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| Groundwater in Burkina Faso, Africa |
Groundwater is one of humans’ most valuable natural resources. Groundwater is the water contained in the rock and soil layers beneath Earth’s surface, and it makes up most of Earth's supply of fresh, liquid water. (The oceans and ice in the North and South Poles contain 99% of Earth's total water supply. Groundwater accounts for almost all of the remaining 1%.)
Throughout history, humans have settled in areas with plentiful and pure groundwater, and have fought to own and protect wells and springs. Today, human water needs in many arid (dry) or heavily populated regions far exceed surface water supplies. Earth’s rapidly-growing human population is becoming increasingly reliant on groundwater.
Groundwater fills wells and city water supplies. Ground-water irrigates (waters) crops, feeds livestock, and produces farm-raised fish. Groundwater is used to cool nuclear reactors that generate electricity, mix concrete, and manufacture millions of consumer products. In short, groundwater plays a vital role in almost every facet of people’s lives, from drinking water, foods, and products people buy to roads and the buildings in which people live and work.
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Groundwater reservoirs: aquifers
Water enters underground reservoirs by soaking in through soils, stream beds, and ponds in areas termed recharge zones. Water flows, often very slowly, through interconnected pore (tiny opening) spaces and then remerges onto the land surface at natural discharge points called springs and seeps.
When discharge from natural springs and/or human wells exceeds the rate of recharge, the groundwater level falls, shallow wells and springs dry, and eventually, the reservoir empties. Many groundwater reservoirs, particularly those beneath arid deserts and semi-arid grasslands, filled with water many centuries ago when regional climate was wetter.
Groundwater reservoirs that yield water for human use are called aquifers. In part, human economics determine which water-bearing units are exploited as aquifers. In regions where clean surface water is plentiful and inexpensive, groundwater may go unused.
In arid regions with scarce or polluted surface water, and in places where human water needs exceed the water supply in streams and lakes, groundwater extraction and purification become economically worthwhile. When conditions change, as during periods of drought (prolonged dry weather) or increased population growth, new groundwater supplies are tapped, thereby elevating them to aquifer status.
Wells
In addition to collecting groundwater from springs, humans extract water from aquifers by digging or drilling wells that extend from the ground surface to the water table, the level below which all the empty space in the rocks and soil are completely full of water (saturated). When a well reaches the water table, groundwater fills the hole like water filling a hole dug in beach sand.
There are a number of ways to construct wells. Some common types of wells are hand-dug, driven, and drilled wells.
- Hand-dug wells: Historically, wells were dug into soil and even rock by hand. Well diggers with shovels or picks would dig a hole below the water table by bailing water faster than it flowed into the well. Once a well was complete, its builders reinforced its walls and fitted it with a bucket system or pump to bring water to the surface. Hand-dug wells are still regularly constructed in many parts of the world, but they are uncommon in developed nations like the United States.
- Driven wells: Driven wells are constructed by forcing or hammering a narrow pipe into soft ground. These wells are inexpensive and can reach very deep aquifers, but can only be used in areas that have loose soil or sediment (particles of sand, gravel, and silt).
- Drilled wells: Today, most water wells are drilled using rotary (turning) or percussion (hammering) machines that are mounted on large trucks. Drilled wells that penetrate loose material are lined with plastic or metal pipe called casing, which keeps the sides of the hole from collapsing. An electric pump is placed at the bottom of the well to bring the water to the surface.
Historical groundwater use
Humans in arid regions such as northern Africa, the Middle East, and central Asia have relied on groundwater to provide drinking water and irrigate crops for thousands of years. Archeologists have discovered the remains of hand-dug wells, oasis (areas in the desert with a source of water) settlements, and groundwater distribution systems throughout the ancient world. Humans have drunk from groundwater springs at the Oasis of Bahariya in the Sahara desert of western Egypt since the early stone age (Paleolithic Age) more than one million years ago.
Knowledge of groundwater supplies and extraction technologies was critical information for ancient desert empires such as Mesopotamia, Sumeria, and Egypt. Nomads (wandering tribes) in the Saharan and Arabian deserts relied upon fiercely guarded knowledge of groundwater springs and seeps to survive.
Egyptians, Mesopotamians, and Chinese who first practiced agriculture dug wells to provide irrigation for water-intensive crops such as rice and cotton, and drinking water for permanent settlements. Groundwater availability affected patterns of conquest and settlement in Greek and Roman Empires. European explorers sought groundwater and white settlers excavated wells that supported settlement and farming throughout North and South America.
Modern groundwater use
Today, people use groundwater for agricultural irrigation, industrial processes, municipal (city) and residential (home) water supplies. In the United States, groundwater accounted for about one quarter (26%) of total water use in the year 2000. (Surface freshwater made up the other 74%.)
Groundwater use, however, varies by location, and many U.S. residents and industries depend almost completely upon water drawn from regional aquifers. More than one-third of U.S.’ 100 largest cities, including Miami Beach, San Antonio, Memphis, Honolulu, and Tucson get all their water from aquifers. Almost all rural households (98%) draw their water from private wells.
Farmers and ranchers in Midwestern and Western states make heavy use of groundwater for irrigation of crops. In the eastern and southern U.S., most drinking and agricultural water comes from lakes and streams, but industries use vast quantities of groundwater for such activities as refining petroleum, aluminum, and other ores; manufacturing steel and chemicals; producing plastics; and mining. Aquaculture (fish farming) is big business and a significant groundwater consumer in Southeastern states like Mississippi, Alabama, and Louisiana.
In the United States, groundwater is particularly important in arid and semi-arid agricultural states in the western half of the nation. Heavily agricultural states such as California, Oregon, and Texas use large quantities of groundwater for irrigation of food crops. The livestock industry also draws heavily upon groundwater supplies in states such as Texas, Nebraska, Kansas, and Colorado.
Water drawn from wells not only fills watering troughs, but also irrigates vast tracts of midwestern cropland that produce material for cattle, poultry, pig, and fish feed. Meat processing plants also require water. (It takes about 13 gallons [49 liters] of water to produce 1 pound (0.45 kilogram) of beef, and about 4 gallons [15 liters] of water go into producing 1 gallon [3.8 liters] of milk!)
