An aquifer is a geologic formation, group of formations, or part of a formation that contains sufficient saturated permeable material to yield significant quantities of water to springs and wells. Aquifers are underground layers of porous rock or sand that allows the movement of water between layers of non-porous rock (sandstone, gravel, or fractured limestone or granite).
How it is formed
Water infiltrates into the soil through pores, cracks, and other spaces until it reaches the zone of saturation where all of the spaces are filled with water (rather than air). The zone of saturation occurs because water infiltrating the soil reaches an impermeable layer of rocks so that it is not able to penetrate any further into the earth (this impermeable layer is known as an aquitard or aquiclude). Water held in aquifers is know as groundwater.
The top of the zone of saturation is known as the water table. The water table typically follows the form of the above ground topography. The depth of the water table is typically greater in regions with low rainfall than in regions with high rainfall. The water table can rise in wet years and fall in dry years.
All aquifers have an impermeable layer beneath them that stops the groundwater from infiltrating further. If an aquifer has an impermeable layer below it but the layers above it are permeable, then it is considered to be an unconfined aquifer. On some occasions a permeable layer can be trapped between two impermeable layers producing a confined aquifer.
Water may become added to aquifers naturally as water infiltrates into the soil. The area over which water infiltrates into an aquifer is known as the recharge zone. The recharge zone above unconfined aquifers is generally the area above the aquifer because water is able to move directly from the surface into the aquifer. However, for a confined aquifer, the recharge zone may be limited to the range where the impermeable layer reaches the surface.
Because water has to infiltrate through layers of soil and rocks in order to reach an aquifer, rates of recharge can be very slow. Some aquifers were formed a long time ago and they no longer are actively recharged.
Groundwater can move through aquifers until it reaches an opening to the surface. In a seep, the water reaches the surface over a large area. In a spring, water flows from the earth at a small point. Because of the pressure of the water above it, water in confined aquifers is generally under high pressure and can result in the production of an artesian spring. Springs and seeps will only continue to flow as long as the water table is higher than they are.