ARCHIVED REPORTS SECOND SEMIANNUAL AND 2013 ANNUAL GROUNDWATER MONITORING REPORT

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2.RESULTS FOR SECOND SEMIANNUAL PERIOD 2013 • One trip blank sample for the cooler of samples (labeled TB-1) ' Sample analyses were conducted within a standard 10-day laboratory turnaround time. 2.1.2 Results iGroundwater Elevation and Flow Groundwater level data collected on September 13, 2013,are presented in Table 2, and elevation contours for these data are shown in Figure 3. These data are included in the tabulation of the eight most recent historical monitoring events in Appendix C. Hydrographs of the historical data are provided in Appendix D. Groundwater Elevations The groundwater elevations and depths to groundwater for the September 13,2013, data were as follows. • Shallow-interval groundwater monitoring wells are constructed with total depths between 14 and 23.5 feet bgs.Calculated elevations for the shallow-interval groundwater monitoring wells ranged from 1.99 feet above mean sea level(msl) at well M-1,located in the north portion of the Site near Trapper Slough, to-9.87 feet msl in well N-19, located in the southeastern portion of the Site. The depth to groundwater ranged from 1.71 feet bgs at well M-1 to 16.32 feet bgs at well 0-1. • 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-10.39 feet msl. The depth to groundwater was at 10.39 feet bgs in the well, above the screened interval. 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 HILL,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 Whiskey Slough Road (approximately 0 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 13,2013,groundwater elevations,all of which are in a west-to-east alignment: • 0.0032 foot per foot(ft/ft) through clayey sediments from well M-3 to the-1-foot-msl elevation contour ' • 0.14 ft/ft through clayey transition zone from the-1-foot to -8-foot-msl elevation contour • 0.00037 ft/ft through sandy sediments from well M-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 ' WBG101513183707SCO/HOLT_2SA13_REPORT_FINAL 2-3