Source Zone Delineation Demonstration Report - Triad Resource ...

vapor extraction does not significant reduce the predicted soil concentration, the concentration contoursare considerable perturbed (see Figure 5-46). Similar results are observed for the predicted concentrationof TCE in the soil gas, especially for depths below 30 feet bgs (see Figure 5-47). This perturbation in thenatural distribution of TCE in both the soil and the soil gas may have influenced the observed TCEconcentration profiles during the step tests conducted at OU 12 near the end of the demonstration,following several days of vapor extraction from the relatively small investigative area.A comparison between the predicted and the measured TCE concentration profiles in the vapor phase atextraction well U12-VP1 for source area 9 is shown in Figure 5-48. Source area 9 is locatedapproximately 8 feet northeast of U12-VP1, and represents a simulated TCE source 5 feet by 5 feet inareal extent and 5 to 10 feet thick. An initial source concentration ranging from 5,000 to 40,000 mg/kgprior to a 15-year or 30-year diffusion period and from 200 to 10,00 mg/kg without diffusion wasinvestigated. Without a pre-extraction diffusion period, the predicted concentration profiles show a sharpincrease in TCE concentration with increased time. This sharp increase in TCE concentration also wasobserved in the measured concentrations during the SVE demonstration at OU 12. Assuming a 5-footthick source zone located at 25 to 30 feet bgs, the predicted TCE concentration profile in the vapor atU12-VP1 becomes independent of soil concentration above 1,000 mg/kg and never rises aboveapproximately 450 ppmv in the vapor phase during the simulated test period. In addition, theconcentration profiles appear relatively linear without the characteristic exponential decline observed inthe measured concentration profiles during the blower shutdown periods. The TCE concentration profilespredicted for the 15-year and 30-year diffusive period scenario do not match the actual profiles obtainedduring the SVE demonstration and appear to resemble a localized, diminishing source or a diffusiondominatedflow regime. Assuming a 10-foot thick source zone containing 400 mg/kg and no preextractiondiffusion, the predicted TCE vapor concentrations at U12-VP1 are in very good agreementwith the measured values; however, during periods of blower shutdown, the predicted concentrationprofiles become relatively flat and do not exhibit the characteristic exponential decline. As extractionoperations are continued following the blower shutdown period, the predicted vapor-phase concentrationssharply decline, apparently from streamtube dilution. This effect was not observed during actual vaporextraction operations at U12-VP1.Although the exponential decline in the concentration of TCE in the vapor phase during blower shutdownperiods was not predicted for a 10-foot thick source containing 400 mg/kg TCE in source area 9, thepredicted concentrations were in good agreement with the measured values for U12-VP1. Thus, acomparison between the predicted concentrations of TCE in the vapor phase with the actual measuredvalues was conducted for all seven extraction wells for this source zone configuration and is shown inFigure 5-49. Although the predicted vapor-phase concentrations and the general concentration profilesare in relatively good agreement for U12-VP1, U12-VP2, U12-VP4, U12-VP5, and U12-VP7, there aresignificant differences between the predicted and measured profiles for U12-VP3 and U12-VP6. Forinstance, the measured concentration of TCE in the vapor phase at U12-VP3 increased from a few partsper-millionby volume to approximately 65 ppmv with increased vapor extraction rates before sharplydecreasing during blower shutdown. However, the concentration of TCE in the vapor phase is predictedto be near zero with no observed increase or decrease for all simulated rates of vapor extraction.March 2003 5-76 OU 12 Demonstration ReportFinal