An aquifer is an underground layer of water-bearing
permeable rock or unconsolidated materials (gravel, sand, or silt) from which
groundwater can be extracted using a water well. The study of water flow in
aquifers and the characterization of aquifers is called hydrogeology. Related
terms include aquitard, which is a bed of low permeability along an aquifer,
and aquiclude (or aquifuge), which is a solid, impermeable area underlying or
overlying an aquifer. If the impermeable area overlies the aquifer pressure
could cause it to become a confined aquifer.
Depth
Aquifers may occur at various depths. Those closer to the
surface are not only more likely to be used for water supply and irrigation,
but are also more likely to be topped up by the local rainfall. Many desert
areas have limestone hills or mountains within them or close to them that can
be exploited as groundwater resources. Parts of the Atlas Mountains in North Africa,
the Lebanon and Anti-Lebanon ranges of Syria, Palestine and Lebanon, the Jebel
Akhdar (Oman) in Oman, parts of the Sierra Nevada and neighboring ranges in the
United States' Southwest, have shallow aquifers that are exploited for their
water. Overexploitation can lead to the exceeding of the practical sustained
yield; i.e., more water is taken out than can be replenished. Along the
coastlines of certain countries, such as Libya and Israel, increased water
usage associated with population growth has caused a lowering of the water
table and the subsequent contamination of the groundwater with saltwater from
the sea.
The beach provides a model to help visualize an aquifer. If
a hole is dug into the sand, very wet or saturated sand will be located at a
shallow depth. This hole is a crude well, the wet sand represents an aquifer,
and the level to which the water rises in this hole represents the water table.
In 2013 large freshwater aquifers were discovered under
continental shelves off Australia, China, North America and South Africa. They
contain an estimated half a million cubic kilometers of “low salinity” water
that could be economically processed into potable water. The reserves formed
when ocean levels were lower and rainwater made its way into the ground in land
areas that were not submerged until the ice age ended 20,000 years ago. The
volume is estimated to be 100x the amount of water extracted from other
aquifers since 1900.
Classification
The above diagram indicates typical flow directions in a
cross-sectional view of a simple confined or unconfined aquifer system. The
system shows two aquifers with one aquitard (a confining or impermeable layer)
between them, surrounded by the bedrock aquiclude, which is in contact with a
gaining stream (typical in humid regions). The water table and unsaturated zone
are also illustrated. An aquitard is a zone within the earth that restricts the
flow of groundwater from one aquifer to another. An aquitard can sometimes, if
completely impermeable, be called an aquiclude or aquifuge. Aquitards are
composed of layers of either clay or non-porous rock with low hydraulic
conductivity.
Saturated versus unsaturated
Groundwater can be found at nearly every point in the
Earth's shallow subsurface, to some degree; although aquifers do not
necessarily contain fresh water. The Earth's crust can be divided into two
regions: the saturated zone or phreatic zone (e.g., aquifers, aquitards, etc.),
where all available spaces are filled with water, and the unsaturated zone (also
called the vadose zone), where there are still pockets of air that contain some
water, but can be filled with more water.
Saturated means the pressure head of the water is greater
than atmospheric pressure (it has a gauge pressure > 0). The definition of
the water table is surface where the pressure head is equal to atmospheric
pressure (where gauge pressure = 0).
Unsaturated conditions occur above the water table where the
pressure head is negative (absolute pressure can never be negative, but gauge
pressure can) and the water that incompletely fills the pores of the aquifer
material is under suction. The water content in the unsaturated zone is held in
place by surface adhesive forces and it rises above the water table (the zero
gauge pressure isobar) by capillary action to saturate a small zone above the
phreatic surface (the capillary fringe) at less than atmospheric pressure. This
is termed tension saturation and is not the same as saturation on a water
content basis. Water content in a capillary fringe decreases with increasing
distance from the phreatic surface. The capillary head depends on soil pore
size. In sandy soils with larger pores, the head will be less than in clay
soils with very small pores. The normal capillary rise in a clayey soil is less
than 1.80 m (six feet) but can range between 0.3 and 10 m (1 and 30 ft).
