Many of Earth's groundwater supplies are threatened, mainly by human population growth and contamination. Groundwater is freshwater that resides underground; it collects following the movement of water from the surface down through soil and rock. As the water moves downward, a zone or line is formed underground, above which the spaces in the soil and rock are filled with air and water (called the zone of infiltration) and below which water occupies every available opening (called the zone of saturation). The water table is at the top of the zone of saturation, and groundwater lies beneath the water table.

In a desert, groundwater is a vital source of life to plants, which can produce roots that are dozens of feet (meters) long, to reach down from the dry surface to the water below. Groundwater is also important to humans for drinking water, growing crops (irrigation), and other uses...