Mobilization of Cd upon acidification of agricultural soils: column study and field modelling

The potential effect of acidification of contaminated sandy soils on Cd transport in the unsaturated zone was assessed. Forty-eight soil profiles were sampled at five depths in a polluted field that was set aside in 1992. The Cd concentration in the top 30 cm of this field was, on average, 10 mg kg(-1). A column experiment was carried out with one of the topsoil samples. Homogeneously packed columns were leached with 0.001 M CaCl2, adjusted to pH 3 or pH 5.7, at a pore water velocity of 6 cm day(-1). The Cd and proton transport was predicted with coupled transport equations. The Cd transport was modelled by assuming local equilibrium and by using sorption parameters derived from batch experiments, while acidification was modelled with a kinetic approach, on the assumption that proton buffering was due to cation exchange and mineral weathering. Organic matter was the main contributor to the cation exchange capacity of these soils. Observed and predicted pH and Cd profiles in the columns agreed well. With the same model, the proton and Cd transport at field scale was calculated for each of the 48 profiles sampled ('grid model'). It was predicted that the field-averaged Cd concentration in the seepage water will increase from 6 mu g litre(-1) at present to 200 mu g litre(-1) over 260 years, which greatly exceeds the maximum permissible concentration (MPC) in groundwater of 5 mu g litre(-1). Predictions of Cd transport using field-averaged soil properties yielded a later breakthrough time and a larger peak Cd concentration than predicted with the grid model, which illustrates the impact of spatial variability on solute transport. Continuation of liming practices is a possible solution to prevent breakthrough of Cd at concentrations far in excess of the MPC.