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------------- <br /> VATC M <br /> - A S S O C I A T E S I N C <br /> -- F <br /> r . 2. Groundwater in the uppermost water bearing zone appears to be confined to semi- �= <br /> confined. Groundwater flow beneath the site is generally northwest with a magnitude of <br /> 0.003 ft/ft, or approximately 16 ft/mile. j <br /> 3. Petroleum hydrocarbon-affected soils are present primarily in the vicinity of the capillary <br /> fringe at approximately 8 to 12 feet bgs. The highest concentrations of TPHg and benzene f <br /> are present in soil northwest of the former location of the gasoline USTs. The horizontal <br /> >; extent of TPHg and benzene, and the presence of these contaminants in the capillary fringe <br /> are indicative of groundwater impact and advection. <br /> 4. Petroleum hydrocarbons in groundwater were detected in samples collected from borings <br /> F <br /> SB 1, SB2, SB3, SB4A, and SB6 and in wells MW-2 and MW-3. The plume of TPHg and <br /> �sl benzene is north and northwest of the former location of the gasoline USTs. TPHg and <br /> =={ benzene were not detected in the groundwater from boring SB7, which was located <br /> immediately northwest of the former USTs. In addition, floating product was detected in <br /> jborings SB2 and FPB-2. The extent of contamination to the south and north is defined. <br /> 1 The extent of contamination to the west and the east are inferred. MTBE was detected in ff <br /> the groundwater from boring SB3 and in monitoring wells MW2 and MW3. The presence I <br /> > of MTBE was confirmed by EPA method 8260. <br /> 5. Based upon geotechnical and geochemical data collected from groundwater and/or soil <br /> �== under/ in the project site intrinsic bioremediation may be occurring in the contaminant <br /> underlymg p J Y g <br /> plume beneath the site. Biodegradation appears to be continuing in a primarily anaerobic <br /> r a environment with aerobic conditions in the vicinity of wells MW2 and MW3. <br /> 6. No potential pathways were identified for petroleum hydrocarbons. The only potential <br /> receptor of the petroleum hydrocarbon plume is the on-site water supply well. No <br /> 1 information regarding the well depth, screen interval or the depth of seal is available. <br /> 7.0 REMEDIAL ALTERNATIVES <br /> Based on the results of previous investigations, ATC has considered three remedial alternatives <br /> for soil and groundwater. Soil and groundwater remedial alternatives include natural attenuation <br /> = with groundwater monitoring, excavation of remaining petroleum impacted soil, and in-situ vapor <br /> extraction with air sparging. A summary of each alternative is presented below: <br /> -' 7.1 Natural Attenuation with Groundwater Monitoring <br /> Natural attenuation, also known as passive bioremediation or intrinsic bioremediation, is a passive <br /> �._ <br /> remedial approach that depends upon natural processes to degrade and dissipate petroleum <br /> constituents in soil and in groundwater. Through biodegradation the actual reduction of <br /> petroleum constituent mass is realized. <br /> f[f4 w:1951221reportslparcap.doc 7 <br /> lJ <br />