Conceptualization of solute transport using time domain reflectometry. A combined laboratory and field study

Abstract: Solute transport in the unsaturated zone is a complex process. The first of the two objectives for this study was to examine how it could be conceptualized under different flow conditions. The second objective was to investigate how the time domain reflectometry (TDR) technique should be used in order to obtain high quality data from the soils studied. The TDR is an accurate instrument for water content (T) and bulk electrical conductivity (ECa) measurement. The highest degree of precision and accuracy of T measurement were found in soils with low organic matter and clay content along with low ECa. Temperature dependent errors exist in TDR measurements. The results presented in this thesis suggest a temperature correction factor of -0.00269 T per degree Celsius for T measurements in sandy soils. In clayey and organic soils the temperature correction factor is normally less and can be positive or negative. The temperature dependence of ECa was found to be very close to the temperature dependence of the soil extract. The effect of temperature on ECa was independent of soil texture. By using the direct calibration approach and relating ECa to soil solution electrical conductivity (ECw), measurements can be made for transient conditions with varying T in sandy soils. In clay soils only the indirect calibration approach should be used due to complications associated with these soils. In the loamy sand soil used in this study, solutes were transported via preferential flow paths or fingers. This was shown in dye and tracer experiments in both the laboratory and the field. Most of the preferential flow paths were consistent during the experiments. The overall heterogeneity evident in the field experiments was greater during low flow compared to high flow. This was probably due to the fact that the fingers became wider with increased flow. This was also reflected in a higher relative velocity in the low flow experiment. Both the convective-dispersive and stochastic-convective concepts can be used to describe local solute transport in all experiments. Global solute transport was, however, difficult to model with these fairly simple, one-dimensional models. The results from the column and field studies show that the magnitude of the flux determines how the solute transport can be conceptualized. This thesis demonstrates that TDR can be used to obtain reliable data suitable for modeling work as well as revealing possible modeling concepts. It also shows that solute transport conceptualization depends not only on soil type, but also the nature of water application (magnitude of the water flux and whether the flow is transient or steady-state) and the location in the soil.