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Water Flux and Electrical Potentials in Water-Saturated Bentonite¹

ABSTRACT

Electrical potential, water flux, and hydraulic gradient were studied for saturated water flow through plugs of sodium bentonite at concentrations ranging from 11.5 to 35.1% clay. A special permeameter with an adjustable piston allowed measurements to be taken with a given mass of clay, but with successively lower clay concentrations as the piston was shifted out to increase the water content of the clay-water mixture. At the lowest clay content, both electrical potential and water flux increased less than proportionally with increasing hydraulic gradient. This less-than-proportional behavior decreased as clay concentration increased, and, at 28.4% clay, proportional response to hydraulic gradient was obtained for both electrical potential and water flux. A less-than-proportional relationship between electrical potential and hydraulic gradient did not cause the water flux to increase more than proportionally with increasing hydraulic gradient, in contrast with what has been reported for nonswelling quartz powder and would be inferred from the classical double-layer theory of streaming potential and electroosmotic flow. The relationship between electrical and hydraulic properties may thus be different for swelling porous media than for nonswelling media. The only indication of a more-than-proportional flux-gradient relationship was found for a 35.1% clay concentration, in the form of a linear relationship displaced to the right from the origin to yield a positive gradient intercept and a negative water flux at zero hydraulic gradient.

¹ Journal Paper no. 5633, Purdue University Agric. Exp. Sta. West Lafayette, Ind. 47907. Contribution from the Department of Agronomy. Partial support was from funds provided by the U.S. Department of the Interior, Office of Water Resources Research, as authorized under the Water Resources Research Act of 1964.

² Postdoctoral Research Associate and Professor of Soils, respectively; first author is now Secretary, National Council for Research and Development, Prime Minister's Office, Jerusalem, Israel.

Received for publication September 11, 1974. Accepted for publication April 30, 1975.