Groundwater is water occupying the saturated pores and fractures of rock and sediment beneath the land surface. It forms an important component of the water cycle, storing water underground and transporting it toward springs, streams, coastal waters, and wells. Unlike the familiar image of underground rivers, most groundwater moves through interconnected spaces within geological materials. Water-bearing formations that transmit useful quantities of groundwater are called aquifers. Groundwater supplies households, agriculture, and industry and contributes to the functioning of many ecosystems. (usgs.gov)
Occurrence and geological setting
The subsurface is commonly divided into an unsaturated zone, where pores contain both air and water, and a saturated zone, where interconnected openings are filled with water. The water table marks the upper surface of the main saturated zone in an unconfined aquifer. Its depth varies with topography, geological conditions, recharge, and water withdrawals; it is not necessarily a flat surface or a fixed distance below ground. (usgs.gov)
Aquifer properties depend on geology. Sand and gravel can store and transmit water through spaces between grains, whereas consolidated rocks may transmit it through fractures. Porosity describes the proportion of a material occupied by voids. Permeability concerns the capacity of connected openings to transmit fluid. High porosity does not automatically imply rapid groundwater movement: the size and connectivity of openings also matter. (pubs.usgs.gov)
An unconfined aquifer has a water table free to rise and fall. A confined aquifer lies beneath a relatively impermeable layer and contains water under pressure. In a well penetrating a confined aquifer, water may rise above the aquifer’s top; if it rises above the land surface, the well flows without pumping. Such conditions are described as artesian. (usgs.gov)
Recharge, movement, and discharge
Recharge occurs when water reaches the saturated zone, commonly through infiltration of precipitation, seepage from streams, or percolation of irrigation water. Not all rainfall becomes recharge: some leaves as surface runoff, while some returns to the atmosphere through evaporation and transpiration. Recharge rates differ considerably between aquifers, and replenishment can be slow where precipitation is limited or the unsaturated zone is thick. (pubs.usgs.gov)
Groundwater flows from higher to lower hydraulic head, a measure of water’s mechanical energy expressed as an equivalent height. Head reflects elevation and pressure, so groundwater can move upward as well as downward. For flow through porous materials, Darcy’s law relates discharge to hydraulic conductivity, flow cross-sectional area, and the hydraulic gradient. Hydraulic conductivity depends on both the geological material and fluid properties; it is therefore distinct from intrinsic permeability. (pubs.usgs.gov)
Groundwater discharges naturally into springs, lakes, and rivers, or is removed by vegetation and wells. Its contribution to streams, often called baseflow, can maintain flow during periods without rainfall. Exchanges also operate in the opposite direction: streams can lose water to aquifers. Groundwater and surface water consequently form interacting parts of a hydrological system rather than independent resources. (pubs.usgs.gov)
Human use and ecological importance
Groundwater is widely used for drinking-water supplies and irrigation, particularly where surface water is scarce or unreliable. The 2022 United Nations World Water Development Report estimated that agriculture accounted for 69% of global groundwater withdrawals, domestic uses for 22%, and industry for 9%. These figures describe the distribution of groundwater abstraction, not groundwater’s share of all water used in each sector. (unesco.org)
Groundwater also sustains springs, wetlands, and streamside vegetation. These groundwater-dependent ecosystems rely on the timing, quantity, and quality of subsurface discharge. Lowering groundwater levels can reduce streamflow and remove water accessible to plant roots, changing habitats even where pumping occurs some distance away. (usgs.gov)
Water quality and contamination
Groundwater acquires dissolved constituents through contact with rock and soil. Because water acts as a solvent, it commonly contains calcium, magnesium, sodium, bicarbonate, chloride, and sulfate. Its composition varies between geological settings, and groundwater is not necessarily fresh: highly saline water occurs in some deeper formations. Clear appearance alone does not establish water quality. (pubs.usgs.gov)
Groundwater contamination can arise from natural processes or human activities. Naturally occurring arsenic and other constituents may enter water from aquifer materials. Agricultural fertilizers, pesticides, leaking fuel tanks, industrial chemicals, and waste-disposal systems can introduce contaminants from the surface. Septic-system leakage can also introduce bacteria. Contaminant movement depends on groundwater flow, chemical properties, and interactions with the aquifer; recovery from historical contamination can take a long time. (usgs.gov)
Depletion and management
Pumping lowers hydraulic head around wells, producing a cone of depression. Persistent withdrawals can cause groundwater depletion, reducing stored water and well yields. Associated effects include greater pumping costs, diminished surface-water discharge, and land subsidence. In coastal aquifers, reduced freshwater pressure can promote saltwater intrusion, drawing saline water toward freshwater wells. (usgs.gov)
Management depends on monitoring withdrawals, water levels, and water quality and on understanding the aquifer’s water budget. Pumping is balanced by changes in storage, recharge, and natural discharge; equating allowable pumping simply with recharge can overlook losses to streams and ecosystems. Managed aquifer recharge deliberately introduces water through infiltration facilities or injection wells. Its applications include increasing underground storage and buffering supplies against drought, with outcomes dependent on local hydrogeological conditions and water quality. (pubs.usgs.gov)