The capillary rise of water in a small diameter tube is this
same physical process. The water table is the level to which water will rise in
a large-diameter pipe (e.g., a well) that goes down into the aquifer and is
open to the atmosphere.
See also: Water content and Soil moisture
Basal aquifer
Basal aquifer or basal water sands aquifers (BWS) describes
"the water-bearing sands, gravel or fractured rock that is found at the bottom
of a geological formation, underlying the bitumen-saturated sands." An
example is the McMurray basal water sands aquifer. The McMurray Formation in
the Athabaska oil sands in northern Alberta, consists of "sandstone and
shale deposited in a transgressive geological sequence", resulting in the
"course grained texture of the basal deposits". Basal water sands
(BWS) aquifers occur when the basal sand is low in bitumen.[6] Although the
water at this depth may be saline, portions of the water-saturated McMurray
basal water sand aquifer are non-saline.
Aquifers versus aquitards
Aquifers are typically saturated regions of the subsurface
that produce an economically feasible quantity of water to a well or spring
(e.g., sand and gravel or fractured bedrock often make good aquifer materials).
An aquitard is a zone within the earth that restricts the
flow of groundwater from one aquifer to another. An aquitard can sometimes, if
completely impermeable, be called an aquiclude or aquifuge. Aquitards comprise
layers of either clay or non-porous rock with low hydraulic conductivity.
In mountainous areas (or near rivers in mountainous areas),
the main aquifers are typically unconsolidated alluvium, composed of mostly
horizontal layers of materials deposited by water processes (rivers and
streams), which in cross-section (looking at a two-dimensional slice of the
aquifer) appear to be layers of alternating coarse and fine materials. Coarse
materials, because of the high energy needed to move them, tend to be found
nearer the source (mountain fronts or rivers), whereas the fine-grained
material will make it farther from the source (to the flatter parts of the
basin or overbank areas - sometimes called the pressure area). Since there are
less fine-grained deposits near the source, this is a place where aquifers are
often unconfined (sometimes called the forebay area), or in hydraulic
communication with the land surface.
Confined versus unconfined
There are two end members in the spectrum of types of
aquifers; confined and unconfined (with semi-confined being in between).
Unconfined aquifers are sometimes also called water table or phreatic aquifers,
because their upper boundary is the water table or phreatic surface. (See
Biscayne Aquifer.) Typically (but not always) the shallowest aquifer at a given
location is unconfined, meaning it does not have a confining layer (an aquitard
or aquiclude) between it and the surface. The term "perched" refers
to ground water accumulating above a low-permeability unit or strata, such as a
clay layer. This term is generally used to refer to a small local area of
ground water that occurs at an elevation higher than a regionally extensive
aquifer. The difference between perched and unconfined aquifers is their size
(perched is smaller).
If the distinction between confined and unconfined is not
clear geologically (i.e., if it is not known if a clear confining layer exists,
or if the geology is more complex, e.g., a fractured bedrock aquifer), the
value of storativity returned from an aquifer test can be used to determine it
(although aquifer tests in unconfined aquifers should be interpreted
differently than confined ones). Confined aquifers have very low storativity
values (much less than 0.01, and as little as 10−5), which means that the
aquifer is storing water using the mechanisms of aquifer matrix expansion and
the compressibility of water, which typically are both quite small quantities.
Unconfined aquifers have storativities (typically then called specific yield)
greater than 0.01 (1% of bulk volume); they release water from storage by the
mechanism of actually draining the pores of the aquifer, releasing relatively
large amounts of water (up to the drainable porosity of the aquifer material,
or the minimum volumetric water content).
Isotropic versus anisotropic
In isotropic aquifers or aquifer layers the hydraulic
conductivity (K) is equal for flow in all directions, while in anisotropic conditions
it differs, notably in horizontal (Kh) and vertical (Kv) sense.
