A model for sorption, flux and plant uptake of cadmium in a soil profile : model structure and sensitivity analysis

Fluxes of cadmium in a soil profile are simulated by coupling a model for linear and nonlinear equilibrium sorption to an existing hydrological model. The aim is to develop a model for Cd transport in soil systems. A separate flow model is used to calculate the water fluxes, which are then used in an equilibrium sorption model, allowing different Freundlich isotherms to be chosen. The model is tested and a sensitivity analysis is made. The variation of soil compartment sizes gave small changes in the results, which is interpreted as a measure of the solution stability. The factor influencing Cd transport most, according to these simulations, is the sorption isotherm. The degree to which Cd sorbs to soil decides how much will be available in the soil water for plant uptake or transport through the soil to the ground water. Other studied factors such as root distribution and hydrological properties influence the result only to a limited degree. With an application of 10 mg Cd/m2 in the given range of Freundlich isotherms, the simulations gave a plant uptake of between 0 and 30% of the applied Cd in two years. At the concentration mainly used in this study (with 10 mg Cd/m2 applied), the nonlinear isotherms found in the literature gave Cd lower mobility than the linear isotherms used for comparison. For high Cd concentrations the situation would be the reverse.