Groundwater Glossary


Groundwater Abstraction
Groundwater Abstraction

  1. The process of taking water from a source, either temporarily or permanently.
  2. The volume of water taken during this process per unit of time.

Synonyms: withdrawal, extraction.


Geological term, indicating an origin related to terrestrial flowing surface water. Example: alluvial sediments or deposits (usually composed of gravel, sand, silt and/or clay), as found within reach of present-day or former stream systems.


A hydraulically continuous body of relatively permeable unconsolidated porous sediments or porous or fissured rocks containing groundwater. It is capable of yielding exploitable quantities of groundwater.

Synonym: water-bearing formation.


Groundwater-filled body of poorly permeable formations, through which still significant volumes of groundwater may move, although at low flow rates.


Groundwater-filled bodies of poorly permeable formations, through which no or almost no flow of groundwater passes.

Arid region in Peru

Arid region

A region of low precipitation, characterized by a severe lack of available water, to the extent of hindering or even preventing the growth and development of plant life. In agriculture this term is used to indicate extremely dry areas where without irrigation no crops can be grown.


Groundwater pressure condition that allows groundwater to rise to above the local ground surface, if a well is drilled to the zone where such pressure is present (‘artesian aquifer’). In other words: the piezometric level of groundwater in an artesian aquifer is located above ground level. A well tapping an artesian aquifer is called a ‘flowing well’ or ‘artesian well’.


The steady flow component of stream flow that continues for a long time after rainfall has stopped. It is mainly fed by the discharge of groundwater systems and is sometimes called ‘dry-weather flow’.


The oldest rocks recognized in a given area, a complex of metamorphic and igneous rocks – usually Precambrian or Palaeozoic in age – that underlies all sedimentary formations.


Solid rock that is relatively unaltered and has not become soft (thus it is rather dense, and usually of low porosity).

Black water

Wastewater containing faeces or other remnants of sanitary use.

Blue water

Natural surface water and groundwater.

Brackish water

Water containing dissolved solids in a concentration between 1 000 and 10 000 milligram per litre.


Hole made in the soil or the earth’s upper crust by drilling. Its diameter is usually small, and its purpose is commonly either geological exploration or the construction of a well for abstracting or injecting liquids (e.g. water).

Cainozoic (also: Cenozoic)

Era in geological history, running from roughly 66 million years ago until the present. Often also used as an adjective, to indicate the age of a geological formation.

Capillary fringe

Subsurface zone immediately above the water table, in which the interstices are completely filled with water, but under pressure less than atmospheric pressure (due to suction forces).

Carbon Cycle, source: BGS
Carbon Cycle, source: BGS

Carbon cycle

Biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere and atmosphere of the Earth.

Carbonate rock

A rock composed of carbonate minerals, especially limestone and dolomite.


Period in geological history, belonging to the Palaeozoic Era. Often also used as an adjective, to indicate the age of a geological formation.

Catchment, catchment area

Total area having a common outlet for its surface water discharge.
Synonyms: watershed, river basin.


Synthesis of local weather conditions, presented as statistics (mean values, variances, probabilities of extreme values, etc.) over a long period of time (usually taken as 30 years).

Climate change

Long-term modification of the climate (manifested by a change in the long-term statistical properties of climatic variables, in particular in averages and/or variability).

Collector well

Well provided with horizontal tubular drains arranged in several radial directions and which increase its effective radius.

Confined aquifer

Fully saturated aquifer (i.e. pressure everywhere greater than atmospheric pressure) directly overlain by an impermeable or almost impermeable formation (confining bed). The confining bed prevents the aquifer from interacting directly with the atmosphere and with surface water bodies (except for surface water bodies that intersect the aquifer).

Confining bed

Impermeable or poorly permeable formation overlying a fully saturated aquifer (see also: ‘Confined aquifer’).

Connate water

Water entrapped in the interstices of a sedimentary rock at the time the rock was formed.


