Most groundwater is fresh water. Many think of groundwater as part of a system of underground lakes and streams. This is true in only a few cases however. Groundwater is usually found in cracks and spaces between rocks and between the soil particles that are under the earth’s surface. These spaces act a bit like a giant underground sponge.
The area found just below the earth’s surface with pore spaces filled partly with water and partly with air is called the unsaturated zone. This groundwater is generally not a reliable source of drinking water. The water in deeper spaces completely filled or saturated with water is called groundwater. The top of this saturated zone is the water table. Water for drinking and other uses is drawn from a saturated zone called an aquifer. About 95% of the U.S. total water supply of fresh water is groundwater. The remaining freshwater is surface water, found in lakes and streams.