TOSC ENVIRONMENTAL BRIEFS FOR CITIZENS

Understanding Groundwater

What Is An Aquifer?

Aquifers typically consist of gravel, sand, sandstone, or fractured rock, such as limestone. These materials are permeable because they have large connected spaces through which water can flow. These aquifers are the sources of wells and springs. There are two main types of aquifers, confined and unconfined.

A confined aquifer is overlaid by a layer of soil or rock material that is impermeable (does not allow water to flow through). If the aquifer is under pressure and water flows naturally to the ground surface, this type of formation is called an artesian aquifer. Because the aquifer is separated from the ground surface by an impermeable layer, pollutants are usually prevented from entering this type of aquifer. Most drinking water wells are drilled into confined aquifers because they typically yield higher quality water and ensure public safety.

An unconfined aquifer does not have an upper confining layer of impermeable soil or rock material. This type of aquifer is dependent on surface water flowing into the soil. Because this type of aquifer does not have a protective layer, it is more vulnerable to contamination from the surface. The top of the saturated zone in an unconfined aquifer forms the water table. The water table may be very close to, or even at, the ground surface or it may be hundreds of feet below the ground. The water table may rise or fall depending on many factors. Heavy rains or melting snow may cause the water table to rise, or an extended period of dry weather may cause the water table to fall.