Groundwater is the water located beneath
the earth's surface in soil
pore spaces
and in the fractures
of rock
formations. A unit of rock or an unconsolidated deposit is called an aquifer when it
can yield a usable quantity of water. The depth at which soil pore spaces or
fractures and voids in rock become completely saturated with water is called
the water
table. Groundwater is recharged from, and eventually
flows to, the surface naturally; natural discharge often occurs at springs and seeps, and can form oases or wetlands.
Groundwater is also often withdrawn for agricultural,
municipal, and industrial use
by constructing and operating extraction wells.
The study of the distribution and movement of groundwater is hydrogeology,
also called groundwater hydrology.
Typically, groundwater is thought of as liquid water flowing
through shallow aquifers, but, in the technical sense, it can also include soil
moisture, permafrost (frozen soil), immobile water in very low
permeability bedrock,
and deep geothermal or oil
formation water. Groundwater is hypothesized to provide lubrication
that can possibly influence the movement of faults.
It is likely that much of the Earth's subsurface contains some water, which may be mixed with
other fluids in some instances. Groundwater may not be confined only to the
Earth. The formation of some of the landforms
observed on Mars may
have been influenced by groundwater. There is also evidence that liquid water
may also exist in the subsurface of Jupiter's moon Europa.
Aquifers
An aquifer is a layer of porous substrate that
contains and transmits groundwater. When water can flow directly between the
surface and the saturated zone of an aquifer, the aquifer is unconfined. The
deeper parts of unconfined aquifers are usually more saturated since gravity
causes water to flow downward.
The upper level of this saturated layer of an unconfined
aquifer is called the water table or phreatic surface. Below the
water table, where in general all pore spaces are saturated with water, is the phreatic
zone.
Substrate with low porosity that permits limited
transmission of groundwater is known as an aquitard. An aquiclude
is a substrate with porosity that is so low it is virtually impermeable to
groundwater.
A confined aquifer is an aquifer that is overlain by
a relatively impermeable layer of rock or substrate such as an aquiclude or
aquitard. If a confined aquifer follows a downward grade from its recharge
zone, groundwater can become pressurized as it flows. This can create artesian
wells that flow freely without the need of a pump and rise to a higher
elevation than the static water table at the above, unconfined, aquifer.
The characteristics of aquifers vary with the geology and
structure of the substrate and topography in which they occur. In general, the
more productive aquifers occur in sedimentary geologic formations. By
comparison, weathered and fractured crystalline rocks yield smaller quantities
of groundwater in many environments. Unconsolidated to poorly cemented alluvial
materials that have accumulated as valley-filling sediments in major river valleys and
geologically subsiding structural basins are included among the most productive
sources of groundwater.
The high specific heat capacity of water and the
insulating effect of soil and rock can mitigate the effects of climate and
maintain groundwater at a relatively steady temperature. In some places where
groundwater temperatures are maintained by this effect at about 10°C (50°F),
groundwater can be used for controlling the temperature inside structures at
the surface. For example, during hot weather relatively cool groundwater can be
pumped through radiators in a home and then returned to the ground in another
well. During cold seasons, because it is relatively warm, the water can be used
in the same way as a source of heat for heat pumps
that is much more efficient than using air.
Groundwater is often cheaper, more convenient and less
vulnerable to pollution than surface water. Therefore, it is commonly used for
public water supplies. Groundwater provides the largest source of usable water storage
in the United States. Underground reservoirs contain far more water than the
capacity of all surface reservoirs and lakes, including the Great Lakes. Many
municipal water supplies are derived solely from groundwater.
The volume of groundwater in an aquifer can be estimated by
measuring water levels in local wells and by examining geologic records from
well-drilling to determine the extent, depth and thickness of water-bearing
sediments and rocks. Before an investment is made in production wells, test
wells may be drilled to measure the depths at which water is encountered and
collect samples of soils, rock and water for laboratory analyses. Pumping tests
can be performed in test wells to determine flow characteristics of the
aquifer.
Polluted ground water is less visible, but more difficult to
clean up, than pollution in rivers and lakes. Ground water pollution most often
results from improper disposal of wastes on land. Major sources include
industrial and household chemicals and garbage landfills, industrial waste
lagoons, tailings and process wastewater from mines, oil field brine pits,
leaking underground oil storage tanks and pipelines, sewage sludge and septic
systems. Polluted groundwater is mapped by sampling soils and groundwater near
suspected or known sources of pollution, to determine the extent of the
pollution, and to aid in the design of groundwater remediation systems.
