Modeling of colloid-associated radionuclide transport in porous

Results are presented which have been obtained in prediction studies on the colloid-associated trans-port of radionuclides through porous groundwater aquifers overlying as geochemical barrier the planned nuclear repository site at Gorleben, Germany. The studies incorporate three-phase retardation factors for 227Ac, 228Th, 232U and 210Pb in site-specific groundwater aquifers containing humic colloids and efficiencies for Brownian deposition of colloidal carriers onto immobile sedimentary grains. It is shown that colloid-bound radionuclide migration can be approximately modeled by using the basic advection-dispersion trans-port equation modified with three-phase retardation coefficients and colloid-chemical attachment factors due to the presence of multiple phases, i.e., fluid, colloid and immobile solid. Moreover, results of site-specific assessment studies indicate that the transport of colloid-associated radionuclides in porous groundwater aquifers overlying the Gorleben salt dome may not occur over long distances, provided that the nuclides are transported by porous flow only. rock salt may reach waste packages so that nuclides might be released (Warnecke et al., 1984, 1994). Therefore, for the performance of a long-term safety assessment the barrier function of the geological formations overlying the salt dome have been studied by batch, column and diffusion experiments (Lieser et al., 1990; Meier et al.