Estimation of Diffusion Coefficients for Salt Transport in Porous Media under Unstable Conditions Using Numerical Modeling

Typical values of diffusion coefficients used to model salt transport under stable conditions cannot be used to model salt transport under unstable conditions since unstable flow also includes the effect of gravity and fingering. Diffusion coefficients used under unstable conditions are usually termed as apparent diffusivity (D). This study determines the apparent diffusivity for salt transport in a saturated porous media (sandy soil, hydraulic conductivity=7.6 m/d), under unstable conditions, by comparing numerically simulated and laboratory measured source depletion curves at the salt source in a semi-infinite, homogeneous, and isotropic porous media. The measured source depletion curves were obtained through eight experiments, conducted over 12 to 15 days, using a rectangular Plexiglass model (1.80 m, 0.61 m, 0.61 m) in which a saline solution was placed over the porous media saturated with freshwater. The initial salt concentrations in the source area ranged from 4.5 kg/m³ to 36 kg/m³ while the source height ranged from 4.5 cm to 11.70 cm. The salt concentrations within the source were periodically measured using a YSI salinity meter. The numerical modeling was conducted using a three-dimensional SEAWAT model. The values of the apparent diffusivity were changed in the numerical model until the numerically simulated source depletion curves matched the laboratory measured curves. The apparent diffusivity values ranged from 2.026*10-09 to 4.282*10-08 m2/s. Analysis showed that there is a direct linear correlation between the initial salt concentration, the source height, and apparent diffusivity (D).