Source Zone Delineation Demonstration Report - Triad Resource ...

The final TCE soil contamination model, shown in Figure 5-9 (100 g/kg iso-surface) and Figure 5-10(1,000 g/kg iso-surface), is based on the analytical results of 599 soil samples taken from the 16 wirelinesampler borings advanced during this investigation. The improved characterization of the 100 g/kg isosurfaceto the west and southwest is clearly evident. To the south, however, more data are needed torefine the delineation of the 100 g/kg iso-surface, which apparently extends beyond the southernboundary of the model. Two additional borings to the south of U12-1808 would have established thesouthern extent of the 100 g/kg iso-surface, but time constraints unfortunately precluded the wirelinesampler contractor from acquiring additional data. The two additional borings would also haveaugmented the characterization of the isolated 1,000 g/kg iso-surface on the southern model boundary.Although data from U12-1817 and additional control to the southwest have eliminated the southwestwardtrend from the 1,000 g/kg iso-surface enclosing (in plan view) U12-1804, U12-1807, and U12-1817, theportion of the iso-surface on the southern boundary is unsupported by data. It is likely an extrapolationartifact, and thus should be discounted as a real indicator of soil contamination.The inferred 10,000 g/kg iso-surface displayed in Figure 5-11 further illustrates the extent of TCEcontamination in soil as delineated by the final model. This iso-surface encloses the soil samples with thehighest measured concentration of TCE. These samples are: from U2-1804, 154,000 g/kg at a depth of27 feet below ground surface and 73,000 g/kg at 29 feet; from U2-1807, 144,000 g/kg at 33 feet; andfrom U2-1817, 131,900 g/kg at 29 feet. In contrast, surrounding borings yielded no soil samples with ameasured TCE concentration greater than 10,000 g/kg.Figure 5-12 is an isometric view of the 100 g/kg iso-concentration surface from the final threedimensionalmodel of TCE contamination in soil. The view is from the southwest corner of the modeldomain, looking to the northeast. The spheres on each of the borings represent soil sample locations,color-coded according to the measured concentration of TCE. The most striking aspect of thisrepresentation of the soil contamination is its layered character of the soil contamination. Broad, layeredareas of soil contamination, connected vertically by narrow throat-like regions, occur at variouselevations.This attribute of the soil contamination is described in more detail in the next section of this report, wherethe co-development of models of the extent of TCE contamination and vadose zone soil architecture isdiscussed.5.1.2 Soil Architecture of the Vadose ZoneModeling of the distribution of soil types in the vadose at OU 12 was performed at two scales (see Figure5-1). The first and larger model was made to establish the architecture of soil types (soil lithology) in thevadose zone over a large portion of the suspected OU 12 source area, and is denoted the site-wide modelin subsequent discussions. The smaller model is spatially coincident with the previously described modelof TCE concentration in soil, and was built for the purpose of understanding the relationship betweenMarch 2003 5-13 OU 12 Demonstration ReportFinal