Groundwater Pumping

Passage One

Groundwater Depletion

Ground water is a valuable resource both in the United States and throughout the world. Where surface water, such as lakes and rivers, are scarce or inaccessible, ground water supplies many of the hydrologic needs of people everywhere. In the United States, it is the source of drinking water for about half the total population and nearly all of the rural population, and it provides over 50 billion gallons per day for agricultural needs. Ground-water depletion, a term often defined as long-term water-level declines caused by sustained ground-water pumping, is a key issue associated with ground-water use. Many areas of the United States are experiencing ground-water depletion.

Excessive pumping can overdraw the ground-water "bank account"

The water stored in the ground can be compared to money kept in a bank account. If you withdraw money at a faster rate than you deposit new money you will eventually start having account-supply problems. Pumping water out of the ground faster than it is replenished over the long-term causes similar problems. The volume of ground water in storage is decreasing in many areas of the United States in response to pumping. Ground-water depletion is primarily caused by sustained ground-water pumping. Some of the negative effects of ground-water depletion:

· Lowering of the water table: For water to be withdrawn from the ground, water must be pumped from a well that reaches below the water table. If ground-water levels decline too far, then the well owner might have to deepen the well, drill a new well, or, at least, attempt to lower the pump. Also, more energy is required to drive the pump making pumping more expensive.

· Reduction of water in streams and lakes: There is more of an interaction between the water in lakes and rivers and ground water than most people think. Some, and often a great deal, of the water flowing in rivers comes from seepage of ground water into the streambed. Ground-water pumping can alter how water moves between an aquifer and a stream, lake, or wetland. The overall effect is a loss of riparian (river bank) vegetation and wildlife habitat.

· Land subsidence: The basic cause of land subsidence is a loss of support below ground. In other words, sometimes when water is taken out of the soil, the soil collapses, compacts, and drops.

^{Source: U.S. Department of the Interior/ U.S. Geological Survey. http://ga.water.usgs.gov/edu/gwdepletion.html}

Passage Two

The case of Phoenix, AZ:

Increased ground-water pumping to support population growth in south-central Arizona (including the Tucson and Phoenix areas) has resulted in water-level declines of between 300 and 500 feet in much of the area. Land subsidence was first noticed in the 1940s and subsequently as much as 12.5 feet of subsidence has been measured. Additionally, lowering of the water table has resulted in the loss of streamside vegetation.

These pictures show a reach of the Santa Cruz River south of Tucson, Arizona. In the 1942 picture vegetation is growing in the riparian (river bank) area the river, indicating that sufficient water in the soil existed at a level that plant roots could access it. The same site in 1989 shows that the riparian trees have largely disappeared as a result of lowered ground-water levels.

Passage Three

On paper, Arizona shouldn't have a water problem. The state gets enough water from the Colorado River alone to serve nearly three times the state's current 5 million population.

But moving water from the Colorado to every corner of the state is expensive and impractical, which is why so many Arizonans get their water from groundwater or from local rivers and streams, especially in rural areas. Groundwater has always been the cheapest and most readily available source; but now, it's the most threatened. ''With the population we have now, we are more than capable of pumping out the supply faster than it can be recharged,'' said Rita Pearson, director of the Arizona Department of Water Resources. Overpumping not only depletes a natural resource that will be needed by future generations, it leaves current users without a backup during drought, and it can lead to land subsidence and poor water quality.

The Valley's biggest cities rely mostly on the CAP (Central Arizona Project Canal) and the Salt River Project (SRP), which manages water from the Salt and Verde rivers. But many smaller communities still use mostly groundwater because it's what they can afford.

^{Source: “Arizona facing water crisis with growth,” U.S. Water News Online. July 2000. http://www.uswaternews.com/archives/arcsupply/tarifac7.html}