Flow in partially saturated porous media, even at moderately unsaturated conditions (e.g., gravity-drained media) takes place through pore sizes smaller than a few micrometers. Hence, practical unsaturated solute transport regimes invariably involve water flow in thin films adjacent to active surfaces. Conventional models for liquid distribution, flow, and solute transport in unsaturated porous media are biased by the geometrical representation of media pore space as a "bundle of cylindrical capillaries". The picture is further distorted by the "empty-filled" approach where a complete desaturation of pores (capillaries), based on the capillary potential, is assumed. Experimental and theoretical evidence suggests a radically different liquid configuration for unsaturated conditions whereby liquid-filled corners and pendular spaces form the backbone for hydraulic connectivity and are further connected through thin liquid films coating exposed solid surfaces. Consequently interfacial forces play a larger role in transport and reactivity than assumed by conventional models. Because the ranges of interfacial forces may exceed film thicknesses to the extent that bulk solution may not be present, hydrodynamic and electrochemical aspects are inseparable and must be considered simultaneously. A new idealized unit cell representing media pore space geometry and its surface area is proposed as an alternative to the bundle of cylindrical capillaries model. To account for film adsorption and capillary condensation, a unitary approach to modeling these two processes in porous media is proposed and applied to the newly assumed basic pore space geometry. Interface science formalism is used to calculate film thickness as a function of thermodynamic conditions, pore space geometry, and specific surface area. The new conceptual framework should improve the representation of liquid configuration at low water contents and the associated hydraulic and transport properties. In addition, it provides a rational basis for considering the important interplay among viscous, electrical and molecular forces operating within thin liquid films. The understanding of these important pore-scale processes is essential to physically-based interpretation of complicated behavior of solutes at larger scales of interest. Details of thin film hydrodynamics and induced interfacial phenomena, and their effects on reactive solute transport at low saturation levels will be a subject of future studies.