Preventing groundwater pollution near potential sources such as landfills
requires lining the bottom of a landfill with watertight materials, collecting
any leachate with drains, and keeping rainwater off any potential contaminants,
along with regular monitoring of nearby groundwater to verify that contaminants
have not leaked into the groundwater.
The danger of pollution of municipal supplies is minimized
by locating wells in areas of deep ground water and impermeable soils, and
careful testing and monitoring of the aquifer and nearby potential pollution
sources.
Water cycle
Relative groundwater travel times.
Groundwater makes up about twenty percent of the world's fresh water
supply, which is about 0.61% of the entire world's water, including oceans and
permanent ice. Global groundwater storage is roughly equal to the total amount
of freshwater stored in the snow and ice pack, including the north and south
poles. This makes it an important resource that can act as a natural storage
that can buffer against shortages of surface
water, as in during times of drought.
Groundwater is naturally replenished by surface water from precipitation, streams, and rivers when this
recharge reaches the water table.
Groundwater can be a long-term 'reservoir'
of the natural water cycle (with residence times from days to millennia), as
opposed to short-term water reservoirs like the atmosphere and fresh surface
water (which have residence times from minutes to years). The figure shows how
deep groundwater (which is quite distant from the surface recharge) can take a
very long time to complete its natural cycle.
The Great Artesian Basin in central and eastern Australia is
one of the largest confined aquifer systems in the world, extending for almost
2 million km2. By analysing the trace elements in water sourced from
deep underground, hydrogeologists have been able to determine that water
extracted from these aquifers can be more than 1 million years old.
By comparing the age of groundwater obtained from different
parts of the Great Artesian Basin, hydrogeologists have found it increases in
age across the basin. Where water recharges the aquifers along the Eastern Divide, ages are young. As groundwater
flows westward across the continent, it increases in age, with the oldest
groundwater occurring in the western parts. This means that in order to have
travelled almost 1000 km from the source of recharge in 1 million years,
the groundwater flowing through the Great Artesian Basin travels at an average
rate of about 1 metre per year.
Recent research has demonstrated that evaporation
of groundwater can play a significant role in the local water cycle, especially
in arid regions. Scientists in Saudi
Arabia have proposed plans to recapture and recycle this evaporative
moisture for crop irrigation. In the opposite photo, a 50-centimeter-square
reflective carpet, made of small adjacent plastic cones, was placed in a
plant-free dry desert area for five months, without rain or irrigation. It
managed to capture and condense enough ground vapor to bring to life naturally
buried seeds underneath it, with a green area of about 10% of the carpet area.
It is expected that, if seeds were put down before placing this carpet, a much
wider area would become green.
Issues
Overview
Certain problems have beset the use of groundwater around
the world. Just as river waters have been over-used and polluted
in many parts of the world, so too have aquifers. The big difference is that
aquifers are out of sight. The other major problem is that water management
agencies, when calculating the "sustainable yield" of aquifer and
river water, have often counted the same water twice, once in the aquifer, and
once in its connected river. This problem, although understood for centuries,
has persisted, partly through inertia within government agencies. In Australia,
for example, prior to the statutory reforms initiated by the Council of Australian Governments
water reform framework in the 1990s, many Australian states managed groundwater
and surface water through separate government agencies, an approach beset by
rivalry and poor communication.
In general, the time lags inherent in the dynamic response of
groundwater to development have been ignored by water management agencies,
decades after scientific understanding of the issue was consolidated. In brief,
the effects of groundwater overdraft (although undeniably real) may take
decades or centuries to manifest themselves. In a classic study in 1982,
Bredehoeft and colleagues modeled a situation where groundwater extraction in
an intermontane basin withdrew the entire annual recharge, leaving ‘nothing’
for the natural groundwater-dependent vegetation community. Even when the
borefield was situated close to the vegetation, 30% of the original vegetation
demand could still be met by the lag inherent in the system after 100 years. By
year 500, this had reduced to 0%, signalling complete death of the
groundwater-dependent vegetation. The science has been available to make these
calculations for decades; however, in general water management agencies have
ignored effects that will appear outside the rough timeframe of political
elections (3 to 5 years). Marios Sophocleous argued strongly that management
agencies must define and use appropriate timeframes in groundwater planning.
