- Groundwater is the
second largest reserve of freshwater on earth. It also makes up
40% of the freshwater used in the U.S. alone. Groundwater is found
within underground aquifers in the "zone of saturation".
A zone of saturation is located where water fills in all of the
spaces that are in the lower layers of soil. The water table is
located at the top of the zone of saturation. These
aquifers need to be recharged by rainwater and other water sources.
Groundwater Works). The recharge rate is slow. In many areas
groundwater is being removed from the aquifer faster than it can
When groundwater is
depleted, the effects on the landscape and the people are drastic.
"Cones of depression" can be formed
if too much water is drawn out of a water table without letting
it recharge. A cone of depression is where the water table sinks
in an area that has been heavily pumped, creating a large area
that has sunken. "Sinkholes" may also form when an underground
cavern or channel collapses and creates a crater in the earth’s
- Another danger is
that aquifers that are located near coastlines can experience saltwater
intrusion, where saltwater mixes with fresh water from the aquifer,
rendering the water unusable. In the end, heavy pumping
of groundwater depletes aquifers until there is little or no fresh
water available to those who depend upon it.
view of a cone of depression. (Cunningham, William P. et. al., “Environmental
Science, 7th edition, McGraw Hill 2003.)
Aquifer in the Great Plains
Even though we know
the dangers of heavily pumping water from aquifers, some regions
are still dealing with these consequences. In the U.S. Great Plains,
the Ogallala Aquifer is a prime example of groundwater depletion.
This aquifer provides water for South Dakota, Nebraska, Colorado,
Wyoming, Kansas, Oklahoma, Texas, and New Mexico. It spans an area
of 800 miles from north to south, and 400 hundred miles from east
level changes in the Ogallala Aquifer (Cunningham, William P.
et. al., “Environmental Science, 7th edition, McGraw Hill 2003.)
- It was first tapped
in 1911 when a farmer dug a well by hand for irrigation purposes.
In the 1950s there were approximately 80 wells a year that were
being dug to tap the aquifer in Colorado alone. There were some
restrictions placed on digging wells to tap the Ogallala Aquifer,
but these limitations did not stop farms and cities from depleting
the aquifer. This aquifer supplies 70% of the water used daily in
- The water pumped from
Ogallala Aquifer is used mostly for irrigation purposes. The land
in the Great Plains is semi-arid and the water that is available
evaporates quickly. Due to the need for greater
amounts of water for irrigation, the aquifer is being depleted because
the recharging process cannot keep up with the withdrawal of water.
Since people had started to rely on the Ogallala Aquifer for irrigation
of their fields, 6% of the aquifer has dropped to an unusable level
that can no longer be pumped. If irrigation continues to draw water
from the aquifer at the same rate, about 6% of the aquifer will
be used up every 25 years. One estimate states that the aquifer
is being depleted at a rate of approximately 12 billion cubic meters
per year. The biggest problem facing people who use the Ogallala
Aquifer is that they do not know how long the water supply will
of Ogallala Aquifer)
Mexico City Groundwater Depletion
- Mexico City may be
one of the worst examples of groundwater depletion. Mexico City
was built on an old lakebed that is surrounded by mountains. It
does not have access to a nearby surface water source, so the city
must rely on the underground aquifer for all of the people’s water
needs, along with some water pumped across the mountains at a very
high cost. There have been other options that the city has looked
at to bring in water from the outside. Since the valley is at a
high elevation, approximately 2,000 meters above sea level, importing
water into the city is too expensive to be a realistic alternative.
The city’s 15.6 million people rely on the only viable water source,
the underground aquifer.
The Mexico City Aquifer
has been depleted since the early 1900s. One study showed that
from 1986 to 1992 the aquifer lowered anywhere from 6 to 10 meters
in heavily pumped areas. This massive depletion of the aquifer
has caused multiple problems for Mexico City. One major problem
is that severe land subsidence has occurred. Land subsidence occurs
when porous formations that once held water collapse, which results
in the surface layer settling. This often occurs where cities
were built on unconsolidated land such as river deltas or lakebeds.
Some areas of Mexico City are rapidly sinking. These areas, such
as the central section of the metropolitan area, have fallen as
much as 8.5 meters. This has caused damage to many of the buildings
as well as ancient monuments that are located in those sections.
The monuments as well as buildings are sinking at angles, causing
damage to their infrastructures. The subsidence is also damaging
the sewer system. This could potentially cause the untreated sewage
to mix with the fresh water in the aquifer. More than 95% of the
hazardous waste generated by companies is dumped directly into
the municipal sewage system. If the waste mixes with the fresh
water in the aquifer it could render the only water supply to
the city unusable until it is cleaned up. This cleanup process
would be slow and expensive.
- Even though the aquifer
is being depleted rapidly, demand for water in the city is increasing.
Not everyone is provided with water on a day-to-day basis. The rapid
population influx from rural areas is causing the supply of water
to be challenging for the city. Even though this challenge is hard
to overcome, Mexico is still using less water than other developed
countries. This means that the demand for adequate water is not
about to be fulfilled anytime soon. (People
Groundwater Depletion Problems
- Even though we often
hear mostly about the Ogallala Aquifer and the present conditions
of Mexico City, groundwater is being depleted at high rates elsewhere
in the world. About one-third of the global population depends on
water from groundwater supplies. India, China, and North Africa
are also feeling the effects of groundwater depletion. In some areas
of India, the water tables have dropped as much as 70 centimeters
(approximately 25 inches). Up to 25% of India’s agriculture may
be threatened by the depletion of groundwater resources. In areas
of northern China, the water table has been dropping as fast as
1.5 meters a year for the last ten years.
- Water tables have
been drastically reduced all over the world, especially in areas
that rely heavily on agriculture for income. Many cities in North
Africa, as well as the Middle East, are experiencing harsh water
shortages. In Iran, villages are being evacuated because wells are
running dry and there is no water supply to support the population
of the village. One estimate reported that the water table had dropped
by 8 meters in 2001 in parts of Iran. In Yemen, on the Arabian Peninsula,
the water tables have been falling on average two meters per year
across the country. It has been predicted that the capital of Yemen
will run out of its water supply within the next ten years.
have led to the inability of the Yellow River in China to reach
the ocean for months at a time. The longest dry-spell has been
200 consecutive days, during which time the river did not flow
into the ocean. Even though many water disasters have been manifested
as surface water depletions, such as the failure of the Colorado
River in the U.S. or the Indus River in Pakistan to reach the
ocean every day, these disasters are a result of groundwater depletion.
These groundwater depletions affect everyone’s lives. The water
supplies to many cities are being drastically reduced. The water
that farmers depend on to irrigate their crops are being depleted.
The depletion of aquifers does not allow the surface waters to
be recharged and they are shrinking as well. Water is a commodity
that will have a profound effect on the world within the next
decade, drastically changing the way in which we view water as
a resource. (People
For more information
on this topic:
of Ogallala Aquifer
Aquifer Depletion in Kansas
of Ogallala Aquifer
W., “Geosystems, 5th edition”. Prentice Hall, 2003.
William P. et.al., “Environmental Science, 7th edition”. McGraw