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Historical Background AGE-NC Project No 99-0559 IPage 2 of 4 ' Laboratory analysis of ground water samples collected from two monitoring wells on the northern and eastern property boundaries appeared to show that dissolved petroleum hydrocarbons extent off- site The vertical and lateral extent of impacted ground water at the site was not defined,further more increasing concentrations of dissolved hydrocarbons in the ground water monitoring wells indicate off-site migration of contaminants The mass of dissolved hydrocarbons and MTBE is significant and required off-site assessment and immediate interim remediation IIn April 2001 three paired soil borings (CPT-1, CPT-2 and CPT-3)were advanced on-site, at each boring location, lithologic data was collected from the initial CPT boring, then an immediately I adjacent twin boring was advanced to collect grab groundwater samples from selected depths Soil borings CPT-1, CPT-2 and CPT-3 were advanced vertically to maximum depths of approximately 155 feet below surface grade(bsg), 162 feet bsg and 158 feet bsg,respectively CPT 1 was advanced ' at the western edge of the site, CPT2 was advanced adjacent to the UST release, and CPT3 was advanced along the southern edge of the site IGenerally, the subsurface of the site is comprised of alternating thick intervals of fine-grained material and thin to thick coarse-grained units Based on the CPT data, AGE identified six . hydrogeologic units (HU1 through HU6), defined as bodies of rock or sediment that has ' hydrogeologic characteristics(i e hydraulic conductivity,pore pressure,etc )that make them distinct from surrounding bodies of rock or sediment(Sanders, 1998),or a part of a body of rock that forms ' a distinct hydrologic unit with respect to the flow of ground water(Maxey, 1964) HU1 was impacted by TPH-g,benzene and MTBE in the former UST area and only by MTBE south ' and west of the former UST area HU4, a thick poorly graded sand to silty sand layer encountered at approximately 92 to 105 feet bsg which was a significant unit of concern,was impacted by TPH-g, benzene and MTBE in the former UST area and toward the west and was slightly impacted by ' MTBE(<5 0 µg/1)south of the former UST area The upper HU5 (130 feet bsg)was impacted in the former UST area by TPH-g, benzene and MTBE, but was less impacted south of the former UST area (TPH-g <100 gg/1), and not impacted west and down-gradient of the former UST area The ' lower HU5, at 150 feet, was significantly impacted by TPH-g, benzene and MTBE in the former UST area,yet was not impacted by TPH-g and BTEX south or west of the former UST HU5 at 150 feet was slightly impacted by MTBE (<5 0 gg/1) south of the former UST area ' The vertical extent of dissolved TPH-g and benzene does not extend to 130 feet bsg in the area of CPT-1 or to 60 feet bsg in the area of CPT-3 The vertical extent of dissolved TPH-g and benzene ' has not been determined in the area of CPT-2, adjacent to the former UST area and extends to the lower HU5 at 150 feet bsg Dissolved MTBE does not extend vertically to 130 feet bsg in the area of CPT-1 The vertical extent of dissolved MTBE has not been determined in the areas of borings ' CPT-2 and CPT-3 The concentrations of dissolved MTBE detected in the former UST area increases . with depth, and highest MTBE detection in bonng CPT2, the former UST area, was in the sample Advanced GeoEnvironmental,Inc