GLOSSARY OF TERMS EXTRACTED BY M.S.N. CARPENTER FOLLOWING TRANSLATION OF "WATER WELLS" (author: Michel DETAY, English language edition published by Masson-Wiley, 1997).

379 keywords in bold;

Syn: synonyms; alternative forms and variants separated by commas;

Abbr: abbreviations or short forms; e.g.: specific examples; 290 definitions tagged with ; contexts in italics; explanations and technical notes in roman type;

cf. cross-references.

absorption test

Well development can be monitored by absorption tests ,

(cf. Lugeon test).


The process of introducing acid down a well into a water-bearing formation,

in order to dissolve part of the carbonate clogging material or rock constituents. It

is also used to remove mud injected during drilling. The general objective of

acidization is to increase the discharge capacity by improving the

permeability, (also known as acidization or acid treatment).

active chlorine

Quantity of chlorine available for a given reaction.

(cf. chlorimetic titre).

active management of groundwaters

Planned control of aquifers involving actions to improve quantity and quality of abstracted waters.


Compounds added during acidization in order to maintain iron and aluminium

oxides in solution, (e.g. Rochelle salts, citric acid, lactic acid, ammonium



Auxiliary chemical substance serving to assist or contribute

to a reaction. Acid pyrophosphate may be used following treatment of the

medium or the product with an adjuvant that raises its pH (e.g. caustic soda

or sodium carbonate).


Said of an environment in which oxygen (or air) is present, of organisms

requiring oxygen for growth or a process that can only take place in the

presence of oxygen

ageing, aging

The progessive change in material properties with increased age; in

catchment structures, usually marked by a deterioration caused by clogging

air-lift pumping Syn: air-lift development

The most commonly used and most efficient method of well development; air

supplied via a line is injected from the base of a tube submerged in the water

well. The emulsion created in this way reduces the density of the water

contained in the tube.

alkalimetric titre Abbr: AT, (cf. alkalinity).


Sum of the concentrations of hydroxides, carbonates and bicarbonates of the

alkali metals and alkaline earths present in a given solution. The alkalinity

of a solution is a measure of its ability to neutralize acidity. On the basis

of the alkalimetric titre or the total alkalimetric titre, it is possible to

derive the proportions of the three main groups of chemical species that give

rise to alkalinity (c.f. alkalimetric titre, total alkalimetric titre).

alpha-emitter radioactivity

Includes activity due to alpha-emitting actinides such as 239-Pu and 241-Am,
but radon is excluded from the definition of overall alpha-emitter radioactivity
given by the ICRP, (c.f. radionuclide, beta-emitter radioactivity).

alpha-emitting actinides

e.g. plutonium isotopes.

alternating pumping

Method of well development, which consists of alternating sudden stops and

starts in order to create brief and powerful variations of pressure on the

water-bearing layer, thus reversing the flow through the screen. This

procedure, also known as rawhiding, facilitates the disruption of sand

bridging, but there is a risk of wear and tear of the pumping equipment, (cf.

controlled overpumping).


An ion NH 4 + or radical NH 4 derived from ammonia by combination with a
hydrogen ion or atom, occurring in salts having properties that ressemble the
alkali metals, and also in quaternary ammonium compounds.

The ammonium ion is very frequently present in groundwaters, resulting in most

cases from the anaerobic decomposition of nitrogenous organic matter ,

(cf. quaternary ammonia compounds).


Said of an environment in which oxygen (or air) is absent, of organisms only

able to grow in the absence of oxygen or a process that can only take place in

the absence of oxygen.

analogue recorder

Device for automatic recording of data in analogue form, e.g. water level recorder,
pressure transducer system. Measurements from chart recorders need to be
digitized prior to data treatment and interpretation, (cf. electronic data recorder).

annulus Syn: annular space

Cylindrical zone surrounding a borehole, used in the estimation of

quantities required for the chemical treatment of a well.


A body of relatively impermeable rock that is capable of absorbing water

slowly but does not transmit it rapidly enough to supply a well or spring.

Aquiclude or semi-permeable terrains show very slow circulation (also known as

confining bed, aquitard).

aquifer Syn: aquiferous formation

Permeable hydrological formation which allows significant discharge of

groundwaters as well as the capture of appreciable amounts of water by

economical means. An aquifer can be defined in terms of the reservoir, the

hydrodynamic, hydrochemical and hydrobiological mechanisms, the part of the

global water cycle involved, as well as the spatial and temporal variability

of these characteristics. The aquifer is recharged by effective infiltration,

i.e. the amount of water actually reaching the groundwater body, whereas its

geometry depends on the characteristics of the geological and hydrodynamic

boundaries, (cf. groundwater body).

aquifer development

Operation designed to improve well performance by enhancing the transmissive

properties of the aquifer; similar to well development, except that the fine

fraction of the aquifer itself is removed (c.f. well development, mechanical


aquifer losses

Reduction of hydraulic head in the immediate vicinity of the well due to

mechanical clogging, (cf. well losses).


1. (adj.) Said of a geological formation or layer with very low hydraulic conductivity
(as in aquifuge or impermeable terrains).

Impermeable formations serve as boundaries to aquifers;

2. (noun) An impermeable body of rock, with no interconnected openings
and thus lacking the ability to absorb and transmit water.

area of pumping depression Syn: contributing region

Area within the zone of influence where all flow lines are directed towards

the well being pumped. Note: do not confuse with radius of influence.

artesian condition

The standing water-level remains virtual as long as a borehole or a piezometer

has not attained the aquifer, lying always above the base of the overlying

impermeable layer. The artesian condition is said to occur when the standing

level remains higher than ground level after a borehole has pierced the base

of the containing bed (c.f. gushing artesian well).

artesian water

Groundwater present in a confined aquifer, which flows into wells without

pumping or gushes out onto the ground surface; the piezometric level is

situated above the ground surface.

artesian well Syn: overflowing well

A well that gushes or flows out onto the surface without pumping.

artificial recharge

Planned operation of transferring water from the ground surface into aquifers.
The quantity, quality, location and timing of artificial recharge are decision variables,
the values of which are determined as part of the management policy of a
given groundwater system.

attenuation zone

Near-field sanitary protection zone surrounding a water well.

Atterberg limits

Series of different behaviours observed with increasing water content of a

soil. Bentonites are characterized by their Atterberg limits, (e.g. liquid

limit, plastic limit, plasticity index).

bacterial corrosion

Chemical attack of metals brought about by the proliferation of microorganisms in water wells.
This phenomenon is due to the presence of bacteria in groundwaters,
(cf. biological corrosion).

bacterial proliferation

Rapid development of bacteria under favourable microenvironmental

conditions. Associated with zones of increased nutrient flux into the well,

(cf. biomass).


Chemical substance capable of destroying bacteria or restricting their



During drilling, the hole is cleaned progressively by the lowering of a bailer

which allows the cuttings to be brought to the surface .

barite Symbol: BaSO 4

Barium sulphate. Barite is used for weighting up the drilling fluid , (also

known as barytes).

base exchange

Chemical reaction whereby a cationic species in solution replaces another

from the solid phase. Secondary phenomena which can bring about modifications

in the chemical composition of the groundwater during its residence time in

the aquifer.

Benoto process

A method for cutting a water well in which the casing penetrates under its

own weight or is rammed by hydraulic jacks. Drilling by cutting is better

known as the Benoto process .

bentonite mud , bentonitic mud

Drilling fluid based on natural smectite-rich clay. This type of mud cannot

be used in water-wells; it has the characteristic property of reacting and

flocculating in the presence of nitrate-rich water.

Berkaloff method

Berkaloff has combined the Hantush and Boulton approaches to give practical

rules of interpretation for pumping tests , (cf. pumping test analysis).

Bernoulli equation

A relation describing the conservation of energy in the laminar flow of an

ideal fluid, stating that: p/d + gz + (v/2) is constant along any stream

line, where p is the fluid pressure, d is the fluid density, v is the

velocity, g is the acceletion due to gravity and h is the height of the

pressure head. To obtain the discharge rate of the pumped water passing

through the plate orifice of a Pitot tube of diameter D, the equation can be rewritten:

Q = C d / h, where Q = discharge rate, C = coefficient of the experimental flow rate depending on d/D,
d = diameter of orifice and h = height of the pressure head.

beta-emitter radioactivity

Includes activity due to I-131, Cs-137 and Sr-90, but tritium is excluded from
the definition of overall beta-emitter radioactivity given by the ICRP,
(c.f. radionuclide, alpha-emitter radioactivity).

biological clogging

Accumulation of gelatinous or viscous material due to bacterial activity in

or around a water well.

biological corrosion

The deterioration of metals as a result of metabolic activity of

microorganisms, (cf. bacterial corrosion)


Mass of living organisms per unit surface or unit volume in a given

environment, (cf. bacterial proliferation).

Boulton method

Procedure used for the analysis of pumping tests in unconfined aquifers, in

which delayed yield is taken into account. Boulton has tabulated the limited

recharges which, for example, can be produced by a secondary aquifer; after an

initial yield from the aquifer due to pressure release, water is gradually

released from unsaturated zone storage by gravity drainage , (cf. pumping test


bridge-slot screen

Frequently used type of well screen with raised ribs; it is constructed

flat, then rolled and welded. Possesses good mechanical resistance and shows

an open area varying between 3 and 27% according to the dimensions of the

perforations, (cf. screen).


