A new micromechanics analysis of solute diffusion in unsaturated granular materials is put forward. This permits predicting the percolation effect. To do so, the pore water is divided up into four phases that account for the different spatial distributions and diffusion properties: interconnected capillary water, isolated capillary water, wetting layer and water film. A parameter denoted as connectivity ratio is introduced to account for the connectivity of the capillary pore water. Our model agrees well with experimental results on unsaturated sands and glass beads from literature. It also permits interpreting the physical meaning of the saturation exponent of Archie's law: The latter is found to be correlated with the connectivity ratio of the unsaturated granular materials.