Modelling of colloid leaching from unsaturated, aggregated soil

The migration of colloids in soils can enhance the leaching of strongly sorbing contaminants. We present a model for the simulation of colloid leaching from unsaturated, aggregated soil media under stationary flow. Transport in the intra-aggregate pores is simulated by convection–dispersion, and transport in the interaggregate pores, and a stagnant layer of water surrounding the aggregates, is simulated by diffusion. The model describes the release of colloids from soil aggregates, sorption and desorption processes at the air–water interfaces, and flocculation and subsequent straining from the flowing water. All three processes were simulated as functions of ionic strength. Transport of ions in intra-aggregate pores was simulated by Fickian diffusion. The model was calibrated against experimental results of colloid leaching from columns packed with natural soil aggregates. The aggregates were of two soils differing in organic matter content. On each soil a single calibrated parameter set could describe the experiments with the three ionic strengths. The parameters for release of colloids from the aggregate surface and the sorption properties of the air–water interface were different for the two soils. The key parameters for leaching were the thickness of the stagnant layer of water surrounding the aggregates, the mechanical dispersion, the maximum concentration of colloids at the surface of the aggregates, the sorption capacity and rate coefficient of the colloids at the air–water interface, and the colloid diffusion coefficient. Simulations were also done with two additional irrigation intensities at one ionic strength. Simulated leaching was greater than measured leaching at both irrigation intensities, but the diffusion-controlled release of colloids from the aggregates was simulated correctly.