Influence of Scale on Chemical Dispersivity in Geomedia

Chemical dispersivity is a common parameter needed for pollutant fate and transport models in geomedia environments. This study estimated the effect of distance on dispersivity measured in columns using X-ray computed tomography. Columns containing natural and homogeneous media were evaluated. The convection- dispersion equation appeared to be appropriate for the column distances used in this study. Dispersivity was found to be a function of distance from the upstream end in the column studies for the natural porous media but not the homogeneous porous media. Dispersivities were estimated using the average breakthrough slope for selected scan planes (mean slope method), using the solution of the convection-dispersion equation fit to the average breakthrough for selected scan planes (curve fit method), and using groups of pixels in the selected scan planes (group pixel method). Values estimated using the mean slope method or the curve fit method were found to be scale dependent; however values with the group pixel method were not. Fractal dimension values appeared within appropriate ranges when estimated for the core samples using the mass versus radius relationship. Results suggest dispersivity may be dependent upon the sampling volume rather than the straight-line length solute travels through media.