Impact of Mobile-Immobile Water Domains on the Retention of Technetium (Tc-99) in the Vadose Zone

The transport of technetium (Tc-99), like many other radionuclides, is of interest due to the potential for human exposure and impact on ecosystems. Technetium has been released to the environment through nuclear weapons testing, nuclear power production, and nuclear fuel reprocessing; as a result, further spreading of Tc-99 is a concern at DOE sites across the US. Specifically, technetium is a contaminant of concern at Hanford, Savannah River, Idaho, and Oak Ridge National Laboratory. The current body of work conducted on Tc-99 has provided a wealth of information regarding the redox relationship and stability of the mineral phases, however little work has been conducted on the physical transport of the highly soluble pertechnetate oxyanion, in the subsurface. Current conceptual models do not explain the presence and persistence of the anion in deep vadose zone environments such as the Hanford site. In an oxic reducing environment with low organic content, the residence time of technetium in the soil would be expected to be near zero, due to its low sorption. Surprisingly, nearly 50 years after the release of contamination at the site, much of the element has persisted in the subsurface in its most mobile form. As new in situ remediation technologies are developed and remediation continues, it is necessary to understand implications for Tc-99 transport and mobilization in unsaturated systems. Specifically, foam and gas phase remediation could potentially alter established flow regimes by changing moisture content and surface reactivity of sediments; therefore, it is imperative to understand fundamental transport properties of Tc-99 in the context of physical and chemical constraints. Using an Unsaturated Flow Apparatus (UFA) we have conducted a series of experiments to examine the impact of mobile-immobile domains on the transport of Tc-99. By varying sand/silt ratios and saturations we examined how changes in pore morphology and moisture content impact the transport of Tc-99 within our experimental system. Results demonstrating the correlation between sediment texture, pore morphology and soil moisture content on physical impediments to Tc-99 transport will be presented.