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SECO 4.0 "A" ZONE 4.1 "A" Zone Groundwater Gradient Groundwater elevations in the "A" zone ranged from 3.54 feet below mean sea level (MSL) in well SP/WC-1 S to 3.01 feet above MSL in well PS/WC-1 S. The groundwater flow direction in the "A" zone is generally toward the southeast. Localized groundwater flow in the "A" zone during the first quarter 2006 was radially inward towards well SP/WC-1S with hydraulic gradients ranging from approximately 0.015 feet per foot (ft/ft) to 0.0166 ft/ft, southeast from well PS/WC-1S at a gradient of 0.007 ft/ft, and to the south on the southern portion of the site at a gradient of 0.009 ft/ft. Groundwater flow direction and hydraulic gradient data are summarized in Table 3. A groundwater elevation contour map for the "A" zone on February 6, 2006 is presented as Figure 2. 4.2 "A" Zone Groundwater Quality Data During the first quarter 2006, groundwater samples were collected from 32 "A" zone wells on February 6 through February 9, 2006 (Table 1). Analytical results are presented in Tables 4 and 5 and are summarized below. "A" zone TPHg, TPHd, benzene, and MTBE isoconcentration maps are attached as Figures 3, 4, 5, and 6, respectively. • • SPH: During the first quarter 2006, " " zone wells TS-4 and AR/MW-11A contained a visible sheen. This is the first quarter visible sheen has been reported in AR/MW-11A. Although visible sheens were reported during gauging activities on February 6, 2006, the wells were inadvertently sampled on February 7, 2006. • Analytical Results Summary: Number of Number of Detections Minimum Maximum Constituents Wells above the Concentration in Concentration in Analyzed MRL Ng/L (Well ID) pg/L (Well ID) TPHg 32 24 58 (TS-1) 65,000 (AR/MW-50) TPHd 32 23 74 (AR/OW-2A) 96,000 (AR/MW-10A) Benzene 32 22 0.64 (TS-10) 9,000 (TO/T-2) Toluene 32 20 0.71 (TS/MW-1A) 5,800 (AR/MW-50) Ethylbenzene 32 21 0.51 (TS-11) 2,600 (AR/MW-50) xylenes 32 21 2.0 (TS-10) 11,000 (AR/MW-244) MTBE 32 21 0.51 (SP/WC-1S) 40,000 (AR/MW-50) TBA 22 14 8.6 (TS/MW-1) 11,000 (AR/MW-11A) DIPE 19 0 not detected above MRL not detected above MRL TAME 19 3 27 (AR/MW-1A) 380 (AR/MW-50) • ETBE 19 Q not detected above MRL not detected above MRL Ethanol 14 0 not detected above MRL not detected above MRL STTC 1Q06 Final Report 5 May 1,2006 770T.43673.12.0002