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EM 1110-1-4001 3 Jun 02 • c Estimating the Impact ofD ffusion Limitations on Remediation Timeframes (1) By measuring rebound rates and using a simple analytical model (Brusseau 1996)some practitioners are able to estimate the impact of rate-limited mass transport on remediation timeframes G� t Praxis Environmental Technologies,Inc developer of the PneulogTM tool,reports that mass transfer �t"� �` f constraints,order-of-magnitude total mass,and time to cleanup can be estimated from historical concentration data and rebound test data Figure 9-2 shows data collected using a PneulogTM device to vertically profile extracted concentrations and flow rates in a single SVE well during a rebound test SVE �3. was performed at this site for 12 days,then paused for four days,and then reinitiated at the same flow rates lr� as before The data show the characteristic rebound in extracted vapor concentrations that is seen in most SVE systems At this site,the rebound in TCE concentration is presumably due to diffusion from a less wpermeable zone into the pathways that transmit most of the air to the extraction well After the pause,a rebound in the extracted vapor concentration of almost 10 percent was observed (2) Figure 9-2 also includes a plot of the results of Brusseau's model,with best-fit parameters, and displays a close match with the measured concentration decay and rebound The model has been developed in the academic literature(Brusseau 1996)but is used infrequently in field practice because the proper data are not collected or practitioners are not famihar with the model The bask simplifying assumptions in the model are • soils are categorized as two-domain,i e,permeable to air flow(mobile)or not(immobile), and • the early flushing rate of the mobile zone by clean air is rapid enough to justify averaging the contaminant concentration in the mobile zone (3) Typical SVE flow rates flush the mobile zone with clean air every few days justifying the averaging assumption Most sites readily meet the conceptual model of mobile and immobile zones for air movement during SVE A third category of low flow zones(e g, silts)can be added if sufficient site data exist Air moves through the mobile soils and contaminants must diffuse out of immobile zones before being extracted The form of the equation describing the vapor concentration in the mobile zone(assumed equal to the extracted concentration)is roughly Cmobile is a function of [Contribution of Mobile Zone]+ [Contribution of Immobile Zone] or {'r rlt r_r C'n"b e = 10 8 +Oze ) (9-1) where 01 =an advective decay constant Oz =a diffusive decay constant ri =a function of air residence time within the soil pores r2 =a function of the compound's diffusivity in the rate-limiting soil type t= elapsed soil venting time 9-11