Field-scale cadmium transport in a heterogeneous layered soil

Heavy metals such as Cd and Zn that have been deposited on soils in the surroundings of non-ferrous industries, are often observed in soil water and groundwater in concentrations above groundwater clean-up values. The present study addresses the field-scale migration of Cd in a heterogeneous 1 m deep layered sandy soil profile. Numerical calculations of field-scale Cd leaching are performed using probability density functions of chemical and physical transport variables in a stream-tube convective-dispersive model approach. The performance of the stochastic model is evaluated by comparing measured and predicted Cd profiles for two types of sorption isotherms. The first isotherm is linear, containing a field-based Cd soil-water distribution coefficient. The second is a nonlinear adsorption isotherm obtained from laboratory batch experiments, which is adapted to field conditions of pH, organic matter content and pore water composition. Numerical calculations made using the nonlinear isotherm better predicted the measured Cd depth profiles than did the linear isotherms. The stochastic simulations further showed that chemical heterogeneity (i.e., due to variations in sorption constant) largely affects the position and the shape of the Cd plume, resulting in an early Cd breakthrough at the bottom of the soil profile and a long-term leaching in concentrations above groundwater clean-up values due to tailing of the breakthrough curve. When calculations were made to investigate the effect of amendment of the topsoil layers, the results indicated that the average time to reach groundwater clean-up values is significantly reduced from approximately 230 to 80 yr.