A new method to trace colloid transport pathways in macroporous soils using X‐ray computed tomography and fluorescence macrophotography

Abstract

The fast and deep percolation of particles through soil is attributed to preferential flow pathways, and their extent can be critical in the filtering of particulate pollutants in soil. Particle deposition on the pore walls and transport between the pores and matrix modulate the preferential flow of particulate pollutants. In the present research, we developed a novel method of combining fluorescence macrophotography and X‐ray computed tomography (CT) to track preferential pathways of colloidal fluorescent microspheres (MS) in breakthrough experiments. We located accumulations of MS by fluorescence imaging and used them to delimit the deposition structures along the preferential colloid pathways by superimposing these images on the 3‐D pore network obtained from CT. Advection–diffusion with transport parameters from the dual‐porosity equation correlated with preferential pathway features across different soil management techniques. However, management did not influence the morphology of the MS preferential pathways. Preferential flow occurred in only a small fraction of the total pore network and was controlled by pores connected to the soil surface and by matrix density.Highlights X‐ray tomography and epifluorescence images were combined to identify preferential pathways in soil. Preferential pathways constituted approximately 1% of the soil bulk volume. Correlations between descriptors of preferential pathways and colloid transport were found. Tillage influenced the morphology of pores, but not the shape of preferential pathways.