Glossary of groundwater and groundwater-related terms

A field of lettuces in the UK

A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R, S, T, U, V, W, X, Y, Z



The process by which substances in gaseous, liquid, or solid form dissolve or mix with other substances.

Abstraction, Q [m³/d, m³/a]

The removal of water from a groundwater reservoir, usually by pumping.


Adherence of gas molecules, ions, or molecules to the surface of the solids.


Mass transport caused by the bulk movement of flowing groundwater in which the mass is dissolved.


A geological stratum or formation that may be capable of storing water but is unable to transmit it in significant amounts.


A rock formation that is sufficiently porous and permeable to yield a significant quantity of water to a borehole, well or spring. The aquifer may be unconfined beneath a standing water table, or confined by an impermeable or weakly permeable horizon.

Arithmetic mean

The sum of all the values of n numbers divided by n.

Artesian Aquifer

Groundwater that is under pressure so when tapped by a well it is able to rise above the level at which it is first encountered. It may or may not flow out at ground level. The pressure in such an aquifer commonly is called artesian pressure, and the formation is an artesian or confined aquifer.

Artificial Recharge

The deliberate replenishment of the groundwater by means of spreading basins, recharge wells, irrigation, or other means to induce infiltration of surface water.


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Bank Storage

The storage of water in an aquifer adjacent to and interconnecting with a surface water body so that a change in a stage of the adjacent surface water body causes a change in storage of water in the aquifer.


Natural discharge of groundwater from an aquifer, via springs and seepages, to rivers. It is baseflow that sustains the low flow of surface steams and rivers during prolonged dry weather.

Baseflow Index, BFI

An estimate of the contribution of groundwater to surface flow, taken as a proportion of total streamflow. BFI will vary with time so comparisons are only valid for the same period of for long-term averages.

Body of groundwater

A distinct volume of groundwater within an aquifer or aquifers as defined under the Water Framework Directive.

Bulk hydraulic conductivity

This term is used to represent the average hydraulic conductivity of a section of aquifer, and is made up of matrix and fracture components.

Bypass flow

Movement of recharge water (usually intermittently) through fractures in the unsaturated zone of a dual-porosity aquifer.


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Stratigraphy that interprets geological history by determining the age and time sequence of the Earth's rock strata.

Clearwater flooding

A form of groundwater flooding caused by the water table in an unconfined aquifer rising above the land surface in response to extreme rainfall.

Coefficient of determination, R²

This is a measure of how well a regression model describes a data set. The closer to 1 the better the independent variable predicts the dependent variable. R² equals 0 when the values of the independent variable do not allow any prediction of the dependent variable from the independent variable.

Cumulative frequency distribution

A curve drawn to represent the percentage of occurrences of a number of observations of a variable less and greater than any given value for an entire sample.

Confined Aquifer

An aquifer whose upper and lower boundaries are low permeability layers which confine the groundwater under greater than atmospheric pressure. These aquifers are sometimes called artesian aquifers, the term first being used where the pressure surface was above ground level resulting in overflow under artesian pressure.

Conjugate joints

The sets of joints that are related in deformational origin, usually compression.

Conjunctive Use

The managed use of both surface and groundwater to meet variable demand. A common feature of conjunctive use schemes is the use of groundwater storage during dry periods to augment surface supplies thus creating more storage capacity to be replenished during the subsequent recharge period.


Originating at the same time as adjacent material.


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One of the measures of sediment particle size used to characterise a sediment. It is the particle size from a sieving process where 50% of the material is finer and 50% of the material is coarser.


Net flux of solutes from a zone of higher concentration to a zone of lower concentration.


The process by which a solute or pollutant moves through an aquifer as a result of molecular diffusion and advection.


The reduction of the pressure head in an aquifer as the result of the withdrawal of free water.

Dry valley

A valley in which the waters of the stream/river that originally cut the valley now flow below the present land surface as groundwater. The groundwater flow direction may no longer parallel that of the former stream.


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Effective Rainfall [mm]

The proportion of rainfall that is available for run-off and groundwater recharge after satisfying actual evaporation and any soil moisture deficit.

Effluent river conditions

A reach of a river is effluent with respect to groundwater if the river gains water from the underlying aquifer.

Ephemeral Stream

An ephemeral stream is one that remains dry during some of the year. Flow can result from a rising water table intersecting the stream-bed or from periods of surface flow.

Evapotranspiration [mm/d, mm/a]

The amount of water that would be lost from the ground surface by evaporation and transpiration from plants if sufficient water were available in the soil to meet the demand is termed Potential Evapotranspiration (PE). The proportion of PE that is actually evapotranspired under the prevailing soil moisture conditions is termed Actual Evapotranspiration (AE).


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The term fracture is used on this site to refer to a parting in a rock. The term does not imply any particular orientation or origin, except that of brittle failure. Thus joints and faults are fractures, but a fracture is only referred to as a joint or fault if the relevant mode of formation is known. The term fissure is commonly used by hydrogeologists but its meaning is imprecise and is not used in the report. Where fractures are thought to have been enlarged by solution they are described as such.

