Diffusion coefficients in gravel under unsaturated conditions

Diffusion coefficients were experimentally determined in unsaturated gravel to evaluate the effectiveness of gravel as a diffusion barrier to ionic transport in the vadose zone. Water contents were fixed by use of an ultracentrifuge with an ultralow constant rate flow pump supplying solution to the sample via a rotating seal. Once the gravel was at hydraulic steady state, the electrical conductivity was measured, and the diffusion coefficient calculated using the Nernst-Einstein equation. Diffusion coefficient values for potassium ion (De) in four types of angular gravel ranged from 1.7 × 10−11 m2/s (1.7 × 10−7 cm2/s) for a 6.3–9.5 mm angular granitic gravel at a volumetric water content of 5.5% to 2.2 × 10−14 m2/s (2.2 × 10×10 cm2/s) in a 4.0–6.3 mm quartzite gravel at a volumetric water content of 0.47%. Variations in De values resulted primarily from differences in water content which depends on gravel type and particle size.