Percolation theory applied to soil tomography

Abstract

This study provides insights on the significance of network features of soil macropores on the transport of solutes and colloids, and in the filtrating capacity of the soil. We applied percolation theory and network analysis to the pore network extracted from X-ray computed tomography (imaged porosity) in intact columns sampled from topsoils with different tillage treatments. Moreover, we developed a procedure to extract the backbone, which is the part of the percolation cluster that controls the direct flow between two boundaries in near saturated conditions, using the ImageJ open source imaging software. We also calculated the percolation threshold of each soil, the probability at which the soil starts to percolate (for the resolution considered).Some backbone characteristics (pore volume, wall surface, circularity, fractal dimension, number of loops and tortuosity) showed significant differences between the treatments. Tilled conventional and organic with high earthworm activity exhibited more complex backbones than no-till soil. Backbone volume, surface, fractal dimension and number of loops are correlated with the surface area of pore walls stained by fluorescent microspheres (MS) used as a colloidal tracer. We also found significant correlations between the tortuosity and the number of end-points of the backbone and the transport model parameters for the microspheres and bromide. These findings reinforce the phenomenology between transport in porous media and percolation theory. Moreover, the properties based on percolation theory allow a complete characterization of the complex soil structure and the development of more accurate transport models.