| Abstract: |
Reliable quantitative prediction of contaminant transport in sub-surface environments is critical to evaluating the risks associated with radionuclide migration. A non-electrostatic surface complexation (NEM SC) approach to radionuclide sorption in conjunction with existing ion exchange models is being used in near-field reactive transport simulations at the Nevada Test Site (NTS). Published radionuclide sorption data were fit to NEM SC reactions and a reaction database was developed for use in reactive transport simulations. For radionuclide-iron oxide sorption, one-site NEM SC reactions adequately fit most data without need for bidentate or ternary surface species. For example, the decrease in U(VI) sorption as a function of carbonate alkalinity was accounted for by aqueous uranyl-carbonate complex formation. Calcite reactions were modeled as NEM surface exchange reactions. Reactive transport simulations using the NEM SC database suggest that migration of Sm, Eu, Np, Pu, and Am will be significantly retarded by sedimentary carbonates. Though iron oxides are usually thought to be the major contributors to radionuclide retardation, carbonates will also be of paramount importance in areas where they are present. |
