The Influence of Non-Linear Sorption on Colloid Facilitated Radionuclide Transport Through Fractured Media
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THE INFLUENCE OF NON-LINEAR SORPTION ON COLLOID FACILITATED RADIONUCLIDE TRANSPORT THROUGH FRACTURED MEDIA Paul A. Smith* *Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland ABSTRACT In the safety assessment of nuclear waste repositories, sorption of radionuclides on the surfaces of colloids may significantly modify transport behaviour where colloid concentration is sufficiently high. In the case of fractured geological media, colloids may be excluded from matrix pores, in which case radionuclides bound to them are not subject to the retarding effects of matrix diffusion and sorption onto matrix pore surfaces. A model is presented describing colloid facilitated transport through fractured media with non-linear sorption. A simple criterion is developed to predict when the presence of colloids will have a significant influence on transport and effects resulting from non-linearity of sorption are described. However, lack of comprehensive sorption data, as well as computational efficiency, mean that the use of a simplified transport model, with linear sorption both on pore surfaces and colloids, is desirable if it can be demonstrated to be conservative. A further criterion is developed to predict where such a model, with linear sorption calculated for the highest concentration encountered along the flow path, would be expected to yield conservative results. INTRODUCTION The modelling of radionuclide transport through the geosphere is an important component in the safety assessment of nuclear waste repositories in geological formations. In most hard rocks, the dominant transport mechanisms are likely to be advection and dispersion of solutes within groundwater flowing through conducting zones or fractures, with matrix diffusion into the adjacent rock. The retardation provided by matrix diffusion is enhanced if radionuclides sorb on pore surfaces within the matrix. However, where the concentration of colloids in the groundwater is sufficiently high, sorption of radionuclides on their surfaces can also significantly modify transport behaviour. Colloids may be excluded from matrix pores, in which case radionuclides bound to them will be retarded less effectively by matrix diffusion and sorption on pore surfaces. Several models for colloid transport in homogeneous porous media [1], [2], [3] and in fractures [4], [5], [6] have been described previously. In these models, linear sorption of radionuclides is assumed on pore surfaces and on colloids. Experiments show, however, that a linear sorption isotherm is the exception rather than the rule and the considerable effects which the non-linearity of sorption can have on the transport of radionuclides in solution has been demonstrated in various modelling exercises [7], [8], [9]. The use of a simple transport model, with linear sorption both on pore surfaces and on colloids, is desirable if it can be demonstrated to be conservative, firstly because the lack of comprehensive sorption data prohibits extensive use of more complex isotherms and secondly because the resulting
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