Gas-solute dispersivity ratio in granular porous media as related to particle size distribution and particle shape

Measurements of solute dispersion in porous media is generally much more time consuming than gas dispersion measurements performed under equivalent conditions. Significant time savings may therefore, be achieved if solute dispersion coefficients can be estimated based on measured gas dispersion data. This paper evaluates the possibility for estimating solute dispersion based on gas dispersion measurements. Breakthrough measurements were carried out at different fluid velocities (covering the same range in Reynolds number), using O2 and NaCl as gas and solute tracers, respectively. Three different, granular porous materials were used: (1) crushed granite (very angular particles), (2) gravel (particles of intermediate roundness) and (3) Leca® (almost spherical particles). For each material, 21 different particle size fractions were used. Gas and solute dispersion coefficients were determined by fitting the advection-dispersion equation to the measured breakthrough curves and in turn used to calculate gas and solute dispersivities as a function of mean particle size (Dm) and particle size range (R) for the 63 particle size fractions considered. The results show that solute and gas dispersivities are related and that their ratio depends on both R and Dm. Based on these observations a simple model for predicting the dispersivity ratio from Dm and R, was proposed.