Fracture flow

The preferential flow of groundwater through dilated cracks, joints, bedding planes or other features of secondary porosity within an aquifer. It does not include preferential groundwater flow through a thin high-permeability horizon of an aquifer.

Flood (see Groundwater flooding)

Flow path or flow pattern

The line or group of lines that indicate the direction of groundwater flow in an aquifer and which reflect the movement of groundwater from a recharge zone to a discharge zone.


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Geometric mean

The nth root of the product of the values of n positive numbers.

Ghyben-Herzberg Principle

A principle that accounts for the existence of a body of fresh water floating on sea water within an aquifer because of the different densities. Generally speaking, fresh water extends to a depth about forty times the height that the fresh water table is found above sea level.

Good groundwater status

The status achieved by a groundwater body when both its quantitative status and its chemical status are at least "good", as defined by the Water Framework Directive.

Groundwater flooding

Groundwater flooding is the emergence of groundwater at the ground surface or the rising of groundwater into man-made ground, through natural processes, under conditions where the 'normal' range of groundwater levels and groundwater flows are exceeded.

Groundwater Protection Zone, GPZ

Delineated area in which groundwater is protected by restrictions on human activities. The most severe restrictions are applied in those zones close to groundwater sources in areas of high vulnerability.

Groundwater rebound

Rising groundwater levels resulting from a reduction in abstraction rates following a period of high abstraction which kept groundwater levels artificially low. The classic scenario is in urban centres overlying major aquifers where groundwater levels were depressed by decades of substantial industrial abstractions. A decline in industrial activities allowed depressed groundwater levels to recover. Groundwater rebound can cause negative effects such as a risk of flooding to subsurface infrastructure, such as tunnels and the basements of buildings, as well as changes in geotechnical and geochemical properties that could result in settlement and corrosion of deeply founded structures.

Groundwater recharge [mm/d, mm/a]

Inflow of water to a groundwater body from the surface. Infiltration of precipitation and its movement to the water table is one form of natural recharge. Many methods have been devised to increase natural recharge to utilise aquifer storage, termed artificial or managed aquifer recharge.


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A hardened layer of chalk, formed by progressive synsedimentary cementation in shallow water areas; reflects reworking of the chalk.

Hydraulic conductivity, k [m/d]

For an isotropic porous medium and homogenous fluid, the volume of water that moves in unit time under a unit hydraulic gradient through a unit area measured at right angles to the direction of flow. Commonly, though imprecisely taken to be synonymous with permeability.

Hydraulic gradient

Slope of the water table or potentiometric surface. The change in static head per unit of distance in a given direction. If not specified, the direction generally is understood to be that of the maximum rate of decrease in head.

Hydraulic head [m]

The height above a datum plane (such as sea level) of the column of water that can be supported by the hydraulic pressure at a given point in a ground water system. For a well, the hydraulic head is equal to the distance between the water level in the well and the datum plane.


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Influent Stream

An influent stream or river is one that loses flow to ground water, i.e., a stream that replenishes ground water reservoirs by percolation through its porous bed. Since influent streams tend to lose substantial amounts of their water, they are usually of an ephemeral nature.


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Limestone terrains produced by dissolution of and attrition by groundwater. Karstic limestone is characterised by the absence of surface drainage and by sinks and rising streams connected underground by flow along major fissures or in cave systems.


Licensed quantity

The volume of water, usually expressed as m³/d, which a user is allowed to withdraw from a groundwater source under the terms of an abstraction license issued by the Environment Agency.


Stratigraphy based only on the physical and petrographic features of rocks; the delineation and classification of strata as three-dimensional, lithologically unified bodies.


As used in the Aquifer Properties database, the term refers to an area encompassing sites within 100 m of each other.


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Matric potential

Total negative pressure potential of pore fluid in the unsaturated zone reflecting the affinity of the pore fluid to the rock or soil matrix.


Pertaining to water of recent atmospheric origin.


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Nodular chalk

Result of partial synsedimentary cementation of the chalk; first stage of formation of hardground.


A rock formation that does not form an aquifer: see aquiclude.


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Packer testing

A field method of hydraulic conductivity testing involving the use of mechanically-, pneumatically- or hydraulically-expanded packers in a borehole to isolate a section of the drilled length. The resultant separated section is then tested in a manner similar to standard pumping tests by injecting or withdrawing water over a period of time.

Permeability K (Specific or Intrinsic Permeability) [mD (milliDarcy)]

The term permeability, used in a general sense, refers to the capacity of a rock to transmit water. Such water may move through the rock matrix (intergranular permeability) or through joints, faults, cleavage or other partings (fracture or secondary permeability).

A more strict definition of permeability is that it is a measure of the relative ease with which a porous medium can transmit a fluid under a potential gradient. It is the property of the medium only and is independent of the fluid. Commonly, but imprecisely, taken to be synonymous with the term Hydraulic Conductivity which implies the fluid is water.


The ratio of the volume of the interstices to the total volume of rock expressed as a fraction. Effective porosity includes only the interconnected pore spaces available for groundwater transmission; measurements of porosity in the laboratory usually exclude any void spaces caused by cracks or joints (secondary porosity).

