LATERAL SOLUTE MIXING IN HOMOGENEOUS AND LAYERED SAND COLUMNS

Lateral dispersion was investigated in water unsaturated columns (leng th 18 or 20 cm and diameter 6 or 11 cm, respectively) at constant flow (q = 28.5 mm h-1). Columns were either packed homogeneously with fine sand and coarse sand, or in horizontal or inclinded layers. A conserv ative dye tracer was applied as a point source in the centre of the co lumn or as a pulse over the entire surface. Lateral mixing of the poin t source was investigated visually by cutting the columns vertically i nto two halves immediately after the dye was detected in the outflow. In non-stratified, homogeneously packed columns lateral solute mixing could be modelled using a convection-dispersion model with lateral dis persion term. In stratified columns, processes at or just above the la yer boundaries significantly modified lateral mixing. These processes depend strongly on the sequence of neighbouring layers. In a fine text ured layer overlying coarse sand streamlines may converge and thereby induce preferential flow. In a coarse layer immediately above a fine t extured one lateral exchange of flowing solutes may be enhanced and th ereby smooth out a moving solute front.