Solute Transfer, with Exchange between Mobile and Stagnant Water, through Unsaturated Sand

Abstract Changes in salt concentration with time were measured at several depths inside and at the exit of a 94‐cm uniform unsaturated sand column which was leached at steady rates with solutions of calcium chloride. Observed salt distributions were compared with salt distributions calculated with a numerical procedure which was based on the dead‐end pore model of Coats and Smith (1964). Values for the dispersion coefficient, the diffusional mass transfer coefficient, and the fraction mobile water used in the model were obtained at different water contents by curve fitting observed and calculated concentrations at one depth. These same values were then used to calculate salt distributions at other depths and at the exit of the column and compared with measured salt distributions. Excellent agreement was obtained. The model presented a good description of the extensive tailing of the salt concentration distributions observed inside and at the exit of the column. The amount of stagnant water was found to increase with decreasing water content from 4% of the total water content of 0.256 cm 3 /cm 3 to 40% of the total water content at a water content of 0.200 cm 3 /cm 3 . The apparent dispersion coefficient decreased from 6 cm 2 /hour to 1 cm 2 /hour at water contents of 0.256 and 0.200 cm 3 /cm 3 , respectively.