|Credit: Environment Canada / USGS|
Aquifers are underground layers of rock that are saturated with water that can be brought to the surface through natural springs or by pumping.
The groundwater contained in aquifers is one of the most important sources of water on Earth: About 96 percent of our liquid freshwater is groundwater, according to the New York Department of Environmental Conservation. The rest is found at the surface in streams, lakes, rivers and wetlands.
Groundwater can be found in a range of different types of bedrock, but the most productive aquifers are found in porous, permeable rock such as sandstone or limestone. Groundwater moves more readily through these materials, which allows for faster pumping and other methods of extracting the water.
Aquifers can also be found in regions where the bedrock is made of denser material — such as granite or basalt — if that bedrock has been cracked or shattered. Dense, impermeable material like clay or shale can act as an "aquitard," i.e., a layer of rock or other material that is almost impenetrable to water. Through groundwater might move through such material, it will do so very slowly (if at all).
An aquitard can put pressure on the groundwater in an aquifer: When an aquifer is confined between two aquitard layers, the pressure on the groundwater can be enough to force the water out of any well that's drilled into that aquifer. Such wells are known as artesian wells, and the aquifers they tap into are called artesian aquifers or confined aquifers.
How groundwater moves
When groundwater moves through an aquifer, it usually flows downward by the force of gravity. Depending on the density of the rock and soil through which groundwater moves, it can creep along as slowly as a few centimeters in a century, according to Environment Canada. In other areas, where the rock and soil are looser and more permeable, groundwater can move several feet in a day.
The water in an aquifer can be held beneath the Earth's surface for many centuries: Hydrologists estimate that the water in some aquifers is more than 10,000 years old (meaning that it fell to the Earth's surface as rain or snow roughly 6,000 years before Egypt's Great Pyramid of Giza was built).
Much of the drinking water on which society depends is contained in aquifers. For example, the Ogallala Aquifer — a vast, 174,000 square-mile (450,000 sq. kilometer) groundwater reservoir —supplies almost one-third of America's agricultural groundwater, and more than 1.8 million people rely on the Ogallala Aquifer for their drinking water.
Similarly, Texas gets almost 60 percent of its water from groundwater; in Florida, groundwater supplies more than 90 percent of the state's freshwater. But these important sources of freshwater are increasingly endangered.
Threats to aquifers
By 2010, about 30 percent of the Ogallala Aquifer's groundwater had been tapped. Some parts of the Ogallala Aquifer are now dry, and the water table has declined more than 300 feet in other areas, according to the Texas Water Development Board.
The same problem is increasingly found throughout the world, especially in areas where a rapidly growing population is placing greater demand on limited aquifer resources — pumping can, in these places, exceed the aquifer's ability to recharge its groundwater supplies.
When pumping of groundwater results in a lowering of the water table, then the water table can drop so low that it's below the depth of a well. In those cases, the well "runs dry" and no water can be removed until the groundwater is recharged — which, in some cases, can take hundreds or thousands of years.
In addition to groundwater levels, the quality of water in an aquifer can be threatened by saltwater intrusion (a particular problem in coastal areas), biological contaminants such as manure or septic tank discharge, and industrial chemicals such as pesticides or petroleum products. And once an aquifer is contaminated, it's notoriously difficult to remediate.