The main drawback in using bentonite mud is that it clogs the water inflow

sections with an excessively thick cake, once drilling is complete, the cake

forms a more or less waterproof crust on the well walls.

cementation , cementing

The operation whereby cement slurry is pumped into a drill hole and forced

in behind the casing for such purposes as sealing the casing to the walls of

the hole, preventing unwanted leakage of fluids into the hole or migration of

fluids from the hole, closing the hole back to a shallower depth, plugging an

abandoned well, etc.

cement slurry

Before undertaking cementation, the exact volume of the cement slurry to be

used must be calculated, then the quantity of cement slurry required to fill

the annular space is introduced at the bottom of the well .


Cylindrical, cage-like device fitted to a well's casing as it is run to keep

the pipe centered in the borehole. Cementing centralizers are made with two

bands that fit the pipe tightly with spring steel ribs that arch out to press

against the wall of the borehole. By keeping the pipe centered, a more uniform

cementing job is assured; centralizers are especially used in deep or deviated


characteristic grain diameter

The characterisitc grain diameter of a material is defined so that, in

relation to the total weight of the sample, 10% of the grains are finer and

90% coarser than this dimension; it represents the x-axis coordinate of the

90-percentile grain-size class , (also known as characteristic diameter).

chelating power Syn: complexing efficiency

The ability of certain bodies to complex cations to form a so-called ring


chemical attack

Chemical weathering of rocks or well casings by hydration, hydrolysis and oxidation.

chemical clogging

Reduction in the porosity of the gravel pack due to processes of

precipitation caused by increased oxygenation, (e.g. carbonate clogging, iron-

manganese clogging).

chemical corrosion

Attack of metals not involving electrochemical processes. In the context of

water wells, better termed non-electrochemical corrosion. This type of

corrosion corresponds to reactions governed by the fundamental laws of

kinetics which occur without being accompanied by an electric current. Note:

avoid the term chemical corrosion; non-electrochemical corrosion is correct

and standardized

chemical development

Declogging procedure involving the use of a chemical treatment. The chemical

reagents used comprise acids and polyphosphate, (e.g. acidification,

polyphosphate dispersion).

chemical treatment

Any procedure for well development making use of chemical methods to loosen

and remove material clogging the catchment structure.


In general, a binary compound containing chlorine combined with another

element or radical.


The dissolved chlorides in water may be derived, among other sources, from

salt formations in potassic evaporite basins. Since chlorides are not absorbed

in the soil, they can be transported over long distances . In addition, they

may originate from excessive pumping near the sea coast.

chlorimetric titre

Quantity of active chlorine present in a unit volume of reagent, expressed

in degrees, where 1 = 3.17 g active chlorine per litre.


The act of bringing the phenomenon of detergency into effect (detersion and

cleaning are terms standardized by ISO).

cleanout pumping

Pumping carried out to remove material brought into well during development.

coefficient of permeability Syn: specific permeability

The volume of mobile water in m 3 transmitted perpendicularly to the flow

direction in unit time (s) through a unit cross-section in m, under the

effect of a unit hydraulic gradient and within the conditions of validity of

Darcy's law. Note: the Meinzer unit is defined as the rate of water flow in

gallons per day through a cross section of 1 square foot under a unit

hydraulic gradient, at the prevailing temperature or at 60 F (16 C).

Also known as the permeability coefficient, Meinzer's coefficient of

permeability (cf. hydraulic conductivity).


A suspension of very small particles (a few microns in size) of various

insoluble substances in a liquid medium. Flocculation is prevented by the high

surface tension and viscosity of water as well as the electric charge on the

surface of the particles. Changes in the pH of the medium or the concentration

of salts will bring about flocculation and precipitation of the solid phase,

(cf. biological clogging).

colloidal deposits

Gelatinous or viscous masses of flocculate associated with the biological

clogging of water wells, (cf. gel).

complexing efficiency Syn: chelating power.

compressed-air treatment (cf. physical treatment).


The current transferred across unit area per unit potential gradient. The

conductivity of water is a relatively reliable indicator of its

mineralization . Knowing the hydraulic conductivity of a reservoir, it is

possible to calculate the discharge rate of an aquifer; conductivity increases

with the concentration of dissolved salts and varies as a function of

temperature. It is expressed in microsiemens/cm, and is the reciprocal of

volume resistivity.

cone of depression , cone of influence, groundwater hole

Concave downward depression in the piezometric (potentiometric) surface of a

groundwater body, defining the area of influence of a well.

confined aquifer , artesian aquifer, pressure aquifer

Aquifer bounded above and below by impermeable beds, or by beds of

distinctly lower permeability than in the aquifer itself (e.g. Bunter

Sandstone, Lorraine; Albian sands, Paris Basin). The water-bearing formation

is saturated throughout its thickness and is capped by a permeable or semi-

permeable layer, (c.f. aquifer, unconfined aquifer, leaky aquifer).

confining pressure Syn: geostatic pressure

Sum of hydrostatic and lithostatic pressures, or total pressure exerted on a

system. An isotropic pressure resulting from the load of overlying rocks, or

hydrostatic pressure resulting from the weight of the water column in a zone

of saturation, (cf. lithostatic pressure).


A measure of the degree to which a climate is affected by continental

influences (or remoteness from maritime influences).

controlled overpumping

Method of well development in which the well is pumped at the physical yield

limit for a short period in order to induce surging. The rapid alternation of

pumping and non-pumping is known as rawhiding, (c.f. alternating pumping,


convection time (c.f. transfer time, residence time).


Complex set of electrochemical or purely chemical phenomena involving attack

of metal casings and screens, the accumulation of carbonate and iron-manganese

precipitates, and even bacterial activity; e.g. uniform corrosion, pitted

corrosion, cracking corrosion, (c.f. electrolytic corrosion, bacterial


critical discharge Symbol: Qc

Well discharge corresponding to the velocity threshold between laminar and

turbulent flow regimes. In practice, the pumping discharge should always be

lower than the critical discharge , (cf. critical velocity).

critical velocity

Corresponds to the velocity at which critical discharge occurs, (cf. Reynold's number).

cutting Syn: holing.

In this type of drilling by sand washing or cutting, the casings penetrate the
formation through the effect of their own weight or the action of a hydraulic jack,

(cf. Benoto process).

Darcy's law

States that the rate of movement of water through porous media is

proportional to the hydraulic gradient; it can be expressed as u=Ki, where u

is the volume of water transmitted through a unit cross-sectional area per

unit time, K is the hydraulic conductivity and i is the unit change in head

through unit length of flow path. The circulation of groundwaters is governed

by Darcy's law , (c.f. hydraulic gradient, discharge).


Removal of encrustations, colloidal deposits or organic material from

various parts of the catchment structure.


Dispersion of colloidal deposits or mud cake to form a suspension, (cf.

polyphosphate treatment).

degrees Baum Abbr: B

Divisions of a scale for measuring the specific gravity of liquids, still

used in French-speaking countries to express the concentrations of solutions

with densities greater than water. Slightly different from the A.P.I. or

Twaddell scale: modulus A.P.I. gravity = [141.5/specific gravity] - 131.5;

modulus degree B = [140/specific gravity] - 130 i.e. 0 B is equivalent to a

specific gravity of 1.0 at 15 C, and each division on the scale represents a

specific gravity increment of 0.004.

delayed storage coefficient

The volume of water released from unsaturated zone storage (under gravity

drainage) per unit surface area of aquifer per unit change in head.


Reduction of nitrate, due to bacterial activity, to form nitrite and then

elemental nitrogen, usually in an anaerobic environment.

density (of drilling fluid)

The density of pure water (at 4 C) is unity. The density of fluid mud can vary

between 0.8 and 2, and is measured with a Roberval balance, or

preferably a Baroid balance.


A measure of water flow at a particular point; in the case of a water well,

the discharge rate is usually given in cubic metres per day.


Process in which a substance looses its cohesiveness, due to the hydrating

power of water, leading to a partial or complete breaking of the various

electrostatic bonds between the atoms and the molecules of the substance

entering solution. Dissolution is said to occur when solvation is complete,

(cf. solvation).

distribution system , mains water supply.

domestic (or drinking) water supply well Abbr: DWS well

Catchment structure providing water for household consumer use, not intended

for irrigation or industrial uses.

down-the-hole hammer drilling Abbr: DTH drilling

A drilling method utilizing impact accompanied by thrust from the tool,

which is itself rotating. A pneumatic hammer fitted with drill bits is

attached to the end of the drill string. The percussive action is driven by

releasing compressed air into the drill string. This process is of great interest

in hydrogeological work, mainly in hard terrains.


