FILM HYDROSTATICS AND FLOW UNDER CONTROL BY CAPILLARITY AND ADSORPTION

Film flow is sometimes invoked as an explanation for observed anomalous transport in the unsaturated zone. Physical considerations can be applied to better identify conditions where this mechanism is likely to be important. This overview of some past and recent studies begins with considerations of constraints imposed by local equilibrium requirements, which help discern when water films are likely to be significant conduits for flow and transport in the unsaturated zone. This equilibrium framework is used to examine some scenarios for droplet formation in fractures, and the possibility for generating fast flow paths through progressively convergent, gravity-driven flow paths through the unsaturated zone. The role of mineral surface microtopography in supporting potentially thick, transmissive capillary water films is reviewed, with emphasis on the importance of constraints imposed by matric (capillary) potential. Some models for how adsorbed water film thickness depends on matric potential are examined to illustrate the high level of hydraulic resistance developed in unsaturated pores and fractures, especially at low (more negative) matric potentials. Some factors imparting high levels of uncertainty in predicting film flow, and therefore representative of areas for future research, are also discussed.