What is an Aquifer?
Aquifers are bodies of well-saturated rocks that make way for the easy movement of water. So, when a saturated rock transmits its water to a well or spring, one can define it as an aquifer. An aquifer needs to be penetrable and porous and it is usually created by rocks like sandstone, limestone, gravel, sand, and conglomerates. While it isn’t as common, certain volcanic rocks like the columnar basalts act as an excellent aquifer. But how exactly is an aquifer created and where can you find it? Let’s take a closer look.
As previously mentioned, aquifers are created via rocks that already contain water. One can drill a well in the aquifer. To do this, you need to drill a hole for the well penetrating the aquifer. Over time, the stored water can be pumped out of the well at a surface level. Precipitation will add more water to the aquifer’s porous rock, recharging the well with fresh water.
It is worth noting that all aquifers aren’t recharged at the same rate and one needs to consider this factor while pumping water from wells. If you pump excess water within a very short timeline, the aquifer won’t function properly. The well too will start yielding less water and within no time, it will end up running dry. Pumping your own well too often may even affect your neighbor’s well, drying it up sooner than anyone expected. This is especially true when both the wells are running on a single aquifer.
Note that as you pump water from the well, its water table will be reduced to a cone creating a depression in the well. So, if you are using an aquifer powered well, it is best to use it in moderation.
Where are Aquifers Found?
While aquifers may be found in various forms of rock, the most useful ones are available from porous and penetrable rocks like sandstones. You might also find them from open cavities or via limestone caves.
Aquifers are commonly found in these rocks, as groundwater moves easily through the said materials. Since this makes way for quicker pumping, extracting water becomes easier. You may also find aquifers in an area where the rock isn’t as porous (like granite, for instance). However, this is only possible when the rock has multiple dentures and cracks.
The oldest known aquifer was found in a Canadian mine. The groundwater was almost two miles deep and it was estimated to be stored for almost 1 to 2 billion years.
Note that most of our regular drinking water is sourced from shallow aquifers. For instance, the Ogallala aquifer is a groundwater repository supplying almost one-third of the agricultural groundwater of America. Almost 1.9 million people depend on this aquifer for drinking water.
The American state, Texas, gets almost 60 percent of its water from groundwater repositories, and in Florida, almost 90 percent of the freshwater is sourced from groundwater.
Types of Aquifers
As with everything else, aquifers may be classified into different types. In this section, we will learn about them in detail.
Unconfined or Phreatic Aquifers
As evident from the name, these variants of aquifers do not have any specific confining layer at the top. They do, however, have a confining surface at the bottom. Unconfined aquifers are often exposed to the atmosphere and their upper area is partially laden with water. The upper area (where the saturation takes place) is known as the water table. Since these aquifers are under persistent atmospheric pressure, they are alternatively known as the phreatic aquifers.
Artesian or Confined Aquifers
Also known as artesian aquifers, a confined aquifer is usually over or underlain via several confining layers. Since this causes the water inside the aquifer to be under immense pressure, these aquifers are often termed pressure aquifers.
When the aquifer’s lateral outcrop is higher than the ground surface, there might be a strong hydrostatic pressure that creates an ideal condition for a water-filled well. Water from these wells will conveniently reach the surface. The best part: you won’t need to pump it out. The assumed level till which the water can potentially rise is known as the piezometric area.
This is a rare instance of an unconfined aquifer. They are created when an impervious or fairly impenetrable layer covering a small area is found in a saturated unconfined aquifer. They are shaped like a lens and the stored water that’s located right on top of the lens is known as the perched table.
A truly confined aquifer is indeed a rare event in nature as the confining layers, albeit impervious, do not offer complete impenetrability. If the aquifer is either over or underlain by a moderately penetrable layer (also known as aquitards), it will immediately lead to the vertical leakage from the said layer. This situation creates a leaky aquifer. At times, these aquifers are also interchangeably known as semi-confined aquifers.
It is worth noting that the semi-confining surface isn’t as permeable as the primary aquifer. Therefore, the water that vertically seeps from the semi-confining zone will be internally diverted to directly move to the primary aquifer.
How Do Aquifers Work?
As you’re probably aware at this point, aquifers tap in groundwater and the groundwater present in the aquifer is an extremely vital source of water. According to the National Oceanic Administration, almost 30 percent of all freshwater is available in the form of groundwater. The remaining may be found in water bodies like streams, rivers, pools, ponds, and wetlands. Almost 69 percent of freshwater is locked in ice caps, glaciers, and mountaintops.
But how does an aquifer work? Well, the working mechanism of most aquifers is simple. The groundwater passes through the penetrable and semi-penetrable rocks to create an underground repository filled with water. As groundwater moves more easily through porous rocks; the dense, non-porous rocks like shale or clay might function like an aquitard. That means, they will operate as a rocky layer that cannot penetrate water. Although groundwater moves through these materials (via creaks or dents), the process is painfully slow.
Over time, aquitards will trap some excess groundwater in an existing aquifer to create artesian wells. As groundwater flows under the aquitard from a high to a low elevation, (like from the slopes of hills/mountains to the floor of valleys), the resultant pressure will be enough to force the groundwater from the well that has already been drilled inside the aquifer.
Every time fresh groundwater is stored in an aquifer, its overall groundwater supply is recharged. This recharge usually happens across mountains as the surface water plunges down from the mountainous slopes into rivers and other water bodies due to the inherent gravitational pull. Depending on the overall density of the rock and the area via which the surface water is moving, it may plunge for only a couple of centimeters over several decades. If, however, the rock and the soil is loose, groundwater will move much faster.
As the water enters an aquifer, it gradually moves towards lower elevations and is finally pulled out by the aquifer in the form of springs. It may also be withdrawn by a well.
Despite our persistent need for freshwater, aquifers and groundwater are now heavily endangered. That is why it is important to take proper steps to conserve and save more water.