ARCHIVED REPORTS_SECOND SEMIANNUAL AND 2011 ANNUAL GROUNDWATER MONITORING REPORT

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L3.ANNUAL REPORT FOR 2011 Lmonitoring for this well began in 2005. The decreases in wells M-10 and 0-1 have occurred to such an extent that well 0-1 is almost no longer in the hydrocarbon plume and well M-10 It. is now almost outside the plume. Before the start of the bioremediation pilot test,no long-term BTEX and TPH concentration trends for extraction well N-17 and nearby monitoring well N-16 were apparent.This was Lpotentially due to extraction at 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 L concentrations in wells N-16 and N-17 have shown similar behavior. Overall,the concentrations of BTEX and TPH constituents began to increase after extraction L 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 BTEX and TPH constituents that were being pulled downgradient from Lthe source area. Since the initial concentration increases after the extraction was shut down, a new equilibrium appears to have been established where the naturally occurring processes are resulting in decreasing concentrations for the BTEX and TPH constituents. L 3.2 Effectiveness of the Current Remediation System LThe 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 1 by the RWQCB, the steady reduction in hydrocarbon mass,as well as BTEX and TPH 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 L Characterization Report(CH2M HILL,2002) and the 3-Year Evaluation of the Bioremediation Pilot Study(CH2M HILL,2007a).The steady reduction in hydrocarbon mass is shown in the time-series concentration charts in Appendix D and Appendix E. i 3.3 Identification of Data Gaps and Potential Deficiencies/ ` Redundancies No data gaps or deficiencies have been identified.Quarterly groundwater monitoring at the Site was implemented in March 1992.To date, the groundwater monitoring program has �.r been performed for approximately 19 years and has provided important time-series information with respect to groundwater elevations and gradients,NAPH thicknesses, and chemical concentrations in groundwater and surface water. Seasonal patterns in L groundwater elevation fluctuations,as well as seasonal and long-term trends in BTEX and TPH concentrations, have been identified,as documented in the Final Site Characterization Report(CH2M HILL,2002) and in subsequent routine groundwater monitoring reports. LRobust data have been collected to demonstrate that the naturally occurring processes are sufficient to result in a continued declining trend of these hydrocarbon constituents without ES102411183308SCOILW3521.DOC)Ul 12970003 33