A modified rate equation for the dissolution of minerals in the unsaturated zone was proposed using a specific solid–water interfacial area (asw) varying as a function of water saturation (Sw). Vadose zone phenomena, such as infiltration, were discussed as example scenarios where such reformulation may be useful. A constitutive relationship for the asw–Sw was derived, and used to formulate an approximate model of the proposed rate equation. Implementation of this new model in the reactive transport code CRUNCH was used to evaluate the influence of a changing asw on reactive transport behavior of silica in the vadose zone. These simulations clearly demonstrate that a changing asw can have a significant impact on the silica distribution profiles in the vadose zone. This is to be expected because changes in asw are substantial in the dry Sw range, whereas they are only moderate in the relatively wet region. These findings have significant implications for contaminant transport in the vadose zone, especially in arid regions. Ignoring the effect of a changing asw is likely to result in an overestimation of the net contaminant fluxes in these cases, which leads to highly conservative risk assessments. In case of nutrient flux calculations coupled with dissolution phenomena, it may lead to an overestimation of nutrient availability. Additional theoretical and experimental verification of the proposed models would be valuable.