Nonequilibrium cadmium leaching in layered sandy soils

Heavy metals such as Cd and Zn are relatively mobile contaminants in acid sandy soils and may be transported from the soil surface to shallow groundwater in a few decades. To investigate the governing transport processes of Cd in heterogeneous soils, we conducted Cd leaching experiments in two 1-m-long × 0.8-m-diameter columns. One column contained an undisturbed dry sandy soil (i.e., Spodosol) developed under oxidizing conditions above a relatively deep groundwater table, whereas the second column was filled with a wet sandy soil that developed under periodically reducing conditions. Both soils were contaminated with heavy metals such as Cd, Zn, and Pb. The dry Spodosol was conceptualized as a two-layered medium consisting of a heterogeneous humic topsoil and a homogeneous subsoil (i.e., C horizon); the wet Spodosol was described in terms of a single layered heterogeneous soil. Physical and chemical heterogeneity caused primarily by the irregular presence of soil organic matter in the profile was accounted for using a deterministic chemical nonequilibrium model dividing the soil into two sorption domains, one at which sorption was instantaneous (i.e., equilibrium sorption sites) and another at which sorption was limited by reaction kinetics (i.e., nonequilibrium sorption sites). Cadmium transport in the top horizons of the dry Spodosol was heterogeneous, as shown by the small fraction of equilibrium sorption sites f_c = 0.097, whereas Cd behaved closer to equilibrium in the subsoil, i.e., f_c = 0.53. The observed Cd transport behavior is consistent with previous results from a nonreactive tracer experiment in the same soil. In particular, the distinct humus B horizon (i.e., Spodic horizon) of the dry Spodosol redistributed solutes from different flow regions of the topsoil through enhanced lateral solute mixing. Below the Spodic horizon, solutes were further homogeneously transported to the deeper soil layers. In the wet Spodosol, transport was assumed to be heterogeneous over the entire length of the soil profile. Cadmium leaching in the wet Spodosol was described adequately with a deterministic nonequilibrium model for a single layered porous medium, having only 18% equilibrium sites. The reaction rate coefficient depended on the flow rate, indicating that diffusion rather than sorption/ desorption kinetics is limiting the Cd transfer from the soil solid phase to soil solution.