Radionuclide Movement during Unsteady, Unsaturated Soil Water Flow

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 piston- front; 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 containing⁶⁰Co²⁺ 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, self- similarity 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.