Advective fluid flow: The flow of fluids through a porous medium; in this case only the fluids move. Advective flow via aquifers is the most efficient mechanism for mass transfer of dissolved solids in the shallow crust. cf. convective flow, groundwater flow.
Aerobic conditions: Reactions that directly utilise available oxygen, the most obvious being respiration in life forms, where oxygen is used in metabolic reactions to generate energy (e.g., from food). In groundwater and in sediments this generally is associated with the metabolic activity of microbes – a distinction is made between these types of reaction and oxidation reactions that do not require intermediary metabolic activity in life forms.
Air sparging: A method of groundwater remediation that uses air forced down a borehole into an aquifer, to volatilize hydrocarbon contaminants. The produced vapour phase is extracted and scrubbed to remove the offending compounds.
Angle of internal friction: A rock or material property that refers to its ability to resist deformation, and is measured as the angle between the normal stress and a resultant stress at the point where shear begins. It is an essential parameter in the quantification of rock deformation. Cf. angle of repose.
Anisotropy: An aquifer or aquitard is considered anisotropic if its permeability or hydraulic conductivity is not the same in all directions; usually specified along three principal orthogonal axes. Most porous aquifer media are anisotropic because of sedimentary bedding, sedimentary structures like crossbedding, fracture and joint networks, or tectonically induced structures like cleavage, folds or faults. Cf. isotropy.
Anoxic conditions: Usually applied to aqueous environments (water masses as well as connate water) where there is none, or insufficient dissolved oxygen for respiration; usually measured at less than 0.5 ml/L. Under these conditions, the sources of oxygen via bacterial reduction are from nitrates and sulphates. Once these sources are depleted carbon dioxide becomes an important source during reduction to methane. Deep waters in lakes where there is no turnover of the water mass, can become anoxic.
Aquiclude: An aquiclude prevents any kind of groundwater flow. Examples include granite-like lithologies, and thick sequences of halite (although even these lithologies have permeability, albeit extremely low. Other aquicludes involve artificial barriers designed to prevent or deflect contaminated groundwater flow.
Aquifer: A porous and permeable medium beneath the surface that permits groundwater flow. In hydrogeology, the definition has a very pragmatic value, where the amount of groundwater flow is usable (as in extraction); everything else is an aquitard.
Aquifer – confined: This term applies to aquifers that are bound above, below, and laterally by aquitards. Confined aquifers are always saturated. Their hydraulic potential is defined by a potentiometric surface.
Aquifer mining: Excess removal of groundwater from a confined aquifer will cause irreversible changes to the structure of the porous medium (commonly sand grains), causing the grains to pack more densely. Not only does this reduce porosity, permeability and therefore water production, it also causes a reduction in the solid volume of the aquifer. Excessive mining can eventually cause land subsidence.
Aquifer – unconfined: The upper boundary of unconfined aquifers is at Earth’s surface. They contain a watertable, above which is an unsaturated zone where pore spaces are air-filled at atmospheric pressures, and a saturated zone below. Drainage of an unconfined aquifer is by gravity alone. Common examples include fluvial and alluvial gravels and sands.
Aquitard: Any rock or sediment unit that retards groundwater flow. Common examples include mudstones and other mud-prone lithologies such as glacial diamictites. An important property of aquitards is their ability to release water by vertical seepage to confined aquifers.
Barometric efficiency: The response of a water level or potentiometric surface in a confined aquifer to changes in atmospheric pressure. Such changes reflect the elastic response to stress on the aquifer – the changes are reversible. The term efficiency refers to how well the aquifer responds – in part depending on the rigidity or compressibility of its framework. An incompressible framework will have a greater barometric response than a compressible medium. The response can also be influenced by tidal effects in wells close to shore.
Baseflow: (Hydrogeology) Baseflow is the subsurface discharge to streams from the watertable. The amount of discharge depends on the hydraulic gradient of the watertable with respect to the stream surface. During dry periods, baseflow may be the only source of water to maintain stream flow.
