Comparison of laboratory methodologies for evaluating radiostrontium diffusion in soils: Planar-source versus half-cell methods

Abstract

A planar-source method, initially designed to obtain diffusion coefficients in compacted clay, is adapted here to determine the apparent diffusion coefficient (D a) of radiostrontium in soils representative of the Spanish territory. Experiments were carried out by varying the moisture content (F moist), and bulk dry density (ρ bulk) of the soil samples, in order to study the influence of these soil packing parameters on D a values. The moisture in the soil samples was established as the percentage of occupancy of each soil's field capacity (OFC). For a similar OFC, D a values in the examined soils ranged by approximately one order of magnitude (e.g. from 6.2 × 10 − 11 to 6.5 × 10 − 12 m 2 s − 1, at 100% of OFC; from 3.0 × 10 − 11 to 3.8 × 10 − 12 m 2 s − 1, at 60% of OFC). For a given soil, D a values increased when water content was increased. F moist, and tortuosity (τ) explained D a variability, with R 2 values usually over 0.9. However, no good simple or multiple regressions between the soil packing parameters and D a were obtained with the whole dataset of all soils, which indicated that soil sorption capacity affects the diffusion of reactive radionuclides in soils. The inclusion of calculated K d values in the multiple regressions improved the correlations in all cases. Finally, D a values were compared with those obtained by the application of a half-cell method. The values of D a obtained by the planar-source methods were systematically lower than the half-cell ones, with a good correlation between the D a derived from both methods (R 2 = 0.98).