Potentiometric Surface

An imaginary surface representing the elevation and pressure head of groundwater and defined by the level to which water rises in a well or piezometer. The water table is a particular potentiometric surface. Replaces the earlier term piezometric surface.

Pressure head

Hydrostatic pressure expressed as the height of a column of water that the pressure can support, expressed with reference to a specific level such as land surface. The hydraulic head is the height of the free surface of a body of water above a given surface or subsurface point.

Pumping test

A field testing procedure to quantify aquifer properties at a site involving pumping water out of (or less commonly injecting water into) an aquifer and measuring the effect on water levels in that aquifer and sometimes in adjacent strata. There are several different procedures employed depending on the physical properties to be quantified. A constant-rate pumping test is conducted at a steady rate of discharge or injection; a step-test increases the discharge in stages to a maximum value; a bailing test is conducted during the drilling process, using the bailer drilling tool as a water withdrawal method.



Any one of three values dividing a frequency distribution into four classes, obtained graphically from a cumulative curve by following the 25, 50 or 75 percent line to its intersection with the curve and reading the value. The latter are percentiles and the interquartile range is the difference between the 25 percentile and the 75 percentile of a range.


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Recharge [mm]

The quantity of water that is added to a groundwater reservoir from areally distributed sources such as the direct infiltration of rainfall or leakage from an adjacent formation or from a watercourse crossing the aquifer.

River Basin District

The area of land and sea, made up of one or more neighbouring river basins together with their associated groundwaters and coastal waters, which is identified under Article 3(1) of the Water Framework Directive as the main unit for management of river basins.


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Saline intrusion

The entry of sea water into a coastal aquifer. It may be caused by over pumping fresh water from the aquifer or insufficient natural head on the fresh water aquifer. Sea water is more dense than fresh water and it may form a wedge beneath the fresh water adjacent to the coast.


A depression in the Earth's surface caused by dissolution of underlying limestone, salt, or gypsum. Drainage is provided through underground channels that may be enlarged by the collapse of a cavern roof.


As used in the aquifer properties database, the term refers to a geographical point from which data are available.


Adhesion or release of molecules or ions on the soil or rock surface.

Specific Capacity Q/s [l/s/m, m²/d, m³/d/m]

The rate of discharge of water from the well divided by the resulting drawdown on the water level within the well.

Specific storage Ss [m-1]

Specific storage of a saturated aquifer is defined as the volume of water that a unit volume of aquifer releases from storage under a unit decline in hydraulic head.

Specific Yield Sy [dimensionless]

The amount of water in storage released from a column of aquifer of unit cross sectional area under unit decline of head. Expressed as a dimensionless proportion of the saturated mass of that aquifer unit. Effectively synonymous with the Storativity in an unconfined aquifer. Equivalent to Effective Porosity.

Storativity (Coefficient of Storage) S [dimensionless]

The volume of water an aquifer releases from or takes into storage per unit surface area of the aquifer per unit change in head.

Swallow hole

The point where a sinking stream goes underground.


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Transmissivity T [m²/d]

The integral of the hydraulic conductivity of an aquifer over its saturated thickness. It relates to the ability of an aquifer to transmit water through its entire thickness.


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Unconfined Aquifer

A partially saturated aquifer which contains a water table which is free to fluctuate vertically under atmospheric pressure in response to discharge or recharge.


A deposit consisting of loose grains that are not held together by cement. River terrace deposits are a typical example of an unconsolidated aquifer.

Unsaturated Zone or Vadose Zone

The zone between the land surface and the water table. It includes the capillary fringe and may contain water under pressure less than that of the atmosphere.

Uniformity coefficient (d40/d90)

A ratio used to characterise a sediment using a plotted grain-size distribution curve. It is the 40% retained size divided by the 90% retained size.


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Volatile organic compounds (VOCs)

VOCs are organic compounds which have a low boiling point, causing them to partition readily into the vapour phase. These compounds can migrate through the unsaturated zone of the subsurface in the vapour (gaseous) phase, as well as in the dissolved (aqueous) phase.


The sensitivity of a groundwater system to contamination. Intrinsic vulnerability takes into account the hydrogeological characteristics of an area, but is independent of the nature of the contaminants and the contaminant scenario. Specific vulnerability takes these latter factors into account.


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Water Framework Directive

The European Water Framework Directive (2000/60/EC), which came into force in December 2000, is the most significant piece of European legislation relating to water management for at least two decades. The Directive provides a framework to pull together existing legislation relating to water and expands the scope of water protection to all waters. The main aims of the Directive are to prevent further deterioration of, and promote enhancement of, the status (quality and quantity) of water bodies and related ecosystems. This includes the progressive reduction in the pollution of groundwater.

Water Table

The surface of a body of unconfined groundwater at which the pressure is equal to that of the atmosphere. The static water level in a well in an unconfined aquifer.


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Yield Q [l/s, m³/d]

The volume of water pumped or discharged from a borehole, well or spring.