Chemical Dispersivity Affected by Homogeneous and Fractal Porous Media

In transport relationships for porous media, chemical dispersivity is an important parameter for estimation of pollutant migration. This study was conducted to evaluate the effects of scale on dispersivity for homogeneous media and fractal media. The approach used to estimate scale was transport distance along the column downstream from the applied chemical. Columns containing homogeneous media and fractal media were evaluated for chemical breakthrough using computed tomography (CT) imaging. The advection-dispersion transport equation appeared to be appropriate for estimating the parameters. Some columns showed that dispersivity was potentially a function of distance from the upstream end of the column. These tests indicated that dispersivity through the fractal media was not consistently dependent upon distance. Estimates were obtained for individual CT image scan pixels or selected groups of pixels. Some values determined from a curve fit method or a mean breakthrough slope method were found to be scale dependent. Results from this experiment suggest dispersivity may be dependent upon the sampling fraction rather than the straight-line length solute travels through media.