K. Prasad Saripalli

A modified rate equation for the dissolution of minerals in the unsaturated zone was proposed using a specific solid–water interfacial area ( a _sw ) varying as a function of water saturation ( S _w ). Vadose zone phenomena, such as infiltration, were discussed as example scenarios where such reformulation may be useful. A constitutive relationship for the a _sw – S _w 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 a _sw on reactive transport behavior of silica in the vadose zone. These simulations clearly demonstrate that a changing a _sw can have a significant impact on the silica distribution profiles in the vadose zone. This is to be expected because changes in a _sw are substantial in the dry S _w 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 a _sw 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.

Determination of aqueous phase diffusion coefficients of solutes through porous media is essential for understanding and modeling contaminant transport. Prediction of diffusion coefficients in both saturated and unsaturated zones requires knowledge of tortuosity and constrictivity factors. No methods are available for the direct measurement of these factors, which are empirical in their definition. In this paper, a new definition for the tortuosity factor is proposed, as the real to ideal interfacial area ratio. We define the tortuosity factor for saturated porous media (τ_s) as the ratio S/S_o (specific surface of real porous medium to that of an idealized capillary bundle). For unsaturated media, tortuosity factor (τ_a) is defined as 2_awla (ratio of the specific air‐water interfacial area of real and the corresponding idealized porous medium). This tortuosity factor is suitably measured using sorptive tracers (e.g., nitrogen adsorption method) for saturated media and interfacial tracers for unsaturated media. A model based on this new definition of tortuosity factors, termed the interfacial area ratio (IAR) model, is presented for the prediction of diffusion coefficients as a function of the degree of water saturation. Diffusion coefficients and diffusive resistances measured in a number of saturated and unsaturated granular porous media, for solutes in dilute aqueous solutions, agree well with the predictions of the IAR model. A comparison of permeability of saturated sands estimated based on τ_s and the same based on the Kozeny‐Carman equation confirm the usefulness of the τ_s parameter as a measure of tortuosity.