| Abstract: |
Reliable quantitative prediction of contaminant transport in subsurface environments is critical to evaluating the risks associated with radionuclide migration. As part of the Underground Test Area (UGTA) program, radionuclide transport away from selected underground nuclear tests conducted in the saturated zone at the Nevada Test Site (NTS) is being examined. In the near-field environment, reactive transport simulations must account for changes in water chemistry and mineralogy as a function of time and their effect on radionuclide migration. Unlike the Kd approach, surface complexation reactions, in conjunction with ion exchange and precipitation, can be used to describe radionuclide reactive transport as a function of changing environmental conditions. They provide a more robust basis for describing radionuclide retardation in geochemically dynamic environments. In a companion report (Zavarin and Bruton, 2004), a database of radionuclide surface complexation reactions for calcite and iron oxide minerals was developed. In this report, a second set of reactions is developed: surface complexation (SC) and ion exchange (IE) to aluminosilicate minerals. The most simplified surface complexation model, the one-site non-electrostatic model (NEM), and the Vanselow IE model were used to fit a large number of published sorption data and a reaction constant database was developed. Surface complexation of Am(III), Eu(III), Np(V), Pu(IV), Pu(V), and U(VI) to aluminum oxide, silica, and aluminosilicate minerals was modeled using a generalized approach in which surface complexation to aluminosilicate >SiOH or >AlOH reactive sites was considered equivalent to the reactivity of aluminum oxide and silica reactive sites. Ion exchange was allowed to be mineral-dependent.
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