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r I' r .' 119 July 2004 AGE -NC Project No 95-0121 ' Page 8 of 9 I, ' It is riot known how effectively the semi-confining/confining intervals (HU1, 17fU3 and HU5) hydrologically separate the informal `flow units' (HU2 and HU4), although the laboratory' analysis of the `grab' ground water samples suggests that the petroleum hydrocarbon -impact has migrated through or around HU1, HU3 and HU5 Geologic cross section A -A' depicts ' the informal hydrogeologic units and petroleum hydrocarbon concentrations beneath the site (Figures 3, 4 and 5) Adsorbed (soil impacted) constituents of concern were detected from CPT -1 through CPT -3 and included minor TPH-g, BTEX and 1,2 -DCA (Table 2), no constituents of concern were ' detected from CPT -4 The adsorbed hydrocarbon -impacted soil plume beneath the site is ; depicted in cross-sectional view A A ' (Figure 4) Hydrocarbon -impacted soil appears to extend to depths of 50 feet bsg beneath the former UST area and well EW -1, and dispersed laterally 50 feet toward the southwest and at least 100 - feet toward the northeast The ' detection of low concentrations of TPH-g/benzene at depths below 50 feet bsg may be attributed to adhering of the contaminants to the soil at these deeper depths after vertical r , ' migration of the dissolved portion of the hydrocarbon plume • Dissolved constituents of concern were detected from CPT -1 through CPT -3, including ' • significant TPH-g, BTEX and 1,2 -DCA (Table 3), minor DIPE and TAME were detected from the Iaboratory analysis No constituents of concern were detected from CPT -4 Hydrocarbon -Impacted ground water appears to extend to depths greater than 100 feet bsg ' beneath the former UST area, and have migrated laterally 80 feet toward the southwest and greater than 100 feet toward the northeast The dissolved hydrocarbon -impacted plume beneath the site is depicted in cross-sectional view A A '(Figure 5) � 1 � ' 4.0. REMEDIATION IMPLICATION I Between February 2000 and present (approximately 4'/4,years), a soil vapor extraction (SVE) system E has been in selective operation at the site to remediate petroleum -impacted soil at depths between ' 10 feet and 20 feet bsg (vadose zone) From the, start-up date'to present, the average SVE influent _ total petroleum hydrocarbon (TPH-g) concentrations have decreased from approximately 490 µg/1 to non detect levels Therefore, SVE appears to have been moderately effective in removing petroleum mass in the vadose zone }� , I- At the same time, an In -situ -air spargmg system (IAS) has been in continuous operation at the site +" 4 to address petroleum -impacted groundwater at depths between 20 and 30 feet bsg, subsequently, identified as HU1 and HU2-First Flow Unit Analytical results of ground water samples from on-site f a monitoring wells suggest that the IAS system has been only marginally effective, at best, in reducing the overall mass of dissolved petroleum plume in HU1 and HU2 Advanced GeoEnvironmental, Inc I '