Ground water is water beneath the surface of the earth. It is the source of water for wells and many springs. Ground water provides about 20 per cent of the fresh water used in the United States. Most rural areas and some cities depend heavily on ground water.
Ground water accumulates chiefly from rain and melted snow that filters through the soil. It also collects from water that seeps into the ground from lakes and ponds. The water settles into the pores and cracks of underground rocks and into the spaces between grains of sand and pieces of gravel. A layer or bed of such porous materials that yields useful amounts of ground water is called an aquifer. Wells are drilled down to aquifers to draw ground water to the surface.
The level of ground water, called the water table, drops when more water is withdrawn than can be replaced naturally. Many regions of the world are using up the ground water faster than aquifers are being recharged (replenished). In some areas, the ground water supply may have to be recharged with water from another area. For example, water can be returned to the earth through special recharge wells, ponds, or ditches that allow surplus water to seep into the ground. Lowering of the water table causes special problems in coastal areas, because salt water from the ocean enters reservoirs of ground water. In some areas, when the water table drops, the land surface cracks and sinks. This action can cause great damage to buildings, roads, pipelines, and other structures.
Pollution of ground water is a serious problem, especially near cities and industrial sites. Pollutants that seep into the ground result from contaminated surface water, leaks from sewer pipes and septic tanks, and chemical spills. Ground water may also be polluted by seepage from landfills, buried radioactive wastes, and the misuse of agricultural chemicals.
