Chemical reaction and Co-60 retardation in unsteady, unsaturated soil water flow: the effect of clay content

The ability of the regolith to adsorb and retard radioactive and other noxious chemicals is important when selecting repository sites. Because of its surface charge and great specific surface, the clay in soil contributes significantly to this retardation. This paper illustrates a novel analysis of unsteady water and solute flow in unsaturated soil that quantifies these effects. Analysis and experiments suggest that the use of a clay-based space-like coordinate permits us to generalise results to account for variation in clay content and soil structure across many materials. Results imply that clay content, mineralogy, and charge permit quantitative material classification according to their ability to retard cationic wastes. The approach appears to offer significant economies in selecting materials where adsorption and retardation of cation pollutants is desired.Extension of that analysis (Smiles 2000) shows that, under these experimental conditions, the moisture ratio (the volume of water per unit volume of soil solid) and Cw will also preserve similarity if a space-like coordinate defined by the distribution of the soil solid volume is used. This latter formulation is strain-independent and applies equally well to non-swelling systems or to systems where the volume changes with water content (Smiles and Rosenthal 1968). This approach is used in this paper, but with the trivial extension that v is replaced by the water mass fraction, θg (gravimetric water content), and the cumulative mass of solid per unit cross-section area is used as the space-like coordinate (Raats and Klute 1968). Consistent with this convention, solute concentrations are expressed per unit mass of solvent (water) and of solid depending on the circumstances. Soil, at an initial water content, θg(i), was packed into a sectioned cylindrical column of internal diameter 20 mm. The soil was added in increments of 2-3 g and packed using a small drop-hammer to ensure uniformity. Saturated CaSO4 solution containing 4.8 kBq/g of H-3 (as tritiated water) and 206 Bq/g of Co-60 was applied at zero water potential to one end of the column. After suitable elapsed times the experiments were terminated and the columns sectioned. The initial water contents, average bulk, densities, and times at which the experiments were terminated are found. Each moist section was weighed and then counted using gamma spectrometry, to determine Co-60 present. Tritium was then measured in a water extract 'based on 2.5 parts water to 1 part soil. Dilutions were tracked gravimetrically and all quantities were referred to the own-dry weight of soil in each section, which was determined following measurement of Co-60 and H-3. Six experiments were performed. Copyright (2001) CSIRO Publishing