Transport of reactive contaminants in heterogeneous soil systems

Abstract

Transport of reactive contaminants was studied in soil systems that exhibit pronounced variability with respect to the flow and sorption parameters and the solute feed function at the inlet boundary. Emphasis was given to the sorption and transport of orthophosphate (P) in soil. An approximate P sorption kinetics model was derived, that is based on a mechanistic description of reaction processes at the microscopic scale. The approximate model, that involves a reversible adsorption according to Langmuir kinetics, and an irreversible diffusionprecipitation reaction, as a function of a concentration scaled time variable, described experimental sorption and desorption data well. Validation by predicting P- transport using independently assessed sorption parameter values, showed a reasonable agreement between experimental and numerical results. With the distributions of sorption model parameters and soil variables found for a field and a watershed, transport was simulated for homogeneous and heterogeneous soil systems at conditions resembling those of the field. It appeared that only a smaller part of sorbed P is subject to desorption, and that transport in many cases conforms to shock front displacement. Transport at field conditions, furthermore, appeared to be largely controlled by P-sorption and to a lesser extent by flow.Assuming soil in the field may be described as a number of parallel, noninteracting columns, and assuming piston type displacement, P-transport was described if the sorption capacity and P-input differ for each column, using stochastic theory. The main trends for P-displacement in such a heterogeneous field were in agreement with experimental data, and showed large differences with the solutions of the convection-dispersion equation for average parameter values. An analytical solution for a more specific case supported these findings, for heavy metal transport, and showed faster breakthrough in the small concentration range than expected using average parameter values. The effect of transversal Interaction between two layers with different properties showed that the loss of solute from the layer with the largest transport velocity may be significant, when at the sharp interface, in the direction of flow, soil properties vary much