A water well is an excavation or structure created in the ground ––by digging, driving, boring or drilling to access water in underground aquifers. The well water is drawn via an electric submersible pump or a mechanical pump (eg from a water-pumping windmill). It can also be drawn up using containers, such as buckets, that are raised mechanically or by hand. Although not essential, a storage tank with a pressure of 40-60 psi is usually added to the system (after the pump), so the pump does not need to operate constantly. To reduce the electricity required to pump up the water, often, a cistern is also added along with a small second pump (see schematic below).
Wells can vary greatly in depth, water volume and water quality. Well water typically contains more minerals in solution than surface water and may require treatment to soften the water by removing minerals such as arsenic, iron and manganese.
A well is made by reaching groundwater in the water table. Groundwater is stored naturally below the earth's surface. Most groundwater originates as rain or snow that seeps into the ground and collects. Groundwater provides about 20 percent of the freshwater used in the United States. Most rural areas, and some cities depend on groundwater as their source for water.
Most rainwater is absorbed by the ground and fills the tiny spaces between soil particles. However, excess water runs over the top of the soil until it reaches a river, stream, or reservoir. Runoff water brings pollutants it encounters along the way to the reservoir.
As water seeps into the ground, it settles in the pores and cracks of underground rocks and into the spaces between grains of sand and pieces of gravel. In time, the water trickles down into a layer of rock or other material that is watertight. This watertight zone collects the groundwater, creating a saturated zone known as an aquifer. Aquifers in the United States are usually made from gravel, sandstone, limestone, or basalt (volcanic rock).
The water in the earth that these wells afford is at a place in the ground known as the water table. The water table is the level of the groundwater below the earth's surface. This table is measured by the depth of the upper limit of the aquifer. The water table can be lowered by lack of precipitation or overdraft.
Overdraft occurs when water is removed from the aquifer at a faster rate than can be naturally replenished by rain or snow. The lowering of the water table causes problems such as land subsidence, surface cracking, sinkholes on the surface, and damage to the aquifer's water producing character due to compaction. For instance, in the Chinese city of Shanghai, the earth was generally soft. People used to pump out groundwater from wells, leading to the eventual sinking of the surrounding strata. Shanghai's city government was forced to seal all wells in the city in the 1960s. In coastal areas, overdraft can lead to saltwater intrusion. Saltwater intrusion occurs in low water tables where drops in water pressure can lead to the ocean backing up into the groundwater.
In a damp area, the water table can be reached simply by digging. In this case the well walls are usually lined with brick, stone, or concrete to keep the sides from caving in on the well. A dug well can be up to deep, and has the greatest diameter of any of the well types. Well water that contains a high number of dissolved minerals is called a mineral well. Except for areas containing Karst formations, underground water is considered fairly clean because soils create a filter that remove toxins with large molecules.
Two broad classes of drilled-well types may be distinguished, based on the type of aquifer which the well is completed in:
Until recent centuries, all artificial wells were pumpless dug wells of varying degrees of formality. Their indispensability has produced numerous literary references, literal and figurative, to them, including the Christian Bible story of Jesus meeting a woman at Jacob's well (John 4:6) and the "Ding Dong Bell" nursery rhyme about a cat in a well.
Such primitive dug wells were excavations with diameters large enough to accommodate men with shovels digging down to below the water table. Relatively formal versions tended to be lined with laid stones or brick; extending this lining into a wall around the well presumably served to reduce both contamination and injuries by falling into the well. The iconic American farm well features a peaked roof above the wall, reducing airborne contamination, and a cranked windlass, mounted between the two roof-supporting members, for raising and lowering a bucket to obtain water.
More modern dug wells may be hand pumped, especially in developing countries.
Note that the term "shallow well" is not a synonym for dug well, and may actually be quite deep - see Aquifer type, below.
Drilled wells can get water from a much deeper level by mechanical drilling.
Drilled wells with electric pumps are currently used throughout the world, typically in rural or sparsely populated areas, though many urban areas are supplied partly by municipal wells.