  1. Geological term: indicates that in principle a rock forms a solid mass, not an accumulation of uncemented loose materials such as gravels, sands, silts and clays (the latter are unconsolidated sediments).
  2. Soil mechanics term: indicates that the volume of an earth layer has decreased (mainly due to reduced porosity) in response to an external load or to the reduction of hydrostatic pressure.

Consumptive use

Part of the water used that is not returned to aquifers, streams or seas, but is either incorporated in products or organisms, or discharged in the form of vapour into the atmosphere.


Period in geological history, belonging to the Mesozoic Era. Often also used as an adjective, to indicate the age of a geological formation.

Darcy’s law

One of the main physical laws in groundwater hydraulics, stating that the volume of flow passing through a section in a porous medium during a given time interval is proportional to the hydraulic gradient and the hydraulic conductivity of the saturated medium.

Amazonas by Andres M.


A land form at the mouth of a river, where this flows into an ocean, sea, estuary, lake, reservoir, another river or a flat arid area, characterized by the deposition of a large share of the sediments carried by that river (alluvial sediments).


Reduction of the stored volume of groundwater in an aquifer (also: reduction of the stored water volume of any other component of the local hydrological cycle).


Geological synonym of sediment.

Domestic water use

Use of water for drinking water and other household purposes, in offices and for public water services outside the agricultural and industrial spheres. The water may be supplied either by municipal or public water supply systems or withdrawn by the users themselves (self-supply).


Natural discharge or artificial removal of excess water.


Lowering of the groundwater level or piezometric surface caused by abstraction of groundwater (including not only pumping, but also outflow from an artesian well or discharge from a spring).


A dynamic complex of plant, animal and micro-organism communities and their non-living environment interacting as a functional unit.


Adjective indicating that water from the area, aquifer or stream concerned is not discharged into the open sea and oceans, but the area’s discharge returns directly to the atmosphere. Endorheic aquifers are either drained by streams in endorheic river basins or by evaporation in closed depressions. Endorheic river or lake basins are usually called ‘closed basins’.


A chemical sedimentary rock consisting of minerals precipitated by evaporating waters. Mainly salt (rock salt) and gypsum.



  1. The physical process of emission of water vapour to the atmosphere.
  2. The amount of water evaporated. (The maximum rate, which is the rate of evaporation from a water surface, is called potential evaporation)


  1. Release of water to the atmosphere through evaporation from the ground and from water surfaces, and also escaping from the leaf surfaces of plants to the atmosphere (transpiration).
  2. The amount of water lost by evapotranspiration. (The maximum rate, which is the rate of evapotranspiration from a fully developed vegetation cover, well supplied with water, is called potential evapotranspiration)


Depletion that has reached the stage of no remaining exploitable resources.


The set of characteristics of a rock that indicates its particular conditions when it was formed and that distinguishes it from other facies in the same rock formation. In the case of sedimentary rocks it indicates its particular environment of deposition, in the case of metamorphic rock the particular range of pressure and temperature under which metamorphism occurred.


See Fracture.


Secondary interstices, which have originated after the rock was formed.


Numerical value of the rate of flow across a given surface.

Flux density

Flux per unit of area.

Folded mountains

Mountains created by an originally planar earth structure (usually a sequence of sedimentary beds) being bent. The deformation may have been produced by horizontal or vertical forces in the crust.


A set of rocks or unconsolidated sediments that are or once were horizontally continuous and that share some distinctive features of lithology.

Fossil groundwater

Stored groundwater that has entered a rock formation during a remote period of time (under climatic and/or geological conditions different from the present day ones) and is not renewed under present-day conditions.
Synonym: non-renewable groundwater.


Cracks in rock masses that develop if forces in the crust exceed a certain critical point (below this critical point folding may occur). There are two categories of fractures: joints and faults. A joint is a crack along which no displacement has occurred. If there is relative displacement of rocks on both sides of the fracture and parallel to it, then the fracture is called a fault.

