Jul 13, 202217 min read

AQUIFERS

The surface of saturated material in an aquifer is known as the water table.

An aquifer is a geologic formation, group of formations, or part of a formation that contains sufficient saturated, permeable material to yield significant quantities of water to wells and springs.

The principal water-yielding aquifers of North America can be grouped into five types: unconsolidated and semiconsolidated sand and gravel aquifers, sandstone aquifers, carbonate-rock aquifers, aquifers in interbedded sandstone and carbonate rocks, and aquifers in igneous and metamorphic rocks.

· Carbonate-rock:

Aquifers in carbonate rocks are most prominent in the central and southeastern parts of the Nation, but also occur in small areas as far west as southeastern California and as far east as northeastern Maine. Most of the carbonate-rock aquifers consist of limestone, but dolomite and marble locally yield water. The water-yielding properties of carbonate rocks vary widely; some yield almost no water and are considered to be confining units, whereas others are among the most productive aquifers known.

Most carbonate rocks originate as sedimentary deposits in marine environments. Compaction, cementation, and dolomitization processes might act on the deposits as they lithify and greatly change their porosity and permeability. However, the principal postdepositional change in carbonate rocks is the dissolution of part of the rock by circulating, slightly acidic groundwater. Solution openings in carbonate rocks range from small tubes and widened joints to caverns that may be tens of meters wide and hundreds to thousands of meters in length. Where they are saturated, carbonate rocks with well-connected networks of solution openings yield large amounts of water to wells that penetrate the openings, although the undissolved rock between the large openings may be almost impermeable.

· Igneous & Metamorphic:

Large areas of the eastern, northeastern, northwest and north-central parts of the Nation are underlain by crystalline rocks. Spaces between the individual mineral crystals of crystalline rocks are microscopically small, few, and generally unconnected; therefore, porosity is insignificant. These igneous and metamorphic rocks are permeable only where they are fractured, and they generally yield only small amounts of water to wells. However, because these rocks extend over large areas, large volumes of groundwater are withdrawn from them, and, in many places, they are the only reliable source of water supply.

Although crystalline rocks are geologically complex, movement of water through the rocks is totally dependent on the presence of secondary openings; rock type has little or no effect on groundwater flow.

Groundwater percolates downward through the regolith which is a layer of weathered rock, alluvium, colluvium, and soil to fractures in underlying bedrock. The water moves from highland recharge areas to discharge areas, such as springs and streams at lower altitudes.

Volcanic rocks have a wide range of chemical, mineralogic, structural, and hydraulic properties, due mostly to variations in rock type and the way the rock was ejected and deposited. Unaltered pyroclastic rocks, for example, might have porosity and permeability similar to poorly sorted sediments. Hot pyroclastic material, however, might become welded as it settles, and, thus, be almost impermeable. Silicic lavas tend to be extruded as thick, dense flows, and they have low permeability except where they are fractured. Basaltic lavas tend to be fluid, and, they form thin flows that have considerable pore space at the tops and bottoms of the flows. Numerous basalt flows commonly overlap, and the flows are separated by soil zones or alluvial materials that form permeable zones. Columnar joints that develop in the central parts of basalt flows create passages that allow water to move vertically through the basalt. Basaltic rocks are the most productive aquifers in volcanic rocks.

In some places, the basaltic-rock aquifers are extremely thick. For example, those of the Columbia Plateau aquifer system in Washington are more than 2,544 meters thick in places, and those of the Snake River Plain aquifer system in Idaho are locally more than 800 meters thick. In most places, however, the thickness of these aquifers is 100 meters or less. Groundwater flow in the basaltic-rock aquifers is local to intermediate. In Idaho, the basaltic-rock aquifers are extremely permeable, and numerous large springs discharge several tens of cubic meters per second from them.

