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 full version of this paper is available on Agronomy Online.