation Pilot • AX 1015181035SCO • •