The Water Cycle
Groundwater refers to water that is located underground in the spaces between soil particles and bedrock, and even in cracks within rocks.
Almost all groundwater originates as surface water. Some portion of rain hitting the earth runs off into streams and lakes, and another portion soaks into the soil, where it is available for use by plants and subject toevaporation back into the atmosphere. The third portion soaks below the root zone and continues moving downward until it enters the groundwater.
Precipitation is the major source of groundwater. Other sources include the movement of water from lakes or streams and contributions from such activities as excess irrigation and seepage from canals. Water has also been purposely applied to increase the available supply of groundwater. Water-bearing formations called aquifers act as reservoirs for storage and conduits for transmission back to the surface.
Groundwater, which is the major source of our drinking water, can accumulate in aquifers over thousands of years. Unconfined aquifers have the water table, or the surface where water pressure equals atmospheric pressure, as their upper boundaries. Confined aquifers often lie below unconfined aquifers and have a layer of rock or other materials as their upper boundaries.
In the United States, the oldest groundwater, known as fossil water, is contained in the Ogallala Aquifer. Lying below about 175,000 square miles (450,000 square kilometers) of eight states in the Great Plains, the Ogallala Aquifer stores about 2,900 million acre-feet (3,600 million kilometers cubed) of water [source: High Plains/Ogallala Aquifer]. The Ogallala Aquifer was formed between 2 and 6 million years ago, when the Rocky Mountain chain was forming. Because the climate of the Great Plains is arid, water in the aquifer is being used faster than it can be recharged. That's why some scientists refer to using fossil water aquifers as water mining.
Groundwater may also exist on other planets. Images from the Mars Global Surveyor spacecraft show what looked like gullies carved out by rivers of water on the surface of the planet. According to NASA, the water is probably 300 to 1,300 feet (100 to 400 meters) below the surface. Europa, one of Jupiter's moons, may also have subsurface water. As our need for water outweighs the Earth's supply, scientists wonder if we may one day mine for water on the other planets and moons in our solar system.
Water has a lot of unique and amazing properties that make it so important to life. They're why we're constantly looking for better ways to obtain and conserve it.
Shanna Freeman "How Water Works" 18 October 2007.
HowStuffWorks.com. <http://science.howstuffworks.com/environmental/earth/geophysics/h2o.htm> 5 February 2016
Groundwater is one of our most valuable resource—even though you probably never see it or even realize it is there. As you may have read, most of the void spaces in the rocks below the water table are filled with water. But rocks have different porosity and permeability characteristics, which means that water does not move around the same way in all rocks below ground.
When a water-bearing rock readily transmits water to wells and springs, it is called an aquifer. Wells can be drilled into the aquifers and water can be pumped out. Precipitation eventually adds water (recharge) into the porous rock of the aquifer. The rate of recharge is not the same for all aquifers, though, and that must be considered when pumping water from a well. Pumping too much water too fast draws down the water in the aquifer and eventually causes a well to yield less and less water and even run dry.
The study of aquifers and the water flows in them is called hydrogeology.
Aquifers can be replenished artificially. For example, large volumes of ground water used for air conditioning are returned to aquifers through recharge wells on Long Island, New York. Aquifers may be artificially recharged in two main ways: One way is to spread water over the land in pits, furrows, or ditches, or to erect small dams in stream channels to detain and deflect surface runoff, thereby allowing it to infiltrate to the aquifer; the other way is to construct recharge wells and inject water directly into an aquifer.
"Aquifers and Groundwater" USGS Water Science School http://water.usgs.gov/edu/earthgwaquifer.html
The Water Cycle
More than half of the world's water supply is contained in just nine countries: the United States, Canada, Colombia, Brazil, the Democratic Republic of Congo, Russia, India, China andIndonesia [source: World Business Council for Sustainable Development].
Most of the world's freshwater -- about 2.4 million cubic miles (10 million cubic kilometers) of it -- is contained in underground aquifers. The rest comes from: