Confined Aquifer vs. Unconfined Aquifer: What is the difference?
An aquifer is a natural underground reservoir of water made of permeable rock or sediment. It can be formed by a variety of sediments and rocks, including gravel, sandstone, conglomerates, and fractured limestone.
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According to National Geographic, about 30% of the available fresh water on our planet is stored in aquifers. This makes them one of the most important water resources on Earth.
An aquifer can be classified in many different ways depending on the characteristics it presents in a given area.
There are many terms used to describe aquifers (like saturated vs unsaturated, isotropic or anisotropic, etc). However, one of the most important ways to classify an aquifer is by its confinement.
Aquifers can be either confined or unconfined in terms of how they have surrounded their environment and how the water pressure is distributed.
In this post, we’ll take a closer look at these two types of aquifers and their defining characteristics.
What is a Confined Aquifer?
A confined aquifer is a geologic formation of sediment or rock that holds a body of water trapped beneath its surface.
The water pressure in a confined aquifer is usually greater than the atmospheric pressure due to the weight of the water column pushing down on the aquifer.
The confining layer can be clay, shale, or a combination of multiple layers of different sediments with low hydraulic conductivity.
The confining layer at the top is named aquitard which slows down the water while it doesn’t stop water from moving through it.
The confining layer at the bottom called is aquiclude which doesn’t let water move through it at all.
This higher water pressure allows for the water to be forced up to the surface through wells, making them a valuable source of water for humans and agriculture.
The water in confined aquifers is under the water table, which means the water isn’t accessible unless wells are drilled or the aquifer is reformed through cracks and fissures in the rocks or sediments.
Since confined aquifers are located below the water table, they are also called “subterranean” aquifers, and they are well protected against drought conditions.
If we drill down to where the water is in a confined aquifer, the water will come bubbling up to the surface. That is why a lot of times these aquifers are also called artesian aquifers.
What is an Unconfined Aquifer?
An unconfined aquifer is a body of water that is not confined between low-permeability material.
It receives direct infiltration from the surface when it rains or snows. With unconfined aquifers, the surface of the saturated zone is the water table. Thus, they are sometimes called “water table aquifers”.
Since these aquifers are close to the surface, their water pressure is typically equal to atmospheric pressure.
They are also more vulnerable to drought conditions than confined aquifers because there is no layer of an impermeable material that can prevent water from being withdrawn.
Unconfined aquifers get contaminated sooner than confined aquifers because they are closer to the surface and are more exposed to pollution and other contaminants.
Because of this, unconfined aquifers aren’t a reliable source of water for human consumption compared to confined aquifers.
Pumping water from a well built on an unconfined aquifer may drop the water table even further, resulting in a drawdown or a cone of depression.
This can alter the balance of the groundwater system and can disturb the aquifer’s natural hydraulic gradient.
As result, biological life can be disturbed or even destroyed by a drawdown sometimes referred to as the “cone of death.”
Using unconfined aquifers as a water source can also cause land subsidence. Withdrawals from unconfined aquifers have caused the ground surface to sink to cause unexpected damage to infrastructures, such as roads and bridges.
Also, groundwater extraction from an unconfined aquifer will cause nearby groundwater sources may flow toward the well to replace the water that was extracted bringing water from local streams and lakes.
This may result in poor water quality by increasing the concentration levels of dissolved solids, nitrates, and pesticides in the groundwater.
As a result, perennial streams and lakes may disappear, and wetland habitats can be destroyed.
The unconfined aquifer is the most common type of aquifer and can be found almost anywhere there is an adequate water supply.
Unconfined aquifers are typically closer to the Earth’s surface than confined aquifers and, as a result, they are more vulnerable to drought conditions.
Confined aquifers are aquifers surrounded by a constraining layer of clay or rock. This layer acts as a barrier to the flow of water in and out of the aquifer, confining it below surface level.
Although the definitions of confined and unconfined aquifers can seem to be clear enough, it is not always easy to classify an aquifer as either.
This can be the case when there are multiple water-table levels or when a water table fluctuates seasonally.
If there is no visible confining layer or if the geology is complex, such as a fractured bedrock aquifer, the value of storativity returned from an aquifer test can be utilized to define confined or unconfined characteristics of an aquifer.
Confined aquifers have really low permeability and porosity, so they store water for long periods of time.
This is why confined aquifers are often called “reservoirs.” The water in these aquifers is often under pressure and can be accessed through a well.
Unconfined aquifers, on the other hand, have high permeability and porosity. This means that water moves through them quickly and they are not good at storing water.
Aquifer Type Comparison Table: Confined vs. Unconfined
| Confined Aquifer | Unconfined Aquifer |
|---|---|
| The common type of aquifer | Not surrounded by an impermeable layer of rock or sediment |
| Higher water pressure than atmospheric pressure | Water pressure is typically equal to atmospheric pressure |
| Can be accessed through wells | Water table is the surface of the saturated zone |
| Water is under the water table | Vulnerable to drought conditions |
| Protected against drought conditions | More vulnerable to pollution and contamination |
| Water can be forced up to the surface through wells | Withdrawal of water may cause drawdown or cone of depression |
| Usually called “subterranean” or “artesian” aquifers | Usually called “water table” aquifers |
| Stores water for long periods of time | Not good at storing water |
| Can be used as a reliable source of water for humans and agriculture | Not a reliable source of water for human consumption |
| Less likely to cause land subsidence | Withdrawals may cause land subsidence |
| Less likely to disturb the aquifer’s natural hydraulic gradient | May disturb the aquifer’s natural hydraulic gradient |
| Can be found almost anywhere | Common type of aquifer |