Hydraulics of Water Films in the Subsurface (Invited)

Water films coating mineral surfaces occur under very wide ranges of conditions in partially saturated porous and fractured media. Thicknesses of water films confined between solid surfaces and the nonwetting fluid phase (in this case, air) in the subsurface commonly vary from nanometers to micrometers, but can become substantially thicker in locations where both pore (aperture) size and local water flow rates are sufficiently large. Diverse flow regimes are associated with this range of film thicknesses; including effectively hydrostatic, laminar, and turbulent conditions. Partly because water films are ubiquitous in the unsaturated zone and also difficult to measure, they are now often invoked to help explain preferential flow despite lack of direct experimental measurements on natural mineral surfaces. Given the variety of conditions where water films occur, it is important to understand how to quantitatively describe film flow. Here, we first broadly categorize different types of film flows, and identify conditions where they can occur. We then consider requirements for film flow to be amenable to description by the Richards equation. The nature of flow associated with water films depends on microscopic (film thickness and mineral surface microtopography) and macroscopic (system size, bulk hydraulic properties, and net infiltration rates) factors. Film transmissivity, the volumetric flow rate per unit surface length (transverse to flow) under unit hydraulic head gradient, depends strongly on film thickness. Therefore, knowledge of how film thickness depends on matric potential and pore (aperture) size is useful for predicting when water films can be significant conduits for flow and transport. The effective film thickness in geologic media is also strongly influenced by mineral surface microtopography, through both capillarity and adsorption on surfaces with scale-dependent roughness. Despite this complexity, physical constraints imposed by characteristics of solid surfaces, water-air interfaces, pores and fractures justify simplifications in many subsurface environments. Based on these constraints, we identify necessary conditions for water films to be important in flow and transport. In principle, the Richards equation is capable of describing film flow under many (but not all) of these conditions. This work was supported by the Director, Office of Science, Office of Basic Energy Sciences, the Chemical Sciences, Geosciences, and Biosciences Division under the Department of Energy Contract No. DE-AC02-05CH11231.