An aquifer is a body of rock which is permeable (such as sandstone and limestone), which means that it allows water to flow through. This type of stone allows for easier and faster pumping of the water. Water, including rain and melted snow, continues downward until it reaches rock that it cannot move through. This water is known as groundwater. The groundwater found in aquifers constitutes one of the most important water sources on Earth. The National Oceanic and Atmospheric Administration estimates that approximately 30% of our freshwater is groundwater. Rivers, lakes, streams and wetlands contain the remaining amount.
There are two types of aquifers – and unconfined aquifer and a confined aquifer. The unconfined aquifer, which is only partly filled, has a level that depends on the water that enters or leaves the aquifer. The confined aquifer is one that sits between two layers of rock that water struggles to flow through. The water trickles in through, and fills the aquifer.
Aquifers have the ability to sustain for many centuries. The oldest groundwater ever discovered was in a Canadian mine, with estimates of it sitting there for 1.5 to 2.64 billion years. However, there are threats to aquifers. The increase in the world’s population puts a strain on existing aquifers, when pumping the water happens at rate that exceeds how quickly the groundwater can be recharged. The water table, or top of the water level of the aquifer, is lowered and can eventually run dry.
Sources:
http://education.nationalgeographic.com/education/encyclopedia/aquifer/?ar_a=1
http://www.livescience.com/39625-aquifers.html
http://imnh.isu.edu/digitalatlas/hydr/concepts/gwater/aquifer.htm