Toxic Substances Hydrology Program
Unsaturated Zone - "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 ... In most areas, the unsaturated zone is composed of horizontal or nearly horizontal layers. The movement of water, on the other hand, is predominantly in a vertical direction." - Heath, 1983
Unsaturated Zone - "The unsaturated zone, which replaces the terms 'zone of aeration' and 'vadose zone,' is the zone between the land surface and the deepest water table. It includes the capillary fringe. Generally, water in this zone is under less than atmospheric pressure, and some of the voids may contain air or other gases at atmospheric pressure. Beneath flooded areas or in perched water bodies the water pressure locally may be greater than atmospheric." - Committee on Redefinition of Ground-Water Terms, 1988
Unsaturated Zone - "The zone between land surface and the capillary fringe within which the moisture content is less than saturation and pressure is less than atmospheric. Soil pore spaces also typically contain air or other gases. The capillary fringe is not included in the unsaturated zone." - U.S. Environmental Protection Agency, 2009
Unsaturated Zone - "The unsaturated zone is characterized by pore spaces that are incompletely filled with water. Pore space not filled by water is filled with gas ... The amount of water present in an unsaturated zone varies widely and is highly sensitive to climatic factors." - Chapelle, 2001
Unsaturated Zone - "The unsaturated zone is frequently divided into three components. The first of these is the soil zone, generally a meter or two thick, which contains living roots and which supports plant growth. The porosity and permeability of the soil subzone is generally higher than that of underlying material, which varies in thickness from place to place, is often referred to as the intermediate zone (not to be confused with the 'intermediate' zones of saturated flow systems) and consists of sediments or rocks that have not been exposed to extensive pedogenic (soil-forming) processes. The boundary between the unsaturated zone and the saturated zone is termed the capillary fringe." - Chapelle, 2001
USGS Information on Unsaturated Zone
Toxics Program Unsaturated Zone Research
Related Fact Sheets
Chapelle, F.H., 2001, Ground-water microbiology and geochemistry (2 ed.): New York, NY, John Wiley & Sons, Inc., 477 p.
Committee on Redefinition of Ground-Water Terms, 1972, Definitions of selected ground-water terms--Revisions and conceptual refinements: U.S. Geological Survey Water-Supply Paper 1988, 26 p.
Heath, R.C., 1983, Basic ground-water hydrology: U.S. Geological Survey Water-Supply Paper 2220, 86 p.
U.S. Environmental Protection Agency, 2009, Glossary of technical terms: U.S. Environmental Protection Agency, access date July 22, 2010.
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