A number of rapidly-setting cements available on the market can

be used to limit the downtime of drilling operations.

drainage basin

Hydrological system bordered by watersheds which delimit the catchment area

of a watercourse and its tributaries. The only input of water into a drainage

basin, which is assumed to be closed, comes from effective precipitation. Also

termed drainage area, catchment, catchment area, catchment basin, gathering

ground, feeding ground or hydrographic basin (cf. hydrological system).

drainage factor

Characterizes the slow downward percolation of waters from the unsaturated

zone into an unconfined aquifer; do not confuse with leakage factor.

drawdown Syn: depletion

Lowering or depression of water-table caused in most cases by pumping.

Corresponds to the difference between dynamic level and standing water-level

for a particular well.

drill collar

One or more very heavy drill collars may be placed above the drill bit to

increase the vertical pressure on the tool, in order to facilitate the

penetration and ensure straightness of the hole.

driller's log Syn: instantaneous log

Record of daily drilling with description of formations encountered, also

containing information on casings, screens and water level in well.

drilling fluid

A suspension of finely divided heavy material, such as bentonite and barite,

pumped through the drill pipe during rotary drilling. The rotary drill

requires the use of a drilling fluid prepared on site which is injected

continuously and under pressure into the hollow drill pipes of the string. The

drilling fluid may be composed of clear water, bentonite mud or mud with a

synthetic biodegradable polymer base; it has several functions: cooling and

lubricating, flushing out of chippings from the geological formations,

consolidation of the borehole with cake, protection against water inflow and

providing useful information on loss of head . The density is adjusted by

weighting up (barite) or lightening (water), while viscosity is adjusted by

thinners or viscosifiers. Can also be called drilling mud, drill fluid, drill

mud, mud fluid, mud flush, circulating fluid, circulation fluid, circulation

medium, driller's mud, mud water, flushing mud, rotary mud (cf. bentonite


drilling log sheet , well data sheet.

drilling rate Syn: rate of penetration (abbr: ROP)

A measure of the speed with which the bit drills into formations; usually

expressed in terms of feet per hour. The rate of penetration in soft terrains

is high and can reach 100-150 m a day.

drill pipe string , string of rods, drilling string

Lengths of pipe, casing or other downhole drilling equipment coupled

together and lowered into a borehole. The whole assembly is made up of the

following elements, from top to bottom: a swivel, a square drill pipe,

ordinary drill pipes, drill collars and a tool. The drill pipes are hollow so

that mud can be injected into the bottom of the borehole.

drill reamer , reamer, drilling reamer, reaming shell, core shell, reamer shell

A short tubular piece designed to couple a bit to a core barrel. The outside

surface of the reaming shell is provided with inset diamonds or other cutting

media set to a diameter to cut a specific clearance for the core barrel

drinking water supply Syn: ?domestic water supply (abbr: DWS)

Distribution network supplying water that is fit for human consumption.

dynamic level Syn: pumping water-level

Height of groundwater level when disturbed by pumping.


Special type of electrical potential method, in which one of the current

input electrodes is placed at a point on the conductor while the other

electrode is placed at infinity. In this context, earthing may be employed as

a strategy in support of prospection. Since the conductor is everywhere at

roughly the same potential, the ore body or underground river is delimited by

equipotential lines.

effective porosity

1: The volume of mobile water, Ve, that a saturated reservoir can contain and

subsequently release when fully drained, divided by the total volume of the

reservoir, Vt; i.e.: ne = Ve/Vt. Since a reservoir is never completely drained

of its water content, effective porosity is more commonly used in hydrogeology

than the more theoretical concept of absolute porosity;

2: the property of a soil or rock containing interconnecting interstices

expressed as a percentage of the bulk volume occupied by these spaces,

(cf. void space volume).

effective precipitation

Amount of water from precipitation that remains available at the ground

surface after subtraction of losses due to true evapotranspiration. Fraction

of precipitation that can infiltrate into the soil or contribute to run off,

(cf. run off, evapotranspiration).

effective transfer capacity , effective velocity, average interstitial velocity

Carrying capacity of waters infiltrating through a medium, (cf. purifying


electrical water-level gauge , tape.

electric log , electrical log

Geophysical survey technique based on resistivity or self-potential;

provides information on the thickness and nature of water-bearing formations

with a view to groundwater extraction. An electric log typically consists of

the spontaneous-potential (SP) curve and one or more resistivity or induction

curves. The Archie equations form the basis for the interpretation of electric

logs, (cf. geophysical log).

electric panel array

Survey method consisting of measuring apparent resistivity between two

electrodes, for different positions of a third electrode. Panel-type arrays

are commonly employed to reveal or confirm the presence of major structural

features, in which case the array is operated at right angles to the inferred

structural trace . Such surveys enable the emplacement of exploratory boreholes

near faults, in order to confirm their role as drains or impervious


electric survey

Electrical prospection based on the conductivity of subsurface formations,

or their capacity to conduct an electric current, whether natural or

artificial; they do not involve measuring the magnetic field. Electric survey

methods make use of: natural (telluric) currents: spontaneous (or self)

polarization and artificial currents: electrical potential (mapping of

potential difference, with earthing, etc), resistivity method (electric

logging, resistivity rectangles).

electric water-level gauge

Tapes with electric contact gauges are satisfactory for measuring the

piezometric level. A good quality probe enables measurements to the nearest

centimetre for absolute values of 30-50 m, (also termed electric water-level


electrochemical corrosion

Form of chemical attack on metals involving galvanic couples or gas

electrode processes, (e.g. hydrogen corrosion, oxygen corrosion).

electrochemical series , electromotive series

List of half-cell reactions arranged in order of decreasing equilibrium

potential with respect to the standard hydrogen electrode; also reflects

increasing susceptibilty of metals to electrolytic corrosion, (cf. standard

electrode potential).


Phenomenon controlling the self-potential measured during an electric log

survey, resulting from the migration of waters towards the middle of a porous


electrolytic cell Syn: galvanic couple

An electrode system in which a potential is set up between two metals having

different tendencies to go into solution; e.g.: Zn + Cu ++ = Cu + Zn ++,

(cf. electrochemical series).

electrolytic corrosion (cf. electrochemical corrosion).

electronic data logger , automatic data aquisition system

Apparatus allowing the acquistion of large quantities of data from a measuring
device in real time. Used to store very precise and closely spaced measurements
of parameters such as water level, pressure, temperature, conductivity

cf. analogue recorder

electroosmosis , electro-osmosis

Electromotive force proportional to the logarithm of the restivitity ratio

between two different electrolytes. It is an important parameter contolling

self-potential in an electric log survey, caused by the transport of water

from one electrode to another, (also known as electroendosmosis in the

biological sciences).


Carrying off of solid particles into a flowing medium.

entrance velocity

Flow rate at which groundwaters pass through the well screen openings, (cf.

uniform inflow distribution).

equilibrium pH Symbol: pHS

Precise value of pH beneath which waters become aggressive and above which

the waters are encrusting.

Euler's constant

Second term in Jacob's equation - equivalent to the well function under a
quasi-steady state regime - that tends towards a constant value when
the radius of influence is infinity.


Process whereby water is lost by the soil to the atmosphere though the

action of evaporation from wet surfaces combined with transpiration, the

exhalation of water vapour by plants, largely from their leaves. Evaporation

takes place from the surface of free bodies of water (oceans, seas, lakes and

rivers) as well as from vegetation. Both of these phenomena - evaporation and

transpiration - are brought together under the term evapotranspiration.

extractable storage Syn: mineable storage

The maximum volume of water that can be economically extracted from the

total storage of an aquifer, (cf. reserve).

ferruginous deposits

Scale or colloidal deposits formed from the accumulation of insoluble iron

hydroxides, usually due to oxygen corrosion or bacterial activity in the zone

of temporary saturation, (cf. iron-manganese hydroxides)

fictitious radius Symbol: Rf

The distance at which the drawdown, as calculated by the Jacob expression,

falls to zero. It is a function of the transmissivity and the storage

coefficient, (also known as the critical radius, "imaginary" radius).

filtrate strength

Solid matter load present in a drilling mud sample, evaluated by measuring

filtrate volume on a Baroid balance, (c.f. mud filtrate, filtrate volume).

filtrate volume

If the filtrate volume is excessive with respect to a given volume of drilling

mud, then the cake is too thin and the walls are not held firm leading to a

real risk of collapse. If the filtrate volume is too weak there is a risk of

the mud clogging the water-bearing formation , (cf. filtrate strength).

fishing, fish job, grappling

Searching or attempting to recover a piece of equipment fallen into a well

float shoe method

Cementation method using a special device which is placed at the lowermost

end of the casing string. The float shoe consists of a plastic ball which acts

as a plug preventing the passage of fluids from the bottom upwards. It is also

in contact with the surface by means of a pipe screwed to the cementation

shoe, thus allowing the cement slurry to pass into the annular space.


Aggregation of clay minerals and other fine particles to form flocs.

