Aquifers

Aquifers are the source of water for approximately 50 percent of the U.S. population. The identification, conservation, and protection of aquifers are important to the future of drinking water supplies in North America and elsewhere in the world.

Principal Terms

aquifer: any porous, permeable geologic structure or rock formation that contains water

cone of depression: the depression, in the shape of an inverted cone, of the groundwater surface that forms near a pumping well

confined aquifer: an aquifer that is completely filled with water and whose upper boundary is a confining bed; also called an artesian aquifer

confining bed: an impermeable layer in the geologic stratigraphy that prevents vertical water movement

groundwater: water found below the ground surface in the zone of saturation

permeability: the ability of rock, soil, or sediment to transmit a fluid

porosity: the ratio of the volume of void space in a given geologic material to the total volume of that material

unconfined aquifer: an aquifer whose upper boundary is the water table; also called a water table aquifer

water table: the upper surface of the zone of saturation

zone of saturation: a subsurface zone in which all void spaces are filled with water

The Water Table

To understand aquifers, one must understand how water occurs beneath the earth’s surface. The world’s water supply is constantly circulating through the environment in a never-ending process known as the hydrologic cycle. Natural reservoirs within this cycle include the oceans, the polar ice caps, underground water, surface water, and the atmosphere. Water on the land surface that is able to infiltrate the ground becomes underground water. “Groundwater recharge” occurs when the infiltration reaches the water table.

Underground water exists in three different subsurface zones: the soil moisture zone, the intermediate vadose zone, and the zone of saturation. The soil moisture zone is found in soil directly beneath the land surface and contains water not confined below a rock stratum so that it is available to plant roots. This zone is generally not saturated unless a prolonged period of rainfall or snowmelt has occurred. Water in this zone is held under tension by the attractive forces between soil particles and water molecules, or surface tension forces. The depth of this zone corresponds to the depth to which plant roots can grow.

Water that infiltrates through the soil moisture zone may pass into the intermediate vadose zone, also called the zone of aeration, before reaching the water table. The vadose zone is always unsaturated, since the pore spaces between particles contain both water and air, and the water it contains is held under tension. The thickness of a vadose zone depends on how close the water table is to the surface.

The water table forms the uppermost surface of the zone of saturation and is characterized by a water pressure equal to atmospheric pressure. It may be only a short distance below the land surface in humid regions and hundreds of meters below the surface in desert environments. In general, the water table mimics the land surface topography. If the water table intersects the land surface, the result is a lake, swamp, river, or spring. Below the water table, in the zone of saturation, geologic materials are completely saturated, and the water pressure increases with depth. Water contained within the zone of saturation is known as groundwater. When groundwater occurs in a particular type of geologic formation known as an aquifer, it can feed a well.

Porosity

An aquifer can be functionally defined as any earth material—rock, soil, or sediment—that yields a significant quantity of groundwater to a well or spring. The definition of “a significant quantity” varies according to the intended use; what constitutes an aquifer for an individual homeowner may be quite different from what constitutes an aquifer for a municipal supply. For a geologic formation to be useful as an aquifer, it must be able both to hold and to transmit water.

The ability of geologic materials to hold, or store, water is known as porosity. Simply stated, porosity is the volume of void space present divided by the total volume of a given rock or sediment. This proportion is usually expressed as a percentage. The higher this ratio is, the more void space there is to hold water. There are various types of porosity. Unconsolidated materials (soil and sediment) have pore spaces of varying sizes between adjacent grains referred to as intergranular porosity. The ratio of pore space to total volume depends on several factors, including particle shape, sorting, and packing. Loosely packed sediments composed of well-sorted, spherical grains are the most porous. Porosity decreases as the angularity of the grains increases because such particles are able to pack more closely together. Similarly, as the degree of sorting decreases, the pore spaces between larger grains become filled with smaller grains, and porosity decreases. Values of porosity for unconsolidated materials range from 10 percent for unsorted mixtures of sand, silt, and gravel to about 60 percent for some clay deposits. Typical porosity values for uniform sands are between 30 and 40 percent.

Rocks have two main types of porosity: pore spaces between adjacent mineral grains and voids caused by fractures. Rocks formed from sedimentary deposits, such as shale and sandstone, may have significant intergranular porosity, but it is usually less than the porosity of the sediments from which they were derived because of the compaction and cementation that take place during the process of transforming sediments into rock. Therefore, although sandstone porosities may be as high as 40 percent, they are commonly closer to 20 percent because of the presence of natural cements that partially fill available pore spaces. Igneous and metamorphic rocks are composed of tightly interlocked mineral grains and, therefore, have little intergranular porosity. Virtually all void space in such rocks is a result of fractures (joints and faults). For example, granite, a dense igneous rock, usually has a porosity of less than 1 percent, but it may reach 10 percent if the rock is fractured.

