Water well
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A water well is an artificial excavation or structure put down by any method such as digging, driving, boring, or drilling for the purposes of withdrawing water from underground aquifers.
Well water may be drawn via mechanical pump (such as an electric submersible pump) from a source below the surface of the earth, or drawn using containers, such as buckets, that are raised mechanically, or by hand. 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.
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[edit] Ground Water
A well is made by reaching ground water in the water table. Ground water is stored naturally below the earth's surface. Most ground water originates as rain or snow that seeps into the ground and collects. Ground water provides about 20 percent of the fresh water used in the United States. Most rural areas, and some cities depend on ground water 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 water tight. This water tight zone collects the ground water, 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 obtain is at a place in the ground known as the water table. The water table is the level of the ground water 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 replaced by rain or snow. The lowering of the water table causes problems such as land subsidence, surface cracking, sinkholes on the surface, 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 ground water 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 salt water intrusion. Salt water intrusion occurs in low water tables where drops in water pressure can lead to the ocean backing up into the ground water.
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 in order to keep the sides from caving in on the well. A dug well can be up to 50 feet 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 large toxins.
[edit] Aquifer classification
- Main article: aquifer
Two broad classes of drilled-well types may be distinguished, based on the type of aquifer which the well is completed in:
- shallow or unconfined wells are completed in the uppermost saturated aquifer at that location (the upper unconfined aquifer); or
- deep or confined wells, which are sunk through an impermeable stratum down into an aquifer which is sandwiched between two impermeable strata (aquitards or aquicludes). The majority of confined aquifers are classified as artesian because the hydraulic head in a confined well is higher than the level of the top of the aquifer. If the hydraulic head in a confined well is higher than the land surface it is a "flowing" artesian well (named after Artois in France).
There clearly are many cases that fall in between these two endmembers; often unconfined wells may be very deep (what is often called a shallow well can be over 150 m deep) and many times wells are completed across all aquifers from their top to their bottom (especially agricultural or industrial wells), being open to both unconfined and confined aquifers.
[edit] Types of water wells
[edit] Dug wells
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 muscle-powered digging 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 undeveloped and third-world countries.
Note that the term "shallow well" is not a synonym for dug well, and may actually be quite deep - see Aquifer type, below.
[edit] Driven Wells
Driven wells consist of a series of pipes with a point at the end. The point is driven into the ground, thus the name driven, to a depth of up to 50 feet.
[edit] Drilled wells
Drilled wells can access water from a much deeper level by mechanical drilling.
Drilled wells with electric pumps are currently used throughout the world, mainly in developing and developed countries, 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 600 feet, but in some areas can go deeper than 3,000 feet.
Rotary drilling machines use a segmented steel drilling string, typically made up of 20 foot 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 & 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 & lower a bit into the bore hole, the 'spudding' of the drill cause the bit to be raised & 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 & drilled into the formation in a similar manner as the bit advancing just below. PVC or plastic is typically welded & then lowered the drilled well, vertically stacked with their ends nested & either glued or splined together. T he sections of casing are usually 20' or more in length, and 6" - 12" in diameter, depending on the intended use of the well and local ground water 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 & 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 & into the acquifer. 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 individuals 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 & 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 & is removed from the well. Rock wells are typically cased with a PVC liner/casing & 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 & a filter media is packed around the screen, between the screen & the borehole or casing. This allows the water to be filtered of unwanted materials before entering the well & pumping zone.
Driven wells may be created in unconsolidated material with a "well point", which consists of a hardened drive point and a screen. The point is simply driven into the ground, usually with a tripod and "driver", with pipe sections added as needed. A driver is a weighted pipe that slides over the pipe being driven and is repeatedly dropped on it. When groundwater is encountered, the well is washed of sediment and a pump installed. This is the cheapest and simplest type of water well known today, however it is only useful at relatively shallow depths and for small capacity wells.[citation needed]
[edit] Use classification
Two additional broad classes of well types may be distinguished, based on the use of the well:
- production or pumping wells, are large diameter (> 15 cm in diameter) cased (metal, plastic, or concrete) water wells, constructed for extracting water from the aquifer by a pump (if the well is not artesian).
- monitoring wells or piezometers, are often smaller diameter wells used to monitor the hydraulic head or sample the groundwater for chemical constituents. Piezometers are monitoring wells completed over a very short section of aquifer. Monitoring wells can also be completed at multiple levels, allowing discrete samples or measurements to be made at different vertical elevations at the same map location.
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.
[edit] Contamination
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 microorganisms 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 microorganisms).
Contamination related to human activity is a common problem with groundwater. For example, BTEX, which comes from gasoline refining, and MTBE, which is a fuel additive, are common contaminants in urbanized areas, often as the result of leaking underground storage tanks. Many industrial solvents also are common groundwater contaminants, which may enter groundwater through leaks, accidental spills or intentional dumping. Cleanup of contaminated groundwater tends to be very costly. Effective remediation of groundwater is generally very difficult.
[edit] Ancient well technologies
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 Linearbandkeramic culture, for example in Kückhoven, dated 5090BC and Eythra), dated 5200BC 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 spoilation. White people call these native wells, soaks or soakages.
In India, stepwells were created at times, sometimes used both for water and for cooling.
From the Iron Age onwards, wells are common archaeological features, both with wooden shafts and shaft-linings made from wickerwork.
[edit] Cultural references
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.
Empty wells are a prominent element in some of the work of Japanese author Haruki Murakami, especially The Wind-Up Bird Chronicle.
There is much folklore in Wales surrounding wells, particularly in relation to their healing properties.
There is a belief that a wish can be made in a well; see wishing well.
Eratosthenes first calculated the radius of the Earth in about 230 BC by comparing shadows in wells during the summer solstice.[1]
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.
[edit] See also
- Groundwater
- Hand pump
- Hydrogeology
- Hydrology
- Pump
- Water resources
- Water table
- Arsenic poisoning of drinking water
[edit] External links
- A Brief History of Wells and Toilets (2005)PDF
- US Geological Survey - Ground water: Wells
- US Geological Survey - Water Science Pictures Flowing Artesian Well
- American Ground Water Trust
- National Ground Water Association
- US Peace Corps Well Page
- Lifewater International Technical Library
- The Internet Sacred Texts Archive: The Folklore of Wells (Wales)
- Driving a well with a well point
- Information on drilling wells
- Information on measuring well depth
