In this thesis, the study of solutes released into unsaturated homogeneous and layered soils has been addressed through physical modelling in a geoenvironmental centrifuge. Because independent long term modelling under a variety of controlled boundary conditions can be performed without the need to establish soil properties, centrifugation is an ideal technique for obtaining data sets for numerical simulation validation.; To facilitate this work, the geoenvironmental centrifuge was brought up to working conditions through a series of modifications to the drive shaft and pulley system. The configuration designed and implemented allows for the operation at a consistent centrifuge speed of 65 rpm which is equivalent to approximately 7g.; Also key to this work was the installation of EC1 SigmaProbes on one of the centrifuges arms. The probes allow for the measurement of pore-water electrical conductivity independent of the effects of probe contact and soil moisture. This is believed to be the first successful use of the SigmaProbes by an independent researcher.; The commercially available program SoilVision was used for the prediction of soil-water characteristic curves. Physical testing in the centrifuge of the hydraulic conductivity and the molecular diffusion coefficient for each soil under prepared conditions was carried out. These values were used to aid SoilVision in the prediction of a correctly calibrated hydraulic conductivity and molecular diffusion functions.; Physical modelling consisted of simulating the release of NaCl solutions into unsaturated soil systems. Pore water concentrations were measured during centrifugation (in-flight) and were determined, along with volumetric water content, at the end of each modelling sequence. Two model series were performed: (1) an unsaturated sandy loam column with release over the entire surface area, and (2) an unsaturated sandy loam column with one silt layer and release along the centre line.; The results of these model series were compared to predictions from the commercially available software SEEP/W and CTRAN/W. SEEP/W was shown to accurately simulate seepage through both saturated and variably saturated individual soils but fell short when dealing with the layered profile. CTRAN/W was shown to simulate advective-dipersive transport in all but the layered profile for which seepage predictions were insufficient.
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