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WEISSASSocurEs MI 2. SUBSURFACE INVESTIGATION The subsurface investigation consisted of collecting soil samples beneath each of the tanks and analyzing the samples for the chemicals stored in each tank. Since chemicals released from the tanks would migrate downward into the underlying soils,analysis of samples collected beneath the tanks may indicate whether the tanks have leaked. To increase the probability of detecting released chemicals, soil samples were collected both within one ft of the tank bottoms and three to four ft beneath the tank bottoms. Since some of the chemicals stored in Tanks 7 and 8 are naturally occurring, samples of uncontaminated native soil were also collected and analyzed. Comparison of the analytic results for these background samples to the analytic results for the samples from beneath the tanks may indicate whether the tanks leaked. 2.1. SOIL BORINGS The objective of the drilling was to describe subsurface conditions and to collect soil samples from below the tanks. Between 13 and 16 December 1988, PC Exploration of Roseville, California drilled nine soil borings beneath the underground storage tanks using Mobile B-53 hollow-stem and Johnson continuous-flight auger drill rigs. The drilling was supervised by Weiss Associates geologist Robert Agnew. All of the borings were drilled at an angle of approximately 45 degrees to horizontal and extended 3.5 to 9 ft below the tank bottoms. Boring logs are presented as Appendix A. Sample locations and borehole orientations are shown on Figures 3a through 3h. Four samples were collected from beneath Tank I from two borings drilled adjacent to the southeastern corner of the tanks. Two samples were collected from each boring within about 1 ft below the tank bottom and about 3.5 ft below the tank. The samples from below Tank 5 were collected from a single boring drilled from the northwestern side of the tank. The samples were collected at about 1.5, 5.0 and 9.0 ft below the tank bottom. A shallow sample was not collected below the southern end of this tank due to limited access and buried utilities. The Tank 6 samples were collected I to 4 ft beneath the tank from two borings drilled from the southwestern side of the tank. The samples beneath Tanks 7 and 8 were collected from boreholes drilled north of Tank 7 and west of Tank 8, respectively, at depths of about 2.5 and 7 ft below the tank bottoms. The background samples were collected at 10.0 ft depth from borings drilled north of Tank 7 and west of Tank 6. After the sampling was completed the borings were filled with cement grout containing about 5% bentonite powder. F:\ALL\331DWG\331L1FE9.WP Page 5