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The Difference Between Groundwater and an Aquifer

The terms "groundwater" and "aquifer" can be confusing. In many instances, it seems as though they can be used interchangeably. This is because most human interaction with groundwater is in the context of aquifers. Put simply, an aquifer is an area of underground rock or sediment that contains enough groundwater to sustain a well or a natural spring.

Groundwater
- Groundwater enters the water table when it seeps down through the soil and into a layer of saturation. A layer of saturation is defined as any area underground containing water under pressure of more than one atmosphere. If a hole is dug into a layer of saturation, it will fill with water to a level matching the top of the layer. However, many zones of saturation do not contain enough groundwater to support a well or spring, and would therefore not be considered aquifers.
Aquifers
- A saturation layer is only considered an aquifer if it can provide a useful amount of water to the surface through a well or spring. Aquifers contain groundwater in the same way lakes and rivers contain surface water. An aquifer can be visualized as an underground lake or river, although it does not usually occupy a hollow space in the rock. Instead, the water seeps or flows through porous material, just as the water from a hose soaks into the soil of a garden.
Permeable and Impermeable Earth
- Groundwater collects in aquifers because the Earth's crust is made up of many different layers and zones of various materials. Some of these materials are porous enough to allow water to seep or flow through them, while others are dense enough that they absorb little or no water. A layer of permeable material situated on top of a layer of impermeable material will form an aquifer. Water will saturate the permeable layer and sit on top of the impermeable layer, forming a reservoir.
Confined and Unconfined Aquifers
- Many of the aquifers tapped by wells and springs are unconfined aquifers. They consist of a layer of permeable, saturated water on top of an impermeable layer of some dense material like granite or basalt. There is no impermeable material on top of them, and they can be easily refilled as rainwater soaks down from the soil above. However, some are surrounded on all sides by impermeable material. This makes a confined aquifer, a saturated area of very old water cut off from the rest of the water table. Confined aquifers are usually under a great deal of pressure, and drilling into one will cause the groundwater to spurt to the surface and form a flowing spring, or artesian well.

Nature