Grid lysimeter study of steady state chloride transport in two Spodosol types using TDR and wick samplers

Solute transport in soils is affected by soil layering and soil-specific morphological properties. We studied solute transport in two sandy Spodosols: a dry Spodosol developed under oxidizing conditions of relatively deep groundwater and a wet Spodosol under periodically reducing conditions above a shallow groundwater table. The wet Spodosol is characterized by a diffuse and heterogeneous humus-B-horizon (i.e., Spodic horizon), whereas the dry Spodosol has a sharp Spodic horizon. Drainage fluxes were moderately variable with a coefficient of variation (CV) of 25% in the wet Spodosol and 17% in the dry Spodosol. Solute transport in 1-m-long and 0.8-m-diameter soil columns was investigated using spatial averages of solute concentrations measured by a network of 36 Time Domain Reflectometry (TDR) probes. In the dry Spodosol, solute transport evolves from stochastic-convective to convective-dispersive at a depth of 0.25 m, coinciding with the depth of the Spodic horizon. Chloride breakthrough at the bottom of the soil columns was adequately well predicted by a convection-dispersion model. In the wet Spodosol, solute transport was heterogeneous over the entire depth of the column. Chloride breakthrough at 1 m depth was predicted best using a stochastic-convective transport model. The TDR sampling volume of 36 probes was too small to capture the heterogeneous flow and concomitant transport in the wet Spodosol.