Aging effects on cadmium transport in undisturbed contaminated sandy soil columns

Limited information is available on the effects of contaminant aging (i.e., the contact time of Cd with the soil) on Cd transport in soils. We conduct ed displacement experiments in which indigenous Cd and freshly applied Cd w ere leached simultaneously from undisturbed samples of three Spodosol horiz ons. Sorption of Cd was described using Freundlich isotherms, whereas trans port was described as a convection-dispersion process. Parameter optimizati on analysis using a mobile-immobile transport model applied to nonsorbing t racer displacement data showed that 16 to 22% of the water in the columns w as immobile. The low dimensionless mass transfer coefficients in the mobile -immobile model were indicative of diffusion-limited transfer between mobil e and immobile water, and hence physical nonequilibrium. A two-site kinetic sorption model could be fitted closely to breakthrough curves of the non-a ged Cd for three soil horizons. No conclusive evidence was found that conta minant aging in soil affects cadmium transport. On the one hand, prediction s of aged Cd leaching, using parameters estimated from displacement experim ents with non-aged Cd, differed from those for the aged Cd in the E horizon . On the other hand, no meaningful differences in transport behavior be twe en aged and non-aged Cd mere found for the humus Bh and Bh/C horizons. The two site kinetic rate coefficient alpha (c) was found to depend on water fl ux, further indicating that mass transfer between sorption sites and the li quid is limited by diffusion rather than by kinetic sorption.