Mass Transfer Processes in Soils Containing Preferential Flow Networks : Experimental and Modeling Results

Concurrent mass transfer processes through preferential flow pathways and other inter-aggregate voids contribute to non-ideal transport and complicate the prediction of water and solute fluxes in agricultural landscapes of the United States Midwest. To better understand these nonequilibrium transport processes in these soils, solute transport studies were conducted at the batch and column scales. In the batch study, bromide diffusion from several soil aggregates was monitored. Large diffusion rates measured in the batch study suggest that the inter-aggregate pores enhance solute diffusion to the interior soil matrix. This supposition was further supported by an estimated diffusion path length that closely matched the half-spacing of the soil peds. In the column-scale study, several miscible displacement experiments were performed at various levels of water saturation and input water flux using undisturbed soil cores from a corn/soybean field. Results show that the diffusive mass transfer at the column-scale is a non-hysteretic process. Estimated mass transfer rates, though still rapid, were slower than those estimated in batch diffusion experiments, most likely a result of more tortuous inter-aggregate pores and less continuity in the pore network. Based on numerical simulation of these experiments, it is concluded that these soils are dominantly comprised of two flow networks. The primary network of inter-aggregate pores and cracks enhance both diffusion and bypass flow at even modest levels of water unsaturation. When the soil approaches saturation, a secondary network of macropores formed from roots channels and earthworm holes overwhelms the preferential transport in the primary network to dominate the transport manifestation of water and solutes.