· Sandstone:

The sandstone aquifers are level or gently dip. Because they are commonly interbedded with siltstone or shale, most of the water in these aquifers is under confined conditions. Groundwater-flow systems in mostly level, relatively thin sandstone aquifers are local to intermediate. Regional, intermediate, and local flow is present in the sandstone aquifers in the western United States, except for those in Oklahoma, where flow is mostly local. Many sandstone aquifers contain highly mineralized water at depths of only a few hundred meters.

In Wisconsin and adjacent states, three Cambrian and Ordovician age sandstone aquifers are combined into an aquifer system that is as much as 650 meters thick. Paleozoic through Cenozoic age sandstones that extend northeastward from Wyoming form the Northern Great Plains aquifer system, which has permeable parts of more than 2,000 meters thick in some places in a deep structural basin. Not all of these thick aquifers, however, contain freshwater.

· Sand & Gravel:

Unconsolidated sand and gravel aquifers can be grouped into four categories: basin-fill aquifers, which also are called "valley-fill aquifers"; blanket sand and gravel aquifers; glacial-deposit aquifers; and stream-valley aquifers which are of generally small extent and not mapped. All four types have intergranular porosity, and all contain water primarily under unconfined or water-table conditions. The hydraulic conductivity of the aquifers is variable, depending on the sorting of aquifer materials and the amount of silt and clay present, but generally it is high. Aquifer thickness ranges from a few meters or tens of meters in the blanket sands along the eastern Atlantic coast of the United States to several hundred meters in the basin-fill aquifers of the southwestern United States. The unconsolidated sand and gravel aquifers are susceptible to contamination because of their generally high hydraulic conductivity. Groundwater in these aquifers flows along relatively short flow paths typical of local flow systems; however, all of the basin-fill aquifers have intermediate flow systems, and the thick basin fill of California's Central Valley aquifer system has a regional flow system. Likewise, the thick blanket sands of the High Plains aquifer and the Mississippi River Valley alluvial aquifer of the central United States have regional flow systems.

Basin-fill or valley-fill aquifers were deposited in depressions formed by faulting or erosion or both. Fine-grained deposits of silt and clay form local confining units in these aquifers, and thick sequences of the unconsolidated deposits become more compact and less permeable with depth. Most basins are bounded by low-permeability rocks, but some in the western United States are hydraulically connected to adjacent carbonate-rock aquifers. Some basin-fill aquifers, such as those in the Central Valley aquifer system of California and in parts of Arizona, have supplied large amounts of water for irrigation and other uses.

Widespread, blanket-like deposits of sand and gravel form aquifers in lowland areas of Alaska, atop lava plateaus in Washington, along the Atlantic and eastern Gulf coasts, along part of the lower reaches of the Mississippi River, and in the High Plains. These aquifers mostly consist of alluvial deposits. They commonly contain water under unconfined conditions, and most groundwater flow in them travels short to intermediate distances from recharge to discharge areas. The High Plains aquifer is the most intensively pumped aquifer in North America.

Glacial-deposit aquifers form numerous local, and some regional, highly productive aquifers in the area north of the line of glaciation. These aquifers consist of outwash, terrace, or ice-contact deposits, and they mostly occupy bedrock valleys or areas of interlobate ice marginal deposition. In places, the valley deposits are buried beneath low-permeability till. Groundwater flow in the glacial-deposit aquifers is primarily local, from recharge areas near stream valley walls to discharge in the streams.

Semi-consolidated aquifers consist of semi-consolidated sand interbedded with silt, clay, and minor carbonate rocks. Porosity is intergranular, and the hydraulic conductivity of the aquifers is moderate to high. The aquifers underlie the Coastal Plains of the eastern and southern United States, and they are of fluvial, deltaic, and shallow marine origin. The aquifers are in a thick wedge of sediments that dips and thickens coastward; in places, the sands of the aquifers are more than 650 meters thick

· Sandstone & Carbonate:

In scattered places in the United States, carbonate rocks are interbedded with almost equal amounts of water-yielding sandstone. In most places where these two rock types are interbedded, the carbonate rocks yield much more water than the sandstone. The larger and majority of these are found in Mid- West Texas and along the Appalachians range, and to a lesser degree in Montana and Wyoming. The aquifers in the Mammoth Cave area of Kentucky are examples of sandstone and carbonate-rock aquifers.

post-depositional change in carbonate rocks is the dissolution of part of the rock by circulating, slightly acidic groundwater. Solution openings in carbonate rocks range from small tubes and widened joints to caverns that may be tens of meters wide and hundreds to thousands of meters long. Where they are saturated, carbonate rocks with well-connected networks of solution openings yield large amounts of water to wells that penetrate the openings, although the undissolved rock between the large openings may be almost impermeable.

