A Two‐Domain Approach Using CAT Scanning to Model Solute Transport in Soil

Abstract

Multi‐region modeling can be used to simulate dynamics of preferential flow in soils. However, the criteria used to determine boundaries between flow regions have been defined arbitrarily up to now. Therefore, there is a need to develop a reliable technique to isolate and characterize flow domains in soil. The primary objective of this study was to develop a reliable method for isolating and characterizing flow domains in a large undisturbed soil column using a computer assisted tomography (CAT) scanner. This approach allows for real‐time examination of flow mechanisms through soil macropores at various depths along the length of soil columns. With the knowledge of the macropore structure and the spatial distribution of the solute, breakthrough in the macropore and matrix flow domains was evaluated. Flow in the matrix domain suggested that part of the matrix contains small pores that are connected to macropore networks. These pores contribute to a rapid tracer buildup in the matrix domain. The breakthrough curves (BTCs) measured in the matrix domain were fitted using the convection dispersion equation (CDE) with CXTFIT 2.0. The macropore domain was divided into two regions, namely the laminar and turbulent regions. A modified version of Poiseullle's law was used to model solute breakthrough in the laminar region. For the turbulent region, a new formula was derived based on Manning's equation. The modifications were done so that these simple models would take into account the distribution density functions of macropore size and hydraulic radius. This approach provides reliable approximation of the overall breakthrough of solutes in the macropore domain.

A review of model applications for structured soils: a) Water flow and tracer transport

“…Only the threeregion model could fit all BTCs and supported the hypothesis that there were two mobile and a diffusion domain in the clayey soils (Kjaergaard et al, 2004). The physical reality of postulated flow regions could be supported by combining Xray CT scans of macropores with measured tracer breakthrough to parameterize a three-mobile-region model (Perret et al, 2000).…”

“…Perret et al (1999) used CT for establishing the 3D arrangement of the geometry and topology of macropores. Perret et al (2000) further applied CT for real-time examination of Potassium iodide movement through the macropores and matrix of a large undisturbed soil column. Based on the CT results, distribution density functions of macropore size and hydraulic radius were assigned to the two (laminar and turbulent flow) macropore subdomains of a three-mobileregion model.…”

“…The breakthrough curves measured in the matrix domain were fitted using the CDE (Toride et al, 1999). Measured potassium iodide breakthrough in the macropores could then be predicted using the three-region model (Perret et al, 2000). Vanderborght et al (2002) explained concentration patterns of two adsorbing fluorescent dye tracers in a soil column with the large 3D pore structure derived from CT imaging.…”

“…intact dry soil columns, 3D reconstruction of macropore networks, solute breakthrough analysis in space and time (Perret et al, 1999(Perret et al, , 2000 UC 3D hydraulic property distribution and diffusivity 3D hydraulic tomography by inversion of travel times of transient pressure propagation and diffusivity measured in hydraulic and pneumatic tests in unsaturated fractured sandstone core. UC θ(h) dependency on micromorphology Micromorphology of soil pore structure was studied on thin sections.…”

Application of x-ray computed tomography in the evaluation of soil porosity in soil management systems

“…This change in the soil structure affects water retention, influencing the dynamics of soil water. PERRET et al (2000) discussed the importance of pore structure complexity in flow dynamics, and described macropore and matrix flow domain characteristics in soil.…”