Our model considers the contaminants as dissolved in the aqueous phase and sorbed to the sediment or sorbed to mobile and immobile colloids. All sorption processes are assumed to be reversible and are modeled as a kinetic-Langmuir reaction. The Europium Migration ExperimentThe column experiments with •52Eu(II!) in a humic-rich groundwater were performed by Klotz and Lazik [1996]. Batch experiments with the same groundwater sediment system were conducted to determine the s0rption .distribution coefficient Rs More than 98% of europium in the injected solution is bound to colloidal humic particles, and the predominant humic colloid size is in the range from 1 to 100 nm.