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claystones and sandstones, is typically unconfined and ranges in thickness from 7 to 10 ft. The general direction of ground water movement is east-northeast. The elevation of ground water beneath the tank excavation is approximately 1,275 ft. Monitor well W-850-05, installed as part of a tritium investigation, is located almost directly downgradient and approximately 50 ft northeast of the tank excavation. Due to the steep slope on the north side of the tank excavation, monitor well W-850-05 is located at the closest possible downgradient location. Ground water samples were collected from well W-850-05 in the third quarter, 1990 and were analyzed by EPA Methods 624 and 8015. The samples did not exceed the detection limits of 0.5 and 50.0 µg/L for volatile organic compounds (VOCs) or total fuel hydrocarbons, respectively (Appendix Q. The nearest surface water to Building 850 is spring 8 (WSPRNG8),located about 800 ft east- southeast of the tank excavation. This spring is cross-gradient of the tank excavation and flows at about 40 gal/hr (Taffet et al., 1990). The nearest intermittent stream channel is located approximately 600 ft northeast of Building 850. Only during heavy winter rains does surface water run through this channel. No water-supply wells are located within one-half mile of Building 850 and surface waters are consumed only by wildlife. Soil Sampling Soil samples have been collected at five different times to determine the spatial extent of the diesel fuel in soils adjacent to the location of the former tank. The dates of the sampling events are summarized in Table 1. Sampling began in 1989 following the expansion of LLNL's Environmental Guidance Division (EGD) tank group. Exploratory borehole logs are in Appendix D. As shown in Figure 4, a total of 45 soil samples have been collected by LLNL personnel. The SJCPHS-EHD requested that LLNL provide independent third-party verification of the sampling. Consequently, personnel from Science Applications International Corporation, a contractor to LLNL,observed each soil sampling event. Table 1. Dates of Soil Sampling Events. LLNL Div. Soil Sampling Event Date Sample No.Series Responsible' One 2/23/89 LP-1-850-34"and EGD LP-2450-45" Two 4/27/89 LP-2-850-55"to EGD LP-5-850-55" Three 5/1546/90 U850-DIU1-06 ERD to-10 Four 9/27-28/90 U850-DIU1-11 ERD to-14 Five 11/5/90 U850-DlU1-ESPI ERD 'EGD denotes Environmental Guidance Division;ERD denotes Environmental Restoration Division. 6