Effectiveness of equilibrium and physical non-equilibrium approaches for interpreting solute transport through undisturbed soil columns

This study observes the dispersion process of chloride through three undisturbed soil columns (sabA, sabB and arg) of different texture under both saturated and unsaturated conditions. Overall, 17 leaching experiments were conducted by means of an equipment designed and built in Portici together with the DITEC of the University of Basilicata. During the experiments, both symmetric and positive asymmetric breakthrough curves of the chloride anion were measured. As expected, manifest asymmetry was obtained especially for soil columns labeled as sabA and arg. In such columns, it was also noted that the degree of asymmetry increased as the flow density increased at the sample surface. The experiments conducted on soil column labeled as sabB resulted in more symmetric BTCs. The experimental data were firstly analysed using the classical two-parameter CD model. The variation of the dispersion coefficient, D, with pore water velocity, v0, and porous medium characteristics was explored in detail. The application to all experimental data of the four-parameter MIM model, which compartmentalises the pore water in the mobile-stagnant domains provided an insignificant different description of BTCs, even of the asymmetric ones. Furthermore, it resulted in ill-defined parameters, especially those related to the partitioning in the mobile-immobile phases. However, for all the three soils examined the two parameter CD model proved to be very powerful even in interpreting asymmetric BTCs. In any case, it should be emphasised that the agreement of outflow experimental data with a mechanistic model does not unequivocally identify the mechanism of solute transport in the soil. In fact, when we applied the CLT model, which is based on a stochastic approach, the BTCs were comparably well predicted.