Modelling of solute transport in a large undisturbed lysimeter, during steady-state water flux

Abstract

Transport models have often been tested in laboratory studies using soil columns, usually of the order of 1dm3 in size. Even if the columns are undisturbed, their small size does not allow water flow and solute transport to occur as they would in the field. We therefore used a 1.7m3 column and applied steady-state flow rates of the order of 1mmh−1, and then applied pluses of tracers Br−, Cl− and 2H2O, and of atrazine (2-chloro-4-ethylamino-6-isopropylamino-S-triazine) to the surface.Classical models for tracers with these boundary conditions are the Convection–Dispersion model (CD), the two-region (mobile–immobile water) model with first-order exchange of solutes (MIM), and the transfer function models, among which the most widely used is the Convective Lognormal Transfer Function model (CLT). Thanks to simple boundary and initial conditions, analytical solutions are available for all these models.The CD model (1 parameter) was not able to fit the tracer elution curves, but use of the MIM model was satisfactory. The CLT model and the CD model (2 parameters) also gave satisfactory fits. To choose the best model we used the parameters fitted to the elution curves to predict vertical concentration profiles in the lysimeter. These predictions are compared to the profile obtained after thorough sampling of the soil when tracers reached about half way down the lysimeter. The MIM model yielded a better prediction. However, accurate predictions would require taking into account the highly stratified characteristics of this soil.Atrazine simulation was done with the CD-based one-site kinetic sorption and first-order decay equation. Again analytical solutions are provided for our experimental conditions. Values of decay and absorption parameters are in agreement with previous studies.