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 springs and wells. Use of the term is often restricted to those water-bearing formations capable of yielding water in sufficient quantity to constitute a usable supply for people's uses.
Formation of Aquifers
|Diagram of an aquifer. (Source: By Hans Hillewaert [CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0) or Public domain], via Wikimedia Commons|
Aquifers are underground layers of porous rock or sand that allows the movement of water between layers of non-porous rock (sandstone, gravel, or fractured limestone or granite). Many people tend to think of aquifers as “underground lakes” which is not the case because water is held between rock particles. Water infiltrates into the soil through pores, cracks, and other spaces until it reaches the zone of saturation where all of the spaces are filled with water (rather than air). The zone of saturation occurs because water infiltrating the soil reaches an impermeable layer of rocks so that it is not able to penetrate any further into the earth (this impermeable layer is known as an “aquitard” or “aquiclude”). Water held in aquifers is know as groundwater.
The top of the zone of saturation is known as the water table. The water table typically follows the form of the above ground topography. The depth of the water table is typically greater in regions with low rainfall than in regions with high rainfall. The water table can rise in wet years and fall in dry years.
Movement of Water Through Aquifers
Two main forces drive the movement of groundwater. First water moves from higher elevations to lower elevation due to the effect of gravity. Second, water moves from areas of higher pressure to areas of lower pressure. Together these two forces make up the driving force behind moving groundwater which is known as the hydraulic head.
Water has the potential to move through four different types of rocks: unconsolidated rock, porous sedimentary rocks, porous volcanic rocks, and fractured rocks. In unconsolidated materials, the particles are not attached to each other in a coherent way (e.g., sand would be unconsolidated but sandstone would be consolidated). Water is able to move through the spaces between particles. Gravel and sand aquifers are common. Because there are more spaces between particles when particles are larger, water moves more quickly through layers of large particles (e.g., gravel) than it does through layers of small particles (e.g., clay).
Carbonate rocks, such as limestone, are brittle so they tend to fracture and these fractures allow some water movement. More importantly, because water will dissolve carbonate rocks once water is able to enter the fractures, the openings in the rock become larger allowing more water movement. Limestone rocks that have large dissolved openings in them are known as karst.
Volcanic rocks such as basalt are produced as a result of volcanic activity. If these rocks cool rapidly, fractures can be produced in the rocks allowing significant water movement. Metamorphic and crystalline rocks such as granite, quartzite, and slate are basically impermeable to water movement. However, fractures that occur in these rocks can allow water movement. The amount of movement through fractured rock depends on the frequency and interconnectedness of the fractures.
Confined and Unconfined Aquifers
All aquifers have an impermeable layer beneath them that stops the groundwater from infiltrating further. If an aquifer has an impermeable layer below it but the layers above it are permeable, then it is considered to be an unconfined aquifer. On some occasions a permeable layer can be trapped between two impermeable layers producing a confined aquifer.
Recharge and Discharge of Aquifers
Water may become added to aquifers naturally as water infiltrates into the soil. The area over which water infiltrates into an aquifer is known as the “recharge zone”. The recharge zone above unconfined aquifers is generally the area above the aquifer because water is able to move directly from the surface into the aquifer. However, for a confined aquifer, the recharge zone may be limited to the range where the impermeable layer reaches the surface.
Because water has to infiltrate through layers of soil and rocks in order to reach an aquifer, rates of recharge can be very slow and low. Some aquifers were formed a long time ago and they no longer are actively recharged (some people refer to these types of aquifers as containing “fossil water”).
Groundwater can move through aquifers until it reaches an opening to the surface. In a seep, the water reaches the surface over a large area. In a spring, water flows from the earth at a small point. Because of the pressure of the water above it, water in confined aquifers is generally under high pressure and can result in the production of an artesian spring. Springs and seeps will only continue to flow as long as the water table is higher than they are. Because of movement of water, the location of the recharge zone may be far from the location of seeps and springs.
Water can also be removed from aquifers by the activity of humans drilling wells. Aquifers have historically been extremely important for humans who have used the water for watering livestock, irrigating crops, powering mills, and as a source of municipal water. If rates of removal of water for human use exceed the very slow, natural rate of recharge, then the total amount of water in the aquifer is reduced which results in a lowering of the water table (aquifer depletion). Lower water tables require deeper wells which greatly increases the cost of pumping water from aquifers and further depletes water from the already slow, natural rate of recharge.
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