A simple two layer model for simulation of adsorbing and nonadsorbing solute transport through field soils

Abstract. While rapid movement of solutes through structured soils constitutes the risk of groundwater contamination, simulation of solute transport in field soils is challenging. A modification in an existing preferential flow model was tested using replicated Chloride and Lithium leachings carried out at constant flow rates through four soils differing in grades and type of structure. Flow rates generated by +10 mm, −10 mm, −40 mm, and −100 mm water heads at the surface of 35 cm diameter 50 cm height field columns. Three well-structured silty clay soils under ponding had concurrent breakthrough of Chloride and Lithium within a few cm of drainage, and a delayed and reduced peak concentration of Lithium with decrease in flow rate controlled by the negative heads. Massive sandy loam soil columns had delayed but uniform breakthrough of the solutes over the flow rates. Macropore flow in well-structured silty clay/clay loam soils reduced retardation, R (1.5 to 4.5) and effective porosity, θ_e (0.05 to 0.15), and increased macropore velocity, v_m (20 to 30 cm cm⁻¹ drainage) compared to the massive sandy soils. The existing simple preferential flow equation (single layer) fitted the data well only when macropore flow was dominant. The modified preferential flow equations (two layers) fitted equally well both for the adsorbing and nonadsorbing solutes. The later had high goodness of fit for a large number of solute breakthroughs, and gave almost identical retardation coefficient R as that calculated by two-domain CDE. With fewer parameters, the modified preferential flow equation after testing on some rigorous model selection criteria may provide a base for future modeling of chemical transport.