What Is Groundwater?

Groundwater is the water held beneath the land surface in the pores, fractures, and other openings of soil and rock, filling those spaces completely in what hydrologists call the saturated zone. Its upper boundary is the water table, above which lies the unsaturated zone where pore spaces hold both air and water. Groundwater is the largest accessible store of liquid fresh water on Earth, far exceeding the volume held in rivers and lakes, and it supplies drinking water for a substantial share of the world's population as well as most irrigation in arid regions.

The study of groundwater sits at the intersection of geology, fluid mechanics, geochemistry, and measurement engineering. The U.S. Geological Survey's groundwater program treats it as a component of the hydrologic cycle rather than a separate reservoir, since surface water and groundwater exchange continuously through streambeds, springs, and wetlands.

Aquifers and Subsurface Storage

An aquifer is a geologic unit saturated and permeable enough to yield useful quantities of water to a well or spring. Aquifers form in unconsolidated sand and gravel, in porous sandstone and fractured limestone, and in weathered or rubbly volcanic rock. Their behavior is described by two properties: hydraulic conductivity, which governs how readily water moves, and storativity, which governs how much water is released per unit decline in head. Unconfined aquifers have a free water table and drain by gravity, while confined aquifers lie beneath a low-permeability layer and are pressurized, so a well tapping one may flow at the surface without pumping. The USGS Water Science School describes this storage as a buffer that sustains streamflow through dry periods.

Recharge, Flow, and Depletion

Recharge occurs where precipitation, irrigation return flow, or surface water infiltrates past the root zone and reaches the water table. Flow through the saturated zone follows Darcy's law, in which discharge is proportional to hydraulic conductivity and the hydraulic gradient, and travel times range from days in a shallow alluvial aquifer to tens of thousands of years in a deep confined system. When withdrawals persistently exceed recharge, water levels fall, wells go dry, streams lose their baseflow, coastal aquifers admit saltwater intrusion, and fine-grained sediments compact irreversibly, causing land subsidence measured in meters in places such as California's Central Valley.

Monitoring and Measurement

Traditional monitoring relies on networks of observation wells with pressure transducers, aquifer pumping tests to estimate hydraulic parameters, and geophysical logging to map lithology. Surface and airborne electromagnetic surveys, seismic reflection, and electrical resistivity tomography extend that picture between boreholes. Since 2002 the Gravity Recovery and Climate Experiment satellites and their successors have added a basin-scale view by detecting the tiny gravity changes that accompany shifts in stored water mass, and the resulting maps of storage trends in the world's largest aquifers show sustained depletion across much of South Asia, the Middle East, and the western United States. Assimilating those measurements into land surface models produces the weekly groundwater and soil moisture drought indicators used in operational drought assessment.

Applications

Groundwater science supports work in a range of fields, including:

• Municipal and rural drinking water supply
• Irrigated agriculture and conjunctive surface water management
• Contaminant transport modeling and remediation of polluted sites
• Geothermal heat pump and aquifer thermal energy storage design
• Geotechnical dewatering for tunnels, mines, and deep excavations
• Satellite geodesy and hydrologic remote sensing