Bernoulli equation: Named after Daniel Bernoulli who in 1738 expressed the conservation of energy in a flowing fluid as:
Total energy E = ½ ρv2 + ρgz + P
Where ρ = fluid density, v = velocity, g = gravity constant, z = elevation with respect to a datum, P = fluid pressure.
The first term ½ ρv2 is kinetic energy; the term ρgh is potential energy; P is fluid pressure, or force per unit area. Because groundwater generally moves very slowly, the kinetic energy term is ignored. The equation allows us to express the potential energy, or hydraulic potential for groundwater flow, commonly referred to as total hydraulic head, in terms of two components – a pressure head, and an elevation head, relative to a datum. Thus hydraulic head can be expressed in terms of some height, or elevation (e.g. metres, feet etc.).
Bioremediation: The use of living organisms to help clean up contaminated sites or aquifers, primarily using naturally available or introduced microbes. For example, certain bacteria will break oil down into manageable compounds like carbon dioxide or methane.
Boundary conditions: Boundary conditions, whether physical or numerical, are applied deliberately and systematically to models because they constrain the model to a limited number of process variables. Groundwater modeling is invariable numerical – commonly used boundary conditions include specifying head values for certain parts of the model (e.g., constant head values that do not vary during the model iterative process), boundaries that permit flow or prevent flow (no flow boundary), or relating the model grid limits to natural boundaries such as drainage divides, lake or marine coastlines.
Brines: Generally used for natural waters more saline than seawater. The main dissolved salt is sodium chloride (NaCl), but calcium and magnesium sulphates are also important constituents, and there are several important trace elements, such as lithium. The primary mechanism for brine concentration in ocean basins and saline lakes is evaporation. The saturation level for NaCl is about 357 ppt (normal seawater is 32 ppt).
Capillary zone: In hydrogeology, also called the capillary fringe. It is a relatively narrow interval above the watertable where surface tension forces on aquifer materials cause water to rise and partly fill pore spaces. The capillary fringe is part of the unsaturated, or vadose zone.
Casing (borehole/well): PVC or metal tubes that are pushed into a newly-drilled borehole, in part to prevent collapse of sediment or bedrock into the well, but also to help secure borehole tools, pumps, and screens. In very deep wells, particularly oil and gas wells, the casing diameter is greatest at the top of the well, decreasing with depth.
Chemical facies: (hydrogeology) This is a useful concept to demonstrate the chemistry of groundwater in relation to aquifer rock-sediment composition, and the evolution of groundwater chemistry as it flows from one rock type to another. For example, flow from sandstone to limestone aquifers will be accompanied by a change in HCO3– and pH, plus the concentrations of cations like calcium and magnesium.
Clints: Fracture networks in limestones formed by surface (meteoric) dissolution. They are common karst landscapes and occur sympathetically with grykes.
Cohesion (Cohesive strength): is a function of material density, the acceleration due to gravity, and a length dimension. It has the units of stress and dimensions ML-1T-2. A dry sand has very low cohesive strength, wet sand a bit more, and muddy sand significantly greater cohesion. The degree of cohesion has a strong influence over kinematic behaviour. It is an important factor in deformation involving slope failure where fluid flow reduces cohesive and shear strength of soils and stratified rock.
Columnar jointing: Regular arrays of joints formed during cooling and contraction of magma. They can occur in lava and hot ignimbrite flows, and intrusive dykes and sills. Cooling begins from the outer surfaces and progresses towards the centre of the magma body where joints are oriented normal to the outer surface. They form as straight to slightly curved columns with 4 to 8 sided polygonal cross-sections. The joints can act to focus fluid flow.
