Effect of kinetic limitations on colloid-facilitated radionuclide transport at the field scale

Sorption onto natural groundwater colloids, a process that may enhance field-scale transport of radionuclides in the subsurface, is typically modeled in studies of potential high-level waste repositories using equilibrium partitioning assumptions. A new streamline-based model suggests that kinetic limitations on radionuclide sorption/desoption may significantly enhance field-scale radionuclide transport in some situations. Specifically, the leading edge of mass breakthrough curves may be enhanced if the kinetic rate constants are in a critical range as quantified by a Damkohler-type number. Application of the model to plutonium transport in the alluvial aquifer near Yucca Mountain, Nevada indicates that the bulk of the uncertain parameter space is in the equilibrium range. However, a small parameter region is in the kinetic regime. Monte Carlo simulations suggest that this kinetic region of the parameter space may cause early breakthrough for a small fraction of migrating plutonium, but the effect on peak breakthrough is minimal.