An aquifer is an underground layer of porous rocks or permeable rocks that store and retain groundwater levels in the soil. The underground aquifer is built with all types of porous or permeable rock materials, such as sand, gravel, or silt, making it a suitable water absorber. The rainwater enters the aquifer through the soil and becomes a part of the groundwater. The groundwater from the aquifers then resurfaces from springs and wells. We can also extract the aquifer water with the help of a water well. The study of groundwater, aquifer, and their property is known as hydrogeology.
Now that we know what an aquifer is, let’s discuss the types of aquifers and their uses in detail.
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Properties and Terms Related to an Aquifer
An aquifer forms near the surface and can also range deeper than 9,000 metres or 30,000 feet. The groundwater aquifers closer to the surface are used for irrigation and drinking purposes. However, there are many other properties of aquifers that we should know. Below we have discussed some of the crucial properties of an aquifer.
Aquitard: An aquitard refers to the different layers of an aquifer that prevent one aquifer’s water from flowing to another. Examples of an aquitard include the clay layer or solid rock layer.
Hydraulic Head: Hydraulic head is referred to the height to which water rises inside the aquifer. It may be measured as the depth below the natural surface. We can also measure the hydraulic head against the sea level.
Hydraulic Gradient: A hydraulic gradient can be defined as the difference between two hydraulic heads in an aquifer divided by the distance between them.
Transmissivity of Aquifer: Transmissivity of an aquifer refers to its capability to transmit groundwater throughout its total saturated thickness. It can also be defined as the rate at which the groundwater flows through an aquifer section under a unit hydraulic gradient.
Hydraulic Conductivity: Hydraulic conductivity refers to the speed or ease with which water moves through the aquifer. It can be calculated by dividing the transmissivity of the aquifer by its thickness.
Above, we discussed the properties and some terms related to an aquifer. Next, let’s discuss the types of aquifers.
Types of Aquifers
Aquifers can be classified into various types depending upon their formation, size, and materials. Let’s discuss each type of aquifer in detail.
Confined Aquifer: A confined aquifer is a part of a rock bed or sand covered by a clay confining layer and prevents the groundwater from moving from one aquifer to another. The water in confined aquifers can have high pressure due to the overlying confining layer. If a bore is created on a confined aquifer, the water level will rise above the aquifer for the high pressure. There are two types of confined aquifers: artesian aquifer and leaky aquifer.
Unconfined Aquifer: An unconfined aquifer is a section of rock bed or sand not covered by a confining layer. An unconfined aquifer is always shallow in-depth, and its top consists of the water table. It acts like a sponge where the water level fluctuates depending on the outflow and inflow of water.
The difference between a confined and unconfined aquifer is that a confined aquifer has a confining layer to prevent water flow. In contrast, an unconfined aquifer does not have a confining layer to prevent water flow.
Saturated Aquifer: Saturated aquifers are those aquifers that are filled with water and have no space to store more water. These aquifers store water with high-pressure heads.
Unsaturated Aquifer: Unsaturated aquifers are those aquifers that contain water but still have some space filled with air and can store more water. These aquifers generally occur above the water table, and their pressure head is negative or less than a saturated aquifer.
Perched Aquifer: Perched aquifer is a type of aquifer that occurs over unsaturated rock formations. These aquifers develop as a result of discontinuous impermeable layers of rock or sand.