Compaction: The process where sediment particles, once deposited, are pushed closer together to form a more tightly knit framework. Compaction begins almost immediately following deposition and continues during sediment burial. The normal compressive stress in this case is applied by the overlying sediment. Because porosity is also reduced, an additional requirement for compaction to take place is the release of interstitial water through aquifers. If fluid cannot escape (for example because of permeability barriers) then the rock body will not compact, and internal fluid pressures will rise – this is called overpressure. Mudrocks can compact to less than a tenth their depositional thickness. More rigid frameworks like sandstones compact far less.
Compaction driven flow: Fluid released by compaction of saturated sediment will flow to regions of lower fluid pressure. In most parts of the crust this will have a strong updip component of flow, depending on potentially competing or additive forces driven by topography and thermal effects.
Component: In the context of Gibb’s Phase Rule, a component is the minimum number of chemicals (aqueous ions, molecules) that combined will make up the chemical species in the equilibrium system under consideration. For example, the equilibria CaCO3(s) = Ca2+ + CO32- has three species but two components – either CaCO3(s) and Ca2+ or CaCO3(s) and CO32-. In each case the third species can be made from combinations of the other two. In this example there are two phases.
Compressibility: The ability of a fluid or rock to change its volume in concert with changing stress, for example changing lithostatic pressures during sediment burial. It is usually expressed as the ratio of relative volume change (V) with pressure (P):
β = 1/V. (δV/δP)
Water has very low compressibility – at 6000 psi (41.4 MPa) (equivalent to 3.2 km water depth) the change in volume is 1.8%. Mudstone is highly compressible; halite is not. Compression results in a loss of porosity and permeability.
Conduction: This is a diffusive process where heat is transferred via molecular vibrations. Conduction does not involve the transfer of mass, cf. convection, advection. It is a less efficient mechanism of heat transfer than convection.
Confined aquifer: see Aquifer-confined.
Confining pressure: For a body of rock at any depth, confining pressure is the combination of hydrostatic and lithostatic pressures. An increase in confining pressure results in an increase in rock strength – the stress required to deform the rock. For normal stress conditions, the confining pressure axes in the stress ellipsoid are σ2 and σ3 (σ2 = σ3).
Constant head boundary: The condition imposed on flow models or flow nets where the hydraulic head is specified and invariant. It is an important condition applied to many groundwater flow models. Some models, like MODFLOW require at least one constant head value to be assigned somewhere in the model grid.
Contaminant: A chemical or substance that we would rather not be present in our environment, food, air, etc., but is present because of either natural occurrences and processes, or human-induced processes. For example, heavy metals like lead, mercury and arsenic can occur naturally concentrated in ore bodies, and released by natural weathering, or by mining, into local surface and groundwaters. Cf. pollutant.
Convection: The flow of fluids en masse resulting from temperature and buoyancy gradients. Convection is the primary mechanism for transferring heat from Earth’s mantle to the lithosphere. Cf. conduction, advection.
Coulomb failure law: Charles Coulomb (1736-1806) discovered his eponymous Law via experiments on friction. The Law is a mathematical statement that defines the height and slope of the failure envelope – the height gives the cohesive strength of a material where the linear envelope intersects the shear stress axis (y axis), and the slope of the envelope gives the internal friction angle: τ = C0 + Tanφ σN where τ is the shear stress, C0 is cohesive strength, and φ the angle of internal friction. Tanφ is the friction coefficient. The Mohr circle expresses this relationship graphically.
Darcy: The unit of permeability normally used by the hydrocarbon industry. Its dimensions are L2. It is defined as (from the Schlumberger Glossary) the flow of one cubic centimeter of fluid having one centipoise of viscosity flowing in one second under a pressure differential of one atmosphere where the porous medium has a cross-sectional area of one square centimeter and a length of one centimeter. Rock permeabilities Are usually quotes as millidarcies (mD). This measure cannot be compared directly with hydraulic conductivity that has dimensions L/T.