· Other:

Rocks identified as "other" include large-to-small areas that are designated "minor aquifer," "not a principal aquifer," or "confining unit"

Such areas are underlain by low-permeability deposits and rocks, unsaturated materials, or aquifers that supply little water because they are of local extent, poorly permeable, or both. Permeability is the relative ease with which water will move through a rock unit; aquifers are more permeable than confining units. Rocks and deposits with minimal permeability, which are not considered to be aquifers, consist of intrusive igneous rocks, metamorphic rocks, shale, siltstone, evaporate deposits, silt, and clay.

Large areas of the eastern, northeastern, and north-central parts of the Nation are underlain by crystalline rock. These igneous and metamorphic rocks are permeable only where they are fractured, and they generally yield only small amounts of water to wells. However, because these rocks extend over large areas, large volumes of groundwater are withdrawn from them, and in many places they are the only reliable source of water supply. Because the crystalline rocks have minimal permeability, they are not mapped as principal aquifers, but they are mapped as other rocks.

Superficial stream valley aquifers or buried principal aquifers are also sometimes in some places categorized as "other". Local stream-valley alluvial aquifers south of the line of continental glaciation that yield small-to-large amounts of water are in the valleys of many major streams that cross principal aquifers, but the stream-valley aquifers are not mapped consistently between states.

Recharging Aquifers:

Water (Rain, Snowbelt, etc) flowing into recharge areas, which is usually land covered with soil and trees, refills the aquifer. Bogs and swamps may absorb and store water that later slowly drains into aquifers.

Some major US Aquifers:

- The Biscayne aquifer in Southeast Florida is an unconfined aquifer, which means that top portion of the aquifer is the water table. It is also a coastal aquifer because it merges with the floor of Biscayne Bay and the Atlantic Ocean. It covers over 4,000 square miles, underlying parts of four Counties, including all of Dade and a majority of Broward. The base of the aquifer slopes seaward about 240’ below sea level.

- The Edwards Aquifer is one of the most prolific artesian aquifers in the world. Located on the eastern edge of Edwards Plateau in the state of Texas, it discharges about 900,000 *acre feet (1.1 km³) of water a year. A water-table aquifer is one in which the water is under atmospheric pressure. Water will not rise above the level of the "table", and the table rises and falls in response to rainfall and recharge. Only a small portion of the Edwards is a water-table aquifer. The water-table portion of the Edwards is the recharge zone, where the Edwards limestone is exposed at the land surface. Here, because there are no confining rock layers on top of the Edwards, the water is under atmospheric pressure. Water will not rise in a well above the level of the water table. Once recharge water works its way by gravity down into the artesian zone, there are other rock formations lying over the Edwards, and water is trapped inside. The artesian zone of the Edwards is confined between two relatively impermeable formations - the Glen Rose formation below and the Del Rio clay on top. The sheer weight of new water entering the Aquifer in the recharge zone puts tremendous pressure on water that is already deeper down in the formation. Flowing artesian wells and springs exist where hydraulic pressure is sufficient to force water up through wells and faults to the surface.

- The Floridan aquifer system underlies an area of about 100,000 square miles in southeastern Mississippi, southern Alabama, southern Georgia, southern South Carolina, and all of Florida. The Floridan is one of the most productive aquifer systems in the world. Florida has 27 springs which discharge 100 cubic feet per second or more, out of 78 in the Nation. All these springs issue from the Upper Floridan aquifer and practically all of them are located in places where the aquifer is exposed at the land surface or is covered by less than 100 feet of clayey upper confining unit. Dissolution of the carbonate rocks of the aquifer in these places has resulted in the development of large caverns, many of which channel the ground water to major spring orifices. Some of the springs are large enough to form the headwaters of surface streams.

