A
water well is an excavation or structure created
in the ground by digging, driving, boring or drilling to access
groundwater in underground
aquifers. The well water is
drawn by an
electric
submersible pump, a vertical turbine pump, a
handpump 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.
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.
Types of water wells
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 men with shovels digging down to below the
water table. They can 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. A more modern method called caissoning uses
reinforced concrete or plain concrete pre-cast well rings that are
lowered into the hole. A well digging team digs under a cutting
ring and the well column slowly sinks into the
aquifer,whilst protecting the well digging
team.
A dug well in a village in Faryab
Province
Hand dug wells provide a cheap and low-tech solution to accessing
groundwater in rural locations, with a
high degree of community participation. Hand dug wells have been
successfully excavated to 60m. Hand dug wells are cheap and low
tech (compared to drilling) as they use mostly hand labour for
construction. Hand dug wells have low operational and maintenance
costs. Even if the hand pump is broken, water can still be
extracted. In many cases, hand dug wells are similar to traditional
abstraction methods and are readily accepted by the host community.
The construction of hand dug wells can incorporate a high degree of
community participation (e.g. pre-fabrication of concrete rings).
Hand dug wells can be easily deepened, if the ground water level
drops, by telescoping the lining further down into the aquifer. The
yield of existing hand dug wells may be improved by deepening or
introducing vertical tunnels or perforated pipes.
Hand dug wells are not suited to hard ground formations and take
time to dig and line. Construction of hand dug wells can be
dangerous due to collapsing soils, falling objects and
asphyxiation. Hand dug well construction generally requires the use
of a trained well construction team. Construction of hand dug wells
can require large capital costs for equipment such as concrete ring
moulds, heavy lifting equipment, well shaft formwork, motorized
de-watering pumps, and fuel.Since most hand dug wells exploit
shallow aquifers, the well may be susceptible to yield fluctuations
and possible surface contamination.
Safety during hand dug well construction is paramount due to the
risk of collapse, falling objects and suffocation from exhaust
fumes from dewatering pumps.
Driven wells
Driven wells may be very simply created in unconsolidated material
with a "well point", which consists of a hardened drive point and a
screen (perforated pipe). The point is simply hammered 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.
Drilled wells
Drilled wells can be excavated by simple hand drilling methods
(augering, sludging, jetting, driven, hand percussion) or machine
drilling (rotary, percussion, down the hole hammer). Drilled wells
can get water from a much deeper level by than dug wells - often up
to several hundred meters.
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 are 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 usually 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.
Classification
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
).
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.
Well Siting
Before excavation, information about the geology, water table
depth, seasonal fluctuations, recharge area and rate must be found.
This is done through
Geophysical
imaging.
Well 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. Dug and driven wells are relatively easy
to contaminate, and dug wells are unreliable in most of the
U.S.
The quality of the well water can be significantly increased by
lining the well, sealing the well head, fitting a self-priming hand
pump, constructing an apron, ensuring the area is kept clean and
free from stagnant water and animals, moving sources of
contamination (latrines, garbage pits) and carrying out hygiene
education. It is important that the well is cleaned with 1%
chlorine solution after construction and periodically every 6
months.
Microorganisms
Most of the
bacteria,
viruses,
parasites, and
fungi that contaminate well water comes from
fecal material from humans and other animals.
Common bacterial contaminants include
E.
coli,
Salmonella,
Shigella, and
Campylobacter jejuni. Common viral
contaminants include
norovirus,
sapovirus,
rotavirus,
enteroviruses, and
hepatitis A and
E.
Parasites include
Giardia
lamblia,
Cryptosporidium,
Cyclospora, and
microsporidia.
Chemicals
Chemical contamination is a common problem with groundwater.
Nitrates from
sewage
or
fertilizer are a particular problem
for children.
Pesticides and
volatile organic compounds from
gasoline,
dry-cleaning, and many other sources are the
most commonly occurring pollutant chemicals in the U.S., and may be
identifiable in more than a third of all U.S. wells, although this
is mostly at levels below U.S. water standards. Other notable
chemical contaminants include the fuel additive
methyl tert-butyl ether (MTBE), and
perchlorate from rocket fuel, airbag
inflators, and other artificial and natural sources. Several
minerals are also contaminants, including
lead
leached from brass fittings or old lead pipes;
chromium VI from electroplating and other
sources; naturally occurring
arsenic,
radon, and
uranium, all
of which can cause cancer; and naturally occurring
fluoride, which is desirable in low quantities to
prevent
tooth decay, but which can cause
dental fluorosis in concentrations
above recommended levels.
Some chemicals are commonly present in water wells at levels that
are not toxic, but which can cause other problems.
Calcium and
magnesium cause
what is known as
hard water, which can
precipitate and clog pipes or burn out water heaters.
Iron and
manganese can appear
as dark flecks that stain clothing and plumbing, and can promote
the growth of
iron and manganese
bacteria that can form slimy black colonies that clog
pipes.
Mitigation
Cleanup of contaminated groundwater tends to be very costly.
Effective remediation of groundwater is generally very
difficult.
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 9 m) to ensure
that the grouted annular space is of even thickness.
Upon the construction of a new test well, it is considered best
practice to invest in a complete battery of chemical and biological
tests on the well water in question. Point-of-use treatment is
available for individual properties and treatment plants are often
constructed for municipal water supplies that suffer from
contamination. Most of these treatment methods involve the
filtration of the contaminants of concern, and
additional protection may be garnered by installing well-casing
screens only at depths where contamination is not present.
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 bring the water to a full boil for one
to three minutes, depending on location. A household well
contaminated by microorganisms can initially be treated by shock
chlorination using bleach, generating concentrations hundreds of
times greater than found in community water systems; however, this
will not fix any structural problems that led to the contamination
and generally requires some expertise and testing for effective
application.
Environmental problems
A possible risk with the placement of water wells could be
soil salination. This problem occurs when the
watertable of the soil begins to drop and salt begins to accumulate
as the soil begins to dry out.
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 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.
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.
In
India
, stepwells were created at
times, sometimes used both for water and for cooling.
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 Turpan
, 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.
From the
Iron Age onwards, wells are common
archaeological features, both with wooden shafts and shaft linings
made from
wickerwork.
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.
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.
In
Lewis Carroll´s
Alice in Wonderland, chapter 7, The
Dormouse tells the history of a family who lived "at the bottom of
a well", made of treacle (see
treacle
mining).
Wishing well at the castle of Zumelle,
Belluro, Veneto, Italy
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.
[604703]
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.
See also
Notes
References
- Driscoll, F. (1986). Groundwater and Wells. St. Paul, MN:
Johnson Filtration Systems, second edition. ISBN
978-0961645601
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