A Non-Electrostatic Surface Complexation Approach to Modeling Radionuclide Migration at the Nevada Test Site: I. Iron Oxides and Calcite

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) project, radionuclide transport away from various 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 K{sub d} approach, surface complexation (SC) 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. The interaction between several radionuclides considered relevant to the UGTA project and iron oxides and calcite are examined in this report. The interaction between these same radionuclides and aluminosilicate minerals is examined in a companion report (Zavarin and Bruton, 2004). Selection criteria for radionuclides were based on abundance, half-life, toxicity to human and environmental health, and potential mobility at NTS (Tompson et al., 1999). Both iron oxide and calcite minerals are known to be present at NTS in various locations and are likely to affect radionuclide migration from the near-field. Modeling the interaction between radionuclides and these minerals was based on surface complexation. The effectiveness of the most simplified SC model, the one-site Non-Electrostatic Model (NEM), to describe sorption under various solution conditions is evaluated in this report. NEM reactions were fit to radionuclide sorption data available in the literature, as well as sorption data recently collected for the UGTA project, and a NEM database was developed. For radionuclide-iron oxide sorption, simple binary NEM reactions adequately fit most data without need for bidentate or ternary surface reactions. For example, the decrease in U(VI) sorption as a function of carbonate alkalinity was accounted for by aqueous uranyl-carbonate complex formation. In some cases, the one-site NEM could not fit sorption data at high and low radionuclide surface loads. This failure results from multiple surface sites with varying sorption affinity as well as electrostatic effects not accounted for by our model. Calcite sorption reactions were modeled as NEM surface exchange reactions. Uncertainty in these surface exchange reactions is relatively high because few radionuclide-calcite sorption data are available. Nevertheless, 1D reactive transport simulations in alluvium comparable to that in Tompson et al. (1999) and based on the NEM database developed in this report, suggest that migration of Sm(III), Eu(III), Np(V), Pu(IV,V), and Am(III) will be significantly retarded by sedimentary carbonates. Though iron oxide minerals are usually thought to be the major contributors to radionuclide retardation, calcite will also be of paramount importance in areas where it is present (e.g. calcareous regions of NTS as well as buried carbonate paleosols at the 200 Area of the Hanford Site (south-central Washington, U.S.A.), calcareous soils of the Marshall Islands, and weathered cementitious materials at waste disposal facilities). The NEM approach provides a simple yet robust mechanistic basis for modeling radionuclide migration with a minimum number of fitting parameters.