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' Draft • TPH-g was detected 1n HB--17 at a depth of 10 to 15 feet bgs at a concentration of 1,100,000 µg/1 The sample collected from HB-17 at a ' depth of 30 to 33 feet bgs contained 3,400 pg/1 of TPH-g and no detectable concentration of benzene (at a detection limit of 2 5 µg/1) This indicates that the vertical extent of impacts is limited to shallow ground water and ' the highest concentration is present near the soil/ground water interface Saturated soil samples collected downgradient of UST-1 also suggest that the vertical extent of TPH-g and BTEX impacts are primarily limited to shallow ground water Soil samples collected from borings immediately ' downgradient of UST-1 (HB-17 through HB-19) at depths between 9 and 21 feet bgs contained TPH-g at concentrations ranging from non-detect to 2,600 pg/l, but decrease with depth Benzene concentrations in these ' samples also decreased with depth and ranged from non-detected to 3 2 pg/1 No TPH-g or benzene was detected in samples collected below 21 ' feet bgs The vertical sampling results are consistent with the hydrogeology of the site As discussed previously, clayey soils extend to a depth of 15 to 20 feet bgs These soils are underlain by sandy soils The elevated concentrations of TPH-g and BTEX compounds are present at depths that ,• correspond to those of clayey soils The concentrations of TPH-g and BTEX in the sandy soils at deeper depths are either not detectable or orders of magnitude less than those in the clayey soils 322 UST-2 ' 3 2 2 Z Lateral Extent The concentration of TPH-g and BTEX compounds downgradient of UST-2 decrease significantly Samples collected from MW-3 in December 1998 contained 90,000 pg/1 of TPH-g and 3,600 pg/1 of benzene At MW-2 ' (approximately 100 feet downgradient), the concentrations decrease to 620 µg/1 of TPH-g and non-detectable concentrations (at 0 5 µg/l detection limit) of benzene In addition, no benzene was detected in ground water samples collected at HB-20 (approximately 20 feet downgradient of the UST-2 excavation) ' 3222 Vertical Extent Samples from HB-20, indicated the presence of TPH-g, ethylbenzene, and • xylenes from the ground water table (approximately 7 feet bgs) to a depth ' of approximately 22 5 feet bgs For the samples collected at depths greater than 22 5 feet bgs, xylenes were the only compounds detected The