There are additional types of porosity that occur only in certain kinds of rocks. Limestone, a rock that is soluble in water, can develop solution conduits along fractures and bedding planes. In the extreme case, solution weathering may lead to the development of a cave, which has 100 percent porosity. The overall porosity of solution-weathered limestone sometimes reaches 50 percent. Rocks created by volcanic eruptions may contain void space in the form of trapped air bubbles (called vesicles), shrinkage cracks developed during cooling (called columnar joints), and tunnels created by flowing lava (called lava tubes). In extreme cases, the porosity of volcanic rocks may exceed 80 percent.

Permeability

The presence of void space alone does not constitute a good aquifer. It is also necessary for groundwater to be able to move through the geologic material in question. The ability of porous formations to transmit fluids, a property known as permeability, depends on the degree to which the void spaces are interconnected. Some high-porosity materials, such as clay, shale, and pumice, do not make good aquifers because the void spaces are largely isolated from one another. Materials that have high permeability include sand, gravel, sandstone, and solution-weathered limestone. Rocks with low porosities, such as shale, quartzite, granite, and other dense, crystalline rocks, have low permeabilities unless they are significantly fractured.

Groundwater moves along tortuous paths through the available void space in a given porous formation. Regardless of the material’s permeability, groundwater flows much more slowly than surface water in a river. The velocity of stream flow may be measured in meters per second, whereas groundwater velocities commonly range between 1 meter (3 feet) per day and less than 1 meter (3 feet) per year, averaging about 17 meters (56 feet) per year in rocks. Underground rivers are uncommon, occurring only in cavernous limestone or lava tubes in volcanic terrane.

The geologic materials that make good aquifers are those that have both high porosity and high permeability. The response of any given aquifer to a pumping well will also depend, however, on its position beneath the surface and its relationship to the water table. Aquifers near the ground surface usually have the water table as their upper boundary. The thickness of these aquifers therefore changes as the water table rises or falls. An aquifer under these conditions is an unconfined aquifer, or water table aquifer. These types of aquifers are the easiest to exploit for a water supply, but they are also the easiest to contaminate. It is important, therefore, to delineate the extent of unconfined aquifers and to take measures to protect them from various forms of pollution.

Because the water table is free to fluctuate in unconfined aquifers, the amount of water supplied to a well reflects the gravity drainage of water from void spaces. The volume of water available from aquifer storage, or the “specific yield,” therefore approaches the upper limit set by porosity. Some groundwater is unable to drain from void spaces under the influence of gravity because it is tightly held by surface tension forces; this retained water, known as “specific retention,” forms a thin film around individual grains. The highest values of specific yield occur in coarse-grained, permeable aquifers, such as sand, gravel, and sandstone.

Confined Aquifers

Some aquifers, usually found at depth or those known as “inclined” aquifers, are completely filled with groundwater and bounded at the top by an impermeable layer called a confining bed, or aquiclude. The water in these confined, or artesian, aquifers is under pressure because of the weight of overlying formations and the fact that the confining bed does not allow groundwater to escape. If a well intrudes into such an aquifer, the water level will usually rise above the upper boundary of the confining bed, creating an artesian well. In some cases, water may rise above the land surface at the point where the well is placed. This condition is known as a flowing artesian well. Water will flow freely out of such wells as long as the aquifer remains under pressure. Many of the Great Plains states (Kansas, Nebraska, and the Dakotas) are underlain by important shale layers. The original pressure in these aquifers was quite high because the sandstone beds are upwarped along the eastern front of the Rocky Mountains and Black Hills, where they receive groundwater recharge.

Confined aquifers supply water to a well not through the gravity drainage of void spaces but through compression of the aquifer as water pressure is reduced during pumping. The volume of water available from storage in a confined aquifer, or the “specific storage,” is only a small fraction of the total volume and is, therefore, always much less than the porosity. When confined aquifers are pumped to the extent that they become dewatered, the accompanying pressure reduction can lead to extensive aquifer compression and land surface subsidence. This problem is most serious in cases where the confining bed is composed of clay because the loss of fluid pressure beneath the clay causes water to be squeezed out of the clay layer by the weight of overlying formations. Once a clay layer is compressed in this way, it will not be able to reabsorb water even if the surrounding materials become saturated again.

