Rocks and Water Series 2
Acknowledgement: United States Geological Survey
All water beneath the land surface is referred to as underground water (or subsurface water).
The equivalent term for water on the land surface is surface water. Underground water occurs in two different zones. One zone, which occurs immediately below the land surface in most areas, contains both water and air and is referred to as the unsaturated zone. The unsaturated zone is almost invariably underlain by a zone in which all interconnected openings are full of water. This zone is referred to as the saturated zone.
Water in the saturated zone is the only underground water that is available to supply wells and springs and is the only water to which the name ground water is correctly applied.
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Recharge of the saturated zone occurs by percolation of water from the land surface through the unsaturated zone. The unsaturated zone is, therefore, of great importance to ground-water study. This zone may be divided usefully into three parts: the soil zone, the intermediate zone, and the upper part of the capillary fringe.
The soil zone extends from the land surface to a maximum depth of three to six feet and is the zone that supports plant growth. It is crisscrossed by living roots, by voids left by decayed roots of earlier vegetation, and by animal and worm burrows. The porosity and permeability of this zone tend to be higher than those of the underlying material. The soil zone is underlain by the intermediate zone, which differs in thickness from place to place depending on the thickness of the soil zone and the depth to the capillary fringe.
The lowest part of the unsaturated zone is occupied by the capillary fringe, the subzone between the unsaturated and saturated zones. The capillary fringe results from the attraction between water and rocks. As a result of this attraction, water clings as a film on the surface of rock particles and rises in small-diameter pores against the pull of gravity.
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