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M gig" M 02 January 2007 AGE-NC Project No. 95-0121 Page 4 of 10 3,306 hours. Extraction flow rates ranged from approximately38 cubic feetperminute (cfin) to73 cfin. Soil vapor extraction data is presented in Table A2. • Between 08 January and 26 January 2004, approximately 0.08 feet of free-phase floating petroleum product was encountered at well VW-1 on three separate occasions; VW- I was hand bailed on each occasion until petroleum product was not observed. Thereafter, free- phase petroleum product was not measured or observed at well VW- I on 07 and 27 April 2004. Hand bailing activities appears to have been effective in removing free-phase floating petroleum product at well VW- I . • The SVE unit operated for approximately 4,392 hours between 22 June and 22 December 2004, recovering a total of 382.6 lbs of TPH-g. Between 22 December 2004 and 04 May 2005, the SVE unit operated for approximately 3, 192 hours. Extraction flow rates ranged from 36 cubic feet per minute (cfu) to 52 cfin. The concentrations of petroleum hydrocarbons detected in soil vapor samples collected at the SVE unit were considered to be acceptable for successful on-going SVE remediation. • 18 January, 16 February, 16 March, 13 April, and 04 May 2005 : TPH-g, BTEX and MTBE were not detected in the influent vapor sample. • Due to consistent non-detections of hydrocarbon constituents in influent air samples taken during the first and second quarters 2005, and as directed in an EHD letter dated 25 April 2005, AGE terminated operation of the SVE system on-site on 11 May 2005 . It was determined that the system had performed to maximum potential remediation; hydrocarbon concentrations had decreased beyond the level at which soil vapor extraction was effective. • Between February 1991 and December 2006, forty-one ground water monitoring/sampling events were performed at the site. During this time, a trend in hydrocarbon concentrations suggested increasing concentrations from the "core" area wells, and decreasing concentrations from the perimeter wells. HYDROGEOLOGIC UNITS Hydrogeologic or hydrostratigraphic units (HUs) are defined as a body of rock or sediment that has hydrogeologic characteristics (i.e. hydraulic conductivity, pore pressure, etc.) 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 flow of ground water (Maxey, 1964). A `flow unit' can 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 in a dissolved phase.