The effects of key geochemical parameters on Kd values for radionuclides in the host rock (pumice, sandstone) of a LLW repository were elucidated through a sensitivity analysis, using a thermodynamic speciation/sorption model for the elements Sr and Ni. The complex mineral assemblage of the rock was approximated by a component-additivity approach. Using published ion exchange and surface complexation parameters, Kd for both Sr and Ni could be well explain-ned by the same model mineralogy and surface chemistry. Model results suggest that pCC>2 can have a significant effect on Kd, and that a correct approximation of groundwater chemistry is a critical component of sorption modeling.
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