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0 <br /> C. Susi Jackson of the Operations and Regulatory Affairs Division to Diane <br /> Hinson, August 3, 1994 (Appendix I). <br /> Field work proceeded as follows: On August 23 we performed additional <br /> excavation in the Building 871 underground tank closure excavation to remove <br /> diesel fuel contaminated siltstone. In mid-afternoon we stopped excavation as <br /> water began entering the excavation. After discussion with the LLNL <br /> Environmental Restoration Division and Diane Hinson of San Joaquin County <br /> Public Health Services, we resumed excavation on August 25, and provided a <br /> pump to keep the excavation dry enough to collect dry siltstone samples at a <br /> depth of twenty feet at each end of the excavation, as possible. When we began <br /> excavation on August 25, the water quickly soaked into the siltstone and was <br /> not encountered again. We believe the water existed in local fractures in the <br /> siltstone. Analyses of siltstone samples collected at a depth of twenty feet <br /> showed 970 mg/kg (west) and 790 mg/kg (east) TPH-D (diesel fuel). By <br /> considering the site specific factors, Diane Hinson completed the "Leaching <br /> Potential Analysis", Table 2-1 of the State Water Resources Control Board <br /> Leaking Underground Fuel Tank Manual. The "Leaking Potential Analysis" for <br /> the Building 871 tank excavation site showed that soil (siltstone) samples <br /> collected in the bottom of the excavation could contain no more that 1000 <br /> mg/kg TPH-D (diesel fuel) and be considered clean (enough) for the site. <br /> Because the two siltstone samples collected in the bottom of the excavation were <br /> shown by the results of analyses to be less that 1000 mg/kg, no further remedial <br /> action was required. <br /> With the completion of excavation, the hole was partially filled with imported <br /> pea gravel in preparation of installation of the new double-wall 1000 gallon <br /> underground diesel fuel tank system. The tank system has been installed and <br /> made operational, and the site has been restored to grade with asphalt and <br /> concrete. <br /> This is the final report of corrective action at the Building 871 underground <br /> tank closure site. <br /> Hydrogeology Of The Building 871 Area. Two hydrologic units have been <br /> identified in the General Services Area (GSA), based on hydraulic, physical, and <br /> chemical characteristics; the Qt-Tnsc1 hydrologic unit (aquifer) and the Tnbs1 <br /> hydrologic unit (aquifer). The 871-DlU1 tank site is in the GSA with a surface <br /> elevation of 550 ft. above mean sea level (MSL). <br /> Quaternary Terrace Deposits - Neroly Middle Siltstone/Claystone (Qt-Tnsc1) <br /> Hydrologic Unit: The Qt-Tnsc1 hydrologic unit is composed of three <br /> stratigraphic units: Quaternary alluvium and terrace deposits (Qt, Qoa). the <br /> Neroly upper blue sandstone (Tnbs2), and the Neroly middle <br /> - 8 - <br />