This will mean calculating groundwater withdrawal permits based on predicted
effects decades, sometimes centuries in the future.
As water moves through the landscape, it collects soluble
salts, mainly sodium chloride. Where such water enters the
atmosphere through evapotranspiration, these salts are left behind.
In irrigation
districts, poor drainage of soils and surface aquifers can result in water
tables' coming to the surface in low-lying areas. Major land
degradation problems of soil salinity and waterlogging result, combined with
increasing levels of salt in surface waters. As a consequence, major damage has
occurred to local economies and environments.
Four important effects are worthy of brief mention. First,
flood mitigation schemes, intended to protect infrastructure built on floodplains,
have had the unintended consequence of reducing aquifer recharge associated
with natural flooding. Second, prolonged depletion of groundwater in extensive
aquifers can result in land subsidence, with associated infrastructure damage – as
well as, third, saline intrusion. Fourth, draining acid sulphate soils, often
found in low-lying coastal plains, can result in acidification and pollution of
formerly freshwater and estuarine streams.
Another cause for concern is that groundwater drawdown from
over-allocated aquifers has the potential to cause severe damage to both
terrestrial and aquatic ecosystems – in some cases very conspicuously but in
others quite imperceptibly because of the extended period over which the damage
occurs.
Overdraft
Wetlands contrast the arid landscape around Middle Spring, Fish Springs National Wildlife
Refuge, Utah
Groundwater is a highly useful and often abundant resource.
However, over-use, or overdraft, can cause major problems to human users
and to the environment. The most evident problem (as far as human groundwater
use is concerned) is a lowering of the water table beyond the reach of existing
wells. As a consequence, wells must be drilled deeper to reach the groundwater;
in some places (e.g., California, Texas, and India) the water table has dropped hundreds of feet because of
extensive well pumping. In the Punjab
region of India, for example, groundwater levels have dropped 10 meters since
1979, and the rate of depletion is accelerating. A lowered water table may, in
turn, cause other problems such as groundwater-related subsidence and saltwater intrusion.
Groundwater is also ecologically important. The importance
of groundwater to ecosystems is often overlooked, even by freshwater biologists
and ecologists. Groundwaters sustain rivers, wetlands, and lakes, as well as
subterranean ecosystems within karst
or alluvial aquifers.
Not all ecosystems need groundwater, of course. Some
terrestrial ecosystems – for example, those of the open deserts and similar
arid environments – exist on irregular rainfall and the moisture it delivers to
the soil, supplemented by moisture in the air. While there are other
terrestrial ecosystems in more hospitable environments where groundwater plays
no central role, groundwater is in fact fundamental to many of the world’s
major ecosystems. Water flows between groundwaters and surface waters. Most
rivers, lakes, and wetlands are fed by, and (at other places or times) feed
groundwater, to varying degrees. Groundwater feeds soil moisture through percolation,
and many terrestrial vegetation communities depend directly on either
groundwater or the percolated soil moisture above the aquifer for at least part
of each year. Hyporheic zones (the mixing zone of streamwater and
groundwater) and riparian zones are examples of ecotones largely
or totally dependent on groundwater.
Subsidence
Subsidence occurs when too much water is pumped out from
underground, deflating the space below the above-surface, and thus causing the
ground to collapse. The result can look like craters on plots of land. This
occurs because, in its natural equilibrium state, the hydraulic pressure of groundwater in the pore
spaces of the aquifer and the aquitard supports some of the weight of the
overlying sediments. When groundwater is removed from aquifers by excessive
pumping, pore pressures in the aquifer drop and compression of the aquifer may
occur. This compression may be partially recoverable if pressures rebound, but
much of it is not. When the aquifer gets compressed, it may cause land
subsidence, a drop in the ground surface. The city of New Orleans, Louisiana is actually below sea
level today, and its subsidence is partly caused by removal of groundwater from
the various aquifer/aquitard systems beneath it. In the first half of the 20th
century, the city of San Jose, California dropped 13 feet from land
subsidence caused by overpumping; this subsidence has been halted with improved
groundwater management.
Seawater intrusion
In general, in very humid or undeveloped regions, the shape
of the water table mimics the slope of the surface. The recharge zone of an
aquifer near the seacoast is likely to be inland, often at considerable
distance. In these coastal areas, a lowered water table may induce sea water
to reverse the flow toward the land. Sea water moving inland is called a saltwater intrusion. In alternative fashion, salt from mineral beds may
leach into the groundwater of its own accord.
