Aquifer
An aquifer is an underground layer of water-bearing permeable rock or unconsolidated materials (gravel, sand, silt, or clay) from which groundwater can be usefully extracted using a water well. The study of water flow in aquifers and the characterization of aquifers is called hydrogeology.
Description of an aquifer system
The diagram on the right shows a cross-sectional view of a simple aquifer system consisting of a confined aquifer below an unconfined aquifer. The two aquifers are separated from each other by a zone called an aquitard, which is a layer of low porosity that restricts the flow of groundwater. The aquitard is said to have "low hydraulic conductivity." The entire aquifer system is surrounded by bedrock known as aquiclude, which has extremely low hydraulic conductivity—that is, it is almost completely impermeable. The arrows in the diagram indicate the direction of flow of groundwater. The water in the aquifer system feeds a stream.
The upper limit of abundant groundwater (in the unconfined aquifer) is called the water table. The zone below the water table is called the zone of saturation or phreatic zone; the zone above the water table is called the unsaturated zone or vadose zone ("vadose" is Latin for "shallow"). Water in the vadose zone is retained by a combination of adhesion and capillary action. It should be noted that aquifers do not necessarily contain freshwater.
Saturated versus unsaturated zones
Groundwater can be found in nearly every part of the Earth's shallow subsurface, to some extent. From this perspective, the Earth's crust can be divided into two regions: the saturated zone (including aquifers and aquitards), where all available spaces are filled with water; and the unsaturated zone (vadose zone), which contains pockets of air that can be replaced by water.
The water table, by definition, is the surface where the pressure head of water is equal to atmospheric pressure (gauge pressure = 0). In the saturated zone, the pressure head of water is greater than atmospheric pressure; and in the unsaturated conditions above the water table, the pressure head is less than atmospheric pressure.
Aquifers versus aquitards
Aquifers are typically saturated regions of the subsurface that produce economically feasible quantities of water to a well or spring. Sand, gravel, and fractured bedrock make good aquifer materials. An aquitard is a zone that restricts the flow of groundwater from one aquifer to another. Aquitards are made up of layers of clay or nonporous rock. "Economically feasible" is a relative term; for example, an aquifer that is adequate for domestic use in a rural area may be considered inadequate for industrial, mining, or urban needs.
In nonmountainous areas, or near rivers in mountainous areas, the main aquifers are typically unconsolidated alluvium. They are typically composed of horizontal layers of materials deposited by rivers and streams. In cross-section, (looking at a two-dimensional slice of the aquifer), they appear to be layers of alternating coarse and fine materials.
Confined versus unconfined
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. Unconfined aquifers usually receive recharge water directly from the surface, from precipitation or from a body of surface water (e.g., a river, stream, or lake) which is in hydraulic connection with it. Confined aquifers have the water table above their upper boundary (an aquitard or aquiclude), and are typically found below unconfined aquifers. A "perched aquifer" occurs when the porous, water-bearing segment of rock is located on top of a layer of non-porous rock.
If the distinction between confined and unconfined is not clear geologically (i.e., if it is not known if a clear confining layer exists, or if the geology is more complex, e.g., a fractured bedrock aquifer), the value of storativity returned from an aquifer test can be used to determine it (although aquifer tests in unconfined aquifers should be interpreted differently than confined ones). Confined aquifers have very low storativity values (much less than 0.01, and as little as 10-5), which means that the aquifer is storing water using the mechanisms of aquifer matrix expansion and the compressibility of water, which typically are both quite small quantities. Unconfined aquifers have storativities (typically then called specific yield) greater than 0.01 (1% of bulk volume); they release water from storage by the mechanism of actually draining the pores of the aquifer, releasing relatively large amounts of water (up to the drainable porosity of the aquifer material, or the minimum volumetric water content).
Misconception about aquifers and groundwater
A common misconception is that groundwater exists in underground rivers (e.g. caves where water flows freely underground). This is only sometimes true in eroded limestone areas known as karst topography which make up only a small percentage of Earth's area. More usual is that the pore spaces of rocks in the subsurface are simply saturated with water — like a kitchen sponge — which can be pumped out and used for agricultural, industrial or municipal uses.
