Radionuclide Movement during Unsteady, Unsaturated Soil Water Flow
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Radionuclide Movement during Unsteady, Unsaturated Soil Water Flow D.E. Smiles1 and G.D. McOrist2 1 CSIRO Land and Water, PO Box 1666, Canberra ACT 2601, Australia 2 Australian Nuclear Science & Technology Organization, PMB 1, Menai NSW 2234, Australia. Email: [email protected] ABSTRACT Absorption of aqueous solution by soil appears to result in piston-like displacement of the soil solution originally present by the invading solution. Diffusive redistribution of non-reactive solutes, relative to the water, occurs about that front when the solutions are of different chemical constitution. Adsorption of solute by the soil solid retards the solute front relative to the pistonfront; it also reduces the effective diffusivity of the solute relative to the water. Such behavior tests prediction of radionuclide transport and retardation during non-steady water flow in unsaturated soil based on measured distribution coefficients. Experiments, where initially uniform soil columns absorbed tritiated water containing 60 2+ Co from a source at constant water potential, illustrate these effects. Columns were sampled at significantly different elapsed times and the transient water content, water soluble salts, tritium and cobalt profiles determined. Theory anticipates, and experiment confirms, selfsimilarity of these profiles when data is graphed in terms of distance divided by the square root of time. Water-, and soil solid-based, material coordinates facilitate data analysis; measurement of absorption isotherms is also discussed. INTRODUCTION Siting and design of low level radioactive waste repositories are affected by the ability of the regolith material to immobilize or retard water soluble nuclides inadvertently released from the waste. Unsaturated, unsteady, water flow experiments in soil materials that characterize the repository permit direct measurement of these properties. These measurements also test surrogate estimates based, for example on adsorption isotherms at arbitrary water contents and solution salt concentrations. This paper describes an analysis of unsteady water and solute flow in unsaturated soil that has not hitherto been used in such studies. The approach uses space-like coordinates based on the distribution of the solid and of the water to provide insights into distributions of nuclides that are conventionally estimated in physical space. Soil material from a depth of 20 m in a potential nuclear repository site in the central-north of South Australia is used to illustrate the approach.
THEORY In porous materials such as soils, space-like material coordinates are often physically more useful and meaningful than are physical coordinates. This is because flow laws do not necessarily describe flux in physical space and also because that domain does not necessarily provide a simple basis for expressing concentrations of entities or for defining material balance. Darcy’s law, for example, describes flow of water, relative to the particles, in soil in response to a space gradient of potential [1]. T
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