Free aquifer

Unconfined aquifer, phreatic aquifer.


Water of relatively low content of dissolved solids (usually less than 1 000 mg per litre).

Geyser field, credits: Andrew Miller
Geyser field, credits: Andrew Miller


A hot spring that forcibly ejects hot water and steam into the air. The heat is thought to result from the contact of groundwater with magma bodies.


Related to the heat energy of the earth’s crust.


Geological term indicating a downthrown block of the earth’s crust, limited by parallel boundaries (known as ‘normal faults’).

Green water

Water originating from precipitation that does not run off or recharge groundwater, but is stored in the soil or temporarily stays on top of the soil or vegetation. Eventually, this part of precipitation evaporates or transpires through plants.

Grey water

Polluted water that results from uses of water other than for sanitary purposes.


Subsurface water in which the pressure is equal to or higher than the local atmospheric pressure. In other words: water below the water table or phreatic level.

Groundwater development, groundwater exploitation

Human activity consisting of abstracting groundwater and making it available for beneficial use.

Groundwater discharge

Outflow of water from a groundwater system (1. Phenomenon; 2. Flux)

Groundwater level

Elevation to which groundwater will or does rise in a piezometer connected to a point in the groundwater domain. It is a time-dependent variable, varies from point to point within the groundwater domain, and indicates the potential energy of groundwater in any point considered (in metres of water column relative to a selected topographic reference level).
Synonyms: piezometric level, piezometric head, hydraulic head, groundwater hydraulic potential.

Groundwater mining

Abstracting groundwater that will not be renewed anymore (fossil or non-renewable groundwater).

Groundwater recharge, groundwater renewal, groundwater replenishment

  1. Process of inflow of water into a groundwater system or aquifer.
  2. Flux related to this inflow process.

Groundwater runoff

Part of stream flow contributed by groundwater (by springs or by diffuse seepage into the stream bed).
See also: base flow (almost a synonym).

Groundwater table

Surface defined by the phreatic levels in an aquifer (i.e. surface of atmospheric pressure within an unconfined aquifer).

Humid region

Region characterized by a surplus of precipitation over potential evapotranspiration. In other words: the available water in the area is sufficient to sustain the growth and development of plant life without irrigation.

Hydraulics, hydraulic

Hydraulics is the applied science dealing with the mechanics of flow of water in pipes, open watercourses and porous media. Hydraulic is the adjective.

Hydraulic conductivity

Capacity of a porous medium to transmit water.
Synonym: permeability.

Hydraulic gradient

Change in water level (piezometric level) per unit of horizontal distance.


Graph of water level or flow rate versus time.

Hydrological Cycle
Hydrological Cycle

Hydrological cycle

The circulatory flow of water at or near the Earth’s surface (in which water is moving between the subsystems atmosphere, surface water, unsaturated zone, groundwater and oceans)
Synonyms: Hydrologic cycle, water cycle, H2O cycle

Hydrological overabstraction

Abstraction from a renewable groundwater resource that is excessive with respect to the hydrological budget of the corresponding aquifer. The abstraction intensity is too high to allow a new dynamic equilibrium to be established in the intermediate or long term. (Some groundwater specialists rank this under ‘groundwater overexploitation’, which may be confusing).


The science of the terrestrial part of the water cycle.


The waters of the Earth, as distinct from the rocks (lithosphere), living things (biosphere) and the air (atmosphere)

Igneous rock

A rock formed by the solidification of a magma.

Impermeable, impervious

Incapable of transmitting water (property of a porous medium).


  1. The process of water entering a porous or fissured formation.
  2. The quantity or rate of infiltrating water.

Infiltration gallery

Man-made construction to tap groundwater and to conduct it to the surface without the need for external energy (the system is based on flow by gravity).
Synonyms: Foggara, qanat, karez, falaj, rhettara, ghail.

