Source Zone Delineation Demonstration Report - Triad Resource ...

The initial TCE contamination model, developed using the concentration of TCE measured in the 294 soilsamples from the first seven CPT wireline sampler borings, is represented in plan-view in Figure 5-5 bythe 100 g/kg iso-concentration surface, and in Figure 5-6 by the 1,000 g/kg iso-concentration surface.In this view the three-dimensional surfaces are projected onto a horizontal plane, so their threedimensionalcharacter is lost except for the shadow effect in the rendering. Each wireline sampler boringis posted with a colored circle. The color of a particular circle will, in general, differ from the othersbecause the circle represents the projection onto the horizontal plane of a sphere, color-coded bymeasured TCE concentration and posted at the upper-most soil sample extracted from the boring. Thesetwo iso-surfaces illustrate the size and shape of the inferred region of soil contamination based on the datafrom the first seven boring locations, and provide a basis for comparison with subsequent models. Itshould be noted that the extent of the modeling domain shown in these figures, which is determined bythe spatial configuration of the boring locations, is smaller than the extent of the final modeling domain.The truncation of the 100 g/kg iso-concentration surface along the western, southern, and easternboundaries of the initial model indicates that insufficient data are available in these regions to characterizethe spatial configuration of the iso-surface. For example, on the eastern boundary of the initial model thekriging algorithm extrapolated TCE concentrations greater than 100 g/kg to the east of borings U12-1804 and U12-1807, because soil samples from these borings did not provide the data needed to constraininterpolation of the 100 g/kg iso-surface. The extrapolation would have continued eastward had themodeling domain been larger to the east. Analogous truncations are evident along the western andsouthern boundaries of the model; in each case, there are insufficient data to achieve closure of the 100g/kg iso-surface within the limits of the modeling domain. Although the 1,000 g/kg iso-surface(Figure 5-6) is fully contained within the domain, it is constrained to the northwest only by U12-1803 andU12-1806, and to the southeast by U12-1805 and U12-1808. The bulge to the east and the elongation tothe southwest are the result of extrapolation. Partly on the basis of Figures 5-5 and 5-6, the next threewireline sampler borings (U12-1809, U12-1810, and U12-1811) were located to the east of the first sevenborings. The decision to step out to the east also was influenced by the location of the drums unearthedduring trenching activities shortly before the field investigation began.A second model of TCE contamination of soil was constructed after receiving the analytical results for thethree new borings. The revised model, shown in Figure 5-7 (100 g/kg iso-surface) and Figure 5-8(1,000 g/kg iso-surface), is based on 10 borings and the analytical results from 419 soil samples. Theadditional borings provide sufficient spatial control to more fully characterize the eastern extent of 100g/kg iso-surface: it no longer extends to (and, by implication, beyond) the model boundary, but is closedto the east. Along the western and southern boundaries of the model, however, the 100 g/kg iso-surfaceremains poorly characterized. The effect of the additional borings on the 1,000 g/kg iso-surface is lessdramatic, but still significant. The extent of the iso-surface to the north and east has been reduced,whereas to the southwest the extent (by extrapolation) of the iso-surface has actually increased. Fiveadditional wireline sampler borings were placed to the west and southwest to address the clear lack ofspatial control. In addition, a sixth boring was placed within the 1,000 g/kg iso-surface to provide morecomplete characterization of the region of highest TCE contamination.March 2003 5-8 OU 12 Demonstration ReportFinal