Pollution
Iron oxide staining caused by reticulation from an unconfined aquifer in karst
topography. Perth, Western Australia.
Water pollution of groundwater, from pollutants
released to the ground that can work their way down into groundwater, can
create a contaminant plume within an aquifer. Movement of water
and dispersion within the aquifer spreads the pollutant over a wider area, its
advancing boundary often called a plume edge, which can then intersect with
groundwater wells or daylight into surface water such as seeps and springs, making the water supplies unsafe for
humans and wildlife. The interaction of groundwater contamination with surface
waters is analyzed by use of hydrology transport models.
The stratigraphy of the area plays an important role in the
transport of these pollutants. An area can have layers of sandy soil, fractured
bedrock, clay, or hardpan. Areas of karst topography on limestone
bedrock are sometimes vulnerable to surface pollution from groundwater. Earthquake
faults can also be entry routes for downward contaminant entry. Water table
conditions are of great importance for drinking water supplies, agricultural
irrigation, waste disposal (including nuclear
waste), wildlife
habitat, and
other ecological
issues.
In the US, upon commercial real estate property transactions both
groundwater and soil are the subjects of scrutiny, with a Phase I Environmental Site
Assessment normally being prepared to investigate and disclose potential
pollution issues. In the San Fernando Valley of California, real estate contracts for property transfer
below the Santa Susana Field Laboratory (SSFL)
and eastward have clauses releasing the seller from liability
for groundwater contamination consequences from existing or future pollution of
the Valley Aquifer.
Love Canal was one of the most widely known examples of
groundwater pollution. In 1978, residents of the Love Canal neighborhood in
upstate New
York noticed high rates of cancer and an alarming number of birth
defects. This was eventually traced to organic solvents and dioxins from an industrial landfill that
the neighborhood had been built over and around, which had then infiltrated
into the water supply and evaporated in basements to further contaminate the
air. Eight hundred families were reimbursed for their homes and moved, after
extensive legal battles and media coverage.
Another example of widespread groundwater pollution is in
the Ganges
Plain of northern India and Bangladesh
where severe contamination of groundwater by
naturally occurring arsenic affects 25% of water wells in the shallower of
two regional aquifers. The pollution occurs because aquifer sediments contain
organic matter that generates anaerobic conditions in the aquifer. These
conditions result in the microbial dissolution of iron oxides
in the sediment and, thus, the release of the arsenic, normally
strongly bound to iron oxides, into the water. As a consequence, arsenic-rich
groundwater is often iron-rich, although secondary processes often obscure the
association of dissolved arsenic and dissolved iron.
Government regulations
In the United States, laws regarding ownership and use of
groundwater are generally state laws; however, regulation of groundwater to
minimize pollution of groundwater is by both states and the federal-level Environmental Protection Agency.
Ownership and use rights to groundwater typically follow one of three main
systems:
- Rule of Capture
The Rule of Capture provides each landowner the ability to
capture as much groundwater as they can put to a beneficial use, but they are
not guaranteed any set amount of water. As a result, well-owners are not liable
to other landowners for taking water from beneath their land. State laws or
regulations will often define "beneficial use", and sometimes place
other limits, such as disallowing groundwater extraction which causes subsidence
on neighboring property.
- Riparian Rights
Limited private ownership rights similar to riparian
rights in a surface stream. The amount of groundwater right is based on the
size of the surface area where each landowner gets a corresponding amount of
the available water. Once adjudicated, the maximum amount of the water right is
set, but the right can be decreased if the total amount of available water decreases
as is likely during a drought. Landowners may sue others for encroaching upon
their groundwater rights, and water pumped for use on the overlying land takes
preference over water pumped for use off the land.
- Reasonable Use Rule (American Rule)
This rule does not guarantee the landowner a set amount of
water, but allows unlimited extraction as long as the result does not
unreasonably damage other wells or the aquifer system. Usually this rule gives
great weight to historical uses and prevents new uses that interfere with the
prior use.
Environmental protection of groundwater
In November 2006, the Environmental Protection Agency
published the Ground Water Rule in the United States Federal Register. The EPA
was worried that the ground water system would be vulnerable to contamination
from fecal matter. The point of the rule was to keep microbial pathogens out of
public water sources. The 2006 Ground Water Rule was an amendment of the 1996
Safe Drinking Water Act.
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