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fringe and in the vadose zone Additionally, oxygen will continue to be supplied to the subsurface environment When volatilization is no longer a significant contributor to mass reduction, operation will shift into a low flow configuration This will allow operation that is tailored toward biodegradation and maintain efficient resou,ce use According to the EPA, this technique is able to treat large volumes of soil effectively and with minimal disruption to property use Advantages of this alternative are that it degrades semi- and non-volatile organic compounds, and it degrades impact in place Also, in biovent mode potential air emissions are significantly reduced On the down side, the EPA recognizes that SVE/biovent ng targets only volatile and biodegradable constituents, and effectiveness is limited in heterogeneous soils The EPA estimates that on average, the lifespan of this tech- nique (reaching approximately 90 percent mass reduction) is between 1 and 4 years It has been PACIFIC's experience that, given a properly designed and operated system, the effectiveness of SVE/bioventing is exhausted within a 3 year period Conditions for terminating SVE are provided below An important aspect of implementing SVE as a remedial action is understanding when the application may be discontinued This should not be confused with terminating remedial action Operation and maintenance of the proposed WQLZ would continue until specific criteria are met General criteria for terminating remedial action (including closing the . proposed WQLZ) is outlined in association with site water quality protection standards and goals Following is a method by which use of SVE may be terminated As SVE proceeds, hydrocarbon concentrations in soul vapor decay At a constant extraction flow rate, concentration decay may be attributed to the decrease in hydrocarbon mixture volatility and mass, and the limitations imposed by mass transfer mechanisms Typically, concentration decay continues until an asymptote concentration is reached It is reasonable to presume the effectiveness of SVE implementation is exhausted when (1)the asymptote concentration is reached, (2)the asymptote concentration is relatively low (biodegradation is controlled by hydrocarbon concentrations and availability), and (3) it is shown that hydro- carbon impact areas were affected SVE influence can be readily documented by collecting subsurface pressure data and there are methods for identifying a zero trend slope (or asymp- tote) An asymptote concentration that does not significantly differ from initial concentrations is also easily identified PACIFIC proposes to terminate implementation of SVE when the effectiveness is exhausted according to the above criteria Showing that a true asymptote has been reached will be accomplished in two stages First, pulsed operation of SVE will be instituted to show that negligible hydrocarbon mass is avail- able, and that the hydrocarbon mixture left in place is relatively immobile Next, a statistical test will be completed to show the slope of the hydrocarbon concentration versus time curve is nearly zero 320133'IB/1918REV 23 August 31, 1995