Source Zone Delineation Demonstration Report - Triad Resource ...

the screened interval in U12-VP1 and U12-VP2 is located in a sandy-silt zone, which likely has a lowerpermeability than the overlying silty-sand region. These observations are manifest in the vapor-phaseTCE concentrations observed during vapor extraction operations from U12-VP1 and U12-VP2 (seeFigures 5-30 and 5-31). The highest levels of TCE concentration in the extracted vapor were detected inU12-VP1 (approximately 1,000 ppmv) and U12-VP2 (approximately 400 ppmv). Because the measuredconcentration of TCE in the extracted soil vapor corresponds to an average vapor concentration for theentire length of the screened interval, it is impossible to delineate the relative contribution to the totalmeasured contamination supplied by the various soil intervals spanned by the screen. However, due tothe close proximity of U12-VP1 and U12-VP2 to high soil contamination and the local soil stratigraphyof their screened intervals, it is surmised that the upper few feet of the intervals for these wells yielded thebulk of the contaminated vapor.In contrast, the screened interval of U12-VP5 is almost entirely within a lens of interbedded sandy-siltand silt, and the top of the screened interval is approximately ten feet below the region of highest soilcontamination. In addition, the horizontal distance is relatively large between U12-VP5 and the region ofhighest soil contamination. These observations may explain the low concentration of TCE(approximately 16 ppmv maximum) measured in the extracted vapor from this well. However, themaximum measured concentration of TCE in vapor extracted from U12-VP3 was approximately 70ppmv, nearly five times higher than in U12-VP5. Higher TCE concentrations were observed at U12-VP3although its screened interval has characteristics much like the screened interval of U12-VP5, and U12-VP3 is more distant from higher soil contamination. This may reflect interbedding of thin, higherpermeability soils above and near the top of the screened interval. As previously discussed in Section5.1.2, such thin layers are beyond resolution of the soil architecture model.The upper four feet of the screened interval of U12-VP6 are located in silty-sand, and the top of theinterval is within elevations of the highest soil contamination. There is evidently a good pneumaticconnection between U12-VP6 and the soil contamination as observed during vapor extraction operations.The highest TCE vapor concentrations at U12-VP6 generally corresponded with increased extractionrates. The top of the screened interval of U12-VP7 is approximately 10 feet below the highest soilcontamination. Although the upper four feet of the screened interval are in sand, the sand isdiscontinuous, and the screened interval is isolated from the high soil contamination by an overlying thinlayer of sandy-silt that appears to be spatially continuous (see Figures 5-37 and 5-38). These factors mayexplain the poor response observed from U12-VP7 during vapor extraction operations.The generally poor pneumatic response of U12-VP4 is difficult to explain. The upper two feet of thescreened interval are in a silty-sand that is apparently areally continuous, and there are no silty layersevident within the screened interval. The poor performance of this vapor extraction well may be due tosoil characteristics beyond the resolution of the model, or possibly to construction problems.March 2003 5-55 OU 12 Demonstration ReportFinal