The colloid and radionuclide retardation experiments performed at NAGRA's Grimsel Test Site in Switzerland are part of an international collaboration program designed to collect ill Situ data on the impacts of colloids oil radionuclide transport. In this work, breakthrough behaviors of trivalent americium (i.e., Am-241 and Am-243) both in the absence and presence of bentonite colloids are analyzed with COLFRAC-a code that models colloid-facilitated Solute transport in discretely-fractured, porous media. Model fits to the experimental results indicate that Am sorbed onto mobile colloids, which enhance Am transport relative to a non-sorbing tracer, I-131. Modelling results suggest that Am is kinetically sorbed onto both naturally Occurring and exogenous bentonite colloids. Results also indicate that desorption of Am from colloids is slow with respect to the duration of the experiment. In addition, early colloid breakthrough compared to a conservative tracer suggests the effects of hydrodynamic chromatography. Overall, Am breakthrough curves suggest enhanced mobility due to co-transport with both naturally occurring and bentonite colloids. (c) 2005 Elsevier Inc. All rights reserved.