The beach is an example of what most aquifers are like. If you dig a hole into the sand at the beach you will find very wet or saturated sand at a shallow depth. This hole is a crude well, the beach sand is an aquifer, and the level to which the water rises in this hole represents the water table.
Human dependence on groundwater
Most land areas on Earth have some form of aquifer underlying them, sometimes at significant depths. Fresh water aquifers, especially those with limited recharge by meteoric water, can be over-exploited and, depending on the local hydrogeology, may draw in non-potable water or saltwater (saltwater intrusion) from hydraulically connected aquifers or surface water bodies. This can be a serious problem especially in coastal areas and other areas where aquifer pumping is excessive.
Aquifers are critically important in human habitation and agriculture. Deep aquifers in arid areas have long been water sources for irrigation (see Ogallala below). Many villages and even large cities draw their water supply from wells in aquifers.
Some aquifers are "riparian aquifers". These are related to rivers, fluvial deposits, or unconsolidated deposits along river corridors, and are usually rapidly replenished by infiltration of surface water. Some municipal well fields are specifically designed to take advantage of induced infiltration of surface (usually river) water, leaving them potentially vulnerable to water quality problems in the surface water body (chemical spills, petroleum spills, and bacteriological problems).
Aquifers that provide sustainable fresh groundwater to urban areas and for agricultural irrigation are typically close to the ground surface (within a couple of hundred meters) and have some recharge by fresh water. This recharge is typically from rivers or meteoric water (precipitation) that percolate into the aquifer through overlying unsaturated materials.
Subsidence
In unconsolidated aquifers, groundwater is produced from pore spaces between particles of gravel, sand, and silt. If the aquifer is confined by low-permeability layers, the reduced water pressure in the sand and gravel causes slow drainage of water from the adjoining confining layers. If these confining layers are composed of compressible silt or clay, the loss of water to the aquifer reduces the water pressure in the confining layer, causing it to compress due to the weight of overlying geologic materials. In severe cases, this compression can be observed on the ground surface as subsidence. Unfortunately, much of the subsidence due to groundwater extraction is permanent (elastic rebound is small). Thus the subsidence is not only permanent, but the compressed aquifer has a permanently-reduced capacity to hold water.
Examples
An example of a significant and sustainable carbonate aquifer is the Edwards Aquifer [1] in central Texas. This carbonate aquifer has historically been providing high-quality water for nearly 2 million people and, even today, is completely full because of tremendous recharge from a number of area streams, rivers and lakes. The primary risk to this resource is human development over the recharge areas.
One of the largest aquifers in the world is the Guarani Aquifer, with 1.2 million km² of area, from central Brazil to northern Argentina.
Aquifer depletion is a problem in some areas, and is especially critical in northern Africa; see the Great Manmade River project of Libya for an example. However, new methods of groundwater management such as artificial recharge and injection of surface waters during seasonal wet periods has extended the life of many freshwater aquifers, especially in the United States.
The Ogallala Aquifer of the central United States is one of the world's great aquifers, but in places it is being rapidly depleted for growing municipal use, and continuing agricultural use. This huge aquifer, which underlies portions of eight states, contain primarily fossil water from the time of the last glaciation. Annual recharge, in the more arid portions of the aquifer, is estimated to total only about ten percent of annual withdrawals.
The Mahomet Aquifer supplies water to some 800,000 people in central Illinois and contains approximately four trillion US gallons (15 km³) of water. The Mahomet Aquifer Consortium [2] was formed in 1998 to study the aquifer with hopes of ensuring the water supply and reducing potential user conflicts.
The Great Artesian Basin is one of the largest groundwater aquifers in the world. It plays a large part in water supplies for remote parts of South Australia.
For more aquifers, see List of aquifers.
External links
See also
| physical aquifer properties used in hydrogeology |
|---|
| hydraulic head | hydraulic conductivity | storativity | porosity | water content |
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