- The Kirkwood-Cohansey aquifer system is in the New Jersey Pine Barrens. It is considered one of the purest sources of water in the US. It receives about 44 inches of precipitation each year. About fifty percent of this water is transpired by vegetation or evaporates back into the atmosphere. A small amount enters streams and rivers as storm runoff. About 17 to 20 inches annually actually enters the ground.

- The Ogallala aquifer (High Plains Aquifer) occupies the High Plains of the United States, extending northward from western Texas to South Dakota. The Ogallala is the leading geologic formation in what is known as the High Plains Aquifer System. The entire system underlies about 450,000 square kilometers (174,000 square miles) of eight states. It is one of the World’s largest aquifers, and approximately 26 percent of the irrigated land in the United States overlies this aquifer system, which yields about 30 percent of the nation's ground water used for irrigation. In addition, the aquifer system provides drinking water to 82 percent of the people who live within the aquifer boundary

- The Mahomet aquifer is the most important aquifer in east-central Illinois. The sand and gravel aquifer is part of the buried Mahomet Bedrock Valley. It underlies 15 counties and ranges from 50 to 200 feet. The Mahomet Aquifer consists of sand and gravel deposited by glacial meltwater flowing westward along the Mahomet Bedrock Valley during the pre-Illinois glacial episode. This bedrock valley forms the western part of the Teays-Mahomet Bedrock Valley System that extends into Illinois from Indiana.

- The Spokane Valley-Rathdrum Prairie aquifer is designated a "sole source aquifer" and extends across an area covering about 325 square miles and provides drinking water for nearly 400,000 people. Originating from the southern end of Lake Pend Oreille (pronounced ponderay), the unconfined aquifer extends beneath the Rathdrum Prairie in Idaho, and Washington's Spokane Valley. Most of the groundwater then flows north up the "Hillyard trough", and discharges as springs along the Little Spokane River.

*It is defined by the volume of one acre of surface area to a depth of one foot. Since the area of one acre is defined as 66 by 660 feet then the volume of an acre-foot is exactly 43560 cubic feet

All of the Coastal Plain aquifers (which include Mississippi embayment and Southeastern Coastal Plain) and aquifer systems are comprised predominantly of poorly consolidated to unconsolidated clastic sedimentary rocks. The distribution and pattern of permeability within the different Coastal Plain aquifer systems are a function of lithology and primary porosity. In general, the most permeable Coastal Plain aquifers consist of sand and some gravel and are separated by silt, clay, marl, or chalk confining units. As these aquifers extend down-dip, most grade to less permeable facies, such as clay or marl, that are part of adjoining confining units.

The surficial aquifer system consists of alluvial aquifers and includes one major and three minor aquifers. In terms of water use and areal extent, the most important aquifer is the highly productive Mississippi River Valley alluvial aquifer. The minor aquifers include the Arkansas River, the Ouachita-Saline Rivers, and the Red River alluvial aquifers. The Arkansas River alluvial aquifer is not as widespread as the other two aquifers, but locally is an important water source.

There are two end members in the spectrum of types of aquifers; confined and unconfined (with semi-confined being in between). Unconfined aquifers are sometimes also called water table or phreatic aquifers, because their upper boundary is the water table or phreatic surface. Typically (but not always) the shallowest aquifer at a given location is unconfined, meaning it does not have a confining layer (an aquitard or aquiclude) between it and the surface. The term "perched" refers to ground water accumulating above a low-permeability unit or strata, such as a clay layer. This term is generally used to refer to a small local area of ground water that occurs at an elevation higher than a regionally-extensive aquifer. The difference between perched and unconfined aquifers is their size (perched is smaller).