Semi-confined aquifers with one or more aquitards work as an
anisotropic system, even when the separate layers are isotropic, because the
compound Kh and Kv values are different (see hydraulic transmissivity and
hydraulic resistance).
When calculating flow to drains or flow to wells in an aquifer, the anisotropy is to be taken
into account lest the resulting design of the drainage system may be faulty.
Groundwater in rock formations
Groundwater may exist in underground rivers (e.g., caves
where water flows freely underground). This may occur in eroded limestone areas
known as karst topography, which make up only a small percentage of Earth's
area. More usual is that the pore spaces of rocks in the subsurface are simply
saturated with water — like a kitchen sponge — which can be pumped out for
agricultural, industrial, or municipal uses.
If a rock unit of low porosity is highly fractured, it can
also make a good aquifer (via fissure flow), provided the rock has a hydraulic
conductivity sufficient to facilitate movement of water. Porosity is important,
but, alone, it does not determine a rock's ability to act as an aquifer. Areas
of the Deccan Traps (a basaltic lava) in west central India are good examples
of rock formations with high porosity but low permeability, which makes them
poor aquifers. Similarly, the micro-porous (Upper Cretaceous) Chalk of south
east England, although having a reasonably high porosity, has a low grain-to-grain
permeability, with its good water-yielding characteristics mostly due to
micro-fracturing and fissuring.
Human dependence on groundwater
Center-pivot irrigated fields in Kansas covering hundreds of
square miles watered by the Ogallala Aquifer
Most land areas on Earth have some form of aquifer
underlying them, sometimes at significant depths. These aquifers are rapidly
being depleted by the human population.
Fresh-water aquifers, especially those with limited recharge
by meteoric water, can be over-exploited and, depending on the local
hydrogeology, may draw in non-potable water or saltwater intrusion from
hydraulically connected aquifers or surface water bodies. This can be a serious
problem, especially in coastal areas and other areas where aquifer pumping is
excessive. In some areas, the ground water can be contaminated by mineral
poisons, such as arsenic - see Arsenic contamination of groundwater.
Aquifers are critically important in human habitation and
agriculture. Deep aquifers in arid areas have long been water sources for
irrigation (see Ogallala below). Many villages and even large cities draw their
water supply from wells in aquifers.
Municipal, irrigation, and industrial water supplies are
provided through large wells. Multiple wells for one water supply source are
termed "wellfields", which may withdraw water from confined or
unconfined aquifers. Using ground water from deep, confined aquifers provides
more protection from surface water contamination. Some wells, termed
"collector wells," are specifically designed to induce infiltration
of surface (usually river) water.
Aquifers that provide sustainable fresh groundwater to urban
areas and for agricultural irrigation are typically close to the ground surface
(within a couple of hundred metres) and have some recharge by fresh water. This
recharge is typically from rivers or meteoric water (precipitation) that
percolates into the aquifer through overlying unsaturated materials.
Occasionally, sedimentary or "fossil" aquifers are
used to provide irrigation and drinking water to urban areas. In Libya, for
example, Muammar Gaddafi's Great Manmade River project has pumped large amounts
of groundwater from aquifers beneath the Sahara to populous areas near the
coast.Though this has saved Libya money over the alternative, desalination, the
aquifers are likely to run dry in 60 to 100 years. Aquifer depletion has been
cited as one of the causes of the food price rises of 2011.
Subsidence
In unconsolidated aquifers, groundwater is produced from
pore spaces between particles of gravel, sand, and silt. If the aquifer is
confined by low-permeability layers, the reduced water pressure in the sand and
gravel causes slow drainage of water from the adjoining confining layers. If
these confining layers are composed of compressible silt or clay, the loss of
water to the aquifer reduces the water pressure in the confining layer, causing
it to compress from the weight of overlying geologic materials. In severe
cases, this compression can be observed on the ground surface as subsidence.
Unfortunately, much of the subsidence from groundwater extraction is permanent
(elastic rebound is small). Thus, the subsidence is not only permanent, but the
compressed aquifer has a permanently reduced capacity to hold water.
