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November 12, 1996 Page 9 1 , itifnnsic bioremediation pararrieterswand found that-present subsurface conditions are _ �• conductive to natural biodegradation of residual petroleum hydrocarbons in round- water This evaluation is based on the measurement of groundwater for the presence of various electron acceptor compounds (dissolved,sulfate, dissolved ferrous iron, dissolved nitrate, and dissolved oxygen) which if absent, lint the degree of biodegradation DISCUSSION OF RESIDUAL PETROLEUM HYDROCARBONS Petroleum Hydrocarbons Above Background Concentrations Residual petroleum hydrocarbons remain in soil and groundwater above background concentrations as discussed in the previous sections Site remediation consisted of removing the product storage and'distnbution equipment, remedial excavation of areas of hydrocarbon-impacted soil, and the operation of an SVE system After nearly 2 years of operation, 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 w}uch 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 "background" 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 horizontal 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 perme- ability to air flow, it is well known that the pnmary variable controlling in-situ remediation is soil permeability For SVE, local variations 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 320 i 337MCLOSURE2