Lateral solute mixing processes - A key for understanding field-scale transport of water and solutes

Lateral mass exchange mechanisms affect the spreading of solutes in the main direction of flow. Modeling vertical solute spreading requires therefore an understanding of the lateral transport across regions of varying velocities. Experimental observations show that the variable extent and rates of lateral mixing cause dramatically different transport regimes, which can neither be predicted nor explained mechanistically in terms of known state variables. In this paper, we show that the various solute flow regimes are sensitive to the relative magnitudes of the vertical and lateral solute particle velocities. Different concepts of lateral mass exchange are discussed, and we postulate that physically meaningful parameters describing lateral mixing would be a clue for deciding a priori which solute transport regime is appropriate for a particular soil or soil horizon.