What Is Groundwater?
QUICK ANSWER
Groundwater is the water found underground in the spaces between soil particles and in cracks and fractures of rocks. It comes from precipitation that soaks into the ground and accumulates in aquifers. About 30% of Earth's accessible freshwater is groundwater. Wells tap groundwater for drinking, irrigation, and industrial uses.
Groundwater is a hidden but enormous water resource, accounting for about 30% of all the world's accessible freshwater. The water beneath our feet flows slowly through aquifers, sometimes taking centuries or longer to travel from where it entered the ground to where it emerges in springs, lakes, or wells. Understanding groundwater explains where most of the world's drinking water comes from and why protecting it matters.
What exactly is groundwater?
Groundwater is the water found below Earth's surface in the spaces between soil particles and in cracks and fractures within rocks. It's distinct from surface water (rivers, lakes, oceans) and from soil moisture (water held in the upper soil zone for plants). The top of the saturated zone where soil pores are completely water-filled is called the water table. Water below the water table is groundwater proper. The depth to groundwater varies from inches in wet regions to thousands of feet in arid areas. Globally, there's about 30 times more groundwater than freshwater in all rivers and lakes combined.
Where does groundwater come from?
Groundwater comes primarily from precipitation that infiltrates the ground rather than running off. As rain or melted snow soaks into the soil, water percolates downward through pore spaces. Initially, soil moisture is held by plant roots and capillary forces. Below the root zone, water continues downward until it reaches the water table. Some groundwater also originates from surface water bodies (rivers, lakes) where water leaks downward through the bed. A small fraction comes from ancient water trapped during rock formation. The age of groundwater varies from days to millions of years depending on the aquifer.
How is groundwater used?
Groundwater is essential for human water supply globally. About 2 billion people worldwide rely on groundwater as their main drinking water source. In the United States, about 50% of the population gets drinking water from groundwater, and almost all rural water comes from wells. Agriculture is the largest user, with about 70% of global groundwater extraction going to irrigation. Industry uses groundwater for cooling, manufacturing, and processing. Many ecosystems depend on groundwater for streamflow during dry periods and for wetland water sources.
How does groundwater move?
Groundwater flows slowly compared to surface water. Typical flow rates are millimeters to meters per day, occasionally faster in highly permeable formations. Water flows from areas of high pressure (often where the water table is high) toward areas of low pressure (often where it discharges to streams, lakes, or wells). The direction is generally downhill but follows the water table gradient rather than the land surface. Water that entered the ground might travel for years, decades, or centuries before emerging again at the surface. Understanding flow paths is essential for protecting groundwater from contamination.
Groundwater is the water beneath Earth's surface in soil pore spaces and rock fractures, accounting for about 30% of Earth's accessible freshwater. It comes from precipitation that infiltrates the ground and flows slowly through aquifers. Used by about 2 billion people for drinking and providing 70% of irrigation water globally, groundwater is one of humanity's most important water resources.
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