| Popis: |
Soil desiccation has been suggested as an economically viable alternative for stabilizing water soluble contaminants in deep vadose zones such as at the Hanford Site. This approach would result in a large volume of the vadose zone being dried out in an effort to eliminate the transport mechanism for aqueous phase contaminants. Protecting the top and sides of the desiccated vadose zone from rewetting is a realistic possibility; however, it would be extremely difficult to restrict the upward migration of water from beneath the desiccated zone. One scenario related to the rewetting of the desiccated soil involves the upward migration of water vapor into the desiccated soil and subsequent adsorption of that vapor onto the soil particles. A series of laboratory experiments was conducted to better understand the processes involved in the migration of water vapor through desiccated soils. Specifically, we considered the relative importance of diffusion and density-driven advection. Water vapor was introduced between a pair of vertically stacked columns filled with desiccated media (empty, BB-, gravel-, and sand- filled columns) to compare the upward and downward migration of the water vapor. Experiments in all the test media confirm that water vapor migrates preferentially in the upward direction relative to the downward direction. Density differences between humid air (less dense) and dry air (more dense) cause the humid air to rise relative to the dry air (i.e., density-driven advection) in the columns. This combines with vapor phase diffusion leading to increased rates of movement in the upper column. The opposite (i.e., decreased rates) occurs when the vapor moves downward, as density-driven advection works against diffusion. These increased rates of vapor migration in the upward direction led to increased wetting of the media over that in the downward direction. |
