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,A c S FL`' EM 1110-1-4001 L}v�";c 1 3 Jun 02 2s R 26 f 2401- 22a--. w 20 • w •• • 0 16 Model c�. • 14 R7 12()- 100- 0 2100 5 10 15 20 25 30 Elapsed Days of Extraction Figure 9-2 Measured and Modeled Extracted TCE Concentrations during SVE(Figure Provided Courtesy of Praxis Environmental Technologies,Inc Burlingame,CA) (a) The first exponential term is dominated by the timescale associated with the total volume of contaminated soil and the air extraction rate This is a convective process The second term is dominated by diffusion from the immobile zone into the mobile zone For the rebound test,when extraction is zero, the magnitude of r, becomes equivalent to that of rZ because all processes are then diffusion dominated Therefore,the rebound concentration yields an estimate for the diffusion rate Additional exponential terms can be added if other sources can be quantified(e g, off-gassing from contaminated groundwater or evaporation of a NAPL)or if low flow zones such as silts make significant contributions As SVE progresses at a site,the model coefficients can easily be revised The immobile fraction of the soil becomes the dominant parameter in reaching cleanup and the longer SVE operates the more accurate this parameter estimate becomes In the past,sites were often described solely by the convective exponential term and ignored a determination of the longer transient associated with the diffusive exponential term Neglecting the contribution of the immobile zone resulted in vastly underestimating the time to reach a specified cleanup goal and the total mass of contaminant at the site (b) Fitting the multi-region model to the extracted concentration will also allow mass estimates to be calculated The cumulative mass removed will be determined by integrating the extracted concentration represented by equation(7-1)over time from zero to the present time and multiplying by the extraction rate Mass Removed= f Q(t')Cmol„1e(f)dt' (9-2) �=o 9-12