Darcy’s Law: Henri Darcy is credited with discovering experimentally the two important relationships:
- Groundwater flux Q is proportional to the difference in hydraulic head between two boreholes (h1 and h2) (he used manometers in his experiments). Thus, Q a h1 – h2, and
- Q is inversely proportional to the distance between the boreholes (L), or Q a 1/L
Q is also proportional to the cross-section area of flow (A). Thus, we can rewrite the two proportionalities, adding a proportionality constant k:
Q = -kA (h1 – h2)/L This is Darcy’s law.
(h1 – h2)/L is the hydraulic gradient. The proportionality constant k is the hydraulic conductivity. The negative sign indicates flow towards lower hydraulic heads.
Darcy velocity: In mathematical terms, hydraulic conductivity is expressed as a velocity, also known as the Darcy velocity. An approximation of true velocity that takes the tortuosity of the porous medium into account is expressed as k/Φ eff – i.e., the hydraulic conductivity divided by effective porosity.
Datum: A point of level somewhere on or in Earth from which measurements of elevation are referenced. The most commonly used datum is sea level (zero elevation), but local datums like lake levels are useful. In detail, sea level can be a problem because it is a dynamic surface – tides, changes globally because of gravitational potential, but mean sea level at a specific location, taken at a specific time has been used for UTM type maps and grids. It is also called base level.
Dewatering: This is the process where interstitial fluids are ‘squeezed’ from sediment during compaction, as sedimentary grains become more closely packed. The process of dewatering increases fluid pressures and promotes fluid flow in aquifer-like deposits. Fluid escape my be diffuse, or focused through narrow pipes and sheets. It is an important stage of mechanical diagenesis, but it also contributes to chemical diagenesis by transferring dissolved mass from one part of the sedimentary column to another. Cf. liquefaction, fluidization, fluid escape structures.
Diffusion: A mechanism for mass transfer, or dispersion of solute beyond that moved by advection. It is the form of dispersion involving molecular transfer and is dependent on the concentration gradient of the dissolved species. The direction of diffusion is from high to low concentrations. The fundamental equation describing diffusion is Fick’s Law – F = -D dC/dx where F = mass flux /unit area/unit time (M/L2/T), D = a proportionality constant called the diffusion coefficient (L2/T), C = solute concentration (M/L3), and dC/dx = concentration gradient (M/L3/L). Diffusion becomes an increasingly important mass transport process at increasing depth in Earth’s crust. cf. mechanical dispersion.
Discharge: See Groundwater discharge.
Discharge lake: A body of water that is maintained by groundwater discharge – there is no discharge to groundwater, but there is likely discharge via evapotranspiration. Many playa lakes are of this type.
Dispersion: In geofluids this is the process where dissolved and insoluble compounds move from their source or point of origin; observed in groundwater flow, diagenesis, and metamorphism. In these contexts there are two primary mechanisms – mechanical dispersion, and molecular diffusion.
Dissequilibrium compaction: Under normal conditions of compaction, fluid that is driven from pore spaces escapes without a significant increase in pore pressure – i.e. hydrostatic conditions prevail. However, rapid deposition of low permeability deposits can impede fluid flow and under these conditions pore pressures increase; this process is called disequilibrium compaction. In many basins, this occurs at about 3km burial depths. Disequilibrium compaction is enhanced by cementation and tectonic compression.
Distributed conduit: Fault zones that contain more than one major fracture plane. Distributed conduits potentially have greater permeability than single fault planes, providing additional pathways for fluid flow.
Drawdown: The measured depth that the water level is drawn in a pumping well or observation well. The amount of drawdown depends on well capacity, pumping rate, and the response of the aquifer and its hydraulic properties during pumping and is usually plotted as a time-drawdown curve.
Drawdown cone: Drawdown in a pumping well (watertable or potentiometric surface in a confined aquifer) produces a 3-dimensional cone-like depression in water levels. The dimensions of the cone can be determined from adjacent observation well water level measurements. In an isotropic and homogenous aquifer the map view of the cone will approximate a circle; for anisotropic aquifers it will be more elliptical.
