Hydraulic properties of adsorbed water films in unsaturated porous media

Adsorbed water films strongly influence residual water saturations and hydraulic conductivities in porous media at low saturations. Hydraulic properties of adsorbed water films in unsaturated porous media were investigated through combining Langmuir's film model with scaling analysis, without use of any adjustable parameters. Diffuse double-layer influences are predicted to be important through the strong dependence of adsorbed water film thickness (f) on matric potential (psi) and ion charge (z). Film thickness, film velocity, and unsaturated hydraulic conductivity are predicted to vary with z(-1), z(-2), and z(-3), respectively. In monodisperse granular media, the characteristic grain size (lambda) controls film hydraulics through lambda(-1) scaling of (1) the perimeter length per unit cross-sectional area over which films occur, (2) the critical matric potential (psi(c)) below which films control flow, and (3) the magnitude of the unsaturated hydraulic conductivity when psi < psi(c). While it is recognized that finer-textured sediments have higher unsaturated hydraulic conductivities than coarser sands at intermediate psi, the lambda(-1) scaling of hydraulic conductivity predicted here extends this understanding to very low saturations where all pores are drained. Extremely low unsaturated hydraulic conductivities are predicted under adsorbed film-controlled conditions (generally < 0.1 mm a(-1)). On flat surfaces, the film hydraulic diffusivity is shown to be constant (invariant with respect to psi).