Quantifying Pore‐Size Spectrum of Macropore‐Type Preferential Pathways

Abstract

Structural pores associated with macropore‐type preferential flow pathways can accelerate chemical transport in unsaturated soils, thereby potentially causing groundwater contamination. To predict chemical transport through these pathways, classical deterministic models depend on soil hydraulic conductivity, which effectively lumps flow contributions from all individual pathways. We contend, however, that quantifying the pore spectrum of preferential pathways, without lumping the contributions of individual pores, is the appropriate method for simulating convective chemical transport through macropore‐type preferential pathways. In this study, we conducted field‐scale experiments by using an improved tile drain monitoring protocol to measure the mass flux breakthrough patterns of conservative tracers. The tails of these patterns suggested that the impact of preferential pathways on contaminant transport can be conceptualized as that occurring through cylindrical capillary tubes. We then proposed a distribution function bracketed by sharp cut‐off points to represent the pore spectrum of these tubes. Finally, we used the measured tracer breakthrough curves (BTCs) as data sources to find the parameters of the proposed function. Our results, based on the best fitting, showed that the preferential pathways are naturally clustered into domains; preferential pathways with a wide range of pore radii could become active simultaneously when infiltration rate increases. Because the derived pore spectra simultaneously satisfy both water movement and solute transport, pore spectra can be used to (i) calculate soil hydraulic conductivity of preferential pathways in deterministic approaches, and (ii) construct multiple probability density functions (PDFs) for the transfer function approach, to accommodate different infiltration patterns.