Unsaturated transport parameters of an aridisol and its components

Radioactive hazardous and mixed-waste disposal, storage, and migration often occurs in the vadose or unsaturated zone, specifically in aridisols, i.e., soils of dry climates having low organic matter, an ochric epipedon, and an extremely low recharge. Predictive modeling of unsaturated contaminant transport in these soils has been hampered by a lack of data on the transport parameters under the low moisture conditions characteristic of these aridisols in the field. This is because traditional experimental methods of investigating unsaturated soils require very long time periods to attain hydraulic steady states using gravity drain. The three primary transport parameters are hydraulic conductivity K(θ), diffusion coefficient D(θ), and retardation factor Rf. The K(θ), D(θ), and Rf, were measured for some aridisols of the Hanford site as a function of water content using the new Unsaturated Flow Apparatus (UFA trademark). The UFA rapidly attains hydraulic steady state in soils over a wide range of water contents in minutes to hours instead of days to months and can easily attain field moisture characteristics. The samples, which exemplify the variations in the aridisol matrix, were various silts, medium to coarse sands, gravelly sands, and gravels of the Hanford site