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Introduction This report documents the investigation of the 850-DlUl underground storage tank (UST) excavation at Lawrence Livermore National Laboratory's (LLNL) Site 300 facility. Figure 1 depicts the location of Building 850 and the tank excavation that once contained the 850-DlUl tank. This tank was formerly designated by LLNL as 850-41D and was registered with the California State Water Resources Control Board (SWRCB) using the Hazardous Substance Storage Statement shown in Appendix A. Site 300 is a high-explosives test area established by LLNL in 1955. It covers approximately 11 square miles and is located about 15 miles southeast of Livermore and 8.5 miles southwest of Tracy, California. Building 850 is located in the remote northwestern section of Site 300 and within San Joaquin County. Site 300 is operated by the University of California for the United States Department of Energy (DOE). Regulatory Requirements The 850-DlUl tank was removed in 1985 prior to the promulgation of UST closure regulations by the San Joaquin County Public Health Services—Environmental Health Division (SJCPHS–EHD). Therefore, initial closure activities for the tank were conducted without the benefit of a closure plan approved by the SJCPHS–EHD. Subsequent investigations of the soils adjacent to the tank excavation have followed the guidelines of the Leaking Underground Fuel Tank (LUFT) Field Manual, Revision 1 (S CB, 1989). The drilling of boreholes for sample collection was approved by the SJCPHS-EHD as Permit#90-430. An initial soil sampling investigation conducted in April 1989 revealed that diesel fuel had been released on to the native soil at the east end of the tank excavation, apparently during the 1985 tank removal operation. Upon receipt of the analytical results, LLNL reported the release in writing on May 25, 1989, to both the SJCPHS-EHD and the Regional Water Quality Control Board (RWQCB)—California Central Valley. Status reports covering the investigation activities have been submitted on a quarterly basis to the SJCPHS–EHD in progress reports and to the RWQCB—California Central Valley in the Environmental Investigation Quarterly reports. Underground Diesel-Fuel System Chronology The 850-DlUl underground diesel-fuel system was installed in 1962 to supply fuel for electric power generation for Building 850. It was taken out of service in 1967. The system consisted of the 850-D lU 1 tank, underground piping, and an aboveground standby emergency generator. The tank was constructed of carbon steel and had a capacity of 1,500 gal of diesel fuel (Appendix A). Since engineering blueprints are not available for the system, the total operational length of the underground piping is inferred from field inspections to be approximately 80 ft. This is the distance from the tank excavation to the present transformer shelter (Figure 2). The generator was probably located in the shelter (R. Ambrosin, personal communication). The exposed piping consists of 0.5-in.-diameter copper tubing with compression fittings. 1