STANDARD WATER-USE TERMS

acre-foot (acre-ft)---the volume of water required to cover 1 acre of land (43,560 square feet) to a depth of 1 foot.

animal specialties---water use associated with the production of fish in captivity except fish hatcheries, fur-bearing animals in captivity, horses, rabbits, and pets. See also livestock water use.

aquaculture---farming of organisms that live in water, such as fish, shellfish, and algae.

aquifer---a geologic formation, group of formations, or part of a formation that contains sufficient saturated permeable material to yield significant quantities of water to wells and springs.

Aquitard is a zone within the earth that restricts the flow of groundwater from one aquifer to another. An aquitard can sometimes, if completely impermeable, be called an Aquiclude or Aquifuge.

commercial water use---water for motels, hotels, restaurants, office buildings, other commercial facilities, and institutions. The water may be obtained from a public supply or may be self supplied. See also public supply and self- supplied water.

consumptive use---that part of water withdrawn that is evaporated, transpired, incorporated into products or crops, consumed by humans or livestock, or otherwise removed from the immediate water environment. Also referred to as water consumed.

conveyance loss---water that is lost in transit from a pipe, canal, conduit, or ditch by leakage or evaporation. Generally, the water is not available for further use; however, leakage from an irrigation ditch, for example, may percolate to a ground-water source and be available for further use.

cooling water---water used for cooling purposes, such as of condensers and nuclear reactors.

delivery/release---the amount of water delivered to the point of use and the amount released after use; the difference between these amounts is usually the same as the consumptive use. See also consumptive use.

domestic water use---water for household purposes, such as drinking, food preparation, bathing, washing clothes and dishes, flushing toilets, and watering lawns and gardens. Also called residential water use. The water may be obtained from a public supply or may be self supplied. See also public supply and self-supplied water.

evaporation---process by which water is changed from a liquid into a vapor. See also evapotranspiration and transpiration.

evapotranspiration---a collective term that includes water discharged to the atmosphere as a result of evaporation from the soil and surface-water bodies and as a result of plant transpiration. See also evaporation and transpiration.

freshwater---water that contains less than 1,000 milligrams per liter (mg/L) of dissolved solids; generally, more than 500 mg/L of dissolved solids is undesirable for drinking and many industrial uses.

ground water---generally all subsurface water as distinct from surface water; specifically, that part of the subsurface water in the saturated zone (a zone in which all voids are filled with water) where the water is under pressure greater than atmospheric.

hydroelectric power water use---the use of water in the generation of electricity at plants where the turbine generators are driven by falling water. Hydroelectric water use is classified as an instream use in this report.

in-channel use---see instream use.

industrial water use---water used for industrial purposes such as fabrication, processing, washing, and cooling, and includes such industries as steel, chemical and allied products, paper and allied products, mining, and petroleum refining. The water may be obtained from a public supply or may be self supplied. See also public supply and self- supplied water.

instream use---water that is used, but not withdrawn, from a ground- or surface-water source for such purposes as hydroelectric power generation, navigation, water-quality improvement, fish propagation, and recreation. Sometimes called nonwithdrawal use or in-channel use.

irrigation district---a cooperative, self-governing public corporation set up as a subdivision of the State government, with definite geographic boundaries, organized and having taxing power to obtain and distribute water for irrigation of lands within the district; created under the authority of a State legislature with the consent of a designated fraction of the landowners or citizens.

irrigation water use---artificial application of water on lands to assist in the growing of crops and pastures or to maintain vegetative growth in recreational lands such as parks and golf courses.

kilowatt hour (kWh)---a unit of energy equivalent to one thousand watthours.

livestock water use---water for livestock watering, feed lots, dairy operations, fish farming, and other on-farm needs. Livestock as used here includes cattle, sheep, goats, hogs, and poultry. Also included are animal specialties. See also rural water use and animal specialties water use.

million gallons per day (Mgal/d)---a rate of flow of water.

