How does groundwater fit between grains of sediment?

Water can lie beneath our feet even when the ground still feels solid. Sand and gravel contain gaps between their grains. These gaps, called pores, can hold water. Cracks in rock can hold water as well.

Above the water table, many pores contain both air and water. Below it, the available spaces in the saturated ground are filled with water. The water table marks the upper surface of this saturated zone; it is not a solid underground floor.

Water moves through connected gaps while the grains stay in place. An aquifer is rock or sediment that can store groundwater and let enough of it move to supply wells or springs. A material can have many pores yet let water through very slowly if the passages are tiny or poorly connected.

Rain that soaks downward can help refill an aquifer. Water also leaves through springs, streams and wells. Replacing water takes time and differs from place to place. Most groundwater is held among solid material, not in a giant underground river: the spaces and their connections make the reservoir work.