Fate and transport of radionuclides [uranium(VI), strontium, cesium] in vadose zone sediments at the Hanford site

Physical and chemical heterogeneities are inherent in subsurface environments due to varying: mineralogy, pore geometry, solution saturation, and solute concentration. The goals of this research were to study the heterogeneities that influence radionuclide fate and transport in the Hanford vadose zone. Hanford was established for nuclear weapons development and has significant subsurface contamination. Specific objectives were: (1) Investigate the influence of secondary precipitate formation on strontium and cesium fate and transport in sand under unsaturated conditions. (2) To evaluate pore scale processes of uranium release from sediment to pore water. (3) To investigate uranium release rates from contaminated Hanford sediments reacted with Columbia River water. (4) To quantify diffusion limited mass transfer of soluble U into micro-fractures.;Waste storage tanks have leaked into the subsurface causing a change in saturation and a solute gradient of leachate into pore water. Simulations displayed non-ideal transport of strontium and cesium in solution unsaturated flow. The incorporation of strontium into secondary precipitates can be fast, leading to retarded transport of strontium. Cesium incorporation into neoformed feldspathoids will occur slowly and depends on mineral transformation and ripening.;Legacy contamination of the Hanford 300 Area resulted in persistent uranium release from capillary fringe sediments. Rising river stages flood these sediments perturbing the chemical state of the system. Simulations showed uranium release after contact with river water was dependent on contact time. Uranium concentrations in micro-pores were higher than in larger pores, causing a diffusion gradient. When solution-to-solid ratio was narrow, the water became supersaturated with respect to uranium, and it precipitated. Initial release of uranium was 6 to 9% of the total. As solution-to-solid ratio widened, portion of uranium released increased steadily, suggesting a diffusion rate limited release of uranium. These results imply that uranium can remain trapped in the sediments for extended periods of time.;Diffusion rate limited release of contaminants due to complex geometry contributes to control of Hanford subsurface contamination. We investigated diffusion of colloids into micro-channels. Colloids (0.2 microm) diffusion into larger factures (10 to 30 microm) could be modeled using the calculated infinite linear coefficient but diffusion into smaller fractures (3 to 5 microm) was reduced by as much as 2.68 times.