This example illustrates how PHREEQC version 3 can simulate a
diffusion experiment, as is now often performed for assessing the properties
of a repository for nuclear waste in a clay formation. A sample is cut from
a core of clay, enveloped in filters, and placed in a diffusion cell (see
Van Loon and others, 2004, for details). Solutions with tracers are
circulated at the surfaces of the filters, the tracers diffuse into and out
of the clay, and the solutions are sampled and analyzed regularly in time.
The concentration changes are interpreted with Fick's diffusion equations to
obtain transport parameters for modeling the rates of migration of elements
away from a waste repository. Transport in clays is mainly diffusive because
of the low hydraulic conductivity, and solutes are further retarded by
sorption (cations) and by exclusion from part of the pore space (anions).
The example can be run using the
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