An introduction to aquifers and groundwater systems

By Darrel Dunn, Ph.D., PG, Hydrogeologist

(View Résumé 🔳)

Introduction to Aquifer Webpage

This is a nontechnical webpage intended to introduce the concepts of groundwater aquifer and groundwater systems to laypeople. 

Aquifer Definition

An aquifer is a water saturated, permeable part of the subsurface that contains and can transmit significant quantities of water (Freeze and Cherry, 1979).  Aquifers are permeable enough for the completion of water wells.  They can transmit significant amounts of water to springs and streams if they are well connected to them.  The water enters the aquifer by seepage of precipitation and other sources of water where the aquifer is present at the ground surface.  The water enters by seepage through overlying aquitards where the aquifer is buried.  The term aquitard refers to less permeable parts of the subsurface where the permeability is not sufficient for completion of water wells.  Aquitards are sometimes called confining beds.  Multiple aquifers may be separated by aquitards.  If an aquifer is not overlain by an aquitard it is called an unconfined aquifer (or water table aquifer).  Figure 1 is a diagram illustrating aquifers and aquitards.  The figure also shows the nature of groundwater flow systems.   Note that groundwater circulating through a deep aquifer may take centuries to return to a discharge location at the surface.  The technical definition of "water table" is the surface where the pressure in the water is atmospheric.  This surface is below the top of the water saturated part of the subsurface.  The difference is that capillary phenomena cause water to rise in the saturated lower part of a capillary fringe above the true water table.  Figure1 illustrates the fact that groundwater does not necessarily "flow downhill."  It may flow down then up in accordance with principles of physics (conservation of mass and energy).  Flow systems may be much more complicated that the diagram in Figure 1.  See the webpage on groundwater velocity partition.  Figure 1 also illustrates how groundwater that would naturally flow to a stream may be diverted to a water well.  Such diversion is related to the concept of water resource sustainability.

Diagram of aquifers and confining beds in a groundwater flow syste.

Figure 1.  Diagram of aquifers and confining beds (aquitards).  Source: U.S. Geological Survey (USGS) Circular 1139.

Aquifer Types

The United States Geological Survey (2021) lists the following types of aquifers:

These types are described in the following subsections.

Sand and Gravel Aquifers

Sand and gravel aquifers are generally composed of loose sand and gravel.  However, some sand and gravel deposits that have been deeply buried may be lightly cemented due to compaction and deposition of mineral substance in the pores.  Sand and gravel aquifers are usually alluvial and are in stream valleys or otherwise deposited by streams (for example, mountain alluvial fans, basin-fill deposits, and buried bedrock valleys beneath prehistoric glacial deposits).  Basin-fill deposits are found in the mountainous areas of the western United States.  They often occur in down-faulted basins between mountain ranges (Figure 2).  The basins of the Rio Grande Rift in Colorado, New Mexico and Texas are examples of such intermontane basins.  There are also extensive sand and gravel aquifers that extend great distances from their source areas.  One example is the Ogallala Aquifer (Figure 3) located in the high plains east of the Rocky Mountains, extending from Nebraska to the Texas Panhandle.  The Ogallala Aquifer serves much irrigated agriculture and has had sustainability issues for decades.   Sand and gravel aquifers tend to be very permeable; and they can yield much water to wells if they have sufficient saturated thickness.  Construction of large capacity wells in these aquifers may require special techniques to prevent caving while drilling and the production of sand in the completed well.

Diagram of an intermontane basing.

Figure 2.  Generalized diagram of basin-fill.  Source: USGS.

Ogallala aquifer cross section.

Figure 3.  Cross section of the Ogallala Aquifer in eastern Colorado.  The vertical scale is greatly exaggerated.  Source: USGS

Sandstone Aquifers

Sandstone aquifers are composed dominantly of sand that has been deeply buried during earlier geologic time. The sand has become compacted and cemented by mineral deposits between the grains to form a hard porous rock.  The rock is fractured, and the fractures form conduits that transmit most of the groundwater.  Sandstone aquifers are often overlain and underlain by siltstone and shale layers, so that they are confined aquifers.  The Dakota Sandstone is an example.  It is present at the surface or at depths shallow enough to yield fresh water at various locations throughout the Rocky Mountains and the Colorado Plateau (located west of the Rocky Mountains in Colorado, Utah, Arizona, and New Mexico), and it extends east of the Rocky Mountains through much of the midcontinent of North America.  Dakota Sandstone water yields are sufficient for domestic and livestock use and in some areas it is used for irrigation and industry.

Carbonate Rock Aquifers

Carbonate rock aquifers are primarily composed of limestone and dolomite rock.  Limestone is mostly calcium carbonate and dolomite is calcium magnesium carbonate.   Most of the groundwater is in open conduits formed as slightly acidic groundwater flowing in fractures enlarges them by dissolving the walls.  In some places the conduits become so large the rocks are cavernous.  Carbonate rock aquifers can yield large volumes of water when wells penetrate solution cavities.  Carbonate rock terrains often have large springs and sink holes.  Figure 4 is a diagram of features of carbonate rock terrains.  The Madison Limestone of the Rocky Mountains and Colorado Plateau is an example of a carbonate rock aquifer.  The Biscayne and Floridan aquifers of south Florida are carbonate rock aquifers that are the sole potable water source for more than 6 million people.  The Biscayne Aquifer is affected by saltwater intrusion.

Diagram of a carbonate aquifer.

Figure 4.  Diagram of features of carbonate rock terrains.  Source: USGS

Sandstone and Carbonate Rock Aquifers

In some areas groundwater is produced from sequences of alternating limestone, sandstone, siltstone, and shale.  Features of sandstone aquifers and limestone aquifers coexist and the combination is adequate for completion of water wells.

Igneous and Metamorphic Rock Aquifers

Igneous aquifers include (1) basaltic lava aquifers that can yield much water to wells and (2) low permeability fractured crystalline and metamorphic rocks, such as granite.  Basalt aquifers tend to have permeable zones that develop between the individual lava flows.  They also yield water from fractures.  The Columbia Plateau in Washington, Oregon, Idaho, California, and Nevada is a vast region of basaltic aquifers which vary greatly in water yielding capability.  Igneous metamorphic rocks are crystalline and yield water only from fractures.  They generally yield only domestic supplies, often from deep wells.  There are, however, highly fractured zones that transmit more groundwater and may produce springs in mountain watersheds. 

Aquifer Technology

This website contains many pages dealing with aquifer technology.  They are listed in the menu that may be viewed from this page.  Click the menu icon (≡).

References for Aquifer Webpage

Freeze, R. A. and J. A. Cherry (1979): Groundwater; Prentice-Hall.

Hunt, C. B. (1974): Natural Regions of the United States and Canada; Freeman.

Sterett, R. J. (2007): Groundwater and Wells 3rd Edition; Johnson Screen Division.

United States Geological Survey, Water Resources Mission Area (2021): Principle Aquifers of the United States;

Winter, T. C. and others (1998): Groundwater and Surface Water, A Single Resource; U.S. Geological Survey Circular 1139. 🔗

Posted March 12, 2024.