Drilled wells are typically created using either top-head rotary style, table rotary, or cable tool drilling machines, all of which use drilling stems that are turned to create a cutting action in the formation, hence the term 'drilling'. Most shallow well drilling machines are mounted on large trucks, trailers, or tracked vehicle carriages. Water wells typically range from 20 to , but in some areas can go deeper than .
Rotary drilling machines use a segmented steel drilling string, typically made up of sections of steel tubing that is threaded together, with a bit or other drilling device at the bottom end. Some rotary drilling machines are designed to install (by driving or drilling) a steel casing into the well in conjunction with the drilling of the actual bore hole. Air and/or water is used as a circulation fluid to displace cuttings and cool bits during the drilling. Another form of rotary style drilling, termed 'mud rotary', makes use of a specially made mud, or drilling fluid, which is constantly being altered during the drill so that it can consistently create enough hydraulic pressure to hold the side walls of the bore hole open, regardless of the presence of a casing in the well. Typically, boreholes drilled into solid rock are not cased until after the drilling process is completed, regardless of the machinery used.
The oldest form of drilling machinery is the Cable Tool, still used today. Specifically designed to raise and lower a bit into the bore hole, the 'spudding' of the drill causes the bit to be raised and dropped onto the bottom of the hole, and the design of the cable causes the bit to twist at approximately 1/4 revolution per drop, thereby creating a drilling action. Unlike rotary drilling, cable tool drilling requires the drilling action to be stopped so that the bore hole can be bailed or emptied of drilled cuttings.
Drilled wells are typically cased with a factory-made pipe, typically steel (in air rotary or cable tool drilling) or plastic/PVC (in mud rotary wells, also present in wells drilled into solid rock). The casing is constructed by welding, either chemically or thermodynamically, segments of casing together. If the casing is installed during the drilling, most drills will drive the casing into the ground as the bore hole advances, while some newer machines will actually allow for the casing to be rotated and drilled into the formation in a similar manner as the bit advancing just below. PVC or plastic is typically welded and then lowered into the drilled well, vertically stacked with their ends nested and either glued or splined together. The sections of casing are usually 20' (6 m) or more in length, and 6" - 12" (15 to 30 cm) in diameter, depending on the intended use of the well and local groundwater conditions.
Surface contamination of wells in the United States is typically controlled by the use of a 'surface seal'. A large hole is drilled to a predetermined depth or to a confining formation (clay or bedrock, for example), and then a smaller hole for the well is completed from that point forward. The well is typically cased from the surface down into the smaller hole with a casing that is the same diameter as that hole. The annular space between the large bore hole and the smaller casing is filled with bentonite clay, concrete, or other sealant material. This creates an impermeable seal from the surface to the next confining layer that keeps contaminants from traveling down the outer sidewalls of the casing or borehole and into the aquifer. In addition, wells are typically capped with either an engineered well cap or seal that vents air through a screen into the well, but keeps insects, small animals, and unauthorized persons from accessing the well.
At the bottom of wells, based on formation, a screening device, filter pack, slotted casing, or open bore hole is left to allow the flow of water into the well. Constructed screens are typically used in unconsolidated formations (sands, gravels, etc.), allowing water and a percentage of the formation to pass through the screen. Allowing some material to pass through creates a large area filter out of the rest of the formation, as the amount of material present to pass into the well slowly decreases and is removed from the well. Rock wells are typically cased with a PVC liner/casing and screen or slotted casing at the bottom, this is mostly present just to keep rocks from entering the pump assembly. Some wells utilize a 'filter pack' method, where an undersized screen or slotted casing is placed inside the well and a filter medium is packed around the screen, between the screen and the borehole or casing. This allows the water to be filtered of unwanted materials before entering the well and pumping zone.
Two additional broad classes of well types may be distinguished, based on the use of the well:
Obviously, a well constructed for pumping groundwater can be used passively as a monitoring well and a small diameter well can be pumped, but this distinction by use is common.
