Geologic materials that can store and freely transmit ground water
are called aquifers. The speed at which water moves through an
aquifer depends on the size of the open void spaces containing
the water and how well these spaces are connected. Where geologic
materials are porous (many void spaces) and permeable (large, well
connected void spaces), water can easily be transmitted to wells
or springs. As with surface water, ground water also is replenished
Ground water in rocks.
In 2000, approximately 21 percent of the freshwater withdrawn
in the United States for public supplies, livestock uses, irrigation,
industry, and mining uses was ground water. Nearly 82 billion gallons
a day were obtained from wells and springs to meet those ground
The U.S. Geological Survey collects and analyzes a variety of
ground water data to provide information about the quantity and
availability of ground water and the potential for contamination.
This includes monitoring ground water levels in thousands of wells;
data are collected and stored either as discrete ground water level
measurements or as a continuous record.