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Hydrodynamic Dispersion During Unsteady, Unsaturated Water Flow in a Clay Soil¹

ABSTRACT

When a 10^-1M calcium chloride (CaCl₂) solution was absorbed by a calcium-saturated clay soil in which both the water content, , and solution salt concentration, C, were initially low, it was found that both and C were uniquely defined by the distance divided by the square root of time.

The "salt front" did not, however, coincide with the piston front that would exist given perfect displacement of the water initially present in the soil by the water being absorbed. The effect was attributed to a water layer of a thickness about 9Å at the clay surface, which was inaccessible to the anion.

Examination of the salt balance indicated that there appeared to be no restriction on the entry of the CaCl₂ into this soil when compared with the water, and an analysis using material coordinates and a constant dispersion coefficient permitted reasonable prediction of the solution salt concentration profile if the inaccessible water was taken into account.

Key Words: anion exclusion • salt movement

¹ Contribution from CSIRO, Australia.

² Chief and Experimental Officer, CSIRO Division of Environmental Mechanics, P.O. Box 821, Canberra City, ACT 2601, Australia.

Received for publication April 22, 1981. Accepted for publication August 31, 1981.