OR/15/028 Glossary

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Ó Dochartaigh, B É, MacDonald, A M, Fitzsimons, V, and Ward, R. 2015. Scotland’s aquifers and groundwater bodies . British Geological Survey Internal Report, OR/15/028.

Abstraction, Q [m³/d, m³/a]
The removal of water from a groundwater reservoir, usually by pumping.

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

A geological 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 overlying impermeable or weakly permeable horizon.

Artesian groundwater
Groundwater in a confined aquifer that is under pressure, so that when tapped by a borehole or 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.

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.

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 varies with time, so comparisons are only valid for the same period of for long-term averages.

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.

Confined aquifer
An aquifer whose upper and lower boundaries are low permeability rocks or unconsolidated deposits that confine the groundwater in the aquifer under greater than atmospheric pressure. Groundwater in these aquifers can become artesian, where the piezometric or potentiometric surface is above ground level, resulting in overflow under artesian pressure. 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.

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

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.

Ephemeral stream
An ephemeral stream is one that remains dry during some of the year. Ephemeral flow may result from a rising water table intersecting the stream bed, or from periods of rainfall and 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).

The term fracture is often used to refer to any 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.

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.

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.

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

Groundwater flooding
Groundwater flooding is the emergence of groundwater at the ground surface or the rising of groundwater, through natural processes, into man-made ground or structures.

Groundwater rebound
Rising groundwater levels resulting from a reduction in abstraction rates following a period of high abstraction during which groundwater levels were kept artificially low. The classic scenario is in cities overlying major aquifers where groundwater levels were depressed by decades of substantial industrial groundwater abstraction, such as London. 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 (e.g. 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.

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 borehole, the hydraulic head is equal to the distance between the water level in the borehole and the datum plane.

Influent stream
An influent stream or river is one that loses flow to groundwater by percolation through its porous bed. An influent stream that loses substantial amounts of their water may be ephemeral.

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 Scottish Environment Protection Agency.

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

Permeability K (specific or intrinsic permeability) [mD (milliDarcy)]
The term permeability, used in a general sense, refers to the capacity of a geological formation to transmit water. Such water may move through the rock matrix (intergranular permeability) or through fractures, fissures, joints, faults, cleavage or other partings (fracture or secondary permeability).

A more strict definition of permeability 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, Permeability is taken to be synonymous with the term Hydraulic Conductivity, which implies the fluid in question is water.

Piezometric surface
See Potentiometric surface.

The ratio of the volume of the interstices to the total volume of rock or superficial deposit 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 borehole or piezometer. The water table is a particular potentiometric surface. An older term is 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.

Recharge [mm, mm/d, mm/a]
Inflow of water to an aquifer from the surface, from sources such as the direct infiltration of rainfall; leakage from an adjacent formation; or leakage from a watercourse overlying the aquifer.

Rest water level
The standing water level in a borehole or well when it is not being pumped.

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.

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.

Specific capacity Q/s [l/s/m, m²/d, m³/d/m]
The rate of discharge of water pumped from a borehole divided by the resulting drawdown of the rest water level in the borehole.

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.

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.

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.

Unconsolidated deposit
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.

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.

Water Framework Directive
The European Water Framework Directive (2000/60/EC; European Parliament, 2000), 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 uppermost surface of a saturated aquifer. The rest (or static) water level in a borehole in an unconfined aquifer is equal to the water table.

Yield Q [l/s, m³/d]
The volume of water pumped or discharged from a borehole, well or spring.