[1] It is commonly assumed a priori that solute transport experiments conducted in homogeneously repacked laboratory columns can be described by the advective-dispersive (AD) or single-rate mobile-immobile (MIM) transport model. To investigate this, nonreactive transport through macroscopically homogeneous repacked unsaturated sand was studied at two water contents using laboratory columns of diameter of 11 cm and lengths of 10, 20, and 40 cm. Non-Fickian behavior was found to dominate transport at this scale, with long breakthrough curve (BTC) tailing persistent over the range of column lengths tested. Measured BTC tails were well described by the single-rate MIM model at each depth; however, over the range of travel distances studied BTC tailing was better explained as a result of stochastic-convective (SC) transport or continuous time random walk (CTRW). The SC model was applied using the BTC measured at 10 cm as the probability density function to make predictions to subsequent depths. The CTRW model spreading parameter (b) remained approximately constant across the range of both travel distances and water contents considered. It is concluded that the assumption of a single-rate MIM model cannot be made a priori for macroscopically homogeneous unsaturated sands. In this case, variability between replicates prevented identification of whether a SC transfer function or CTRW provided the best description of transport. These results demonstrate the variability in transport as a result of heterogeneities in column packing, even for macroscopically homogeneous sand, and emphasize the importance of studying transport over a range of travel distances in order to allow prediction of transport.

It is expected that solute dispersivity is determined by the physical properties of the transport medium. However, experimental investigations to date have not identified such a relationship. In this work, we examine the relation between soil physical parameters, experimental parameters and dispersivity. Experimental data were compiled for 291 repacked, saturated homogeneous laboratory column experiments reported in the literature. Using multiple stepwise regression and classification and regression tree analysis, the relationships between soil physical parameters (sand, silt and clay content, bulk density, water content), experimental parameters (pore-water velocity, column length and diameter), and transport parameters (dispersion coefficient and dispersivity) were examined. Regression tree analysis showed that clay content was the most important factor controlling dispersivity, followed by column diameter. Columns of length less than approximately 10 cm generally resulted in greater dispersivities than longer columns. A scale effect appeared significant with increasing column diameter, but was not apparent with column length. These results suggest that it may only be possible to relate dispersivity to intrinsic soil properties by explicitly accounting for the experimental design, including column geometry, inlet dead volume and soil packing.

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