Intensive development of groundwater

Groundwater development to such a degree that the natural flow in the aquifer or aquifer system has been significantly changed (significant change of flow regime).


Contact surface between two distinct bodies, e.g. between saline and fresh groundwater. Depending on local conditions, the interface between saline and fresh groundwater may be rather sharp or consist of a thick transition zone (zone where fresh and groundwater are mixed).

Interstices (voids)

Open spaces within subsurface unconsolidated sediments and rock formations (i.e. the portion not occupied by solid matter), allowing fluids (air, water, oil, gas, etc.) to flow or be stored underground. Interstices may be subdivided into original or primary interstices (mainly pores) and secondary interstices that have originated after the rock was formed (mainly fissures, sometimes widened by dissolution). The corresponding sediment/rock categories are indicated as porous or fissured, respectively. The term porosity is used to indicate the percentage of the bulk volume of the rock that is occupied by interstices (pores and fissures combined).

Irrigation, source: AgriLife Today
Irrigation, source: AgriLife Today


Artificial application of surface water or groundwater to the land or soil, usually with the objective of promoting crop growth.


See Fracture.


Phenomenon that causes irregularities in land surface and drainage features (both internal and external) as a result of dissolution of solid rock, mainly limestone.

Land subsidence

Sinking elevation of the ground surface, which may result from compression of a sediment due to reduced water pressures, which may occur when water is drained or abstracted from subsurface strata, e.g. from an aquifer.


  1. Type of rock.
  2. Branch of earth science dealing with the distinction and description of different types of rock and their physical properties.


Geological term, indicating an origin related to seas or oceans.

Marine regression

Sea-ward shift of the boundary between sea and land, lasting for a geologically significant period of time.

Marine transgression

Land-ward shift of the boundary between sea and land, lasting for a geologically significant period of time.


Era in geological history, running from roughly 250 to 66 million years before present. Often also used as an adjective, to indicate the age of a geological formation.

Metamorphic rock

A rock whose original mineralogy, texture or composition has been changed due to effects of pressure, temperature, or the gain or loss of chemical components.

Mineral water

Natural water with a content of dissolved salts greater than a defined threshold and often considered to have therapeutic properties.


Depleting the stock of a non-renewable natural resource.

Groundwater Monitoring
Groundwater Monitoring


Systematically repeated observation of a certain variable or phenomenon.

Natural resource

A natural resource is commonly defined as something (usually a substance or ‘raw material’) found in nature
and necessary or useful to humans. Typical examples are solar energy (perpetual resource), forests and water (renewable resources), or fossil fuels and minerals (non-renewable resources).

Non-renewable groundwater

A groundwater body or unit that under current climatic conditions receives no or negligible quantities of recharge.
Synonym: fossil groundwater


Intensive exploitation considered to be excessive. Among groundwater specialists there is no generally shared interpretation of groundwater overexploitation, and in particular it is often not explicitly specified with respect to what criterion the exploitation is considered excessive. Here, overexploitation is interpreted as intensive exploitation characterized by a less favourable balance between benefits and negative side effects than would have been the case at a lower rate of exploitation. (See also: Hydrological overabstraction).

Palaeozoic (also: Paleozoic)

Era in geological history, running from roughly 590 to 250 million years before present. Often also used as an adjective, to indicate the age of a geological formation.

Permeable, pervious

Capable of transmitting significant quantities of water (property of a porous medium).


Capacity of a porous medium to transmit water.
Synonym: hydraulic conductivity.


Soil or rock layer in which the temperature has been continuously below 0oC for some years at least.

Phreatic aquifer (‘water-table aquifer’)

Aquifer in which the upper boundary of the groundwater mass forms a surface (water table) that is in direct contact with the atmosphere. This condition favours the aquifer being actively involved in the water cycle.
Synonyms: water-table aquifer, unconfined aquifer, free aquifer.