Influx from gypsiferous terrains can lead to flocculation .

floccule Abbr: floc

A loose fluffy or foamy mass formed by the aggregation of fine suspended


flow meter, micro-current meter

An instrument used to measure velocity of currents flowing in a water well,

(fluid meter, flow indicator, flow-measuring device).

flow net

A graphical representation of flow lines and equipotential (piezometric)

lines used in the study of seepage phenomena, (also known as hydraulic flow


flow superposition

The respective and simultaneous effects of several different causes of

change in groundwater level at a given point. In porous media, these different

water flows can be added together algebraically.

food-grade approved

Said of materials suitable for use in food packaging or drinking water

supply applications.

fracture permeability

Ability to transmit water due to the presence of fracture porosity. Within

limestone massifs, the fractures are commonly open, thus providing channels

which allow the very rapid circulation of groundwaters; a non-porous rock can

behave as a reservoir if sufficiently fractured, (cf. karst aquifer).


Development technique making it possible to widen existing fractures or

create new ones in order to improve the specific capacity of a well. There are

two types of artificial fracturing: hydraulic fracturing (or hydrofracturing)

and fracturing by explosives.

free plug method

A destructible cementation plug is introduced into the casing string to be

sealed before or after injection of the cement slurry.

gamma-gamma log

Downhole profile of induced radioactivity showing the bulk density of rocks

and their contained fluids. A porosity log of the wall-contact type indicating

formation density by recording the backscatter of gamma rays, (also known as

density log, scattered gamma-ray log, densilog).

gas electrode process

Half-cell reaction involving a gaseous species (e.g.: Pt, H 2/H +).


Gaseous compounds held in solution in water (e.g. oxygen, carbon dioxide,

methane, hydrogen sulphide). The concentration of dissolved gases is governed

by Henry's law, and is an important indication of water quality, (cf. Henry's



Gelatinous material formed from the coagulation of a colloid, (cf. colloidal


geophysical log

Downhole recording of properties in a well or in adjacent geological

formations (e.g. self-potential, resistivity, gamma ray, gamma-gamma,

neutron). The three main categories are: formation, structural and fluid logs,

(cf. electric log).

grab bucket

A grab bucket is used to empty progressively the inside of the casing as long

as it is situated above the standing water-level .

granulometric analysis

The choice of the gravel used in the pack is based on granulometric analyses

of the aquifer formations that are to be tapped. These analyses make it

possible to define certain parameters such as the characteristic diameter, the

fineness index or the sorting coefficient.

gravel pack

A mass of very fine gravel that is placed around a well screen. Gravel

packing is a method of well completion in which a slotted or perforated liner

is placed in the well and surrounded by a very fine-mesh gravel. Different

methods such as gravel packs, sand consolidation and well screens are used to

control the influx of sand.


Geophysical survey technique based on the force of attraction between

masses, giving rise to an acceleration g - due to gravity - which affects all

bodies placed near the surface of the globe.

groundwater Syn: underground water

Subsurface waters contained in aquifers beneath the soil-water (or

unsaturated) zone.

groundwater basin

That part of a drainage basin situated beneath the ground surface.

A groundwater basin can be made up of one or more aquifers, whose boundaries
are constrained by geological structures; its recharge takes place through the

infiltration of effective precipitation , (cf. hydrological system).

groundwater body , water-sheet

Mass of subsurface water contained within permeable geological formations,

(cf. aquifer).

groundwater level

The upper surface of groundwater, or the level below which an unconfined

aquifer is permenently saturated with water, (also known as water-table,

piezometric level).

groundwater level contour

Line of equal piezometric level (also: line of equal hydraulic head,

hydraulic head contour, groundwater contour, groundwater table contour).

gushing artesian water Syn: overflowing artesian water.

half-cell reaction

An electrode process forming part of a galvanic couple, in which a potential

is set up between a given metal species and the electrolyte, e.g.: Zn - 2e =

Zn ++ (at the anode), Cu + 2e = Cu (at the cathode).

Hantush method

Technique for analysing pumping tests in a non-steady state regime, in which

a semi-log relationship between drawdown and time is established for a given

observation well. Hantush has tabulated the high recharges which bring about

permanent stabilization of the observation wells after the initial drawdown ,

(cf. pumping test analysis).

Henry's law

The amount of a gas dissolved by a given amount of liquid at a given

temperature is proportional to the temperature, the volume dissolved being

independent of pressure.

heterogeneous aquifer

One of the two main types of groundwater reservoir, showing fracture

permeability and generally composed of limestones but also volcanic,

metamorphic and granitic rocks, (cf. homogeneous aquifer).


If the wall cake does not detach itself, hexametaphosphate is used to clear

out the hole .

historical data

Records of technical information acquired during testing, operation and

development of a well.

homogeneous aquifer

One of the two main types of groundwater reservoir, showing pore-space

permeability and made up of sands, gravels and sandstones. Homogeneous

aquifers are associated with alluvial deposits occupying valley floors and

account for part of the groundwaters of major sedimentary basins; the

groundwater discharge rates are generally low , (cf. heterogeneous aquifer).


Chemical combination of water with another substance. The weathering of

rocks and minerals involves penetration of water into the lattice of

crystalline solids; do not confuse with hydrolysis.

hydraulic conductivity

A measure of permeability with dimensions of length per unit time, denoted

as K in Darcy's law. It corresponds to the coefficient of permeability only

when the fluid is water at moderate temperatures; hydraulic conductivity has

the dimensions of velocity and is expressed in m/s. Also known as coefficient

of hydraulic conductivity or hydric conductivity, (cf. Darcy's law, coefficient of permeability).

hydraulic gradient

Change in head through unit length of flow path; the h/l ratio is denoted as

i in Darcy's law, where h is the head (height of the water column) in metres,

proportional to the weight of the water column and l is the height of the

cylinder in metres, (cf. Darcy's law).

hydraulic head Syn: total head

The difference in piezometric level between the recharge and discharge areas

of a hydrological system, calculated as the sum of elevation potential energy

possessed by a given mass of water, its pressure head and velocity head. Also

termed potentiometric head, total static head. Note: not to be confused with

pressure head, (cf. static head).

hydraulic vibrator

Device used to facilitate the lowering or extraction of casings in a water


hydrochloric acid Symbol: HCl

A solution of hydrogen chloride gas in water. A poisonous, pungent liquid

forming a constant-boiling mixture at 20% concentration in water, widely used

as a reagent, (muriatic acid and spirit of salt are obsolete synonyms).

hydrogen corrosion

Chemical attack of metals in unaerated media, incorrectly termed chemical corrosion.

hydrogen potential Symbol: pH, (cf. pH).

hydrogen sulphide , hydrogen sulfide; symbol: H 2S

Gaseous compound giving rise to aggressive and corrosive properties when

dissolved in water. Waters containing H 2S will attack steel and form iron

sulphide encrustations. It is characterized by a smell of rotten eggs, (also

known as hydrosulphuric acid, sulphuretted hydrogen).


Study of the chemistry, physics and environmental aspects of water at the

Earth's surface, including the mechanisms of storage and flow in groundwaters.

Also used in the more restricted sense of "ground-water geology" only


hydrographic basin , catchment area, drainage area, drainage basin

A region or area bounded by a drainage divide and occupied by a drainage

system. Otherwise termed a catchment basin, gathering ground, feeding ground,

(cf. river basin).

hydrological cycle Syn: water cycle

Natural cycle in which water evaporating at the Earth's surface - mostly

from the oceans - passes into the atmosphere and falls back as precipitation.

hydrological system

Dynamic system which corresponds to a resource - variable in space and time

- forming part of the global water cycle, (e.g. drainage basin, groundwater

basin, aquifer).


Decomposition of mineral salts brought about by the action of water, owing

to the dissociation into H + and 0H- ions which leads to exchange reactions

with crystalline solids. Plays an important role in the destructive action of

water on silicates and the formation of hydroxyl-bearing minerals such as

micas and chlorite. Also known as hydrolytic alteration or hydrolytic

decomposition, (cf. hydration).

hydrostatic pressure

Pressure exerted by the overlying water, (cf. confining pressure).

impervious fault

A structural trap forming a lateral boundary to a groundwater body.


The natural or artificial introduction (recharge) of water into the ground.

Infiltration involves flow into a substance, in contradistinction to

percolation, which implies passage through a porous substance (e.g. a bed of

solid absorbant). In general, the process consists of the slow laminar

movement of water through the pore spaces of soil and rock, while flow in

large openings such as caves is not included.

infiltration basin Syn: seepage basin

Engineered structure used for the artificial recharge of an aquifer.

infiltration rate

Flow rate of water infiltrating through a medium. The infiltration capacity

is evaluated by cumulating the rates over a period of time.

ion-exchanged bentonite

Calcic bentonite can be transformed into sodic bentonite by the addtion of

sodium carbonate; the swelling of these bentonites can vary between

10 and 15 times.

iron bacteria

Microorganisms capable of metabolizing iron present as minerals or organic

complexes (e.g. Gallionella, Toxothrix). Siderophile bacteria are present in

most aquifers and use iron as a source of energy; some iron bacteria are

chemicolithotrophic (e.g. sulphate-reducing bacteria, siderocapsaceans) and do

not use ferrous iron at all, while others metabolize manganese instead (e.g.

