Fate and Transport of Radionuclides Beneath the Hanford Tank Farms: Unraveling Coupled Geochemical and Hydrological Processes in the Vadose Zone

Although the accelerated transport of {sup 99}Tc, {sup 137}Cs, and {sup 235}U within the vadose zone beneath the 200-West Area of the Hanford tank-farm area has been recognized, the mechanisms responsible for the vertical migration of the radionuclides is unclear. Does horizontal stratification enhance the lateral movement of contaminants, which in turn enhances vertical preferential flow due to perched water dynamics? Do physical heterogeneities, such as stratification and pore regime connectivity, influence the retardation and degree of geochemical nonequilibrium during contaminant transport? Recent modeling efforts of the problem have failed to yield answers to this question since they are inadequately parameterized due to the lack of sufficient quality data. Fundamental experimental research is needed that will improve the conceptual understanding and predictive capability of radionuclide migration in the Hanford tankfarm environment. Since geochemical reactions are directly linked to the system hydrodynamics, coupled geochemical and hydrological processes must be investigated in order to resolve the key mechanisms contributing to vadose zone and groundwater contamination at Hanford. Our research group has performed extensive investigations on time-dependent contaminant interactions with subsurface media using dynamic flow techniques which more closely simulate conditions in-situ. Of particular relevance to this proposal is the work of Barnett et al. (2000) who showed that U(VI) transport through Hanford sediments was highly retarded and extremely sensitive to changes in pH and total carbonate. What remains elusive are the geochemical mechanisms for uranium retention-necessary information for accurately simulating transport-and are thus the focus of this study. The experimental and numerical results from this research will provide knowledge and information in previously unexplored areas of vadose zone fate and transport to support EM's performance/risk assessment and decision-making process for Tank Farm restoration. By unraveling fundamental contaminant transport mechanisms in complex porous media, we provide an improved conceptual understanding and predictive capability of a variety of vadose issues within the DOE system