Anion and cation leaching through large undisturbed soil cores under different flow regimes. 2*. Simulation results

In the first part of this study, the movement of anions and cations during sequential leaching of CaSO4 and KBr solutions under different flow regimes in 2 large undisturbed cores was simulated using a mobile-immobile convect ion-dispersion model (MIM). By using parameters from the literature or inde pendently measured, and with 75% of the adsorption/exchange sites allocated to the immobile water volume, the model was able to simulate the leaching of bromide and sulfate in both cores, even when flow rates differed by more than 2 orders of magnitude. The leaching of cations (Ca2+ and K+) in the s low flow core, where cation exchange occurred, was also simulated reasonabl y well. But the simulation of cation leaching in the fast flow core, where the surface-applied solutes appeared very rapidly in the effluent, was poor . Sensitivity analysis of the model output showed the most critical paramet ers were the mobile water volume theta(m), the fraction of the adsorption s ites associated with the mobile water f(s), and the mobile-immobile transfe r coefficient alpha. Caution is needed when choosing these parameters for m odelling purposes. The values used here gave good simulations of cation and anion leaching at flow rates of 3-20 mm/h, but were not satisfactory at ve ry high flow rates (350 mm/h).