Solute diffusion in variably saturated porous media plays an important role in the transport of contaminants and dissolved chemicals in the vadose zone. Various empirical and semiphysical models have been proposed to describe the saturation dependence of solute diffusion. In this study, we applied concepts from percolation and effective medium theories to develop a theoretical framework to model solute diffusion. Percolation theory, which has been successfully applied to describe saturation-dependent air permeability, electrical conductivity, and gas diffusion in porous media, provides a universal scaling, a power law in the water content (less a threshold value) with an exponent of 2.0. To evaluate our universal scaling, we used a database including 106 experiments (766 data points) from published studies and found good agreement between theory and measurement.

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