A spring is a point of exit at which groundwater from an aquifer flows out on top of Earth's crust (pedosphere) and becomes surface water. It is a component of the hydrosphere. Springs have long been important for humans as a source of fresh water, especially in arid regions which have relatively little annual rainfall.
Springs are driven out onto the surface by various natural forces, such as gravity and hydrostatic pressure. They vary in volumetric flow rate from nearly zero to more than 14,000 litres per second (490 cu ft/s) for the biggest springs.
Springs are formed when groundwater flows onto the surface. This typically happens when the groundwater table reaches above the surface level. Springs may also be formed as a result of karst topography, aquifers, or volcanic activity. Springs also have been observed on the ocean floor, spewing hot water directly into the ocean.
Springs formed as a result of karst topography create karst springs, in which ground water travels through a network of cracks and fissures—openings ranging from intergranular spaces to large caves, later emerging in a spring.
The forcing of the spring to the surface can be the result of a confined aquifer in which the recharge area of the spring water table rests at a higher elevation than that of the outlet. Spring water forced to the surface by elevated sources are artesian wells. This is possible even if the outlet is in the form of a 300-foot-deep (91 m) cave. In this case the cave is used like a hose by the higher elevated recharge area of groundwater to exit through the lower elevation opening.
Non-artesian springs may simply flow from a higher elevation through the earth to a lower elevation and exit in the form of a spring, using the ground like a drainage pipe. Still other springs are the result of pressure from an underground source in the earth, in the form of volcanic activity. The result can be water at elevated temperature such as a hot spring.
The action of the groundwater continually dissolves permeable bedrock such as limestone and dolomite, creating vast cave systems.
Spring discharge, or resurgence, is determined by the spring's recharge basin. Factors that affect the recharge include the size of the area in which groundwater is captured, the amount of precipitation, the size of capture points, and the size of the spring outlet. Water may leak into the underground system from many sources including permeable earth, sinkholes, and losing streams. In some cases entire creeks seemingly disappear as the water sinks into the ground via the stream bed. Grand Gulf State Park in Missouri is an example of an entire creek vanishing into the groundwater system. The water emerges 9 miles (14 km) away, forming some of the discharge of Mammoth Spring in Arkansas. Human activity may also affect a spring's discharge—withdrawal of groundwater reduces the water pressure in an aquifer, decreasing the volume of flow.
Springs fall into three general classifications: perennial (springs that flow constantly during the year); intermittent (temporary springs that are active after rainfall, or during certain seasonal changes); and periodic (as in geysers that vent and erupt at regular or irregular intervals).
Springs are often classified by the volume of the water they discharge. The largest springs are called "first-magnitude", defined as springs that discharge water at a rate of at least 2800 liters or 100 cubic feet (2.8 m3) of water per second. Some locations contain many first-magnitude springs, such as Florida where there are at least 27 known to be that size; the Missouri and Arkansas Ozarks, which contain 10 known of first-magnitude; and 11 more in the Thousand Springs area along the Snake River in Idaho. The scale for spring flow is as follows:
|Magnitude||Flow (ft3/s, gal/min, pint/min)||Flow (L/s)|
|1st magnitude||> 100 ft3/s||2800 L/s|
|2nd magnitude||10 to 100 ft3/s||280 to 2800 L/s|
|3rd magnitude||1 to 10 ft3/s||28 to 280 L/s|
|4th magnitude||100 US gal/min to 1 ft3/s (448 US gal/min)||6.3 to 28 L/s|
|5th magnitude||10 to 100 gal/min||0.63 to 6.3 L/s|
|6th magnitude||1 to 10 gal/min||63 to 630 mL/s|
|7th magnitude||2 pint to 1 gal/min||8 to 63 mL/s|
|8th magnitude||Less than 1 pint/min||8 mL/s|
|0 magnitude||no flow (sites of past/historic flow)|
Main article: Mineral spring
Minerals become dissolved in the water as it moves through the underground rocks. This mineral content is measured as total dissolved solids (TDS). This may give the water flavor and even carbon dioxide bubbles, depending on the nature of the geology through which it passes. This is why spring water is often bottled and sold as mineral water, although the term is often the subject of deceptive advertising. Mineral water contains no less than 250 parts per million (ppm) of tds. Springs that contain significant amounts of minerals are sometimes called 'mineral springs'. (Springs without such mineral content, meanwhile, are sometimes distinguished as 'sweet springs'.) Springs that contain large amounts of dissolved sodium salts, mostly sodium carbonate, are called 'soda springs'. Many resorts have developed around mineral springs and are known as spa towns. Mineral springs are alleged to have healing properties. Soaking in them is said to result in the absorption of the minerals from the water. Some springs contain arsenic levels that exceed the 10ppb World Health Organisation standard for drinking water. Where such springs feed rivers they can also raise the arsenic levels in the rivers above WHO limits.
