WATER AND SALT MOVEMENT IN RELATIVELY DRY SOILS (VAPOR TRANSPORT)

Water and solute movement in relatively dry soils is of interest to agronomists and engineers working in arid regions of the world. This interest derives from a need to understand nutrient transport to plant roots and to provide for the containment of hazardous wastes, among other applications.; The primary focus of the research presented here is the development and demonstration of experimental techniques capable of defining the minimum solution content at which salt transport is possible, and determination of the liquid and vapor transport coefficients. One technique involves a relatively simple procedure using a single-source gamma attenuation system for measuring isothermal liquid-vapor diffusivities in the dry water-content range of a variety of soils. Liquid-vapor diffusivity functions are presented for five different materials ranging in texture from fine sands to a clay loam. Based on their diffusivity functions and vapor adsorption isotherms, two soils (a loamy sand and a silt loam) were chosen as representative of soils demonstrating relative extremes in vapor transport phenomena and vapor adsorption characteristics, and subjected to further study.; The second experimental technique utilized a dual-source gamma attenuation system capable of measuring changing solution contents and salt concentrations in unsaturated soils. Application of this technique to the loamy sand and silt loam illustrated substantial movement of constituent water out of solution by evaporation and subsequent condensation in the silt loam, and the comparative absence of this phenomena in the loamy sand. Salt adsorption complexities were also present in the silt loam, but tests using adsorbing and nonadsorbing tracer salts yielded similar results.; The principal finding relates to identification of the significant water vapor transfer-adsorption abilities of the silt loam. In addition, it was found that no salt movement was detected at solution contents below a particular finite value, suggesting the possibility of immobile liquid films at solution contents below this value. Conversely, salt movement was detected at all measured solution contents greater than the minimum detection limit of solution content in the loamy sand. Various ramifications of these findings are discussed with respect to application of transport equations, adsorption experiments, and solution displacement investigations.