Preferential transport of solute through soil columns containing root channels, earthworm holes, and simulated macropores

Preferential flow has been increasingly recognized as an important mechanism for water and solute transport in field soils. Little is known, however, about preferential transport of solutes through different types of macropores under unsaturated conditions. In this study, solute transport through three types of macropores was investigated using nitrate as a tracer, and three commonly used solute transport models were evaluated with respect to their ability to describe solute transport in the macropore systems. Each type of macropore was generated separately in large plexiglass columns uniformly packed with a Matapeake silt loam, a silica sand, or an Evesboro loamy sand. The soil columns without macropores served as controls.; Preferential solute transport occurred in the root channel or earthworm holes even at relative fluxes less than 0.05 Ksm, where Ksm is the saturated hydraulic conductivities of soil columns containing the macropores. The occurrences of preferential flow were attributed to the establishment of well connected macropores, as supported by the significant increases in the saturated hydraulic conductivities, the drainable pore-water fractions, and the dispersion coefficients of the packed soil columns after the root channels or earthworm hoes were developed.; Pronounced preferential flow also occurred in the simulated macropore systems packed with Matapeake or Evesboro soils, but at a lesser degree. At fluxes far from saturation, no preferential flow took place and the breakthrough curves (BTCs) displayed single peaks, indicating that only the matrix contributed to the solute transport. As the flux increased, the BTCs exhibited double peaks, with the relative magnitudes and the positions of the two peaks being proportional to the flux. These two curves may represent the solute transported by the macropores and the matrix, respectively. Little preferential flow occurred in the silica sand system even at fluxes exceeding the saturated hydraulic conductivity of the sand. This was ascribed to the porous and conductive nature of the medium.; The Physical Nonequilibrium Model provided a better description of the experimental data than the Convection-Dispersive Equation and the Stochastic Models. However, none of the models was adequate to describe the solute transport in the macropore systems.