The unsaturated zone is that portion of the subsurface in which the intergranular openings of the geologic medium contain both water and air. The unsaturated zone, also known as the vadose zone or the zone of aeration, extends downward from the land surface to the top of the underlying saturated zone . Water in the pores of this zone is at a pressure that is lower than atmospheric pressure . Most of the water that eventually recharges the saturated zone must first pass through the unsaturated zone.
The movement of water in the unsaturated zone is dominated by capillary action. This tendency of a liquid to be drawn into interstices, is the result of cohesion of water molecules and adhesion of those molecules to the solid material forming the void. Smaller voids produce greater capillary forces frequently great enough to resist the downward force of gravity .
Water originating as precipitation at the land surface infiltrates into the unsaturated zone and forms a film on the surface of the material surrounding the pore. The adhesion of the water molecules nearest the solid material is greatest. As precipitation events occur, the thickness of the film increases with greater availability of infiltrating water, the capillary force is reduced in magnitude, and water molecules on the outer portion of the film may begin to flow under the influence of gravity. It is only through these transient variations in water availability, film thickness, and capillary pressure that water is able to migrate within the unsaturated zone.
Most plants utilize water from the unsaturated zone. Capillary water may be drawn from the pores by the plant's roots until the capillary forces can no longer be exceeded. This is the wilting point of the plant. The remaining capillary water can only be displaced through evaporation .
Research into the characteristics and dynamics of flow within the unsaturated zone is very active. A variety of human activities are controlled by, or may impact, the unsaturated zone. These include agriculture, subsurface pipeline and tank emplacement, and waste disposal . The thick unsaturated zone beneath Yucca Mountain in southern Nevada is currently slated for placement of the nation's high-level nuclear waste. The extremely thick unsaturated zone at the site would allow placement of the waste approximately 1,000 ft (305 m) above the saturated zone and nearly the same distance below the land surface. The nature of the unsaturated zone and its ability to isolate the waste from human contact is the subject of detailed investigation.
See also Hydrogeology; Porosity and permeability; Water table