Comparison of numerical simulation of solute transport with observed experimental data in a silt loam subsoil

Solute transport experiments were conducted on loamy soils of north-eastern Iowa, USA, and the results were compared with a numerical solution of a cl assical advection-dispersion transport model developed in this study. Flow experiments in the laboratory on undisturbed soil columns showed a flow rat e of water much higher than was estimated from the soil properties and grai n-size analysis data, suggesting preferential flow regime in the soil. In c ontrast, the relative concentration peaks of Cl- and Br- in the effluent we re only approximately 70% of those predicted by the classical advection-dis persion equation (ADE). In addition, the experimental breakthrough curves ( BTCs) showed greater tailings of these ions than the model solution. These observations suggest a loss of solute mass during transport from the dynami c flowing regions to a stagnant, immobile water phase in the soil matrix. E xperiments in small disturbed soil columns showed that movement of Cl- and Br- is in good agreement with predictions of the classical ADE when the tra cers are applied as a continuous source. However, in the case of a pulse so urce, the BTCs of Cl- and Br- matched the model only in ascending part of t he curves. Such variation indicates greater retardation of these ions than that of simulation, probably caused by the decrease in soil permeability du e to cation exchange reactions in the soil involving monovalent and divalen t cation pairs such as K+-Ca2+ and K+-Mg2+. In addition, retardation occurr ed as a result of the continuous saturation of soil columns which seemed to have caused an expansion of clay minerals, thus resulting in decreased soi l permeability. In both the continuous and the pulse-source experiments, K was not detected in the effluent samples, which seemed to have been lost i n exchange reactions and adsorption.