Aquifer
An aquifer is an underground layer of water-bearing material consisting of permeable or fractured rock, or of unconsolidated materials. Aquifers vary greatly in their characteristics. The study of water flow in aquifers and the characterization of aquifers is called hydrogeology. Related concepts include aquitard, a bed of low permeability along an aquifer, and aquiclude, a solid and impermeable region underlying or overlying an aquifer, the pressure of which could lead to the formation of a confined aquifer. Aquifers can be classified as saturated versus unsaturated; aquifers versus aquitards; confined versus unconfined; isotropic versus anisotropic; porous, karst, or fractured; and transboundary aquifer.
Groundwater from aquifers is sustainably harvested by humans through the use of wells. This groundwater is mainly used for agricultral purposes but is used for other reasons such as home use, industrial use and is sometimes used as a source of renewable energy. Groundwater is a major source of fresh water for many regions, although it can present various challenges for the environment, such as overdrafting, groundwater-related subsidence of land, and the salinization or pollution of the groundwater.
Properties
Depth
Aquifers occur from near-surface to deeper than. Those closer to the surface are not only more likely to be used for water supply and irrigation, but are also more likely to be replenished by local rainfall.Although aquifers are sometimes characterized as "underground rivers or lakes," they are actually porous rock saturated with water.Many desert areas have limestone hills or mountains within them or close to them that can be exploited as groundwater resources. Part of the Atlas Mountains in North Africa, the Lebanon and Anti-Lebanon ranges between Syria and Lebanon, the Jebel Akhdar in Oman, parts of the Sierra Nevada and neighboring ranges in the United States' Southwest, have shallow aquifers that are exploited for their water. Overexploitation can lead to the exceeding of the practical sustained yield; i.e., more water is 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 a lowering of the water table and the subsequent contamination of the groundwater with saltwater from the sea.
In 2013 large aquifers containing offshore freshened groundwater were discovered under continental shelves of Australia, China, North America and South Africa. They contain an estimated half a million cubic kilometers of "low salinity" water that could be economically processed into potable water. The reserves formed when ocean levels were lower and rainwater made its way into the ground in land areas that were not submerged until the ice age ended 20,000 years ago. The volume is estimated to be 100 times the amount of water extracted from other aquifers since 1900.
Groundwater recharge
Classification
An 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. Aquitards are composed of layers of either clay or non-porous rock with low hydraulic conductivity.Saturated versus 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 or phreatic zone, where all available spaces are filled with water, and the unsaturated zone, where there are still pockets of air that contain some water, but can be filled with more water.Saturated means the pressure head of the water is greater than atmospheric pressure. The definition of the water table is the surface where the pressure head is equal to atmospheric pressure.
Unsaturated conditions occur above the water table where the pressure head is negative and the water that 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 by capillary action to saturate a small zone above the phreatic surface at less than atmospheric pressure. This is termed tension saturation and is not the same as saturation on a water-content basis. Water content in a capillary fringe decreases with increasing distance from the phreatic surface. The capillary head depends on soil pore size. In sandy soils with larger pores, the head will be less than in clay soils with very small pores. The normal capillary rise in a clayey soil is less than but can range between.
The capillary rise of water in a small-diameter tube involves the same physical process. The water table is the level to which water will rise in a large-diameter pipe that goes down into the aquifer and is open to the atmosphere.
Aquifers versus aquitards
Aquifers are typically saturated regions of the subsurface that produce an economically feasible quantity of water to a well or spring.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 contain layers of either clay or non-porous rock with low hydraulic conductivity.
In mountainous areas especially near rivers, the main aquifers are typically unconsolidated alluvium, that is horizontal layers of materials deposited by water processes such as rivers and streams. In cross-section, they appear as layers of alternating coarse and fine materials. Coarse materials, because of the high energy needed to move them, tend to be found nearer to their source, such as mountain fronts or rivers, whereas the fine-grained material travels farther, 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, those aquifers, also known as the fore-bay area, are often unconfined or in hydraulic communication with the land surface.
Confined versus unconfined
An unconfined aquifer has no impermeable barrier immediately above it, such that the water level can rise in response to recharge. A confined aquifer has an overlying impermeable barrier that prevents the water level in the aquifer from rising any higher. An aquifer in the same geologic unit may be confined in one area and unconfined in another. Unconfined aquifers are sometimes also called water table or phreatic aquifers, because their upper boundary is the water table or phreatic surface. Typically the shallowest aquifer at a given location is unconfined, meaning it does not have a confining layer 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. Confined aquifers are aquifers that are overlain by a confining layer, often made up of clay. The confining layer might offer some protection from surface contamination.If the distinction between confined and unconfined is not clear geologically, the value of storativity returned from an aquifer test can be used to determine it. Confined aquifers have very low storativity values, 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 greater than 0.01 ; they release water from storage by the mechanism of actually draining the pores of the aquifer, releasing relatively large amounts of water.
Isotropic versus anisotropic
In isotropic aquifers or aquifer layers the hydraulic conductivity is equal for flow in all directions, while in anisotropic conditions it differs, notably in horizontal and vertical sense.Semi-confined aquifers with one or more aquitards work as an anisotropic system, even when the separate layers are isotropic, because the compound Kh and Kv values are different.
When calculating flow to drains or flow to wells in an aquifer, the anisotropy is to be taken into account lest the resulting design of the drainage system may be faulty.
Porous, karst, or fractured
To properly manage an aquifer its properties must be understood. Many properties must be known to predict how an aquifer will respond to rainfall, drought, pumping, and contamination. Considerations include where and how much water enters the groundwater from rainfall and snowmelt, how fast and in what direction the groundwater travels, and how much water leaves the ground as springs. Computer models can be used to test how accurately the understanding of the aquifer properties matches the actual aquifer performance. Environmental regulations require sites with potential sources of contamination to demonstrate that the hydrology has been characterized.Porous
Porous aquifers typically occur in sand and sandstone. Their properties depend on the depositional sedimentary environment and later natural cementation of the sand grains. The environment where a sand body was deposited controls the orientation of the sand grains, the horizontal and vertical variations, and the distribution of shale layers. Even thin shale layers are important barriers to groundwater flow. All these factors affect the porosity and permeability of sandy aquifers.Sandy deposits formed in shallow marine environments and in windblown sand dune environments have moderate to high permeability while sandy deposits formed in river environments have low to moderate permeability. Rainfall and snowmelt enter the groundwater where the aquifer is near the surface. Groundwater flow directions can be determined from potentiometric surface maps of water levels in wells and springs. Aquifer tests and well tests can be used with Darcy's law flow equations to determine the ability of a porous aquifer to convey water.
Analyzing this type of information over an area gives an indication how much water can be pumped without overdrafting and how contamination will travel. In porous aquifers groundwater flows as slow seepage in pores between sand grains. A groundwater flow rate of 1 foot per day is considered to be a high rate for porous aquifers, as illustrated by the water slowly seeping from sandstone in the accompanying image to the left.
Porosity is important, but, alone, it does not determine a rock's ability to act as an aquifer. Areas of the Deccan Traps, a basaltic lava formation in west-central India, are examples of rock formations with high porosity but low permeability, making them poor aquifers. Similarly, the micro-porous Chalk Group of south east England, although having a reasonably high porosity, has a low grain-to-grain permeability, with its good water-yielding characteristics mostly due to micro-fracturing and fissuring.