ARCHIVED REPORTS_SECOND SEMIANNUAL 2018 GROUNDWATER MONITORING REPORT

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Second Semiannual and 2018 Annual Groundwater Monitoring Report JACOBS® i 3. Annual Report for 2018 This section presents the 2018 annual report and includes a summary of groundwater monitoring results, • an evaluation of the effectiveness of the current remediation system, and an evaluation of potential data gaps and deficiencies/redundancies in the MRP. 3.1 Groundwater Monitoring Results Two semiannual groundwater monitoring events were performed for the 2018 annual reporting period • (October 2017 through September 2018) in accordance with the previous MRP No. R5-2007-0831 (October 2017 through March 2018) and the new MRP No. R5-2018-0806 (March 2018 through September 2018). • The first semiannual 2018 event was conducted in March 2018; results are documented in the semiannual monitoring report submitted to the RWQCB on April 30, 2018 (CH2M, 2018). Tables and figures from that report showing tabular and graphical summaries of the data collected are included in Appendix F of this report for reference. • The second semiannual 2018 event was conducted in September 2018; results are documented in Section 2, and in the following tables and figures, of this report: — Table 2— Groundwater Depths and Elevations — Table 3— Hydrocarbon Concentrations and Field Parameter Results — Figure 3 —Groundwater Elevation Contour Map • — Figure 4 — Dissolved-Phase TPH-g Concentration Map — Figure 5—Dissolved-Phase Benzene Concentration Map • Current data are also presented in Appendix C as part of the tabulation of the 10 most recent quarters of collected data: — Table CA — Historical Groundwater Elevation Data — Table C.2— Historical Groundwater Concentration Data Appendix D provides a graphical summary (time series charts) of the water level data and select chemical w monitoring data since quarterly monitoring was implemented in 1992. A hydrograph is provided for all monitoring wells. Time series concentration charts are also provided for wells that have consistently detectable TPH and BTEX concentrations (wells 0-1, M-10, N-7, N-8, N-15, N-16, N-17, and RW-1). Appendix E provides time series concentration charts for these wells with the time scale expanded to 2050 to help assess potential long-term concentration trends. • 3.1.1 Groundwater Elevations and Flow Average rainfall typically is approximately 13.76 inches per year as recorded by the City of Stockton rain gauge. Historically, during normal rainfall years, the water table increases during the spring semiannual event following the winter rainy period and then decreases during the fall semiannual event following the i dry summer months, as shown in the hydrograph in Appendix D. The correlation between higher groundwater elevations during the winter and lower groundwater elevations after the summer has been • observed since monitoring began in 1992. As shown on Figure 3 of this report, and Figure 3 from the previous report(Appendix F), groundwater flow patterns were consistent for the March 7, 2018, and September 7, 2018, data. These groundwater flow patterns are also consistent with historical monitoring events before and after the active remediation • systems (GWETS and SVETS)were shut down in February 2004 for the 3-year bioremediation pilot study. Groundwater flow is described in Section 2. i 0 AX 1015181035SCO