Laboratory experiments to characterize radiochloride diffusion in unsaturated soils

Diffusion transport of 36Cl was examined in seven soils under unsaturated conditions in tubes packed with two portions of each soil having different 36Cl activity concentrations. Apparent diffusion coefficients (Da) derived from diffusion profiles varied within a narrow range (from 3×10−10 to 7×10−10 m2 s−1) confirming the minor effect of soil properties on the diffusion of a non-reactive radionuclide like 36Cl. Instead, packing conditions had a major effect. Solid–liquid distribution coefficients (Kd) derived from Da (0.02–0.2 L kg−1) were systematically lower than those obtained from batch experiments (0.6–1.0 L kg−1), but with a similar variation pattern among soils. The low values of Kd (Cl) confirmed an almost negligible radiochloride–soil interaction.