ARCHIVED REPORTS_XR0002669 - Laserfiche WebLink

Home Browse Search

T As stated earlier, mass removal of petroleum hydrocarbons by the SVE system had reached an T 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 factional 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 permeability to air flow, it is well known that the primary 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 • Lower weight molecular components of gasoline volatilize more readily than the higher molecular weight components As a result, the volatilization rate of TPH-g decreases with tune even though hydrocarbon-impacted soils remain within the radius of influence of the SVE wells • Clays, silts, and organic carbon, components of soils found beneath the site, tend to hold hydrocarbon impact in place This phenomenon limits the fraction of soil bound hydrocarbons that may be removed by physical means 2.5 Exposure Pathway Since gasoline constituents have been detected in groundwater beneath the site, it is reason- able to expect groundwater flow to be the primary exposure pathway The most likely expo- sure point would be a downgradient water-supply well located near the site In 1989, EA 3201337B/1918REV 7 August 31, 1995