Movement of CaCl₂ solutions in an unsaturated clay soil: the effect of solution concentration

Abstract

The effect of solution salt concentration on the movement of calcium chloride during unsteady, unsaturated flow in a structurally stable, calcium-saturated, heavy clay soil is described. Solutions of CaCl, were supplied at a head of -10 mm of water to horizontal columns of soil containing solution with a different concentration of CaCl, at a low water content. The measured apparent solution concentrations near the inlet were less than the applied solution concentrations, and the identifiable salt front moved more rapidly than the 'piston-front' of the water. The magnitude of both effects was dependent on solution concentration. The observations are consistent with exclusion of anions from the diffuse double layer at the clay surface. The relationship between exclusion volume (or inaccessible water content) and solution concentration, obtained from the measurements, was affected by the interaction of double layers at low solution concentration and may also be affected by water content. An expression was derived which permits prediction of the position of the salt front from the properties of the initial and invading solutions.