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Groundwater

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What are the trends in extent and condition of ground water and their effects on human health and the environment?

A large portion of the world’s fresh water resides underground, stored within cracks and pores in the rock that makes up the Earth’s crust. The U.S. Geological Survey estimates that there are approximately 1 million cubic miles of ground water within one-half mile of the Earth’s surface—30 times the volume of all the world’s fresh surface waters.2 Many parts of the U.S. rely heavily on ground water for human uses (e.g., drinking, irrigation, industry, livestock), particularly areas with limited precipitation (e.g., the Southwest), limited surface water resources, or high demand from agriculture and growing populations (e.g., Florida). Half of the U.S. population (51 percent) relies on ground water for domestic uses.3

Ecological systems also rely on ground water. For example, some wetlands and surface waters are fed by springs and seeps, which occur where a body of ground water—known as an aquifer—reaches the Earth’s surface. While the contribution of ground water to stream flow varies widely among streams, hydrologists estimate that the average contribution of ground water is 40 to 50 percent in small- and medium-sized streams. The ground water contribution to all stream flow in the U.S. may be as large as 40 percent.4

The extent of ground water refers to the amount available, typically measured in terms of volume or saturated thickness of an aquifer. The condition of ground water reflects a combination of physical, biological, and chemical attributes. Physical properties reflect patterns of flow—i.e., the volume, speed, and direction of ground water flow in a given location. Biologically, ground water can contain a variety of organisms, including bacteria, viruses, protozoans, and other pathogens. Ground water can also contain a variety of chemicals, which may occur naturally or as a result of human activities. Chemicals that may occur in ground water include nutrients, metals, radionuclides, salts, and organic compounds such as petroleum products, pesticides, and solvents. These chemicals may be dissolved in water or—in the case of insoluble organic contaminants—exist as undissolved plumes.

Many stressors can affect the extent of ground water, including patterns of precipitation and snowmelt and human activities that change or redistribute the amount of ground water in an aquifer. One major way humans influence ground water extent is by withdrawing water for drinking, irrigation, or other uses (e.g., ground water extracted to lower the water table for mining operations). Other human activities can increase ground water levels, such as surface irrigation runoff recharging a shallow aquifer, or water pumped directly into the ground in order to store surface waters for future use, or to aid in oil and gas extraction. Human activities can affect ground water extent indirectly, too; for example, impervious paved surfaces may prevent precipitation from recharging ground water. In some cases, changes in ground water extent may be caused by a combination of these human and natural factors—for example, droughts that require humans to withdraw more water from the ground (e.g., for irrigation), while at the same time providing less precipitation for recharge. Some aquifers are more susceptible than others to changes in extent. For example, some deep aquifers may take thousands of years to recharge, particularly if they lie below highly impermeable confining layers.

Aquifer depletion—i.e., decreased extent—can adversely affect the humans and ecosystems that directly or indirectly depend on ground water. Less ground water available for human or ecological use can result in lower lake levels or—in extreme cases—cause perennial streams to become intermittent or totally dry, thus harming aquatic and riparian plants and animals that depend on regular surface flows. An area with a high water table may have plant communities that tap ground water directly with their roots, so even a slight lowering of the aquifer could affect native species—which in turn could benefit invasive species.5 In addition, lower water table levels may lead to land subsidence and sinkhole formation in areas of heavy withdrawal, which can damage buildings, roads, and other structures and can permanently reduce aquifer recharge capacity by compacting the aquifer medium (soil or rock). Finally, changes in the ground water flow regime can lead to consequences such as salt water intrusion, in which saline ground water migrates into aquifers previously occupied by fresh ground water.

Although aquifer depletion can have serious effects, the opposite, far less common problem—too much ground water—can also be detrimental. Too much ground water discharge to streams can cause erosion and can alter the balance of aquatic plant and animal species, as has been reported in association with some mining sites.6

Like extent, condition is influenced by both natural sources and human activities. Some ground water has high levels of naturally occurring dissolved solids (salinity), or metals such as arsenic that can be present as a result of natural rock formations. Land use can affect the condition of ground water; for example, pesticides, fertilizers, and other chemicals applied to the land can leach into ground water, while waste from livestock and other animals can contribute contaminants such as nutrients, organic matter, and pathogens. Shallow and unconfined aquifers are particularly susceptible to this type of contamination. In addition, landfills may leach metals, solvents, and other contaminants into ground water (particularly older landfills that do not have liners and leachate collection systems). Mining operations can mobilize toxic metals, acidic compounds, and other substances that can impact the condition of ground water. Finally, chemical or biological contaminants may enter aquifers as a result of unintentional releases, including chemical spills on land, leaks from storage tanks, sewers or septic systems, and unplugged abandoned wells that allow a direct route of entry for contaminants.

Stressors that affect ground water condition ultimately affect the condition of water available for drinking, irrigation, or other human needs. In some cases, treatment may be needed to ensure that finished drinking water does not pose risks to human health. Because drinking water can come from many different types of water bodies, and because of the many complex issues associated with treatment and regulation of drinking water, this topic is addressed in greater detail in its own section of this report, Section 3.6. The condition of ground water also can affect ecological systems. For example, many fish species depend on cold, clear spring-fed waters for habitat or spawning grounds.7,8 In some cases, aquifers themselves may constitute ecosystems. For example, caves and sinkholes are home to many types of aquatic fauna, including invertebrates and fish adapted to life underground.9 Ground water can also affect the condition of other environmental media. For example, volatile ground water contaminants can potentially migrate into indoor air via soil vapor intrusion.

In many ways, extent and condition are intertwined. For example, stressors that affect extent—such as withdrawal or injection—can also alter physical parameters of the ground water flow regime, such as velocity and direction of flow. These physical alterations can affect patterns of discharge to surface waters, as well as the movement of water and contaminants within the ground (e.g., salt water intrusion).

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