Unsaturated Zone

The unsaturated zone is the zone between the land surface and the regional water table. It includes the capillary fringe and may also include localized perched ground water. Pore-water pressure in the unsaturated zone is less than atmospheric, except for perched ground water. Except for the capillary fringe and perched ground water, pores in the unsaturated zone contain both water and air. The unsaturated zone differs from the saturated zone, in which pores contain water at greater than atmospheric pressure and are almost always completely filled with water. The unsaturated zone is also called the vadose zone and the zone of aeration.

The unsaturated zone is a major focus of many contaminated-site investigations because contaminants often have to pass through the unsaturated zone before they get to the ground-water resources or the surface environment. Furthermore, many contaminants are accidentally placed or intentionally disposed of in the unsaturated zone.

Unsaturated Zone Headlines

• Ground-Water Recharge Affects Fate of Petroleum Contaminant Plumes
• Desert Plants Reveal Contaminant Transport Pathways
• New Model Improves Understanding of the Transport of Carbon Isotopes in the Unsaturated-Zone
• Using Plants To Detect Tritium Contamination
• High Nitrate in the Desert? What's Going On?
• Are Deserts Still Drying Out Since the Ice Age?
• The Atmosphere - A Potential Source of MTBE to Ground Water

Toxics Program Unsaturated Zone Research

• Waste Disposal and Contaminant Migration in the Arid Southwest - Amargosa Desert Research Site, Nevada
• Unsaturated Zone Field Studies - Unsaturated Zone Transport Models
• Unsaturated Zone Flow Project
• Unsaturated Zone Flow Project Lab Methods
• Mathematical Models For Simulating Flow and Transport in the Unsaturated Zone

More Information

• General Facts and Concepts about Ground Water, Sustainability of Ground-Water Resources: USGS Circular 1186
• Unsaturated-Zone Flow: Definitions and Details
• Ground Water Issues: Unsaturated Zone
• USGS Unsaturated Zone Interest Group
• Geology School Definitions: Unsaturated Zone, Short QuickTime© video clips from the USGS Coastal and Marine Geology Program InfoBank

Related Fact Sheets

• Simulating Transport of Volatile Organic Compounds in the Unsaturated Zone using the Computer Model R-UNSAT, USGS Fact Sheet FS-019-98
• Waste Burial in Arid Environments--Application of Information From a Field Laboratory in the Mojave Desert, Southern Nevada, USGS Fact Sheet FS-179-95
• Effects of recharge on the transport of agricultural chemicals at the Princeton, Minnesota Management Systems Evaluation Area, 1991-92: USGS Open-File Report 93-79

References

• Heath, R.C., 1983, Basic ground-water hydrology: U.S. Geological Survey Water-Supply Paper 2220, p. 84.
• Lohman, S.W., and others, 1972, Definitions of selected ground-water terms—revisions and conceptual refinements: U.S. Geological Survey Water-Supply Paper 1988, 21 p.
• Meinzer, O.E., 1923, The occurrence of ground water in the United States with a discussion of principles: U.S. Geological Survey Water-Supply Paper 489, p. 321.
• National Research Council, 2001, Science and Technology for Environmental Cleanup at Hanford: Washington, D.C., National Academy Press, 180 p.

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