Many tropic soils exhibit double- or even triple-porosity features as reflected by heterogeneous pore- and/or particle-size distributions. While the amount of clay in tropical soils is generally relatively high, cementation of the finer particles into larger grains make field soil often behave macroscopically more like coarse-textured media. Natural aggregation may further enhance preferential flow paths for water and contaminant transport. The same can be observed in some sedimentary rocks (coquinas) composed partially or entirely of transported, abraded and/or mechanically-sorted fragments of the shells of mollusks, trilobites, brachiopods or other invertebrates. Such coquinas often contain large interconnected pore networks that directly or indirectly influence fluid flow and contaminant transport processes. Analysis of their pore properties (such as their size, shape and connectivity) as estimated from three-dimensional images (3D) provides a way to link microscale pore structures with their macroscopic functioning, and how all this may affect overall fluid flow processes in such media. We evaluated the applicability of an integrated characterization approach involving 3D microtomography, measurements of the pore-size distribution (PSD) using 3D images of two tropical soils, evaluating the double porosity nature of undisturbed soil samples, and modelling flow and transport using the PoreFlow pore network model. The procedure combines the use of commercial software such as Avizo, with in-house software developed at Utrecht University (Netherlands) and the University of Rio de Janeiro (Brazil). Images were obtained with a benchtop X-ray microtomography system at spatial resolutions of 3.6, 6, 12, and 30 microns by varying the size of the samples from 0.5 cm to about 4 cm in diameter. Pore-size distributions obtained at each image resolution were fitted with lognormal distribution functions. Results showed that large pore sizes are better represented in low resolution images of relatively large samples, while proper characterization of the smaller pores require higher resolutions of by necessity smaller samples. We also analyzed the PSDs obtained after skeletonization of the samples having different sizes. The image-based approach was found to correlate well with PSDs measured using mercury intrusion porosimetry. Saturated hydraulic conductivity estimates obtained with the pore network model were further compared with theoretical values based on soil texture.
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