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I 3212 Vertical Extent i HB-17 was advanced approximately 20 feet downgradient of MW- Ground water samples collected at HB-17 revealed concentrations of TPH-g Benzene was not detected above the detection limit of 2 5 mg/1 TPH-g was detected in HB-17 at a depth of 10 to 15 feet bgs at a concentration of 1,100,000 µg/l The sample collected from HB-17 at a depth of 30 to 33 feet bgs contained 3,400 µg/I 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 µg/l, but decrease with depth Benzene concentrations in these ' samples also decreased with depth and ranged from non-detected to 3 2 µg/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 3.2.2 UST-2 3221 Lateral Extent Figures 11 and 12 present the distribution of TPH-g and benzene in ' ground water at UST-2, respectively As shown, 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 4g/1 of TPH-g and 3,600 µg/I of benzene At MW-2 (approximately 70 feet downgradient), the concentrations decrease to 620 µg/1 of TPH-g and non-detectable concentrations (at 0 5 4g/1 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) 9