Exactly what is an aquifer? Aquifers are geologic formations, a group of formations, or a part of a formation that contains enough saturated permeable material to result in significant quantities of water for wells and springs.
A two-dimensional map representation of the principal aquifers in the United States is shown below. Derived from the Ground Water Atlas of the United States, this map shows the extent of our uppermost principal aquifers on a national scale. A principal aquifer is defined as a regionally extensive aquifer or aquifer system that has the potential to be used as a source of potable water.
Those closer to the surface are more likely to be used for water supply and irrigation but are also more likely to be partially replenished by rainfall. Many desert areas contain limestone hills or mountains that act as groundwater resources. The Sierra Nevada and neighboring ranges in the Southwest United States have shallow aquifers that are exploited for their water. Over-exploitation can result in more water being taken out than can be replenished. Along the coastlines of certain countries such as Libya and Israel, increased water usage associated with population growth has caused lowering of the water table and the subsequent contamination of the groundwater with saltwater from the Mediterranean and Red Seas.
If a hole is dug in sand, very wet or saturated sand will be located at a shallow depth. This hole is a crude well, the wet sand represents an aquifer, and the level to which the water rises in this hole represents the water table. Aquifers are typically water-saturated areas below the ground's surface that produce an economically-feasible quantity of water for a well or spring. Sand, gravel or fractured bedrock can make good aquifer materials.
An aquitard is a zone within the earth that restricts the flow of groundwater from one aquifer to another. A completely impermeable aquitard is called an aquiclude or aquifuge. Aquitards are made up of layers of clay or non-porous rock with low hydraulic conductivity, the ease with which a fluid can move through pore spaces or fractures.
In mountainous areas or near rivers in mountainous areas, the main aquifers are typically alluvium composed mostly of horizontal layers of materials deposited by rivers and streams. Because of the high energy needed to move them, the larger rocks in the material tend to be found nearer mountains or rivers, whereas the fine-grained material will move farther from the source. Since there are fewer fine-grained deposits near the source, this is a place where aquifers are often unconfined.
(photos above and below: Mammoth Cave, KY, a karst aquifer)
These are termed as either confined, unconfined, and semi-confined. Unconfined aquifers are sometimes called water table or phreatic aquifers because their upper boundary is the water table. Typically, but not always, the shallowest aquifer at a given location is unconfined which means it does not have a confining layer between it and the surface. The term perched refers to ground water accumulating above a low-permeability area such as clay. This term is generally used to refer to a small, local area of groundwater that occurs at an elevation higher than a regional aquifer. The difference between perched and unconfined aquifers is their size. Perched is smaller. Confined aquifers are aquifers with a confining layer on top. The confining layer may provide some protection from surface contamination.
Studies are necessary in order to predict how an aquifer will respond to rainfall, drought, pumping, contamination, where and how much water enters the groundwater from rainfall and snowmelt, how fast and what direction the groundwater traveled, how much water leaves the ground as springs or evaporation, how much water can be sustainably pumped out, and how quickly a contamination incident could reach a well or spring. Computer models can be used to test how accurately the aquifer properties match the actual performance. Environmental regulations require sites with potential sources of contamination to demonstrate that the hydrology has been characterized.
Porous aquifers typically occur in sand and sandstone. Karst aquifers typically develop in limestone. Surface water containing natural carbonic acid moves down into small fissures in limestone. This acid gradually dissolves limestone and widens the fissures. The enlarged fissures allow more water to enter which leads to a progressive enlargement of the openings. Abundant small openings store a large quantity of water. The larger openings create a conduit system that drains the aquifer to springs.
Aquifers are critically important
Deep aquifers like the Ogallala have long been water sources for irrigation. Many villages and even large cities draw their water supply from wells in aquifers. Municipal, irrigation, and industrial water supplies are provided through large wells. Multiple wells for one water supply source are called wellfields. Using ground water from deep, confined aquifers provides more protection from surface water contamination.
Aquifers that provide sustainable fresh groundwater to urban areas and for agricultural irrigation are typically close to the ground surface and can have some recharge of fresh water from rivers or precipitation that percolates into the aquifer.