What are the trends in the extent and condition of ground water and their effects on human health and the environment?
Importance of Ground Water
A large portion of the world's fresh water resides underground, stored within cracks and pores in the rock that make up the Earth's crust. Half of the U.S. population relies on ground water for domestic uses. In many parts of the United States, people rely on ground water for drinking, irrigation, industry, and livestock. This is particularly true in areas with limited precipitation, limited surface water resources, or high demand from agriculture and growing populations. Some ecological systems, such as wetlands or surface waters fed by springs and seeps, also rely on ground water.
Ground Water Extent
The extent of ground water refers to the amount available, typically measured in terms of volume or saturated thickness of an aquifer (body of ground water). Concerns related to extent include aquifer depletion and excessive ground water in aquifers.
- Aquifer depletion. Stressors that can deplete aquifers include changes in precipitation and snowmelt patterns; withdrawal of ground water for drinking, irrigation, and other human uses; and impervious paved surfaces that prevent precipitation from recharging ground water. Some deep aquifers may take thousands of years to replenish. Some consequences of aquifer depletion include:
- Lower lake levels or—in extreme cases—intermittent or totally dry perennial streams. These effects can harm aquatic and riparian plants and animals that depend on regular surface flows.
- Land subsidence and sinkhole formation in areas of heavy withdrawal. These changes can damage buildings, roads, and other structures and can permanently reduce aquifer recharge capacity by compacting the aquifer medium (soil or rock).
- Salt water intrusion. Changes in ground water flow can lead to saline ground water migrating into aquifers previously occupied by fresh ground water.
- Too much ground water. Some human activities, such as pumping water into the ground for oil and gas extraction, can cause an aquifer to hold too much ground water. Too much ground water discharge to streams can lead to erosion and alter the balance of aquatic plant and animal species.1
Ground Water Condition
The condition of ground water reflects a combination of physical, biological, and chemical attributes, which are influenced by both natural sources and human activities. Physical properties reflect patterns of flow—the volume, speed, and direction of ground water flow in a given location. Biological contaminants can include bacteria, viruses, protozoans, and other pathogens. Ground water can contain chemicals that occur naturally or that result from human activities.
- Stressors that affect ground water condition include application of pesticides and fertilizers to the land, waste from livestock and other animals, landfills, mining operations, and unintentional releases such as chemical spills or leaks from storage tanks. Some ground water has high levels of naturally occurring dissolved solids (salinity), or metals such as arsenic found in natural rock formations. These stressors can ultimately affect:
- The quality of water available for drinking, irrigation, or other human needs. Treatment may be needed to ensure that finished drinking water does not pose risks to human health.
- Ecological systems. Many fish species depend on spring-fed waters for habitat or spawning grounds.2,3 Aquifers themselves can constitute an ecosystem, such as caves and sinkholes that support invertebrates and fish adapted to life underground.4
- The extent and condition of ground water are often intertwined. Stressors that affect the extent of ground water—such as withdrawal or injection—can change ground water velocity and flow. These physical changes can affect patterns of discharge to surface waters and the movement of water and contaminants within the ground.
- The first indicator describes levels of nutrients and pesticides in ground water in a variety of regions and hydrogeologic conditions across the U.S. It comes from a multi-decade U.S. Geological Survey (USGS) study that has collected and analyzed untreated water samples from wells in agricultural, urban, and mixed land-use areas.
- The second indicator describes the amount of water that Americans withdraw from various sources every year, including ground water. This indicator relates to the extent of ground water, but it is not a direct measurement of extent, which would involve tracking the height of the water table (water levels) over time. Some regional studies have been conducted, but there are no programs that measure groundwater levels nationwide.
 United States Department of the Interior. 2002. Hydrologic impacts of mining. Chapter 1. In: Permitting hydrology, a technical reference document for determination of probable hydrologic consequence (PHC) and cumulative hydrologic impact assessments (CHIA) (PDF) (197 pp, 1.3 MB, About PDF). Washington, DC. Accessed November 8, 2003.
 Prichard, D., J. Anderson, C. Correll, J. Fogg, K. Gebhardt, R. Krapf, S. Leonard, B. Mitchell, and J. Stasts. 1998. Riparian area management: A user guide to assessing proper functioning condition and the supporting science for lotic areas. Technical reference 1737-15. Denver, CO: U.S. Department of the Interior, Bureau of Land Management, National Applied Resource Sciences Center.
 Boyd, M., and D. Sturdevant. 1997. The scientific basis for Oregon's stream temperature standard: Common questions and straight answers. Portland, OR: Oregon Department of Environmental Quality.
 Elliott, W.R. 2000. Conservation of the North American cave and karst biota (PDF) (27 pp, 4.4 MB, About PDF) . In: Wilkens, H., D.C. Culver, and W.F. Humphreys, eds. Subterranean ecosystems. Amsterdam, The Netherlands: Elsevier (Ecosystems of the World series). pp. 665-689.