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WFISS ASSOCw7£S 6 Gordon Boggs / 10 June 1988 / Page 4 As reported in our correspondence dated 30 March 1988, EPA Method 8015 analysis of four native soil samples, collected beneath the slab, showed an average of 28 ppm TFHC (Figure 1). Comparison of the analytic results for- the samples collected from the perimeter borings and the excavation side-wall to the analytic results for the samples collected below the slab indicates that hydrocarbons are present in significantly higher concentrations in soil immediately adjacent to the slab than in soil below the slab or in soil from the perimeter borings. This suggests that the majority of the hydrocarbons from the leaking fuel tanks migrated laterally and downward away from the edges of the slab and are contained in soil adjacent to the present excavation. 2) LATERAI. MOVEMENT OF HYDROCARBONS As di's:.'.,ssLd dl,uvc, I,ydtvLai l,V115 appear lu L)C vvitfhell iv soil immediately adjacent to the excavation and, other than in the vicinity of the southwestern corner, there is little indication of lateral migration of hydrocarbons in unsaturated soil. 3) NATIVE SOIL SAMPLES FROM THE SOIL,/WATER INTERFACE Since ground water occurs at a depth of about 17 to 18 feet in the three ground water monitoring well#, Lin situ and the bottom of the extsvytien occurs at vllly 14 feet below ground surface, the soil samples from the excavation side-walls and from beneath i i the club were unoaturated. Eamples telleeted front dzptl:s 61, 15.7 fl, %1%11 ; � tltillirrg ul` borings B-3 and B-7, from 15.8 ft in the borings for wells MW-1 and MW-2 and from 18.3 ft in the boring for well MW-3, were collected close to the soil./water interface. As detailed in our 10 June and 13 October 1987 reports, no hydrocarbons were detected in these samples. i I 4) DOWNGRADIENT MONITORING WELL I Three ground water monitoring wells have been installed at the site and sampled for ! hydrocarbons. Borehole stratigraphy, construction characteristics, ground water sampling Procedures and analytic results for the wells are detailed in our report dated 15 October 1987. According to water level data recorded on 28 March 1988, we11 MW-2 lies directly 1 f