Convective solute transport in clayey soils and sediments is driven by gradients in hydraulic and osmotic pressure. Chemical osmosis arises if a gradient in equilibrium concentration of salt is present in clayey material. Due to the surface charge of the particles a clayey layer may act as a semi-permeable membrane. Semi-permeability is of importance for the 1:2 type (swelling) clays. Since clays are not perfectly semi-permeable membranes a reflection coefficient has to be taken into account (Mitchell, 1993).
In preliminary laboratory experiments with 100% Wyoming Na-bentonite subjected to a gradient in equilibrium salt concentration a water flux was induced in agreement with estimated hydraulic conductivity and reflection coefficient of the material (Fritz, 1986; Mitchell, 1993).
A storage facility for dredging sludge in coastal water contains a 5 m thick layer of consolidated illite clay. The layer is bounded by salt pore water at the bottom and river water at the top. The swelling pressure across the layer is calculated to be less than 150 kPa. At the steady state when swelling pressure gradients are eliminated, the osmotic pressure gradient across the layer is about 400 kPa m-1. The reflection coefficient estimated by different models (Bresler, 1973; Fritz, 1986) varies between 0.1 and 0.67. At an estimated hydraulic conductivity (Mitchell, 1993) of 10-9 m sec-1, a water flux between 12 and 80 cm yr-1 is induced by the osmotic pressure gradient.
Chemical osmosis has to be considered in risk assessments of contaminants in confined aquifers, clayey sludges and man-made clay barriers in coastal or estuarine regions. If the flux of solution from a clayey sludge depot in such regions should be minimized for environmental reasons, this cannot be done without taking osmotic pressure gradients into account.
Bresler, E., Soil Sci. Soc. Am. Proc. 37, 663-669 (1973).
Fritz, S. J., Clays and Clay Minerals 34, 214-223 (1986).
Mitchell, J. K., Fundamentals of Soil Behavior 2nd Edn. Wiley & Sons, New York (1993).