ARCHIVED REPORTS_SECOND SEMIANNUAL 2018 GROUNDWATER MONITORING REPORT

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Second Semiannual and 2018 Annual Groundwater Monitoring Report JACOBS® • The deeper-interval groundwater monitoring well (N-24) is constructed at a total depth of 42 feet bgs. The calculated groundwater elevation for well N-24 was -10.89 feet msl. These data are consistent with historical data, as shown in the hydrographs in Appendix D. • Groundwater Flow Groundwater flow depends on the physical characteristics of the subsurface materials and the groundwater gradients. A detailed analysis of groundwater flow for the fourth quarter 2001 groundwater elevation data is presented in the Final Site Characterization Report(CH2M, 2002). In general, • groundwater flow occurs in the direction of maximum potential hydraulic gradient, or perpendicular to the lines of equipotential head. Groundwater elevation is highest in the clayey sediments northwest of S Whiskey Slough Road (approximately -1 foot msl) and lowest in the sandy sediments southeast of Whiskey Slough Road (approximately -9 feet msl). The horizontal gradient magnitude at the site is characterized by the individual gradients across the following three transects for the September 7, 2018, groundwater elevations, all of which are in a west-to-east alignment: S0 0.0044 foot per foot(ft/ft) through clayey sediments from well M-3 to the -1-foot-msl elevation contour • 0 0.10 ft/ft through the clayey transition zone from the-1-foot-msl contour to the -9-foot-msl elevation • contour • 0.0016 ft/ft through sandy sediments from well N-8 to well N-19 Sitewide, horizontal groundwater flow occurs from the recharge areas of Trapper Slough and the area • with standing water, through the clayey sediments northwest of Whiskey Slough Road, toward the sandy sediments southeast of Whiskey Slough Road. The water table drains from the clayey sediments laterally • into the sandy sediments at the band shown by water table elevation contours at-1 foot and -9 feet msl. This flow results in a focusing effect, where groundwater in the sandy sediments southeast of Whiskey Slough Road is naturally directed toward a common area in the vicinity of the two former extraction wells, • RW-1 and RW-3. Locally, the former release area is bounded on three sides by a constant head—to the north and west by Trapper Slough and to the south by the area with standing water. These boundaries result in locally focused groundwater flow toward the area between the two access roads (of the former pipeline and the bypass pipeline), which then drains to the southeast. 2.1.2.2 Groundwater Analytical Results Analytical results for the groundwater and blank samples collected on September 7, 2018, are presented in Table 3. TPH-g and benzene concentration contours for these data are shown on Figures 4 and 5, respectively. The data are further presented in the following appendixes: • Appendix C –Data are tabulated with the most recent eight monitoring events. • Appendix D–Concentrations are shown in time series charts for wells with consistently detectable • constituents (wells M-10, N-7, N-8, 0-1, N-15, N-16, N-17, and RW-1). • Appendix E–Concentrations are plotted on long-term trend charts by expanding the future scale of the charts in Appendix D to 2050 to better see the long-term concentration trends. Shallow Groundwater Laboratory analytical results of shallow groundwater samples are consistent with data reported during previous quarters, as shown in the time series charts in Appendix D. The data indicate that the lateral extent of petroleum-affected shallow groundwater is stable and shrinking. The lateral extent of petroleum- affected groundwater is defined by wells M-1, M-8, N-18., N-19, and N-20. Petroleum hydrocarbons were not detected in samples collected from wells M-8, N-18, N-19, and N-20 on September 7, 2018. • AX1015181035SCO