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vicinity of the former USTs was estimated to be 26,000 pounds mass prior to SVE system operation, currently TPPH-g mass in the vicinity of the former USTs is estimated to be 4,000 pounds mass Based on these figures, a total reduction of TPPH-g mass of approxi- mately 85 percent was achieved during the time period between SVE system activation and the present Since SVE system operational data indicate that approximately 12,000 pounds of petroleum hydrocarbons were removed by the SVE system, the remaining 10,000 pounds were removed as a result of natural attenuation processes The migration potential has there- fore been reduced by the reduction of total mass beneath the site This conclusion recognizes the TPPH-g mass influence on migration transport mechanisms As stated earlier, mass removal of petroleum hydrocarbons by the SVE system had reached an asymptotic condition when it was deactivated in November 1992 It is recognized that while SVE was an effective remediation technology which contributed considerably to the remedial effort at the site, there were environmental and system design factors that contributed to the decline of system effectiveness prior to achieving acceptable soil concentrations These factors are presented below • Vapor flow to the SVE wells is affected by frictional resistance along the well screen, the difference between horizontal and vertical soil permeability to air flow, and general variations in soil permeability to air flow Screen intervals for each of the SVE wells extend from 14 to 44 feet bgs, but because of frictional resistance along the well screen, most of the vapor flow occurs over the upper portion of the well screen Additionally, the effects of frictional resistance are compounded due to the fact that horizon- tal permeability to air flow is typically an order of magnitude greater the vertical permeability The two factors mentioned would cause mass removal to be localized near the top of the screen interval which is at 14 feet bgs Data suggest the greatest petroleum hydrocarbon impact currently occurs at the 25- to 30-foot bgs interval With respect to general variations in soil permeability to air flow, it is well known that the primary variable controlling in-situ remediation is soil permeability For SVE, local varia- tions in permeability create preferred flow channels which restrict direct contact between soil vapor flow and hydrocarbon impact In the end, soil permeability will dictate the pace and completeness of active remediation • Lower weight molecular components of gasoline volatilize more readily than the higher molecular weight components As a result, the volatilization rate of TPPH-g decreases with time even though hydrocarbon-impacted soils remain within the radius of influence of the SVE wells • 3201337B/CAP—AMD DOC 7 November 30, 1995