Saltwater intrusion
Aquifers near the coast have a lens of freshwater near the
surface and denser seawater under freshwater. Seawater penetrates the aquifer
diffusing in from the ocean and is denser than freshwater. For porous (i.e.,
sandy) aquifers near the coast, the thickness of freshwater atop saltwater is
about 40 feet (12 m) for every 1 ft (0.30 m) of freshwater head above sea
level. This relationship is called the Ghyben-Herzberg equation. If too much
ground water is pumped near the coast, salt-water may intrude into freshwater
aquifers causing contamination of potable freshwater supplies. Many coastal
aquifers, such as the Biscayne Aquifer near Miami and the New Jersey Coastal
Plain aquifer, have problems with saltwater intrusion as a result of
overpumping.
Salination
Example of a water balance of the aquifer
Aquifers in surface irrigated areas in semi-arid zones with
reuse of the unavoidable irrigation water losses percolating down into the
underground by supplemental irrigation from wells run the risk of salination.
Surface irrigation water normally contains salts in the
order of 0.5 g/l or more and the annual irrigation requirement is in the order
of 10000 m³/ha or more so the annual import of salt is in the order of 5000
kg/ha or more.
Under the influence of continuous evaporation, the salt
concentration of the aquifer water may increase continually and eventually
cause an environmental problem.
For salinity control in such a case, annually an amount of
drainage water is to be discharged from the aquifer by means of a subsurface
drainage system and disposed of through a safe outlet. The drainage system may
be horizontal (i.e. using pipes, tile drains or ditches) or vertical (drainage
by wells). To estimate the drainage requirement, the use of a groundwater model
with an agro-hydro-salinity component may be instrumental, e.g. SahysMod.
Examples
The Great Artesian Basin situated in Australia is arguably
the largest groundwater aquifer in the world (over 1.7 million km²). It plays a
large part in water supplies for Queensland and remote parts of South
Australia.
The Guarani Aquifer, located beneath the surface of
Argentina, Brazil, Paraguay, and Uruguay, is one of the world's largest aquifer
systems and is an important source of fresh water. Named after the Guarani
people, it covers 1,200,000 km², with a volume of about 40,000 km³, a thickness
of between 50 m and 800 m and a maximum depth of about 1,800 m.
Aquifer depletion is a problem in some areas, and is
especially critical in northern Africa; see the Great Manmade River project of
Libya for an example. However, new methods of groundwater management such as
artificial recharge and injection of surface waters during seasonal wet periods
has extended the life of many freshwater aquifers, especially in the United
States.
The Ogallala Aquifer of the central United States is one of
the world's great aquifers, but in places it is being rapidly depleted by
growing municipal use, and continuing agricultural use. This huge aquifer,
which underlies portions of eight states, contains primarily fossil water from
the time of the last glaciation. Annual recharge, in the more arid parts of the
aquifer, is estimated to total only about 10 percent of annual withdrawals.
According to a 2013 report by research hydrologist, Leonard F. Konikow, at the
United States Geological Survey (USGC), the depletion between 2001–2008,
inclusive, is about 32 percent of the cumulative depletion during the entire
20th century (Konikow 2013:22)." In the United States, the biggest users
of water from aquifers include agricultural irrigation and oil and coal
extraction. "Cumulative total groundwater depletion in the United States
accelerated in the late 1940s and continued at an almost steady linear rate
through the end of the century. In addition to widely recognized environmental
consequences, groundwater depletion also adversely impacts the long-term
sustainability of groundwater supplies to help meet the Nation’s water
needs."
An example of a significant and sustainable carbonate aquifer
is the Edwards Aquifer in central Texas. This carbonate aquifer has
historically been providing high quality water for nearly 2 million people, and
even today, is full because of tremendous recharge from a number of area
streams, rivers and lakes. The primary risk to this resource is human
development over the recharge areas.
SUBSCRIBERS - ( LINKS) :FOLLOW / REF / 2 /
findleverage.blogspot.com
Krkz77@yahoo.com
+234-81-83195664
No comments:
Post a Comment