Study of Aquifers

The first step in studying aquifers is to utilize data that have already been collected, such as geologic maps. These maps show the distribution of various geologic formations on the land surface and are, therefore, valuable tools for delineating the outcrop patterns of potential aquifers. If cross-sections are also available, they can aid in the estimation of potential aquifer thicknesses and the identification of possible confined aquifers. Geologic maps, however, serve only as a preliminary tool in aquifer study. Any interpretations made from maps need to be verified by field descriptions and, if possible, pumping tests.

Topographic maps can be used to make generalizations about the groundwater flow system. Springs, lakes, streams, and swamps may indicate areas of groundwater discharge. Because the water table is usually a subdued version of the land surface, it may be possible to infer groundwater flow directions from the local topography. That can provide a clue as to where recharge areas occur. Other indications of recharge areas are topographic high points and a general lack of surface-water features. For groundwater recharge to occur, however, permeable materials must be exposed at the land surface.

Reliable estimates of aquifer properties require fieldwork. Often, samples are taken from the field for the purpose of determining aquifer properties, such as grain-size distribution or permeability, in the laboratory. Such tests, however, are performed only with small samples and may not be representative of the overall aquifer unit. That is particularly true in fractured rock structures, where the movement of groundwater may be very difficult to predict. In such situations, the injection and monitoring of tracer dyes has proved helpful in understanding groundwater flow.

The most direct way to study aquifers is by boring holes and installing wells. By drilling, geologists are able to discover the exact nature of the subsurface materials. Detailed drilling logs are kept of the different layers and the depths at which they were encountered. Using their knowledge of geologic materials, properly trained geologists can predict which formations will constitute the best aquifers.

Once wells have been installed, additional information about the aquifer can be learned by conducting pumping tests (pumping the wells at known rates for extended periods). Because water moves relatively slowly through an aquifer, the pumping of a well removes groundwater faster than it can be replaced. The resulting water-level drawdown is called a cone of depression; it is a zone of dewatering near the pumping well that resembles an inverted cone. The exact shape of this cone is a function of the pumping rate and the aquifer properties. Therefore, certain aquifer characteristics, such as permeability, are discovered by studying the cone of depression created by a known pumping rate. Monitoring wells must be placed near the pumped well in order to identify the shape of the cone and detect drawdown at various distances.

The prediction of long-term well yields requires not only an understanding of aquifer properties but also a knowledge of the groundwater recharge rates. To determine the amount of water in a particular drainage basin that is available for groundwater recharge, it is necessary to develop a hydrologic budget for that basin. Hydrologic budgets attempt to account for all water inputs and losses from the basin in question. Inputs include precipitation, surface-water inflow, groundwater inflow, and water imported by human activities (as for irrigation). Losses include surface-water runoff, groundwater outflow, evaporation, transpiration by plants, and water exported from the basin by human activities. The difference between these inputs and losses equals the amount of water gained or lost by the groundwater reservoir.

Significance

The term “aquifer” is generally not precise because its definition depends somewhat on the intended use. For an individual homeowner who requires only 7 to 20 liters (2 to 5 gallons) per minute from a well, fractured bedrock may serve as a suitable source of water. By comparison, only a few geologic materials are capable of delivering the larger water supply demanded by a municipality or industrial plant—often more than 1,500 liters (396 gallons) per minute. In the case of high-capacity well requirements, the term “aquifer” is restricted to highly permeable geologic materials, such as unconsolidated sands and gravels, sandstone, and solution-weathered limestone.

Most private wells draw water from unconfined aquifers, which have the water table as an upper boundary. The saturated thicknesses of these aquifers can fluctuate as the water table rises or falls. Therefore, shallow wells completed in unconfined aquifers can be pumped dry as the water table declines during periods of prolonged drought. Because of their proximity to the land surface, these aquifers are also susceptible to groundwater contamination. Confined aquifers do not comply to a water table because they are fully saturated and capped with an impermeable confining bed. The natural groundwater quality in confined aquifers is not necessarily superior to that found in unconfined aquifers, but the presence of a low-permeability confining bed may help to protect these aquifers from being contaminated by materials descending from the surface.

An understanding of aquifer characteristics is important to the proper use of groundwater. The determination of maximum sustainable well yields requires a knowledge of how the aquifer stores and transmits water. Such information is also needed to estimate how close adjacent wells can be to one another without causing interference through the formation of overlapping cones of depression. Pumping water out of an aquifer at rates exceeding the rate of groundwater recharge will eventually cause the depletion of that aquifer as a valuable water supply. Many arid regions in the western United States are facing this problem because of years of groundwater mismanagement. The overpumping of confined aquifers is especially troublesome because if these aquifers become dewatered, they compress, leading to subsidence of the land surface. This has been especially noted in California, where groundwater withdrawal has resulted in subsidence of 9 meters (30 feet) or more in the San Joaquin Valley and the formation of deep, chasm-like cracks in the level grounds of Rogers Lake.