Shallow pumping wells can often supply drinking water at a very low cost, but because impurities from the surface easily reach shallow sources, a greater risk of contamination occurs for these wells when they are compared to deeper wells. In shallow and deep wells, the water requires pumping to the surface; in artesian wells, conversely, water usually rises to a greater level than the land surface when extracted from a deep source.
Well water for personal use is often filtered with reverse osmosis water processors; this process can remove very small particles. A simple, effective way of killing micro organisms is to boil the water (although, unless in contact with surface water or near areas where treated wastewater is being recharged, groundwater tends to be free of micro organisms). Alternately the addition of 1/8 teaspoon (0.625 mL) of bleach to a gallon (3.8 L) of water will disinfect it after a half hour
Contamination of groundwater from surface and subsurface sources can usually be dramatically reduced by correctly centering the casing during construction and filling the casing annulus with an appropriate sealing material. The sealing material (grout) should be placed from immediately above the production zone back to surface, because, in the absence of a correctly constructed casing seal, contaminated fluid can travel into the well through the casing annulus. Centering devices are important (usually 1 per length of casing or at maximum intervals of 30 feet/9 m) to ensure that the grouted annular space is of even thickness.
The earliest wells are known from the Neolithic. In the submerged Pre-Pottery Neolithic B settlement of Atlit Yam in Israel, dated to 8100-7500 BC, a well has been found, which so far is the oldest known. Other PPNB wells (7-8 m deep) are known from Kissonerga-Mylouthkia on Cyprus and maybe shallower examples from Shillourokambos as well.
Wood-lined wells are known from the early Neolithic Linear Pottery culture, for example in Kückhoven, dated 5090 BC and Eythra), dated 5200 BC in Germany and Schletz in Austria. The early Mesolithic site of Friesack in Germany has yielded a shallow pit with the remains of a birch-bark container that may have been a shallow artificial well as well.
Australian Aborigines relied on wells to survive the harsh Australian desert. They would dig down, scooping out sand and mud to reach clean water, then cover the source with spinifex to prevent spoilage. Non-aborigines call these native wells, soaks or soakages.
A karez well system is a model of an ancient water collection system made up of a series of wells and linked underground water channels that collects flowing water from a source usually a distance away, stores it, and then brings the water to the surface using gravity. This system of wells was most fully developed in Turfan, China, an important trading center on the ancient Silk Route, that owed its prosperity to the water provided by its karez well system.
In Egypt, shadoofs and sakiehs are used. When compared to each other however, the Sakkieh is much more efficient, as it can bring up water from a depth of 10 meters (versus the 3 meters of the shadoof). The Sakieh is the Egyptian version of the Noria.
Lately however, the described wells/pumps are no longer very efficient and can be replaced by either handpumps or treadle pumps. Another alternative is the use of self-dug wells, electrical deep-well pumps (for higher depths). Appropriate technology organizations as Practical Action are now supplying information on how to build/set-up (diy) handpumps and treadle pumps in practice.
Springs and wells have had cultural significance since prehistoric times, leading to the foundation of towns such as Wells and Bath in Somerset. Interest in health benefits led to the growth of spa towns including many with wells in their name, examples being Llandrindod Wells and Royal Tunbridge Wells.
There is a belief that a wish can be made in a well; see wishing well. There is much folklore in Wales surrounding wells, particularly in relation to their healing properties. In Scotland and Ireland, there is a Celtic tradition of leaving cloth offerings for healing at Clootie wells. In the Peak District of England, there is a tradition of Well dressing which has persisted from Pagan to Christian religion, possibly related to plague.
Eratosthenes first calculated the radius of the Earth in about 230 BC by comparing shadows in wells during the summer solstice.
In Western Ukraine, water wells were traditionally centers of social life, and the community came together to build them using a traditional process. Local stories often emphasize the social and cultural values of wells. The wells were decorated and had a wooden wheel attached to raise the bucket. Wells are still used in many Ukrainian towns and cities.
The same is true with the early Israelites, as depicted in the Hebrew Bible and in the [Christian] New Testament. Many Bible stories take place around wells, such as the finding of a wife for Isaac in Genesis and Jesus's talk with the Samaritan woman in the Gospels.