Solute transport and water content measurements in clay soils using time domain reflectometry

Clayey and saline soils have been shown to be problematic for time domain r eflectometry (TDR) measurements. This study presents some of these problems and discusses solutions to them. Thirteen solute transport experiments wer e carried out in three undisturbed soil columns of swelling clay soil from Tunisia, labelled S1, S2, and S3 respectively. The columns were collected a t three different physiographical regions within a catchment. Water fluxes ranged from 1.2 to 7.2 cm day(-1). The large solute transport heterogeneity and large tailing indicated that preferential flow was most pronounced in SI. The preferential flow took place in voids between structural elements a nd in wormholes. In S3, preferential flow was also evident, but not to the same extent as in S1. In S2, the solute transport was more uniform with lit tle preferential flow. The heterogeneity of the solute transport increased with the water flux in S1 and to a smaller extent in S3, whereas it remaine d constant in S2. In a previous dye experiment in the field, preferential f low in cracks was observed at those sites where SI and S3 were collected. I n the column experiments, preferential flow in these cracks was less due to the higher initial water content compared to the dye experiments, indicati ng that the desiccation cracks were closed by the swelling clay.