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' 20 January 2005 AGE-NC Project No 97-0343 ' Page 5 of 21 ' eight-hour test OV readings ranged from 108 to 363 ppm Induced vacuum measured at the extraction well (VW-2) ranged from 54 to 72 Inches of water • As expected, the greatest vacuum was measured in the observation point nearest the ' extraction well(VW-1) The lowest vacuum was measured in MW-6,approximately 66 feet north of the extraction point The maximum vacuum measured at the observation points ' during the pilot test was plotted versus the distance from the extraction well The effective radius of influence was determined by drawing a best-fit line though these data points to correlate distance to vacuum data At a vacuum potential of 0 10 inches of water,the radius ' of influence is approximately 47 feet, at an induced vacuum potential of 10 inches of water, the radius of influence is approximately 30 feet, at 10 0 inches of water the radius of influence is approximately 13 feet Based on the United States Environmental Protection ' Agency(EPA)-prepared document,How to Evaluate Alternative Cleanup Technologies For UST sates the radius of influence is considered to be the distance from the extraction well at which a vacuum of at least 0 1 inches of water is observed Based upon an effective vacuum ' potential of 0 1 inches of water, the calculated effective radius of influence at the site will be approximately 47 feet 4 0. SOIL CHARACTERIZATION IThe soil profile beneath the site is composed dominantly of sands and silts from surface grade to approximately 96 feet bsg,and finer-grained silts and clays from 96 feet bsg to total depth of 134 feet bsg Figures 4 and 5 illustrate subsurface geologic profiles at the site Boring logs are included in Appendix A 41 HYDROGEOLOGICAL MODEL I Hydrogeologic or hydrostratigraphic units(HUs)are defined as a body of rock or sediment that has hydrogeologic characteristics that make it 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 Iflow of ground water (Maxey, 1964) A `flow unit' may be defined as a hydrogeologic unit and is characterized by significantly greater hydraulic conductivity values than overlying and underlying units, a flow unit is capable of serving as a preferred migration route, and could potentially allow migration of large contaminant masses over great distances I The model presented is interpretational and will be modified as more data is collected The four vertical zones identified are as follows (depths are approximate and can vary by several feet across the site) IAdvanced GeoEnvironmental,Inc I