Estimating transport parameters in an undisturbed soil column using time domain reflectometry and transfer function theory

Modelling solute transport in the unsaturated zone is a complex problem given the inherent soil heterogeneity. Using time domain reflectometry (TDR) as described in this paper can improve data quality to be used in predictive models. The TDR technique requires appropriate calibration if it is to be used for solute concentration measurements under constant and varying water content. Two ways of determining the parameters in the relationship between bulk soil electrical conductivity, sigma(a) soil solution electrical conductivity, sigma(w) and volumetric water content, theta, were used. Among the two calibration methods tested here, the one which involved eluting the soil with several pore volumes of water of known electrical conductivity under transient flow conditions in an undisturbed soil column gave a linear sigma(w)-sigma(a)-theta relationship with depth dependent parameters. The other calibration method involved the use of disturbed soil under static conditions and resulted in a curvilinear sigma(w)-sigma(a)-theta relationship. The former calibration was subsequently used to determine solute concentrations in breakthrough experiments. Two solute transport experiments were carried out in the same undisturbed soil column; one for steady-state and one for transient flow condition. Parameters of the convective log normal transfer function (CLT) model were determined using time-integral-normalized resident concentrations. It was shown that the stochastic-convective approach described the experimental data well. Horizontal heterogeneity was more important during the transient experiment, where the topsoil was periodically ponded during a short period, than in the steady-state experiment, where unsaturated conditions were maintained throughout the experiment. This increased heterogeneity during transient flow was also reflected by the fact that preferential solute flow was only observed in the column's outlet under transient conditions.