Dispersivity-saturation relationship in a lab-scale soil column

Abstract

Solute transport through porous media is affected by variations in pore-flow velocity which cause meandering of flow paths. This causes the solute to spread in both longitudinal and transversal directions. This spreading is defined as hydrodynamic dispersion in the advection-dispersion equation (ADE). Dispersivity is a property of a porous medium that is linearly related to the hydrodynamic dispersion through pore-water velocity. In this study the relation between dispersivity and soil water content is investigated by conducting solute displacement experiments in a soil column under saturated and unsaturated conditions. A pulse of a CaCl2 tracer solute was injected into a soil column and the concentration was measured at three depths along the column using electrical conductivity sensors. The resulting breakthrough curves (BTCs) were analyzed to determine the dispersivity, using the solution of the ADE by the CXTFIT2 software code. Results show the dispersivity values that were an order of magnitude larger for the unsaturated soils than for the saturated soils. This demonstrates the large dependency of dispersivity on soil water content. However, the results did not allow a clear relation to be determined. This is caused by a lack of data points and a lack of a mobile-immobile transport model, which could probably describe solute transport in unsaturated soils more accurately than the ADE.