Effects of rock fragments on water movement and solute transport in a Loess Plateau soil

Calcium carbonate concretions are present in the soil of the Loess Plateau as a result of pedogenesis. The presence of these small rock fragments can have a great impact on soil bulk density, structure and water storage properties, as well as on soil water movement and solute transport processes. We studied the effects of different gravimetric rock fragment contents in a soil (Rw) (0, 10, 20, 30, 40, 50, and 60%) on infiltration, saturated hydraulic conductivity (Ks) and solute transport. Both infiltration rates and the saturated hydraulic conductivity initially decreased with increasing rock fragment content to minimum values for Rw = 40%, and then increased. The Peck–Watson and Bouwer–Rice equations predicted Ks for low rock fragment contents but failed to forecast the observed trends. Cumulative infiltration over time was described well by a power function. Solute transport processes, determined using CaCl2 as a tracer, were accurately described by both the convection–dispersion equation (CDE) and the two-region model (T-R) although the T-R model fitted the experimental data a little better than the CDE, which is possibly more convenient to use. When Rw was about 40% solute transport parameters indicated that relatively more advection occurred in this mixture where immobile regions occupied the greatest proportion of the columns.