Water from springs is usually clear. However some springs may be colored by the minerals that are dissolved in the water. For instance, water heavy with iron or tannins will have an orange color.
In parts of the United States a stream carrying the outflow of a spring to a nearby primary stream may be called a spring branch, spring creek, or run. Groundwater tends to maintain a relatively long-term average temperature of its aquifer; so flow from a spring may be cooler than other sources on a summer day, but remain unfrozen in the winter. The cool water of a spring and its branch may harbor species such as certain trout that are otherwise ill-suited to a warmer local climate.
Springs have been used for a variety of human needs - including drinking water, domestic water supply, irrigation, mills, navigation, and electricity generation. Modern uses include recreational activities such as fishing, swimming, and floating; therapy; water for livestock; fish hatcheries; and supply for bottled mineral water or bottled spring water. Springs have taken on a kind of mythic quality in that some people believe, falsely, that springs are always healthy sources of drinking water. They may or may not be. In order to know how to use a spring appropriately, whether for a mineral bath or drinking water one needs to access a comprehensive water quality test. Springs that are managed as spas will already have such a test.
Springs are often used as sources for bottled water. When purchasing bottled water labelled as spring water one can often find the water test for that spring on the website of the company selling it.
Springs have been used as sources of water for gravity-fed irrigation of crops. Indigenous people of the American Southwest built spring-fed acequias that directed water to fields through canals. This method was later used by the Spanish missionaries.
Main article: Holy well
A sacred spring, or holy well, is a small body of water emerging from underground and revered either some religious context: Christian and/or pagan and/or other. The lore and mythology of ancient Greece was replete with sacred and storied springs—notably, the Corycian, Pierian and Castalian springs. In medieval Europe, pagan sacred sites frequently became Christianized as holy wells. The term "holy well" is commonly employed to refer to any water source of limited size (i.e. not a lake or river, but including pools and natural springs and seeps), which has some significance in local folklore. This can take the form of a particular name, an associated legend, the attribution of healing qualities to the water through the numinous presence of its guardian spirit or of a Christian saint, or a ceremony or ritual centred on the well site. Christian legends often recount how the action of a saint caused a spring's water to flow - a familiar theme especially in the hagiography of Celtic saints.
Main article: Hot spring
The geothermally heated groundwater that flows from thermal springs is greater than human body temperature, usually in the range of 45–50 °C / 110–120 °F, but they can be hotter. Those springs with water cooler than body temperature, but warmer than air temperature are sometimes referred to as warm springs.
Hot springs or geothermal springs have been used for balneotherapy, bathing and relaxation for thousands of years. Because of the folklore surrounding hot springs and their claimed medical value, some have become tourist destinations and locations of physical rehabilitation centers.
Hot springs have been used as a source of heat for thousands of years. In the 20th century they became a renewable resource of geothermal energy to heat homes and buildings. The city of Beppu, Japan contains 2,217 hot spring well heads that provide the city with hot water. Hot springs have also been used as a source of sustainable energy for greenhouse cultivation and the growing of crops and flowers.
Springs have been represented in culture through art, mythology and folklore throughout history. The Fountain of Youth is a mythical spring, which was said to restore youth to anyone who drank from it. It has been claimed that the fountain is located in St. Augustine, Florida, and was discovered by Juan Ponce de León in 1513, though it has not demonstrated the power to restore youth and most historians dispute the veracity of Ponce de León's discovery.
Pythia, also known as the Oracle at Delphi was the high priestess of the Temple of Apollo. She delivered prophesies in a frenzied state of divine possession that were "induced by vapours rising from a chasm in the rock". It is believed that the vapors were emitted from the Kerna spring at Delphi.
The Greek myth of Narcissus describes a young man who fell in love with his reflection in the still pool of a spring. Narcissus gazed into "an unmuddied spring, silvery from its glittering waters, which neither shepherds nor she-goats grazing on the mountain nor any other cattle had touched, which neither bird nor beast nor branch fallen from a tree had disturbed." (Ovid)
The early 20th century American photographer, James Reuel Smith created a comprehensive series of photographs documenting the historical springs of New York City before they were capped by the city after the advent of the municipal water system. Smith later photographed springs in Europe leading to his book, Springs and Wells in Greek and Roman Literature, Their Legends and Locations (1922).
The 19th century Japanese artists Utagawa Hiroshige and Utagawa Toyokuni III created a series of wood-block prints, Two Artists Tour the Seven Hot Springs (Sōhitsu shichitō meguri) in 1854.
The Chinese city Jinan is known as "a City of Springs" (Chinese: 泉城), because of its 72 spring attractions and numerous micro spring holes spread over the city centre.
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