Metallogenium ), (cf. sulphate-reducing bacteria).

iron-manganese clogging

Precipitation of iron and manganese compounds, forming encrustations on

various parts of the well structure.

iron-manganese hydroxides

Compounds generally responsible for iron-manganese clogging, formed in the

zone of temporary saturation if the screen is dewatered, (cf. ferruginous


Jacob method

Method for the evaluation of pumping test data under constant-discharge

conditions, (cf. pumping test analysis).

jet cleaning

Method in which a tool with pressurized water jets is rotated as it is moved

past the section of the screen to be treated. The fine particles penetrate

into the screen where they are recovered by pumping or with a bailer; the

efficiency of this procedure depends on the type of screen, being optimal for

screens of the Johnson type, (also known as high-pressure jetting).

jet cleaning head Syn: jetting head

Downhole tool fixed to the drill stem of a rotary rig, used for high-

pressure jetting of the screen face.

Johnson-type screen

Screen with a continuous horizontal opening running the whole length of the

screen. The main advantages of such a screen are: the regularity and precision

of the opening, the very low risk of clogging and the highest open area

coefficient of all the screen types, (cf. screen).


Type of topography developed in limestone terrains, formed as a result of

variable degrees of dissolution by meteoric waters, (cf. karst aquifer).

karst aquifer

Water-bearing formation where most of the storage is in solution channels

and fracture porosity, (cf. fracture permeability, karst).

Kjedahl nitrogen

Incorporates all nitrogen present in organic or ammoniacal form; it should not
be confused with total nitrogen which also includes nitrates and nitrites.

laminar flow

Fluid flow characterized by the gliding of fluid layers (laminae) past one

another in an orderly fashion. As a preliminary hypothesis, it is necessary to

assume that groundwaters exhibit laminar flow over most of their transport

path, (cf. Reynolds number).

Landsat D

Recent satellite in a series carrying multispectral scanners and thematic

mappers, which provides a better resolution at ground level (30, 20 or 10 m).

As a result, such images can reveal just as must detail as aerial photographs

at a scale of 1:50,000, (cf. remote sensing).

Langelier's index

The value obtained by subtracting the saturation pH (pHs) from the measured

pH of a water sample. The Langelier index can be used to determine whether a

water is scale-forming or aggressive.

leakage Syn: interformational flow

Draining or passage of fluids from one formation to another.

leakage coefficient Syn: leakance factor

Parameter with the dimensions of reciprocal time, equivalent to the

hydraulic conductivity divided by the thickness of the semi-pervious

interlayer. It is a measure of the capacity of a leaky interlayer to transmit

water vertically

leakage factor

Parameter with the dimensions of length used to characterize the effects of

leakage in a semi-confined aquifer. Not to be confused with leakage


leaky aquifer Syn: semi-confined aquifer

Aquifer overlain and/or underlain by a relatively thin semi-pervious layer,

through which flow into or out of the aquifer can take place. The confining

bed and/or substratum of an aquifer is often made up of a semi-permeable

hydrological formation. Under certain favourable hydrological conditions

(difference of pressure head), the semi-permeable layer allows the exchange

of water with the overlying or the underlying aquifer. This phenomenon,

which is known as leakage, implies the presence of an aquifer containing

semi-confined groundwaters. (c.f. aquifer, confined aquifer, unconfined



Lightening of a drilling mud is achieved by adding water .

linear loss of head Symbol: BQ

Reduction of hydraulic head generated by laminar flow of water into the

aquifer close to the well. It is constrained by the hydrodynamic parameters of

the aquifer and increases with the duration of pumping, (also known as linear

well loss, linear head loss).

lithostatic pressure Syn: lithostatic load

Pressure exerted by overlying ground or strata, (cf. confining pressure).

louvre-slot screen

Screen with horizontal rectangular perforations in the form of a hood.

Mechanically resistant but with a low open area, (cf. screen).

Lugeon test

Type of absorption test providing a value for hydraulic conductivity at a

given point in the well. The hydraulic conductivity so obtained arises from

fracture porosity; 1 Lugeon permeability unit corresponds to the absorption of

1 litre water per metre depth in the well per minute at a pressure of 1 MPa,

(cf. absorption test).


An instrument for measuring pressure, but which can be adapted to measure


manometer tube

A waterproof piezometric pipe which goes down into the well and which is

fixed along the pump column.

maximum acceptable drawdown

Criterion determined by physical and technical constraints of the aquifer

complex/catchment structure, expressed by the critical discharge, Qc, and the

corresponding critical drawdown, Sc, as measured from well tests.

mechanical clogging

Movement of fine particulate matter into the near-field well environment and

gravel pack, leading to aquifer losses. Particularly important in overpumped

wells, (cf. aquifer development).

mechanical declogging Syn: swabing

Physical method of development based on surging water backwards and forwards

by means of a plunger. Used to clean screens, gravel pack and aquifer matrix.

mechanical weathering , disintegration, crumbling

Disaggregation of rock masses under the effect of weathering, leading to the

release of debris of various different grain sizes.

mud filtrate

Fluid remaining after deposition of mud cake. If the drilling fluid is too

thin, filtrate may be driven into the surrounding permeable formations.

Measurements are taken with a Baroid press, in which is placed a sample of the

mud to be strained, (cf. filtrate strength).

mud loss

Leakage of drilling fluid through well walls into the surrounding terrain. A

fairly frequently encountered situation arises from total loss of mud which

places the well structure in real danger .

natural polymers

Organic products derived from Guar gum, used to enhance viscosity during

drilling operations (e.g. Revert, Foragum). For the same amount of matter with

the same viscosity, their molecular structure enables them to produce ten

times more gel than a bentonitic mud.

90-percentile class Syn: 0.9 quantile

The sorting coefficient is expressed as the quotient of the diameters of the

40- and 90-percentile classes on the cumulative grain-size curve.

non-steady state regime

Regime which takes account of the observed fact that the size of the cone of

depression increases as a function of pumping time. Also termed non-

equilibrium regime, (cf. steady-state regime).

nutrient flux

Inflow of dissolved minerals essential for bacterial proliferation.

Increased nutient flux is often linked to high entrance velocities across the


observation well Syn: piezometer

A generally narrow well or tube designed to measure the water-table level or

hydraulic head at a particular point. In theory, observation wells are set out

along two rectangular axes centred on the control well and at incresing

distances away from it, the distance of the last one being close to the

estimated radius of influence; observation wells must have a very low response


open area coefficient Syn: open area

Open area coefficient: C = f / f + 1 where f is the dimension of the slot

between two coils and l is the width of the encasing wire. Fundamental

parameter controlling the inflow of water from the aquifer towards the well.

operation (of water wells)

The techniques or actions used for extracting groundwaters from aquifers.

Planning the extraction of groundwaters depends on the evaluation of water

reserves and resources. More generally, the extraction of mineral resources or

Earth materials from the surface or subsurface, (the term abstraction is used

for groundwaters, and mining for mineral ores).

organic compounds

Dissolved organic matter present in water (e.g. polyaromatic hydrocarbons,

phenols, carbon tetrachloride, pesticides).

organic emulsifier

Muds with emulsified oil are obtained by adding diesel fuel and an organic

emulsifier to standard mud.

orthophosphoric acid Symbol: H3PO4

Water-soluble, transparent crystals, melting at 42 C; phosphoric acid is

obtained cormmercially from phosphate rock.

overextraction , overdevelopment, groundwater overdraft

Abtraction of groundwaters at a rate above the safe or sustainable yield,

not to be confused with overpumping.


Simplest method of well development, which consists of pumping at a rate far

above the estimated water extraction capacity. There are risks of irregular

development as a result of vertical variations in the permeability of the

terrain. This type of development can provoke a compaction of the fine

sediments, which causes a reduction in permeability, while sand bridging

formed by unidirectional flow may also lead to a lowering of yield, (cf.

controlled overpumping).


This reaction particularly concerns oxides that are deficient in oxygen, as

well as sulphides such as pyrites and also organic matter. It is a phenomenon

of considerable importance in highly oxygenated infiltration zones, but its

effect is also felt to a lesser degree at greater depth

oxygen corrosion

Chemical attack of ferrous metals in aerated waters, due to to a gas

electrode process involving oxygen. Characterized by the formation of ferric

hydroxide blisters which accumulate around the anode.

partial cementation

It is possible to cement only a part of the cased section by carefully

estimating the volume of cement to be employed. In this way, the lower or

upper part of catchment structure can be cemented, this procedure is

particularly useful for separating water-bearing horizons superimposed one

above the other


A process involving the elevation of temperature for an appropriate period

of time, for the purpose of either inactivating microorganisms, particularly

pathogens, or decreasing their number to a specified level.

peptization power Syn: dispersive capacity.

percussion drilling

Method that consists of raising a heavy tool (churn drill bit) and letting

it fall onto the terrain to be traversed, the height and frequency of the drop

being varied according to the hardness of the formation. Also known as

percussion boring, percussion system or percussive drilling.

permanent storage

Fraction of the total reserve that is not replenished. In the case of

unconfined groundwater, its upper surface corresponds to the mean mininum

water table. For confined bodies, the permanent storage is very similar to the

total groundwater storage, (cf. reserve).

permeability Syn: perviousness.


e.g. simazine, atrazine.


