Global ground water supplies contain a greater volume of fresh water than all rivers, lakes, and wetlands combined. They provide most drinking water and most water used for agricultural irrigation. Ground water collects beneath the surface in permeable geological formations known as aquifers. The water table — the uppermost limit of the water-saturated zone — rises and falls according to whether water is added to or withdrawn from an aquifer.

Aquifers are naturally replenished, or recharged, very slowly as precipitation seeps between soil particles and enters the aquifer. Water is withdrawn from aquifers through wells, pumps, and springs that escape to the surface. In some places, over a billion liters (hundreds of millions of gallons) of water are tapped from aquifers each day for irrigation. When ground water is depleted, the effects on the landscape and the people can be significant. If too much water is drawn out of an aquifer before it is able to recharge, the water table falls and overlying surface layers can collapse, or subside.

The effects of ground water depletion are evident in many major cities around the world, including Bangkok, Thailand, and Jakarta, Indonesia. Mexico's largest city may offer one of the most dramatic examples. Mexico City was built on an old lakebed surrounded by volcanic mountains. Because it lacks access to a surface water source, the city relies on a huge underground aquifer to provide water to nearly all of its 20 million inhabitants. As water is pumped out from beneath the city, it causes the land to subside, or sink. This is particularly problematic because the land is experiencing subsidence also due to the weight of buildings on the soft, unconsolidated land of the lakebed. Nowhere is this more obvious than at the Metropolitan Cathedral, which has actually been sinking ever since construction commenced. In 1993, workers began digging shafts under the cathedral to try to straighten and level the building. By filling these shafts with concrete, they intended to give the massive structure a solid base on which to rest.

The dangers of land subsidence extend beyond buildings to encompass roads, sewer systems, and underground pipelines. For example, if sewage waste or fuel oil were to leak from a pipe damaged by subsidence and mix with the fresh water in the aquifer, the city's main source of water would become unusable. Using less water is one obvious way to slow ground water depletion and lessen its consequences. Another is recycling water. The city of Tucson, Arizona has taken steps to reduce the potential for subsidence in its metropolitan area and to lessen dependence on its primary aquifer by having fresh water diverted in from an alternate source: the Colorado River.

Print Background Essay