What is an Aquifer?
What is an Aquifer?
An aquifer is a large underground storage space for water. They can be located right at the ground surface or very deep underground where they are impossible to access. They can be very large (e.g. some aquifers can span a province) or quite small.
Aquifers can have high permeability, so water can flow easily in this layer. There is always a barrier below an aquifer that will not let water pass through to lower layers. This barrier, called an aquitard, is a layer with very low permeability. It is very difficult for water to pass through an aquitard, so it helps contain the water in the aquifer. Every aquifer has an aquitard below it and many also have an aquitard above. When an aquifer has an aquitard on top of it, it is called a confined aquifer. If an aquifer only has an aquitard below it and does not have an aquitard above then it is called an unconfined aquifer. Unconfined and confined aquifers are shown in Figure 1.
Unconfined aquifers are directly connected to the surface. When water infiltrates into the ground it passes through the unsaturated zone. The unsaturated zone has mostly air in it and is highly permeable. When the water passes the unsaturated zone it reaches the saturated zone. The saturated zone is filled with water. The boundary between the saturated zone and the unsaturated zone is called the water table. When water seeps into the earth it will travel down the soil layers until it reaches the water table. The water table moves up and down depending on how much water is in the unconfined aquifer. Since unconfined aquifers are connected to the surface, they recharge quickly, and are prone to contamination.
Confined aquifers are confined because they have an aquitard above and below the aquifer. These aquifers do not recharge quickly because it takes a long time for water to pass through the top aquitard. In some cases, confined aquifers contain high quality water because they are not directly impacted by human activity on the surface. Confined aquifers can contain groundwater that is very old. Water can stay in a confined aquifer for several millennia.
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| Figure 1: Confined and unconfined aquifers: This figure shows the two types of aquifers, confined and unconfined, within an impermeable bedrock basin. The dark blue layer between the aquifers is an aquitard [1]. |
Types of Aquifers
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Fast Fact: Groundwater makes up the largest source of useable freshwater in the world. A major aquifer like the Guarani aquifer in South America holds almost double the amount of water as major surface water reservoirs, such as the North American Great Lakes. In Alberta only 0.01% of groundwater is recoverable. Source: Rivera, A. (2014). Canada’s Groundwater Resources. In print. |
Aquifers are categorized as confined or unconfined, but there are many types of aquifers that are classified by where they are located in the earth and the material of which they are comprised. There are three types of aquifers: unconsolidated deposit aquifers, bedrock aquifers and quaternary aquifers.
Unconsolidated Deposit Aquifers
An unconsolidated deposit aquifer is an aquifer that is made up of loose sediment such as gravel and sand. These aquifers are close to the surface and are almost always unconfined. This type of aquifer is commonly found near rivers in a floodplain. Unconsolidated deposit aquifers are formed as the result of old rivers that no longer exist, by glaciers that have moved the sediment or by deposition at the bottom of a lake. The water in an unconsolidated deposit aquifer is directly connected to the surface water system.
Bedrock Aquifers
| Fast Fact: Most of the sand and gravel that lies on top of bedrock in Alberta was placed there by glaciers over 10,000 years ago! |
Bedrock is the hard rock that lies below all the sand, gravel and soil near the ground surface. A bedrock aquifer is an aquifer that is confined within hard bedrock layers. Water can travel through porous bedrock, or through cracks, fractures and crevasses in the hard bedrock. In Alberta, 84% of groundwater wells draw from bedrock aquifers [2]. These aquifers are easily accessible in areas where the bedrock is near the earth’s surface, such as in southern Alberta.
In Alberta, there are three types of bedrock aquifers: carbonate aquifers, sandstone aquifers, and fractured shale aquifers.
Carbonate aquifers are made of rocks such as limestone and usually contain saline water. Sandstone aquifers are made of sandstone, a highly permeable rock, and can contain either saline or freshwater. The largest aquifer in Alberta, the Paskapoo Aquifer, is a sandstone aquifer. One third of groundwater wells in Alberta are located in the Paskapoo Aquifer [3]. Shale is a rock that is similar to sandstone, but is less permeable. For shale to be an aquifer, it must be fractured, or cracked, so water can flow into it. Fractured shale aquifers are relatively rare in Alberta. The wells that draw from this type of aquifer do not produce as much water.
Quaternary Aquifers
Quaternary aquifers are aquifers that were created by glaciers. They are located between bedrock and the earth’s surface. These aquifers can be confined or unconfined. There are two types of quaternary aquifers: buried valley aquifers and alluvial aquifers.
| Fast Fact: Coal can also be an aquifer. Water can easily flow through the porous and permeable layers of coal in the rock layers. |
Buried valley aquifers are confined aquifers that can be directly above bedrock or higher up in the rock layers. These are ancient valleys that are filled with permeable sand and gravel. Unconfined sand and gravel aquifers are located at the surface or near the surface. An alluvial aquifer is a specific type of unconfined aquifer which has a river flowing through it. The river is the main source of recharge. Quaternary aquifers generally contain freshwater.