Liquid fresh water occurs either as surface water or groundwater. Surface water is water located atop Earth’s surface (such as in a river or lake) and groundwater is water beneath the surface that resides within pores in soil or rock. Any precipitation reaching Earth’s land surface that does not evaporate, flow into waterways, or get taken up by organisms infiltrates (sinks into) the surface.


Groundwater flows slowly beneath the surface from areas of high pressure to areas of low pressure and can remain underground for a long time, in some cases for thousands of years. Groundwater makes up one-fifth of Earth’s freshwater supply and plays a key role in meeting human water needs. Groundwater is contained within aquifers: porous, spongelike formations of rock, sand, or gravel that can hold water. An aquifer’s upper layer contains pore spaces partly filled with water. In the lower layer, the spaces are completely filled with water.


The boundary between these two zones is the water table. Picture a sponge resting partly submerged in a tray of water; the lower part of the sponge is saturated, whereas the upper portion contains plenty of air in its pores. Any area where water infiltrates Earth’s surface and reaches an aquifer below is known as a recharge zone.