Unsaturated flow is defined as the flow of water in that portion of the Earth that is between the water table and the land surface (Figure-1) (Izbicki, 2000). In the unsaturated zone, not all of the void (pore) space is filled with water. Instead some pores are filled with water, and others with air. Unsaturated flow is primarily driven by capillary and gravitational forces. The gravitational force is constant near the Earth's surface and independent of material properties. On the other hand, capillary forces are dependent on fluid properties, solid properties, and the geometry of the pore space (i.e., holes). The magnitude of capillary forces is thus variable, and can be much smaller or much larger than the gravitational force. An example of capillary forces exceeding gravity is capillary rise in a thin glass tube, where water is lifted upwards against the force of gravity. Most Earth materials are hydrophilic, which means that water adheres to them rather than being repelled. As a result, water in the unsaturated zone is preferentially drawn into small spaces over large ones, which tend to be occupied by air. In un-fractured rock, water occupies a three-dimensional network of interconnected small pores that occur between the grains that make up the rock matrix. The size and nature of these pores varies substantially between different rock types, and between specific units. .
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Unsaturated Flow and Toxic Waste Transport.
Unsaturated flow in fractured rocks is of considerable interest with respect to the protection and development of water supplies in semi-arid regions. These concerns are due to diminishing supplies and possible contamination from toxic waste. For example, there is a considerable amount of controversy regarding the proposed high level radioactive storage site at Yucca Mountain, Nevada, which centers on fractured volcanic rock. Opponents to this high level nuclear storage facility fear radioactive contamination may leak from storage containers, and infiltrate the ground water aquifer through the complex fracture network within the volcanic tuffs (National Research Council, 1992).