mining water use---water use for the extraction of minerals occurring naturally including solids, such as coal and ores; liquids, such as crude petroleum; and gases, such as natural gas. Also includes uses associated with quarrying, well operations (dewatering), milling (crushing, screening, washing, floatation, and so forth), and other preparations customarily done at the mine site or as part of a mining activity. Does not include water used in processing, such as smelting, refining petroleum, or slurry pipeline operations. These uses are included in industrial water use.

off-stream use---water withdrawn or diverted from a ground- or surface-water source for public-water supply, industry, irrigation, livestock, thermoelectric power generation, and other uses. Sometimes called off-channel use or withdrawal.

per capita use---the average amount of water used per person during a standard time period, generally per day.

public supply---water withdrawn by public and private water suppliers and delivered to users. Public suppliers provide water for a variety of uses, such as domestic, commercial, thermoelectric power, industrial, and public water use. See also commercial water use, domestic water use, thermoelectric power water use, industrial water use, and public water use.

public-supply deliveries---water provided to users through a public-supply distribution system.

public water use---water supplied from a public-water supply and used for such purposes as firefighting, street washing, and municipal parks and swimming pools. See also public supply.

reclaimed wastewater---wastewater treatment plant effluent that has been diverted for beneficial use before it reaches a natural waterway or aquifer.

recycled water---water that is used more than one time before it passes back into the natural hydrologic system.

residential water use---see domestic water use.

return flow---the water that reaches a ground- or surface-water source after release from the point of use and thus becomes available for further use.

reuse---see recycled water.

rural water use---term used in previous water-use circulars to describe water used in suburban or farm areas for domestic and livestock needs. The water generally is self supplied, and includes domestic use, drinking water for livestock, and other uses, such as dairy sanitation, evaporation from stock-watering ponds, and cleaning and waste disposal. See also domestic water use, livestock water use, and self-supplied water.

saline water---water that contains more than 1,000 milligrams per liter of dissolved solids.

self-supplied water---water withdrawn from a surface- or ground-water source by a user rather than being obtained from a public supply.

standard industrial classification (SIC) codes---four- digit codes established by the Office of Management and Budget and used in the classification of establishments by type of activity in which they are engaged.

surface water---an open body of water, such as a stream or a lake.

thermoelectric power water use---water used in the process of the generation of thermoelectric power. The water may be obtained from a public supply or may be self supplied. See also public supply and self-supplied water.

transpiration---process by which water that is absorbed by plants, usually through the roots, is evaporated into the atmosphere from the plant surface. See also evaporation and evapotranspiration.

wastewater---water that carries wastes from homes, businesses, and industries.

wastewater treatment---the processing of wastewater for the removal or reduction of contained solids or other undesirable constituents.

wastewater-treatment return flow---water returned to the hydrologic system by wastewater-treatment facilities.

water-resources region---designated natural drainage basin or hydrologic area that contains either the drainage area of a major river or the combined drainage areas of two or more rivers; of 21 regions, 18 are in the conterminous United States, and one each are in Alaska, Hawaii, and the Caribbean. (See map on inside of front cover.)

water-resources subregion---the 21 designated water-resources regions of the United States are subdivided into 222 subregions. Each subregion includes that area drained by a river system, a reach of a river and its tributaries in that reach, a closed basin(s), or a group of streams forming a coastal drainage system.

water transfer---artificial conveyance of water from one area to another.

water use---1) in a restrictive sense, the term refers to water that is actually used for a specific purpose, such as for domestic use, irrigation, or industrial processing. In this report, the quantity of water use for a specific category is the combination of self-supplied withdrawals and public-supply deliveries. 2) More broadly, water use pertains to human's interaction with and influence on the hydrologic cycle, and includes elements such as water withdrawal, delivery, consumptive use, wastewater release, reclaimed wastewater, return flow, and instream use. See also offstream use and instream use.

watthour (Wh)---an electrical energy unit of measure equal to one watt of power supplied to, or taken from, an electrical circuit steadily for one hour.

withdrawal---water removed from the ground or diverted from a surface-water source for use. See also offstream use and self-supplied water.