Phreatic level

Level in a phreatic aquifer where water pressure is exactly equal to the local atmospheric pressure. It is the level observed in a shallow well in such an aquifer.


Water-loving plants supplied permanently or intermittently by groundwater through their roots reaching the capillary fringe.


An area lying at the foot of a mountain or mountain range and formed by its erosion products.


Device or well that is designed to measure the potential energy level in a point of a water body (stream, lake, aquifer, aquitard, etc.).

Piezometric level

Elevation to which water will rise in a piezometer connected to a point in an aquifer.
Synonyms: potentiometric level, piezometric head, hydraulic head.

Piezometric surface

Surface defined by the piezometric levels corresponding to a predefined layer or topographic level inside an aquifer.
Synonym: potentiometric surface.



Extensive relatively flat and horizontal upland area, usually higher than the surrounding area and bounded at least at one side by steep slopes.
Synonyms: meseta, altiplano (the latter usually restricted to a highland intermontane plateau).


Extensive relatively flat and horizontal area, usually higher than the surrounding area.

Pores, porous, porosity

See Interstices.

Potential evaporation, potential evapotranspiration

See Evaporation and Evapotranspiration


Eon in geological history, running from roughly 4 000 to 590 million years before present. Often also used as an adjective, to indicate the age of a geological formation.


  1. Rain, hail, snow, dew and other liquid or solid condensation products of water vapour falling from clouds or deposited from the air on the ground.
  2. Amount of water produced by rain, hail, snow, etc. falling on a unit of horizontal surface per unit of time.


Most recent period in geological history, belonging to the Cenozoic Era. Often also used as an adjective, to indicate the age of a geological formation.
Synonym: Anthropogen.

Recharge area

Area where most or all of the recharge of an aquifer occurs.


Any solid material lying on top of bedrock, which includes soil, alluvium, and rock fragments weathered from the bedrock.

Renewable groundwater

Body of groundwater that is replenished under current climate and geological conditions.


Volume stored.


Ability of a system to recover from an unsatisfactory state (in particular, the ability to remain in or return to a state of dynamic equilibrium).

Return flow

Any flow of abstracted water that returns after use to a stream channel or to the groundwater reserves.

Mekong River, credits: Anthony Bouch
Mekong River, credits: Anthony Bouch

River basin

Catchment area of a river.


The part of precipitation that appears as stream flow. Often three components are distinguished: (a) surface runoff, resulting from overland flow during and after a storm or precipitation event, and caused by that event; (b) interflow, which is a delayed flow passing at least partly through the upper soil layers; and (c) groundwater runoff, which is the outflow of groundwater to streams (forming the essential part of baseflow).

Runoff coefficient

Ratio of runoff volume and the volume of rainfall during the event that caused runoff.

Safe yield

Flux of groundwater that can be withdrawn from an aquifer without causing undesirable side effects (outdated

Saline water

Water containing dissolved solids in concentrations of more than 10 000 milligram per litre.

The referenced media source is missing and needs to be re-embedded.


Concentration of dissolved salts in water.

Salt water intrusion, seawater intrusion

Invasion of seawater (usually seawater) into a body of freshwater (either a surface water or a groundwater body).


Section of the decomposed/weathered zone (regolith) of crystalline basement aquifers, rich in clay minerals. It has low hydraulic conductivity but significantly higher storage capacity than the more permeable fissured zone below it (saprock).

Saturated zone

Part of the subsurface water-bearing formation in which all interstices (voids), large and small, are filled with water.


Ratio of distance on a map compared to distance in the real world (e.g. 1: 50 000 or 1:1 000 000). For a given map size, a small-scale map may present a relatively large area, but with limited detail.

Secondary groundwater resources

Groundwater resources due to recharge in addition to the natural recharge, such as irrigation water losses or return flows (by-product) and artificial recharge.


Any of a number of materials (usually erosion products) deposited by natural processes on the Earth’s surface.