The negative logarithm of hydrogen ion concentration, a measure of the

acidity or the basicity of a solution ranging on a scale from 0 (acidic) to 7

(neutral) to 14 (basic). pH measurements are of considerable interest since

they reveal contamination by cement or by water from the aquifer, and also

indicate the risks of flocculation of the drilling mud , (cf. hydrogen potential)


A rapid and cheap method for drawing up a structural or geological sketch map,
thus representing a valuable adjunct to geological and soil surveys which provide
the essential information for choosing the location of a borehole.
Photographic interpretation may be applied to conventional aerial photogrammetry
and satellite imagery, (cf. remote sensing).

physical treatment

Pneumatic or hydraulic technique of well development/restoration used to

break up mechanical clogging in the gravel pack, (e.g. controlled overpumping,

air-lift pumping, compressed-air treatment).


A generally narrow observation well or tube designed to measure the
water-table level or hydraulic head at a particular site.

piezometric level Syn: groundwater level.

piezometric surface Syn: potentiometric surface

The level to which water in a confined aquifer will rise in observation

wells, mapped by interpolation between piezometer measurements.

Pito(t)meter Syn: Pitot tube

A measuring device consisting of a delivery main entering a rigid conduit

with a diameter ensuring that flow takes place over the full cross section and

along a minimum length. At the end of the conduit, there is a section of tube

of diameter identical to that of the intake. The end of this tube is obstructed by
a metal plate with a circular orifice at the centre. On the side of the tube, there
is a transparent manometer tube for the direct reading of discharge rate.

plasticity index (cf. Atterberg limits).

plutomium isotopes (cf. alpha-emitting actinides).

pneumatic development

A procedure based on the same principle as surging. Makes use of the forward

and return flow of groundwaters around the screen brought about by the large

volume of air introduced into the borehole. Two distinct methods can be

adopted: the open hole method and the closed hole method.

polymer mud

Product containing chemical compounds with a high molecular weight resulting

from the association of several simple molecules having a low molecular

weight. Can be used directly as a drilling mud or as an additive to bentonic

mud, (cf. natural polymers).

polyphosphate treatment

Addition of a solution of polyphosphate in order to break down the

cohesiveness of clay. Used to disperse mud cake, infiltrated mud or the clay

present in cutting samples, (cf. deflocculation).


Phenomenon involving the displacement of fluids towards the well in order to

equilibrate the pressure. Appears during the recovery in water-level after

drawdown or cessation of pumping.

potentiometric surface , piezometric surface, water table

The level to which water in a confined aquifer will rise in observation

wells. It is also the top surface of a groundwater body defined by the set of

piezometric levels measured at different points at a given date. Since this

surface corresponds to the upper boundary of the aquifer, it is the

hydrodynamic limit of the system, mapped by interpolation between piezometer


pressure head Syn: piezometric head

Potential energy of a unit mass of water at any point compared with a

pressure of one atmosphere at the same elevation. Note: do not confuse with

static head.

protected perimeter

e.g. attenuation zone, remedial action zone, well field management zone,

(cf. sanitary zone of well protection).

producing well

An exploratory borehole having an adequate diameter with respect to the fixed

objective can be subsequently equipped for water extraction use. Otherwise,

the hole can be re-bored so that it can be fitted out with a larger diameter

and transformed into a producing well.

pump discharge column Syn: rising main

Tube used for extracting water from a well, connects the submersible pump

with the distribution network.

pumped water discharge

The rate at which water can be pumped out of a well.

pumping discharge measurements Carried out by reading a calibrated flow

rate gauge (Pitot tube) or by measuring the time to fill a known volume.

pumping discharge

Yield obtained from a well during pumping.

pumping discharge head Syn: total static head.

pumping test analysis

Procedure for interpreting data from pumping tests, usually based on the

graphical representation of drawdown vs. time curves and a comparison with

tabulated well functions, (e.g. Berkaloff, Boulton, Hantush, Jacob, Theis and

Walton methods). The experimental drawdown vs. time curve is superimposed on

standard curves in order to assess departures from the ideal state, (cf.

pumping test).

pumping tests , aquifer tests

Procedure for measuring well efficiency, well performance or aquifer

characteristics, (e.g. step drawdown test, constant discharge test, absorption

test). Such tests make it possible to determine: the characteristics of an

aquifer/catchment structure; the hydrodynamic parameters; the operating

conditions of the well and the evolution of drawdown. Pumping tests performed

before the start of water abstraction or after a regeneration phase allow the

satisfactory completion of cleaning and development procedures, including

natural development, (cf. pumping test analysis).

purifying capacity

A measure of the ability of a given terrain to absorb a pollutant from the

groundwater, equivalent to the path length required for complete removal of

the pollutant from the liquid phase, (cf. effective transfer capacity).

quadratic head loss Syn: turbulent well loss. Symbol: CQ2

Non-linear reductions in hydraulic head arising from turbulent flow within

the well structure, screen and casings. Quadratic head losses become all the

more important as the drawdown curve becomes increasingly convex .

quasi steady-state regime

An eventual stabilization of the hydrological system in which the aquifer

restores its water balance.

quaternary ammonium compounds

Strongly alkaline substances that are derived from ammonium by replacing one

or more of the four hydrogen atoms with organic radicals, (cf. ammonia).

radionuclide Syn: radioactive isotope.

Radioactive isotope tracers enable the estimation of turnover rates and circulation
flow rates for groundwaters, thus leading to the basic concept of residence time for
waters in an aquifer
, (e.g. alpha-emitting actinides), (cf. radioactivity).

radius of influence (of well pumping) Syn: area of influence

Zone within which water-levels are influenced by pumping. Corresponds to the

area of diversion in the restricted sense of TOLMAN, (cf. recharge area).

rainfall deficit , drought

rate of penetration Syn: drilling rate.

readjustment Syn: matching.

The use of gamma-ray logs is often necessary in order to enable a calibration
with the geological cross section.

recharge area Syn: intake area

Zone located between radius of influence of well pumping and upstream

boundary of the hydrological system. Corresponds to undisturbed aquifer, (cf.

radius of influence).

redox potential Symbol: Eh

A scale of electric potential measured in volts indicating the ability of a

substance or solution to cause reduction or oxidation reactions under a given

set of conditions. The higher the Eh, the more oxidizing are the conditions

and the greater is the capacity of the medium to accept electrons.

redox potential vs. pH diagram

Plot of redox potential versus hydrogen potential used to display stability

of minerals or solutions in natural systems.


Reverse of oxidation, being equally important in aerated groundwaters as in

surface environments.

regulating storage

The volume of mobile water contained in the temporary zone of saturation,

(cf. reserve).

remedial protection zone

Immediate protected perimeter for pollution control around a water well.

remote sensing

Technique based on data obtained from artificial satellites in Earth orbit.

The periods of the shots and the repetitivity of the information makes it

possible to select the images of most interest. Satellite imagery leads to a

better integration of major fractures (on the scale of several km), but the

lack of relief on these images hinders satisfactory correlation with the

ground truth, and the user is then obliged to fall back on conventional

methods. Where high groundwater discharges are sought, satellite imagery
can prove to be a valuable guide to the hydrogeologist in the selection of borehole


reserve Syn: storage

Quantity of water contained in a hydrological system at a given date or

stored over a period of time, expressed in terms of volume (hm 3 or km 3). Four

types of storage are distinguished: total groundwater storage, regulating

storage, permanent storage and extractable storage. The groundwater storage is

evaluated using the volume of the slice of aquifer in question and either the

effective porosity (in the case of an unconfined aquifer) or the storage

coefficient (for a confined aquifer).

residence time

The average time that formation waters remain in contact with a given volume

of rock, directly linked to the rate of infiltration of water within the massif,
(c.f. convection time, transfer time).


The coefficient r is proportional to the electrical resistance R of a

homogeneous conductor of length l and cross-sectional area s; if the current

density parallel to l is uniform and the temperature is constant, then R = r

l/s. The units of r are ohm.metre. The measurement of resistivity allows an

estimate to be made of the concentation of dissolved salts (or salinity) in a

water sample at a given temperature. It is also possible to determine the

amount of dissolved matter in a water/sediment mixture. Continuous on-line

recordings are commonly carried out with a resistivity meter, often in

combination with a nephelometer for turbidity measurement.


Quantity of water that can be extracted from a defined volume over a given

period of time. Evaluation of the resource is based on the hydrodynamic and

hydrochemical behaviour of the aquifer, the resource is measured in terms of

mean discharge rate (m 3/s, hm 3/yr or km 3/yr).

restoration , regeneration, rehabilitation.

reverse circulation

System used for drilling large-diameter water wells in unconsolidated

formations. In the cleaning of holes with large diameters, the mud can be

injected into the annular space to bring up the cuttings inside the drill


Reynolds number

A dimensionless parameter which represents the ratio between forces of

inertia and forces of viscosity, i.e.: Re=v.d /V, where Re is the Reynolds

number, v is the Darcy velocity, d is the characteristic length and Vr the

kinematic viscosity. Serves to calculate the linear loss in head occurring in

a conduit under an established steady-state regime, assuming that the conduit

is rectilinear, the discharge rate is relatively stable and the diameter of

the conduit is homogeneous. The transition between laminar and turbulent flow

is a function of a large number of parameters, but it is generally accepted

that the laminar flow regime ceases for Reynolds numbers greater than unity,

(c.f. laminar flow, c.f. turbulent flow).

river bank effect

Uptake of pollutants onto particulate matter as waters percolate through the

first few metres of river bank deposits towards the aquifer. Associated with

bacterial activity.

