Aquifer depletion

An aquifer is water-saturated geological layer that easily releases water to wells or springs for use as a water supply. Also called ground water reservoirs or water-bearing formations, aquifers are created and replenished when excess precipitation (rain and snowfall) is held in the soil . This water is not released through runoff nor is removed by the surface flows of rivers or streams. Plants have used what they need (transpiration ) and little is evaporated from non-living surfaces, such as soil. The remaining excess water slowly percolates downward through the soil and through the air spaces and cracks of the surface overburden of rocks into the bedrock. As water collects in this saturated area or recharge zone , it becomes groundwater . The uppermost level of the saturated area is called the water table .

Groundwater is especially abundant in humid areas where the overburden is relatively thick and the bedrock is porous or fractured, particularly in areas of sedimentary rocks such as sandstone or limestone. Aquifers are extremely valuable natural resources in regions where lakes and rivers are not abundant. Groundwater is usually accessed by drilling a well and then pumping the water to the surface.

In less-moist environments, however, the quantity of precipitation available to recharge groundwater is much smaller. Slowly recharging aquifers in arid environments are easily depleted if their groundwater is used rapidly by humans. In some cases, groundwater sources may exist in water-bearing geologic areas that make pumping nearly impossible. Moreover, increased irrigation use has led to heavy pumping that is draining aquifers and lowering water tables around the world. Aquifer depletion is a growing problem as world populations increase and the need for increased food supplies.

Large, rapidly recharging aquifers underlying humid landscapes can sustain a high rate of pumping of their groundwater. As such, they can be sustainably managed as a renewable resource. Aquifers that recharge very slowly, however, are essentially filled with old, so-called "fossil" water that has accumulated over thousands or more years. This kind of aquifer has little capability of recharging as the groundwater is used because the groundwater is depleted so rapidly for human use. Therefore, slowly recharging aquifers are essentially nonrenewable resources, whose reserves are mined by excessive use.

In 1999, the Worldwatch Institute reported that water tables were falling on every continent in the world, mainly because of excessive human consumption. Ground-water in India, in particular, is being pumped at double the rate of the aquifer's ability to recharge from rainfall. The aquifer under the North China Plain is seeing its water table fall at 5 feet (1.5 meters) a year.

In the United Sates, it is similar. The largest aquifer in the world, known as the Ogalalla Aquifer, is located beneath the arid lands of the western United States. The Ogallala aquifer is very slowly recharged by underground seepage that mostly originates with precipitation falling on a distant recharge zone in mountains located in its extreme western range. Much of the groundwater presently in the Ogalalla is fossil water that has accumulated during tens of thousands of years of extremely slow infiltration . Although the Ogalalla aquifer is an enormous resource, it is being depleted alarmingly by pumping at more than 150,000 wells. Most of the groundwater being withdrawn by the wells is used in irrigated agriculture, and some for drinking and other household purposes. In recent years, the level of the Ogalalla aquifer has been decreasing by as much as 3.2 feet (1 meter) per year in intensively utilized zones, while the recharge rate is only of the order of 1 mm/yr. (or a little over 1/32 of an inch). Obviously, the Ogalalla aquifer is being mined on a large scale.

Aquifer depletion brings with it more than the threat of water scarcity for human use. Serious environmental consequences can occur when large amounts of water are pumped rapidly from ground water reservoirs. Commonly, the land above an aquifer will subside or sink as the water is drained from the geologic formation and the earth compacts. In 1999, researchers noted that portions of Bangkok, Thailand and Mexico City, Mexico were sinking as a result of overexploitation of their aquifers. This can cause foundations of buildings to shift and may even contribute to earthquake incidence. Large cities in the United States like Albuquerquer, Phoenix, and Tuscon lie over aquifers that are being rapidly depleted.

Unfortunately, the current solutions to aquifer depletion are to drill wells deeper or abandon irrigated agriculture and import food. Both are costly choices for any country, both in dollars and in economic independence.

[Bill Freedman Ph.D. ]

RESOURCES

BOOKS

Freeze, R.A. and J.A. Cherry. Groundwater. Inglewood Heights, NJ: Prentice Hall, 1979.

Opie, J.Ogallala: Water for a Dry Land. Lincoln, NB: University of Nebraska Press, 2000.

Robins, N. (Ed.). Groundwater Pollution, Aquifer Recharge and Vulnerability. Special Publication Number 130, London, UK: Geological Society Publishing House, 1998.

ORGANIZATIONS

World Resources Institute, 10 G Street, NE (Suite 800), Washington, DC USA 20002 (202) 729-7600, Fax: (202) 729-7610, Email: [email protected], http://www.wri.org/

Worldwatch Institute, 1776 Massachusetts Ave., N.W., Washington, D.C. USA 20036-1904 (202) 452-1999, Fax: (202) 296-7365, Email: [email protected], http://www.worldwatch.org/