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Appendix A - Historical Background AGE-NC Project No 02-0964 Page 6 of 7 HU7 One sample(CPT3-150)was collected from westernmost boring CPT3, target analytcs were not detected in this sample HU8 Samples CPTI-150, CPT2-150, CPT4-145 and CPT5-150 were collected from this HU TPH-g was detected in samples CPT1-150, CPT2-150 and CPT5-150 at a maximum concentration of 11,000 µg/1 (CPT2-150), in addition,TPH-d was detected in samples CPT1-150 and CPT2-150 at a maximum concentration of 5,300 µg/I (CPT1-150) BTEX compounds were detected in CPT1-150, CPT2-150 and CPT5-150 at maximum concentrations of 35 µg/1 benzene, 590 µg/l toluene, 360 µg/1 ethylbenzene and 1,700 µg/1 xylenes, in sample CPT2-150 In addition, the fuel additive 1,2-DCA was detected in sample CPT2-150 at 2 8 µg/I No other analytes were detected in the samples analyzed An additional grab ground water sample (CPT2-185) was collected at a depth of 185 feet bsg, approximately 25 feet below the limit of CPT soil data TPH-g was detected in this sample at a concentration of 130 µg/1 In addition,BTEX compounds were detected at concentrations of 17µg/1 benzene, 2 3 µg/1 toluene and 2 2 µg/1 xylenes No other analytes were detected SITE CONCEPTUAL MODEL • The site stratigraphy is composed predominantly of fine-grained soil consisting of silt,clayey silt, silty clay and clay, with the exception of five significant sand lenses or layers at depths of approximately 50 feet bsg, between 60 and 75 feet bsg, between 95 and i 10 feet bsg, between 120 and 130 feet bsg and between 140 and approximately 160 feet bsg CPT pore pressure dissipation tests in the sandy silt, sand and gravelly sand indicated that hydraulic conductivities were typical for sand mixtures • TPH-g and BTEX impacted soil was detected at low concentrations at 55 feet bsg, but at significant concentrations at 78 feet bsg Based on the vertical distribution of contaminants in soil from the present and previous site investigations,AGE delineated two distinct zones of contaminated soil, in fine-grained soil (HUI) at between 30 and 40 feet bsg beneath the Chevron (northeastern-most) former dispenser island, and in fine-grained soil (HU3) at 80 feet bsg The soil contamination at 30 to 40 feet bsg, which included diesel as well as gasoline products,may have originated from the overlying former Chevron dispenser island area However,based on the available data,the source for the deeper contamination at 80 feet bsg is uncertain, the former Standard Oil USTs cannot be ruled out • Petroleum hydrocarbons were detected in grab ground water samples collected from the northeastern portion of the site, in the area of CPT1, CPT2 and CPT5,to approximately 180 feet bsg Based on the vertical distribution of contaminants in ground water from the present and previous site investigations, the highest concentration of contaminants appears to be at between the upper ground water surface at boring A/MW-1 and at 40 and 60 feet bsg in the area of CPT1 and CPT2 The concentrations of contaminants appear to decrease vertically, towards a minimum in clay-rich fine grained soil at a depth of between 80 and 100 feet bsg Advanced GeoEnviroumental