Rochelle salts (cf. additive).

rotary drilling

A tool (drill bit) is attached to the end of a drill pipe string, and the

assembly is driven with a rotary movement at variable speeds and under vertical

compression. The rotary movement is transmitted to the drill string and to the

tool by a motor situated at the well-head. The drill pipes are hollow so that

mud can be injected into the bottom of the borehole.

run off, runoff

Part of the precipitation flowing towards drainage systems into free bodies

of water.

Ryznar's index

Chemical parameter commonly used in corrosion problems, equivalent to 2 pHS

- pH.


Chemical compounds formed by the reaction between an acid and a base, (e.g.

sulphates, chlorides). Ionic species present in water are derived in part from

the dissolution of minerals such as calcite, dolomite, gypsum and halite,

during their residence in the subsurface. Groundwaters take up a certain

number of mineral substances into solution, including limestone, dolomite,

gypsum, sodium chloride and potassium chloride, (cf. water chemistry).

sand bridging

Clustering of grains in the gravel pack due to unidirectional flow, leading to reduced permeability.

sand content

The presence of sand in the mud can be harmful because of its abrasive action.

It also increases the mud density and, where there are significant deposits towards
the bottom, can cause blocking of the drill string; the permitted maximum sand content
in a mud is generally evaluated at 5%. Sand content can be measured with an elutriometer.

sanding up

Influx of sand into a well due to inappropriate design, screen corrosion or

overextraction. Can be treated by air-lift pumping, well scraping, surging,


sanitary zone of well protection , catchment protection zone

cf. protected perimeter.

saturated aquifer formation

An aquifer whose reserve has been fully recharged. A saturated aquifer

formation will bring about dilution of the drilling fluid.

scale Syn: encrustation

scouring Syn: water flush

Process by which well is cleaned by injection of water using compressed air,

(cf. air-lift development).

scraping Syn: brushing

Method of well development making use of wall scratchers (e.g. porcupine



A slotted or perforated liner placed in the well to control sand influx. The

screens should be placed adjacent to the points of greatest water inflow and,

generally speaking, over the entire thickness of the tapped aquifer zone, (cf.

bridge-slot scren, louvre-slot screen, Johnson-type screen).

sediment influx

Movement of sediment particles into the well across the screen face, arises

from screen corrosion, overextraction or poor well design, (cf. sanding-up).

seismic reflection

Geophysical surveying method based on the recording of reflected seismic

waves, whose travel-times depend on the nature and structure of the geological

formations traversed. Although relatively little used in hydrogeology, small-scale
seismic reflection surveys can provide a valuable adjunct to seismic refraction,
covering a range of investigation from a few tens of metres to some hundred
metres depth, (cf. seismic refraction).

seismic refraction

Seismic surveying based on the study of travel-times of fully refracted

waves, depending on the nature and structure of geological formations.

Although seismic refraction suffers from some limitations, its use in

hydrogeology is preferred to seismic reflection because better results are

obtained in the depth range 0-200 m and the implementation is easier (cf.

seismic reflection).


A process in which fine-grained material is washed away from the immediate

vicinity of the well. As a result, the flow velocity in the aquifer increases

near the well, thus leading to increased drawdown and enhanced performance. If

the 90-percentile is greater than 0.25 mm, then it is considered that the

formation can be developed naturally. This phenomenon is also termed


sequestering power

Capacity of a given metal salt to combine with soluble species to form a

complex with covalent properties.

sodium hypochlorite Syn: Javel water.

solubility of gases

Volume of a gas that can be absorbed by a volume of water depending on the

pressure and concentration of gases in the medium. Waters enter into contact

with gases especially in the zone of infiltration (e.g. only certain gases,

such as carbon dioxide, ammonia or hydrogen sulphide, exhibit high

solubilities). Being governed by Henry's law, gas solubility in water

decreases with increasing temperature and increasing salt concentration, (cf.

Henry's law).

solutizing power

Capacity of certain substances to form soluble complex salts, thus

incorporating insoluble species into their structure and promoting solvation.

This property is useful in solubilizing carbonate scale.


Process in which the hydrating power of water (which is a strongly bipolar

molecule) leads to a partial or complete breaking of the various electrostatic

bonds between the atoms and the molecules of the substance entering solution,

thus forming new bonds and structures in the liquid state. Dissolution is said

to occur when solvation is complete, (cf. dissolution).

sonic log, acoustic well logging.

sorting coefficient Abbr: SC

Quotient of the diameters of the 40- and 90-percentile classes on the

cumulative grain-size curve.

specific capacity (of a well)

Discharge rate obtained per unit height of drawdown; it has the dimensions

of surface/time and is denoted as Q/s, (cf. specific drawdown curve).

specific drawdown Symbol: s/Q

Drawdown level measured in the well divided by the pumped discharge; it is

expressed in m/m 3/h, being obtained from the equation of C.E. Jacob.

specific drawdown curve

Plot of specific drawdown versus discharge used to analyse step drawdown

tests. Provides an estimate of aquifer and well losses, in addition to

information on transmissivity and storage coefficients, (cf. specific


specific yield (of an aquifer) Syn: effective porosity

In the case of recharge, the volume of water that fills the available void

space of the porous medium per unit volume; in an unconfined aquifer, the

volume of water drained under the effect of gravity from the porous medium per

unit volume.


Recent satellite that provides an improved image resolution at ground level

(30, 20 or 10 m) and offers the possibility of obtaining stereoscopic views.

As a result, such images can reveal just as must detail as aerial photographs

at a scale of 1:50,000.

square kelly

A square drill pipe.

stabilized foam

Physical mixture of liquid and gaseous components, in which the bubbles are

maintained in stable suspension. The foaming solution is sometimes

supplememented with polymers or bentonite in order to increase its density and

improve its viscosity qualities and also stabilize the borehole walls. The

foam must maintain a certain consistency, comparable to that of shaving foam.

standard electrode potential , equilibrium potential

The potential of a couple in which the left-hand half cell is the standard

hydrogen electrode and the right-hand cell is the electrode system in question

(using 1M solutions at 1 atmosphere H 2 pressure, e.g.: Zn/Zn ++ (1M)//Cu ++

(1M)/Cu). Oxidation takes place at the left-hand half cell, (cf. electrochemical series).

standing water-level , rest water-level, standing level

The level of the free water surface in an unpumped well or the piezometric

level of an unconfined aquifer. It corrseponds to the supposedly stable upper

surface of an undisturbed aquifer. In the case of an unconfined aquifer, the

standing water-level is always found beneath ground level; in the case of a

confined aquifer, the standing water-level remains virtual as long as a

borehole or a piezometer has not attained the aquifer, lying always above the

base of the overlying impermeable layer. It is also incorrectly termed static


static head , hydrostatic head

Pressure exerted at any point in a liquid, represented by the height of the

overlying water column. Equivalent to pressure head plus elevation potential

energy, (cf. hydraulic head).

steady-state regime , equilibrium regime

Regime in which, after a short pumping time, the geometry of the cone of

depression remains constant. In strict terms, the steady-state regime does not

exist except under exceptional conditions. In reality, the appearance of a

quasi-steady state regime is accepted for the purposes of calculation, (cf.

non-steady state regime).

step drawdown test

Used to assess the efficiency and performance of a well, and to measure the

response of performance to varying discharge rate, (cf. pumping test).

Stoke's law

A sphere of radius r moving with velocity v through a large expanse of fluid

of viscosity n will experience an opposing force, or viscous drag, F, such that

F = 6p r n . The equation describes the rate of settling of solids particles in a
suspension or dispersion, but is valid only for restricted conditions (laminar flow
and low Reynolds number). In practice, corrections need to be made for boundary conditions.

storage coefficient

Parameter which characterizes an aquifer. Also known as storativity or bed


subartesian well

A well where water rises from the confined aquifer but does not reach the

ground surface without pumping, (cf. artesian well).

sulphate-reducing bacteria

Strict anaerobes that obtain their source of energy by converting sulphate

to sulphide (e.g. Thiobacillus). These bacteria are ubiquitous in

groundwaters, soils and muds, but can only develop in the absence of oxygen

and at very low Eh values, (cf. iron bacteria).

sulphate reduction

Chemical reaction between an electron-acceptor and sulphate ions to form

sulphide. A secondary phenomenon which can bring about modifications in the

chemical composition of groundwaters and lead to biological clogging; it is

associated with bacterial activity in de-oxygenated aquifers, (cf. sulphate-

reducing bacteria).


Mainly derived from gypsum and anhydrite, as well as from the oxidation of

pyrites. Certain sulphates of magnesium or sodium can also be present in



A surface-active substance used for its detergent properties.

surge block Syn: desanding plunger.

surging Syn: clearance pumping.

suspended particulate matter Abbr: SPM

Sediment particles which do not settle out under normal conditions of flow.

swelling marls

The degree of swelling of marls increases with the alkalinity of the drilling



A swivel enables the bit to rotate on itself with each stroke.


