Effects of Different Rock Fragment Contents and Sizes on Solute Transport in Soil Columns

The presence of rock fragments in soil can have a great impact on solute transport processes, as well as on soil bulk density, soil structure, and water storage properties. In the laboratory, we studied the effects of different gravimetric rock fragment contents (R-C) and different rock fragment sizes (R-S) on the solute transport processes in soil columns. Using CaCl2 as a tracer, the solute transport processes were accurately described by both the convection-dispersion equation (CDE) and a two-region model, although the latter model fitted the experimental data slightly better than the CDE. Smooth breakthrough curves were obtained, even for a large R-S or R-C, although the presence of rock fragments impeded solute transport to different degrees depending on the content and size of fragments in the columns. The two models produced the same trends in dispersivity, but the values determined by the two-region model were smaller than those from the CDE. The R-C and R-S values had a significant effect on the mobile-immobile partition coefficient and the mass transfer coefficient.