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An aquifer is an underground layer of water-bearing permeable rock, or permeable mixtures of unconsolidated materials (gravel, sand, silt, or clay) (see also groundwater). Some productive aquifers are in fractured rock (carbonate rock, basalt, or sandstone). The study of water flow in aquifers and the characterization of aquifers is hydrogeology.
Saturated vs. unsaturated
Groundwater can be found at nearly every point in the earth's shallow subsurface, to some degree; although aquifers do not necessarily contain fresh water. The earth's crust can be divided into two regions: the saturated zone (e.g., aquifers, aquitards, etc.) and the unsaturated zone (also called the vadose zone). Saturated means the pressure head of the water is greater than atmospheric pressure (it has a gauge pressure > 0). The definition of the water table is surface where the pressure head is equal to atmospheric pressure (where gauge pressure = 0). Unsaturated conditions occur above the water table where the pressure head is negative (absolute pressure can never be negative, but gauge pressure can) and the water which incompletely fills the pores of the aquifer material is under suction. The water content in the unsaturated zone is held in place by surface adhesive forces and it rises above the water table (the zero gauge pressure isobar) by capillary action to completely saturate a small zone above the phreatic surface (the capillary fringe) at less than atmospheric pressure. The capillary head depends on the porosity of the soil, in sandy soils with larger pores the head will be less than in clayey soils with very small pores.
The capillary rise of water in a small diameter tube is this same physical process. The water table is the level to which water will rise in a large diameter pipe (e.g. a well) which goes down into the aquifer, and is open to the atmosphere.
Aquifers vs. aquitards
Aquifers are typically saturated regions of the subsurface which produce an economically feasible quantity of water to a well or spring (e.g., sand and gravel or fractured bedrock often make good aquifer materials). Aquitards (sometimes, if completely impermeable, called an aquifuge) are saturated regions, which due to lower hydraulic conductivity, do not yield a sustainable amount of water in a economic fashion (e.g., clay, silt or fresh bedrock often form aquitards). Economically feasible is a relative term; for example, an aquifer that is quite adequate for local domestic use, as in a rural area, might be considered an inadequate aquitard for industrial, mining, or urban water supply.
In non-mountainous areas (or near rivers in mountainous areas), the main aquifers are typically unconsolidated alluvium. They are typically composed of mostly horizontal layers of materials deposited by water processes (rivers and streams), which in cross-section (looking at a two-dimensional slice of the aquifer) appear to be layers of alternating coarse and fine materials. Coarse materials, due to the high energy needed to move them, tend to be found nearer the source (mountain fronts or rivers), while the fine-grained material will make it farther from the source (to the flatter parts of the basin or overbank areas - sometimes called the pressure area). Since there are less fine-grained deposits near the source, this is a place where aquifers are often unconfined (sometimes called the forebay area), or in hydraulic communication with the land surface.
Confined vs. 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.
If the distinction between confined and unconfined is not clear geologically (it is not known if a clear confining layer exists, or the geology is more complex, i.e., fractured bedrock), 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).
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 exessive.
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 are 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.
An example of a significant and sustainable carbonate aquifer is the Edwards Aquifer  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 kmē of area, from central Brazil to northern Argentina.
Aquifer depletion is a global problem, and is especially critical in northern Africa. 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 is being rapidly depleted, primarily for agriculture use. This huge aquifer, which underlies portions of eight states, contain primarily fossil water from the time of the last glaciation. Annual recharge is estimated to total only about ten percent of annual withdrawals.
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, subsidence due to groundwater extraction is permanent.
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