  1. Slow movement of water in a porous medium
  2. Loss of water as a result of water entering a porous medium (infiltration) or emerging from such a medium along a line or surface (exfiltration).

Semi-arid region

A region of low precipitation during a significant part of the year, characterized at least seasonally by a lack of available water, to the extent of hindering the growth and development of plant life, at least during the dry season. In agriculture this term is used to describe dry areas where land cultivation is impracticable or much less profitable without irrigation.

Semi-confined aquifer

Confined aquifer, where either the confining bed on top of the aquifer or the poorly permeable basal formation underneath the aquifer have sufficient permeability (aquitards) to allow the exchange of water between the aquifer and the domains above or below it.

Semi-permeable bed

Earth layer with limited capacity to transmit groundwater.
Synonym: aquitard.


Depression in the ground surface in karst terrain, acting as point of disappearance of surface water flow. It usually develops into a funnel-shaped cavity at the ground surface by dissolution of karstified limestone or dolomite (doline).
Synonyms: ponor, swallow hole.

Soil moisture

Water present in soil pores above the water table.


Science dealing with caves.

Missing media item.


Location of concentrated natural groundwater outflow.

Storage, stored volume, stock

Volume of water present in a component of the local hydrological system (e.g. aquifer, stream, lake, soil).


Body of water flowing in a natural surface channel. Generic term that includes rivers, rivulets, brooks and other natural water courses.

Subsidence, land subsidence

Sinking elevation of the ground surface, due to natural processes or caused by human activities. It results from compression of a sediment due to reduced water pressures, which may occur – inter alia – when water is drained or abstracted from subsurface strata, e.g. from an aquifer or an aquitard.

Surface water

Water located on the surface of the Earth, such as in streams, rivers and lakes.

Sustainable yield

Flux of groundwater that can be withdrawn from an aquifer without causing undesirable side effects, in particular without causing a permanent state of non-equilibrium of the hydrological budget of an aquifer.
(see also: Safe yield, Hydrological overabstraction).


Related to land masses (as opposed to marine and atmospheric).


Period in geological history, belonging to the Cainozoic Era. Often also used as an adjective, to indicate the age of a geological formation.

Texture, rock texture

Physical properties of a rock related to the size, shape and arrangement of the mineral constituents it is composed of.

Transboundary aquifer

An aquifer that spans two or more political entities, separated by political boundaries.

Unconfined aquifer

Phreatic aquifer.

Unconsolidated sediments

Sediments consisting of a matrix of loose, uncemented particles.

Fern Springs, credits: Todd Petrie

Unsaturated zone

Zone below ground surface where the interstices are partly filled with water and partly with air. This zone is located above the water table or above the top of a confined aquifer. The water pressure in the unsaturated zone is lower than atmospheric pressure.

Virtual water

Water used in the production of goods or services.
Synonym: embedded water.

Water budget

A specification of inflows, outflows and change of stored volume of water for a defined water system (river basin, aquifer, soil system, area) during a specified period. In principle, the change of stored volume equals the difference between inflows and outflows.
Synonym: water balance.

Water cycle

The circulatory flow of water at or near the Earth’s surface (in which water moves between the subsystems atmosphere, surface water, unsaturated zone, groundwater and oceans).
Synonyms: hydrologic cycle, hydrological cycle, H2O cycle.

Water footprint

The total volume of freshwater used to produce the goods and services consumed by an individual or community or produced by a business.


A man-made construction – usually of vertical cylindrical form – used to get access to groundwater and abstract it by lifting it to the surface.

Nanticoke River Wetlands [credit: Chesapeake Bay Program |]


An area where the ground is permanently or seasonally saturated with water (swamp, marsh, peatland, shallow lake)


  1. The process of taking water from a source, either temporarily or permanently.
  2. The volume of water taken during this process (usually expressed as a volume per unit of time).

Synonym: abstraction.