Characterization of Water Flow and Solute Transport Driven by Preferential Flow in Soil Vadose Zone

The vadose zone acts as a buffer zone between the ground surface and the aquifers underneath and controls the transmission of infiltrating water and contaminants, for example, pesticides and chemical spills. Therefore, understanding the flow and transport processes that dominate the vadose zone is important. Macropores are ubiquitous and particularly found in abundance in the vadose zone. These macropores facilitate preferential flow, through which water travels rapidly deep into the soil, bypassing most of the porous matrix. Preferential flow and transport have environmental significance as their processes impact hydrology, ecology, agriculture, subsurface contamination, and waste management sectors. Thus, the overall objective of this work is to understand flow and solute transport behaviors in vadose zone in the context of preferential flow and transport mechanisms. This was accomplished through a series of laboratory-scale flow and transport experiments in soil columns with complex crack networks monitored using imaging techniques, such as a one-dimensional (1D) gamma-ray spectroscopy system (gamma scanner) used to monitor the movement of tracers and a three-dimensional (3D) pre-clinical x-ray Computed Tomography (CT) imaging system used to monitor flow. The desiccation crack networks in laboratory-scale soil columns were formed naturally by a packing methodology explicitly developed to accomplish this work.\nTwo sets of transport experiments are reported here. The objective of the first set of transport studies was to develop a novel methodology to quantify the effective diffusion coefficient of gamma-emitting radionuclides (i.e., 22Na) in saturated porous media using a 1D gamma scanner. One-dimensional diffusion modeling in COMSOL Multiphysics produced an excellent fit (i.e., r2 = 0.98) in terms of predicting the activity concentrations for both Ottawa sand and Savannah River Site (SRS) wetland soil used in this study. The second set of transport studies tested a hypothesis that macropores in soil can contribute to the net upward transport of solutes and investigated the effect of different kinds of macropore structures on solute transport. We found that macroporous soils dominated by a vertical crack network can produce downward flows during wetting that bypass the soil matrix (i.e., where salts are stored) while maintaining an equivalent amount of upward transport during drying relative to non-macroporous soils. The result is an enhancement of the net upward movement of salts in macroporous compared to non-macroporous soils. In addition, we found that horizontal macropores can act as capillary barriers to restrict the upward movement of solutes during drying. The flow experiments monitored by 3D x-ray CT further investigated macropore flow mechanisms observed and hypothesized in transport studies. The CT-derived two-dimensional (2D) water content distribution images and 1D averaged water content profiles illustrate different flow behaviors, such as flow activation in macropore and lateral water imbibition from macropore to matrix. A conceptual model is presented based on the CT observations, which is found to be consistent with the conceptual models presented in the literature. A 1D numerical flow and transport model was used to elucidate and quantify influential factors impacting preferential flow and transport behaviors. The performances of three conceptually different 1D flow models, i.e., single-domain, dual-porosity, and dual-permeability, were compared against the CT-derived water content observations in homogeneous and macroporous media. Both the dual-porosity model and dual-permeability model produced a good fit in predicting matrix water content. Although both of these models appear to be able to capture the kinematics for the migration of the wetting front in the macropore., neither of them was able to fit the macropore water content data.\nThis work has the potential to influence and inform future work on preferential flow and transport or the broader hydrology community, particularly in relation to soil salinization issues. The results are especially relevant to arid or semi-arid regions, where long drying periods and desiccation cracks are prevalent. Our work finds that macropores can be both harmful and beneficial regarding soil salinization issues. While vertical macropore networks can facilitate salt transport to the soil surface to form soil crust and destroy the fertility of the land, the horizontal macropores can act as barriers to prevent the upward transport of salt.

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