Characterization, visualization and quantification of soil macropores and preferential flow using SPECT and X-ray CAT scanning

The presence of soil macropores provides an opportunity for water and associated chemicals to move preferentially through the vadose zone. Most studies on the characterization of macropore flow consider the soil system to be a “black-box”. Thus, efforts to describe macropores in quantitative terms have not yet resulted in a comprehensive theoretical framework that allows for a complete representation of their geometry and their effect on preferential flow.; Macropore networks in four large undisturbed soil columns (850 mm x 77 mm diameter), were quantified and visualized, both in 2-D and 3-D, using X-ray Computer Assisted Tomography (CAT). Macropore quantification included two-dimensional parameters (i.e., number of macropores, macropore size, hydraulic radius, rectangularity and circularity), and three-dimensional and topological parameters (i.e., macropore length, volume, 3-D hydraulic radius, tortuosity, numerical density, coordination number, and connectivity). Pores larger or equal to 1.0 mm in equivalent diameter were readily detected, visualized and quantified. It was found that the average macroporosity of four soil columns varied between 2.1 and 3.8%. From the hydraulic radius distribution, it was inferred that about 20% of macropore throats have a diameter of 1.6 mm. It was found that the majority of macropore networks had a length of 40 mm, a volume of 60 mm 3 and a wall area of 175 mm2.; Breakthrough of potassium iodide was monitored with X-ray CAT scanning. This approach allowed for real-time examination of flow mechanisms through the macropore and matrix flow domains at various depths along the soil column. Flow in the matrix domain suggested that part of the matrix contains small pores (mesopores) that were connected to macropore networks. It is suggested that the matrix domain should be subdivided into two regions: mesopores and micropores. In addition, the macropore domain should be defined both in terms of macropore geometry and its ability to convey the tracer preferentially.; Single Photon Emission Computer Tomography (SPECT) was also used for the real-time analysis. both in 2-D and in 3-D, of radioactive tracer flow patterns in four soil columns. Our results characterized preferential flow very clearly in soil columns. This technique has never been applied to soil physics and opens new avenues for tracer studies in soils.; Efforts were made to relate the 3-D geometry of macropores to their water and solute conveying abilities. A new multi-region modeling approach was developed to simulate macropore flow in a soil under saturated conditions using an analogy between macropore flow and pipe flow. The macropore domain was divided into two regions, namely the laminar and turbulent regions. A modified version of Poiseuille'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. Modifications were made so that these models took into account macropore tortuosity and the distribution density functions of macropore size and hydraulic radius. This approach provides a reliable approximation of the overall tendency for breakthrough in the macropore domain.