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

Transport models have often been tested in laboratory studies using soil co lumns, usually of the order of 1 dm(3) in size. Even if the columns are und isturbed, their small size does not allow water flow and solute transport t o occur as they would in the field. We therefore used a 1.7 m(3) column and applied steady-state flow rates of the order of 1 mm h(-1), and then appli ed pluses of tracers Br-, Cl- and (H2O)-H-2 and of atrazine (2-chloro-4-eth ylamino-6-isopropylamino-S-triazine) to the surface. Classical models for tracers with these boundary conditions are the Convect ion-Dispersion model (CD), the two-region (mobile-immobile water) model wit h first-order exchange of solutes (MIM), and the transfer function models, among which the most widely used is the Convective Lognormal Transfer Funct ion model (CLT). Thanks to simple boundary and initial conditions, analytic al solutions are available for all these models. The CD model (1 parameter) was not able to fit the tracer elution curves. b ut 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 concentrati on profiles in the lysimeter. These predictions are compared to the profile obtained after thorough sampling of the soil when tracers reached about ha lf way down the lysimeter. The MIM model yielded a better prediction. Howev er, accurate predictions would require taking into account the highly strat ified characteristics of this soil. Atrazine simulation was done with the CD-based one-site kinetic sorption an d first-order decay equation. Again analytical solutions are provided for o ur experimental conditions. Values of decay and absorption parameters are i n agreement with previous studies. (C) 1999 Elsevier Science B.V. All right s reserved.