Delineating the extent of aquifers and particularly of their recharge zones is critical to the development of policies that protect groundwater supplies from pollution as well as depletion. In the case of confined aquifers, recharge zones may be restricted to areas where the confining bed is absent, so contaminants entering the ground through a relatively small zone could affect a large number of downgradient wells. Moreover, because groundwater moves very slowly through the subsurface, it takes a long time to be renovated once contaminated.

In terms of depletion, over-allocation is a serious threat to many rivers throughout the world. In the Rio Grande–Rio Bravo area, the border between Texas and Mexico, surface water was 150 percent overallocated as of 2021, and groundwater was similarly overallocated. These issues were forseen by specialists for decades. In 1974, the United States passed the Federal Safe Drinking Water Act (SDWA), which was a farreaching bill that addressed the contamination, overuse, and sustainability of water as well as the management and contingency of water and its distribution means, like aquifers. States have also passed individual laws to ensure water is protected and properly managed. However, in areas like Rio Grande–Rio Bravo, these laws may not be enough, and thus contingencies are needed. As climate change impacts rainfall and weather patterns, further attention will need to be paid to water supplies worldwide.

Bibliography

Ahmed, S., R. Jayakumar, and Abdin Saleh. Groundwater Dynamics in Hard Rock Aquifers: Sustainable Management and Optimal Monitoring Network Design. Dordrecht: Springer, 2008.

“Aquifers.” National Geographic Society, 19 Oct. 2023, education.nationalgeographic.org/resource/aquifers/. Accessed 3 Aug. 2024.

Batu, Vedat. Applied Flow and Solute Transport Modeling in Aquifers. Boca Raton, Fla.: CRC Press, 2006.

‗‗‗‗‗‗‗‗‗‗. Aquifer Hydraulics: A Comprehensive Guide to Hydrogeology Data Analysis. New York: Wiley, 1998.

“Federal Laws on Groundwater Protection.” Environmental Health & Safety, Michigan State University, ehs.msu.edu/enviro/water/whpp/wh-05law.html. Accessed 3 Aug. 2024.

Fetter, Charles W. Applied Hydrogeology. 4th ed. Upper Saddle River, N.J.: Prentice Hall, 2000.

Gilbert, Janine, Dan L. Danielopol, Jack A. Stanford, et al., eds. Groundwater Ecology. San Diego, Calif.: Academic Press, 1994.

Glennon, Robert. Unquenchable. America’s Water Crisis and What to Do About It. Washington, D.C.: Island Press, 2009.

Gorelick, Steven M., et al., eds. Groundwater Contamination: Optimal Capture and Contamination. Boca Raton, Fla.: Lewis, 1993.

Guymon, Garry L. Unsaturated Zone Hydrology. Upper Saddle River, N.J.: Prentice-Hall, 1994.

Hamblin, Kenneth W., and Eric H. Christiansen. Earth’s Dynamic Systems. 10th ed. Upper Saddle River, N.J.: Prentice Hall, 2003.

Hermann, Emily. “What Is Groundwater and Why Is It So Important?” World Wildlife Federation, 21 Mar. 2022, www.worldwildlife.org/stories/what-is-groundwater-and-why-is-it-so-important. Accessed 3 Aug. 2024.

Kasenow, Michael. Aquifer Test Data: Analysis and Evaluation. Highlands Ranch, Colo.: Water Resources Publications, 2006.

Misstear, B. D. R., David Banks, and Lewis Clark. Water Wells and Boreholes. New York: John Wiley & Sons, 2006.

Nielson, Kurt Ambo. Fractured Aquifers: Formation Evaluation by Well Testing. Vancouver: Trafford Publishing, 2007.

Nonner, Johannes C. Introduction to Hydrogeology. London: Taylor & Francis Group, 2006.

Omer, Sevil. “Global Water Crisis.” World Vision, 6 Mar. 2024, www.worldvision.org/clean-water-news-stories/global-water-crisis-facts. Accessed 3 Aug. 2024.

Randolph, John. Environmental Land Use Planning and Management. Washington, D.C.: Island Press, 2004.

Todd, David K. Groundwater Hydrology. 3d ed. New York: John Wiley & Sons, 2004.