An aquifer is an underground layer of water-bearing rock. Water-bearing rocks are permeable, meaning they have openings that liquids and gases can pass through. Sedimentary rock such as sandstone, as well as sand and gravel, are examples of water-bearing rock. The top of the water level in an aquifer is called the water table.
An aquifer fills with water from rain or melted snow that drains into the ground. In some areas, the water passes through the soil on top of the aquifer; in others, it enters through joints and cracks in rocks. The water moves downward until it meets less permeable rock.
Aquifers act as reservoirs for groundwater. Water from aquifers sometimes flows out in springs. Wells drilled into aquifers provide water for drinking, agriculture, and industrial uses. Aquifers can dry up when people drain them faster than nature can refill them. Because aquifers fill with water that drains from the surface of the Earth, they can be contaminated by any chemical or toxic substance found on the surface.
There are two types of aquifers. An unconfined aquifer is covered by permeable rock and can receive water from the surface. The water table of an unconfined aquifer rises or falls depending on the amount of water entering and leaving the aquifer. It is only partly filled with water.
In contrast, a confined aquifer lies between two layers of less permeable rocks and is filled with water. Water trickles down through cracks in the upper layer of less permeable rock, a nearby water source, such as an underground river or lake, or a nearby unconfined aquifer.
An artesian well is a type of confined aquifer that flows upward to the Earth's surface without the need for pumping. The artesian well sits below the water table at the bottom of U-shaped aquifers. Pressure from water in the long sides of the aquifer pushes the water up the well shaft.
Great Artesian Basin
The world's largest known aquifer is the Great Artesian Basin, in Australia, at more than 1.7 million square kilometers (661,000 square miles).
Term Part of Speech Definition Encyclopedic Entry agriculture Noun
the art and science of cultivating the land for growing crops (farming) or raising livestock (ranching).
Encyclopedic Entry: agriculture aquifer Noun
an underground layer of rock or earth which holds groundwater.
Encyclopedic Entry: aquifer artesian well Noun
type of confined aquifer that flows to the Earth's surface without the need for pumping.
confined aquifer Noun
layer of water-bearing rock between two layers of less permeable rock.
to poison or make hazardous.
small stones or pebbles.
water found in an aquifer.
Encyclopedic Entry: groundwater industrial Adjective
having to do with factories or mechanical production.
allowing liquid and gases to pass through.
Encyclopedic Entry: rain reservoir Noun
natural or man-made lake.
Encyclopedic Entry: reservoir sand Noun
small, loose grains of disintegrated rocks.
common sedimentary rock formed by grains of sand compacted or cemented with material such as clay.
sedimentary rock Noun
rock formed from fragments of other rocks or the remains of plants or animals.
precipitation made of ice crystals.
top layer of the Earth's surface where plants can grow.
small flow of water flowing naturally from an underground water source.
unconfined aquifer Noun
layer of water-bearing rock covered by permeable rock.
water-bearing rock Noun
rock that can hold water in tiny pores.
water table Noun
underground area where the Earth's surface is saturated with water. Also called water level.
Encyclopedic Entry: water table well Noun
a hole drilled in the Earth to obtain a liquid or gaseous substance.