Effective porosity: The component of porosity that permits significant flow. Microporosity (intergranular, intercrystalline) is commonly excluded from this porosity value.
Effluent streams: (Gaining stream) Streams that gain water from the local groundwater system. In this situation, a river level coincides with the watertable such that variations in watertable level will directly affect stream levels. This type is most common in temperate and tropical regions. Cf. Influent streams.
Elastic behaviour: This rheological behaviour describes materials that respond to stress by deforming but can return to their original state when the stress is removed. The principle was developed by Robert Hooke– Hookes Law (1660); the classic physics experiment involves a spring. The principle can also be applied to most sediments and rocks. The level of stress at which deformation becomes irreversible is called the elastic limit. Confined aquifers that have been pumped excessively begin to deform irreversibly when granular materials pack closer together – this process is called aquifer mining.
Electroneutrality: Used to check the integrity of groundwater chemical analyses of ionic species. The concentrations of cations and anions in a sample are converted to milliequivalents/litre (meq); The sum of the cation meq values should be within 1-2% of the sum of the anion values.
Elevation head: see hydraulic head
Equilibrium constant: For a specific reaction, equilibrium constants are the ratio of product activities (or concentrations) divided by reactant activities; they can be determined experimentally (assuming a reaction is at equilibrium) or using thermodynamic considerations (where activities must be used). The general expression for a reaction involving ionic species in solution is: aA + bB ↔ cC + dD, where a, b, c, and d are the stoichiometric values for each ion (e.g. 2H+).
K = cC + dD/ aA + bB at equilibrium.
In a real aqueous solution, we can determine whether a reaction will proceed to the left or right: if cC + dD/ aA + bB is <K the reactants will convert to products (the reaction goes to the right. The opposite occurs if cC + dD/ aA + bB >K. K is strongly dependent on temperature and pressure.
Equipotential: In hydrogeology, a line or plane of equal hydraulic head on a potentiometric surface, or on a hydrogeological cross-section. Equipotentials are determined primarily from well water level data. Equipotential contours allow interpolation of water levels at any point on the potentiometric surface.
Evaporative pumping: In arid regions, intense evaporation at the surface creates a hydraulic gradient in shallow subsurface aquifers, inducing lateral groundwater and/or seawater flow to replace lost fluid. Vertical capillary flow through the unsaturated zone (above the watertable) transfers these saline fluids from the aquifer to the surface.
Evapotranspiration: The transfer of water from Earth’s surface to the atmosphere takes place primarily via direct evaporation from water bodies, soils, and the groundwater capillary zone (above the watertable), plus transpiration of water vapour through plants. It is one of the most important mechanisms of groundwater discharge and needs to be accounted for in mass balance estimates and groundwater flow models. Regions of peak discharge are commonly vegetated by phreatophytic plants.
Fault breccia: Angular blocks of bedrock produced by crushing and grinding during faulting. A distinction is sometimes made between a breccia made up of clasts >1 mm and <0.5 m, and megabreccia with clasts >0.5 m. An important difference among fault breccia, gouge, and cataclastite is the high degree of induration in the latter. Cf. cataclastite, gouge.
Fault conduit: The open, dilational part of a fault between fracture planes. Conduit width, or aperture, is measured normal to fracture surfaces. The width can vary considerably along the length of a fault. Fault conduits provide access for fluid flow.
Fault core: In hydrogeology, this is the primary zone along the fault plane, and can be presented as an open conduit, a zone of fractured rock and gouge, or a zone of mud-shale lithologies that have been smeared along the fault plane during fault shear. The permeability of the core will depend on the relative proportions of these attributes.
Fault damage zone: The zone either side of the fault plane or fault core that where the host rock is damaged by fracturing and cataclasis. The degree of damage decreases with increasing distance from the core. The intensity of deformation depends primarily on the magnitude of fault displacement.
Fault gouge: Very fine-grained (silt-clay size) material formed by intense shear of rock and sediment during faulting., generally <0.1 mm. Cf. fault breccia.
