Limpopo River, Africa

Groundwater Drawdown
Kally Worm wormka@uwec.edu
 
Part of Water is Life, a class website on water privatization and commodification, produced by students of Geography 378 (International Environmental Problems & Policy) at the University of Wisconsin-Eau Claire, USA, Spring 2004.
 
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. (See How Groundwater Works). The recharge rate is slow. In many areas groundwater is being removed from the aquifer faster than it can be replenished.

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 surface.

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.
 

Pictorial view of a cone of depression. (Cunningham, William P. et. al., “Environmental Science, 7th edition, McGraw Hill 2003.)

 

Ogallala 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 to west.

Water 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 Kansas.
 
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 last. (Overview 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 and Planet)
 
Other 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.

Groundwater depletions 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 and Planet)

 

Sources

For more information on this topic:

People and Planet

Earth Policy Institute

Ogallala Aquifer Depletion

History of Ogallala Aquifer

Ogallala Aquifer Depletion in Kansas

Overview of Ogallala Aquifer

Christopherson, Robert W., “Geosystems, 5th edition”. Prentice Hall, 2003.

Cunningham. William P. et.al., “Environmental Science, 7th edition”. McGraw Hill, 2003.