Solute Transport During Absorption of Water by Soil: Laboratory Studies and Their Practical Implications

An experimental study of solute transport during absorption into uniform horizontal soil columns is reported. Three pairs of experiments were performed. Each pair had a common initial moisture content (0.04, 0.1, or 0.15), with one experiment involving displacement of concentrated soil solution by dilute solution and the other, vice versa. In all experiments, both the soil solution content and the salt concentration preserved similarity in terms of distance divided by square root of time. This confirmed earlier observations that the longitudinal dispersion coefficient for the conditions of these experiments is effectively independent of volume flux density. These results were found to be consistent with the studies of Saffman (1959) and Pfannkuch (1963) on transport during steady flow. The inferred dispersion coefficient is essentially independent also of the volumetric moisture content (at least in the range 0.18–0.28). It is approximately equal to the product of the volumetric moisture content and the molecular diffusivity of KCl in water. The practical implications of the work for field problems are explored, use being made of scaling theory for flow processes involving capillarity and viscous flow. It is concluded that, for the majority of field soils, solute transport during unsaturated flow of any practical duration may be described by a velocity-independent dispersion coefficient. This represents a very great simplification of the formulation, analysis, and prediction of solute transport in such systems. The observed pistonlike displacement of the initial water in the columns by the absorbed water suggests that there is no basis for subdividing the water in the system into mobile and immobile fractions. Further theoretical and experimental work is under way on solute transport during infiltration, and during absorption into very coarsetextured soils (for which the dispersion coefficient is not velocity independent).