Vapor Diffusion as a Mechanism for Moisture Redistribution in Unsaturated Flow Systems with Variable Salinity

Understanding movement of saline waste solutions is important for assessing the contaminant migration near leaking waste storage tanks in the unsaturated zone at the Hanford site (Washington, USA). Laboratory experiments presented by Selker and collaborators at AGU 2000 Fall Meeting have shown that vapor diffusion can be an important mechanism for redistributing moisture in unsaturated flow systems with salinity gradients. The effect arises from the dependence of vapor pressure on salinity. Vapor pressures of salt solutions generally decrease with salt concentration, which results in a vapor pressure gradient and induces vapor diffusion from low to high salt concentration regions. Vapor then condenses in the high concentration regions and increases the liquid water saturation there. We have performed numerical experiments to study this salinity-driven moisture redistribution. Systematic simulation studies use different values of thermal conductivity, permeability, and temperature, to illustrate conditions and parameters controlling these processes. Results indicate that significant effects occur rapidly (hours) over rather small spatial scales (mm to cm), requiring very fine space discretization. The rapid occurrence is consistent with laboratory experiments of Selker and collaborators that show that significant transport of water of pre-wetted sand into highly saline NaNO3 solution plumes takes place in a matter of hours. Heat conduction plays a very important role in this salinity-driven vapor diffusion by maintaining a nearly constant temperature. The smaller the permeability, the more water is transferred into the saline region. Effects of permeability on water flow are complicated by effects of capillary pressure and tortuosity. The salinity driven-fluid flow is more significant at higher temperature. This work was supported by the U.S. Department of Energy under Contract No. DE-AC03-76SF00098 through Memorandum Purchase Order 248861-A-B2 between Pacific Northwest National Laboratory and Lawrence Berkeley National Laboratory.