Fault permeability: The permeability along the plane of the fault, primarily through the fault conduit and damage zone, and normal to a fault plane. Faults in this context provide a focus or barrier to fluid flow.
Flow lines: Lines indicating (usually inferred) fluid flow directions, added to potentiometric and watertable maps. The lines are drawn at right angles to hydraulic head contours, assuming isotropic conditions.
Flow net: A 2D cross-section or 3D model of equipotential lines or planes that describe aquifers and their associated aquitards. It is basically a representation of hydraulic potential. Flow lines can be constructed based on assessed hydraulic gradients, to show the directions of groundwater flow.
Flow-through lake: A more or less permanent water body that receives water from the local groundwater system, and releases water to the same groundwater system. In this system, lake levels reach an approximate steady state.
Fluid pressure: The pressure within a fluid (liquid and gas phases), usually expressed as a compressive stress – in its simplest form: P = ρgz
where P is the pressure of interstitial fluids at some depth measured vertically, ρ is the density of the fluid, g = the gravitation constant, and z the depth from the surface to the point of interest. Fluid pressures generally increase with depth in the crust. Cf. hydrostatic pressure, lithostatic pressure.
Fluidization: The process where sedimentary particles are suspended, or float in the interstitial fluid by the upward flow of fluid. In contrast, the fluid in a liquefied sediment is largely static. Fluidization in sediment may be caused by escaping, overpressured fluids (dewatering).
Flux melting: A term derived from welding and glass making. A flux is a substance that lowers the melting point of solids. It applies to magma generation in the mantle where water, derived by dehydration of mica, glaucophane, and serpentinite minerals, lowers melting points by 200°C and more. Flux melting is a critical stage in the formation of partial melts.
Force: Force causes objects to move, move faster (accelerate), or stop moving. Force cause rock deformation, tectonic plates to move – all real-world actions and reactions. Gravity is an ever-present force. The classical Newtonian expression of force (F) is F = m.a where m is mass, and a acceleration. The standard SI unit is the Newton (N) – the force required to accelerate a mass of one kilogram by one metre/second/second (Kg.m.s-2); the dimensions of N are MLT-2. NB. Stress ≠ Force.
Fracture networks: In hydrogeology this refers to the three-dimensional array of joints and faults for which there is interconnected permeability.
Fracture porosity: The pore space permitting fluid flow through rock fractures and joints. Fracture and joint networks are oriented according to ancient stress fields, hence the porosity will also be focused at these orientations. It tends to occur in hard rock. In crystalline or volcanic rock (the latter includes columnar joints) it is the only effective porosity.
Fumaroles: Also known as Solfataras. Geothermal gas and steam vents where temperatures are >/= 100°C. The proportion of liquid water is low. They tend to form when the watertable is deep. , Hot springs are more common where watertables. are shallow.
Gaining streams: See Effluent streams.
Geofluids: Below the watertable (local or regional) all sediment and rock is saturated with fluid – aqueous, or non-aqueous. Geofluids include:
- Fresh and saline water (aqueous fluids in aquifers and aquitards) and hydrocarbons (oil and gas).
- Depth of flow ranges from near surface to the deepest parts of the crust.
- Rates of fluid flow rates range from cm/second near the surface, to cm/million years deep in the crust.
- Aqueous fluids are involved in all chemical reactions and distribute dissolved mass through the crust, including those that form rocks, hydrocarbons, and ore deposits.
- Fluids play an important role in how the earth deforms by reducing shear strength and elevating fluid pressures.
- Hydrous igneous melts have lower melting points.
Geostatic pressure: An alternative term for lithostatic pressure.