Apart from fulfilling the required physico-chemical criteria, drinking water

should not have any odour or taste at a dilution factor of 2 or 3 .

Theis method , Theis's bi-logarithmic method

Technique for the analysis of pumping tests assuming a fully confined

aquifer and no leakage from or into the aquifer. Used when the pumping time is

short, when the distance between the borehole and the observation well is very

large or in the case of pumping from an unconfined aquifer with a small

drawdown; in such circumstances, the data are difficult to treat with the

Jacob method, (cf. pumping test analysis).


e.g. a sodium polyacrylate base used in a 30% aqueous solution.


Capacity for a mixture of particles in suspension to pass from a solid state

to a liquid state while being agitated and to return to its initial state when

the agitation ceases.

total alkalimetric titre Abbr: TAT, (cf. alkalinity).

total capacity

Maximum flow rate physically obtainable from a water well, being a function

of the local characteristics of the aquifer and the groundwater recharge. Also

known as potential yield or physical yield limit, (cf. water-yield).

total groundwater storage

Quantity of mobile water contained in a volume comprised between the

substratum and the upper boundary of the aquifer. The mean total groundwater

storage is bounded at its top by the mean annual piezometric surface, (cf.


total static head , hydrostatic head, pumping discharge head

Pressure exerted at any point in a liquid, represented by the height of the

overlying water column. It is equivalent to pressure head plus elevation

potential energy. The nature of the pumping equipment will vary according to

the expected discharge and the total static head and remains dependent upon

the diameter of the equipped borehole , (cf. hydraulic head).

trace elements

Chemical elements present in water whose concentration per unit volume does

not exceed a few tens of parts per million (ca. 0.01%), e.g.: Al, Fe, Mn, Ni,

Cr and Pb.

transfer time

The time taken for a tracer to travel from an intake point to a given

discharge in a hydrological system, (cf. convection time).


A measure of the discharge of a water-bearing layer over its entire

thickness per unit width, subject to a unit hydraulic gradient. The discharge

capacity of a well in an aquifer depends on the hydraulic conductivity K and

the thickness e of the aquifer, the discharge capacity of a well can also be

estimated by means of a parameter T, denoting transmissivity, using the

equation: T = Ke, where: T is expressed in m/s, K in m/s and e in m.

Transmissivity should not be confused with hydraulic conductivity, which is

calculated over unit thickness. Long pumping tests are used to calculate the

transmissivity, based on measurements of drawdown and recovery both in the

control well and also in observation wells.


Cloudiness or opacity of fluid media caused by dispersion or absorption of

transmitted light, due to the presence of suspended particulate matter (clay,

silt, organic matter, plankton) or coloured chemical pollutants. The turbidity

of water becomes even greater with increasing amounts of colloid in

suspension; in practice, turbidity is evaluated by means of a nephelometer

which measures the intensity of light diffused laterally by the water sample.

The unit of measurement is the IU (International Unit), which corresponds to 1

mg of formazine per litre.

turbulent flow

Turbulent flow may sometimes occur in the immediate proximity of a well, where

it arises from increased velocity of water circulation , (cf. Reynold's


turbulent well losses Syn: quadratic well losses.

turnover rate

Mean annual recharge of the aquifer, IE, expressed in volume, divided by the

mean total groundwater storage, WM.

turnover time

Theoretical duration needed for the cumulative recharge volume of an aquifer

to become equal to its mean total storage, WM, which is equivalent to the

discharged volume of underground waters over the long term, QW; the turnover

time is expressed in years: WM/IE=WM/QW.

unconfined aquifer

A groundwater body having an upper boundary corresponding to a water table

that is free to fluctuate in response to hydrodynamic factors, (e.g. Champigny

Limestone, Beauce Limestone, Alsace valley alluvium, Lorraine karstic

limestone). The water-bearing formation is not saturated throughout its entire

thickness, (c.f. aquifer, confined aquifer, leaky aquifer).

unconfined groundwater body Syn: unconfined aquifer.

uniform inflow distribution

A regular pattern of flow into a well, usually achieved by installing a

second screen with openings that ensure constant entrance velocity throughout

the tapped aquifer zones, (cf. entrance velocity).

unsaturated zone Syn: vadose zone

Soil-water zone situated between the ground surface and the water table.

Water percolates rapidly downwards through this zone, and is not easily

extractible due to capillary forces, (cf. zone of infiltration).

unstable terrain , running ground

Uncohesive formations with quicksand-like behaviour. Soft or unconsolidated

formations require feed pipes during drilling to prevent collapse.

verticality Syn: verticalness.

video camera inspection Syn: CCTV inspection.


e.g. viscosifier with a sodium acrylamide/acrylate copolymer base, biodegradable viscosifier.

viscosity (of drilling mud)

An appropriate choice of viscosity ensures a clean bit as well as the

efficient return to the surface and rapid settling out of drill cuttings.

Furthermore, it leads to a reduction in pressure-head loss in the drill

string. The viscosity of a mud can be measured with a Marsch viscometer (on

site) or a Stormer viscometer (in the laboratory).

void space volume

Volume of air contained in pores, (cf. effective porosity).

Walton method

A method proposed by B. Walton makes it possible to characterize the

condition of a well in terms of its C value.

water balance Syn: water budget

A measure of the difference between input and output flow rates in natural

hydrological systems. The global water budget is the amount of water involved

in the hydrological cycle each year. The calculation of a water balance

provides a means of checking the consistency of data with respect to the

recharge and flow behaviour within groundwater basins.

water chemistry

Compositional specification of dissolved salts and gases present in water.

The composition of a particular water results from dissolution and chemical

reactions with substances present in the formations traversed by the water.

The concentation of dissolved substances in water may be expressed in terms of

the amount of gases, salts or ionic species per unit volume, (c.f. salts,

dissolved gases).

water colour

The colour of pumped out water is a parameter that can be readily estimated,

It is measured by comparing the tint of the sample with calibrated known

reference solutions. The results are expressed in colorimetric units or in

degrees Hazen.

water cycle Syn: hydrological cycle

Natural cycle in which water evaporating at the Earth's surface - mostly

from the oceans - passes into the atmosphere and falls back as precipitation.

Vast circulation process occurring at the surface of the Earth, including the

movement of water within the soil and subsoil; the origin, storage and flow of

groundwaters is controlled by the functionning of the hydrological cycle,

which can be represented as an equation taking account of the water balance:

P=E+R+I, where: E=evapotranspiration, P=precipitation, R=runoff and


water flush Syn: scouring

Procedure for cleaning a well by water injection.

water hardness , hardness of water

The quantity of calcium and magnesium salts contained in a water sample,

expressed as the equivalent mass of calcium carbonate dissolved in 1 litre.

The hardness of water is due mainly to the presence of calcium and magnesium

salts in the form of bicarbonates, sulphates and chlorides; a water hardness

of 1 corresponds to 10 mg/l on the French scale, 0.7 on the British scale.

water inflow Syn: influx

Movement of water from tapped aquifers into a well.

water-level recorder , gauge

e.g. water-level dippers, pressure transducer system.

water-producing zone Syn: tapped aquifer zone

Section of an aquifer that can be tapped by a well.

water quality

Standards set by regulating authorities for the control of physical,

chemical and microbiological pollution in the drinking water supply (e.g.

colour, turbidity, taste, physico-chemical properties, undesirable substances,

toxic substances, bacteriology, pesticides). For each listed parameter, water

operators must ensure that the maximum permissible dose or level is not


water-retention agent

Product used to control loss of circulation.

water-yield Syn: capacity

Flow rate of a well. Various types are defined, including safe yield,

potential yield and economic yield, (cf. total capacity).

weighting up

Weighting up of mud is achieved by adding barite.

well development

Operation designed to repair damage to the well face and aquifer matrix

caused by drilling. Mainly aimed at removing cake from well walls and

improving the transmissivity of the near-field well environment in order to

restore the well's performance, (cf. aquifer development).

well effect

Changes in permeability due to the influence of drilling mud. Becomes

apparent during phases of pumping. It is commonly observed in oil-wells, which

are almost always drilled with a rotary system and show productive capacities

that are relatively low. In the oil industry, it is customary to refer to this

phenomenon as the "skin effect."

well field

Group of wells designed for the abstaction of groundwaters from an aquifer.

well field management zone

Far-field protected perimeter for pollution control around a well or group of wells

well head

The well head should be built with great care since this determines the

staunchness of the catchment works, which represents a very important factor

in avoiding accidental pollution by surface waters. The well head should be

sufficiently ventilated so as to avoid condensation, which could also be a

source of pollution.

well losses

e.g. linear and quadratic.

yield-depression curve

Plot of well discharge versus drawdown obtained from pumping tests.

zone of infiltration

Unsaturated zone containing some air in its void space volume. It is located

between the water table and the ground surface, or up to the base of the

overlying aquitard where this exists, (cf. unsaturated zone).

zone of water-table fluctuation Syn: temporary zone of saturation

Belt of oscillation of the free upper surface of a groundwater body.