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The results of a diffusion-sorption experiment on a sample of Darley Dale sandstone, using simulated groundwater spiked with a mixture of 125I, 85Sr and 137Cs, are modelled by a one-dimensional porous medium approach in which sorption is described by Freundlich isotherms. The governing equations are solved analytically for the special case of a linear isotherm, and numerically using the computer code RANCHDIFF for non-linear isotherms. A set of time-dependent, ordinary differential equations is obtained using the Lagrange interpolation technique and integrated by Gear's variable order predictor-corrector method.
It is shown that the sorption behaviour of 85Sr can be modelled successfully by a linear isotherm, using a sorption parameter consistent with batch-sorption tests. The behaviour of 137Cs may be modelled by a non-linear isotherm, but the amount of 137Cs sorbed is less than that anticipated from batch-sorption tests. 125I is assumed to be non-sorbing and is used to determine the porosity of the sandstone.