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Whereas the transport of solutes in nonreactive porous media can mostly be described by diffusion driven by the concentration gradients in the external bulk water phase, the situation for dense clays and clay rocks has been less clear for a long time. The presence of fixed negative surface charges complicates the application of Fick's laws in the case of ionic species. Here we report the seeming uphill diffusion of a (22)Na(+) tracer in compacted sodium montmorillonite, that is, transport directed from a low to a high tracer concentration reservoir. In contrast to the classical through-diffusion technique the present experiments were carried out under the conditions of a gradient in the background electrolyte and using equal initial (22)Na(+) tracer concentrations on both sides of the clay sample. We conclude that the dominant driving force for diffusion is the concentration gradient of exchangeable cations in the nanopores. Commonly used diffusion models, based on concentration gradients in the external bulk water phase, may thus predict incorrect fluxes both in terms of magnitude and direction.

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