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-2- C 14 0 I 1 borings were drilled in an approximate grid pattern to help characterize the site. The borings were located with a 100-foot tape and plotted on a site map (Figure 1 ) . The samples were collected with a modified California split-tube sampler fitted with three clean brass liners. The brass liner selected for geochemical analysis (either the lowermost or middle liner) was covered with aluminum foil , capped and placed on ice for delivery to the laboratory. Borings B1 through B4 were spaced at intervals greater than 100 feet across the site. Each of these borings was drilled to a depth of three feet and sampled . The lowermost brass liners were collected from a depth of 4-4 . 5 feet below ground surface. A silty sand and gravel lay below the surface to a depth of approximately one foot and appeared to be fill material . The soil below this porous material is a brown , silty clay with disseminated, thin stringers of whitish calcium carbonate. The white material reacts to } hydrochloric acid and is probably equivalent to an early stage of caliche formation. Two grassy depressed areas indicate the probable location of the backfilled tank pits as suggested by MSV' s agent. Five borings, B5--B9 were drilled in the pits. Boring B5 was drilled near the assumed downgradient end of the southwest pit , the tank pit farthest from the gate. The augers were advanced to a depth of ten feet and sampled at 9-10 . 5 and 10 . 5-12 feet . The top foot and one--half. appeared to be fill material but the rest of the soil in the boring looked like native material : an olive-brown colored, silty clay. We concluded that this boring was not in the actual tank pit. The next boring (B6) was therefore moved closer to what appeared to be the middle of the same tank pit. The augers were advanced through what resembled fill material to a depth of six feet and then sampled continuously in an attempt distinguish fill material from native soil. Apparently two types of fill material were used to backfill the pits : a loose sandy silt with small gravel and a silt and clay mixture containing concrete fragments. The inferred interface, between the fill material and native soil below, occurs at approximately nine feet below grade. One more boring (B7) was drilled and a sample was obtained from the southwest pit . Two borings and samples (B8 and B9) were collected from the adjacent, east- west oriented pit closest to the entrance gate. The top of an olive silty clay, below the fill in these borings , correlated well with with the nine foot depth for assumed native material in the other (southwest) pit. All of the samples from both of the pits were collected below the fill in this horizon. Samples from the underlying native soil provide the best estimate for possible contamination that may have leaked below the underground tank (see "DISCUSSION" section) . The resultant small volume of cuttings were mixed with bentonite chips and returned to the bottom 1-2 feet of the borings ; the remaining open holes were sealed to the surface with bentonite chips as required by San Joaquin County (SJC) . A SJC inspector visited the site as the final seal was completed and approved the work.