Ghyben-Herzberg equation: (hydrogeology)
z = h.ρf / ρs – ρf
where z is the depth to the interface from sea level, h the watertable elevation, ρs (1.025 gm/cc) and ρf (1 gm/cc) the densities of seawater and freshwater respectively, such that:
z = 40h
This relationship is an important approximation of the interface between freshwater and seawater in coastal aquifers. The equation states that for every unit decrease or increase in watertable depth (h) there will be a corresponding 40 unit rise or fall respectively in the interface between seawater and freshwater. In practice, it provides a reasonable approximation of potential seawater intrusion into coastal aquifers that have been over-produced.
Gibb’s Phase Rule: To solve problems of chemical equilibrium we need to know what variables are involved, those for which we have data and those we do not (unknowns), plus parameters like solubility constants and species activities. Solutions are found by solving simultaneous equations that express known and unknown variables. Gibbs Phase Rule is one way to formulate the problem. It is written as: F = C – P + 2. F is the degrees of freedom, which indicates the number of independent constraints needed to define the system in question (such as temperature, pressure, activity, concentration, electrical neutrality); C is the number of components, and P the number of phases. If F = 2 then two simultaneous equations are required to solve for the two unknown constraints.
Groundwater: Water that resides in porous and permeable sediment and rock beneath the surface. The term applies equally to fresh and saline waters, in aquifers and aquitards at any depth in the crust. The term does not apply to chemically bound water, although such water may be released to groundwater during diagenesis and metamorphism. See also aquifer, hydraulic gradient.
Groundwater discharge: Natural discharge of groundwater as springs, seeps, or baseflow, at the surface or in streams, lakes, or the sea floor. Discharge occurs where the watertable (unconfined aquifers) or potentiometric surface (confined aquifers) intersect the land or water body surface, and the hydraulic gradient is sufficient to drive flow. Cf. Groundwater recharge.
Groundwater flow systems: A concept stated by J Tóth (1962; PDF available) where small scale local systems where recharge occurs at a topographic high and discharge in the adjacent topographic low; intermediate systems that has several local topographic highs and lows between its main areas of recharge-discharge; regional systems that are recharged at major highs and discharged at major lows. The smaller-scale flow systems are nested within the larger systems. The concept is important for illustrating the connectivity of groundwater at different depths and ages. It is usually portrayed on 2D cross-sections.
Groundwater recharge: The infiltration of water from precipitation into an aquifer. For unconfined aquifers this recharge occurs at the watertable. For confined aquifers recharge occurs by slow seepage from the confining aquitards.
Groundwater residence time: The time from recharge (usually at the surface) to discharge. Residence times are briefest in unconfined aquifers, ranging from days to years. In regional groundwater flow systems these times are measured in 105 to 106 years. Groundwater dating utilises trace compounds such as fluorocarbons, isotopes like ³H (tritium from atmospheric atomic device testing), and cosmogenic isotopes such as Carbon-14, and Beryllium-10.
Grout: Grouting is used to seal sections of a well-borehole to prevent potentially contaminated water from entering. Cement and bentonite pellets are commonly used (bentonite is a clay that swells as it absorbs water). The top of a borehole is commonly grouted to prevent surface contamination from entering. Grouting may also be used to isolate certain sections of a borehole that are being used to sample groundwater (i.e., above and below the sample interval).
Grykes: Elevated blocks of limestone bound by fracture networks, or clints. They are common in karst landscapes.
Halophytes: In arid and semi-arid regions where evapotranspiration is high, the local groundwater can become increasingly saline – the kinds of phreatophytic plants that thrive there are called halophytes. See phreatophytes.
Heat flow: The transfer of heat from Earth’s core and deep mantle to the surface, primarily by conduction and convection. It is expressed as milli-Watts per square metre (mWm-2).
Heterogeneity: Homogeneity is defined as the dissimilarity of properties from point to point and in the same direction. It is the most likely condition in real world rock and sediment.
Homogeneity: Unlike isotropy that considers the similarity of properties in any direction from a point, homogeneity is defined as the similarity of properties from point to point and in the same direction. It is generally viewed as an ideal condition, commonly assumed to simplify groundwater model properties.
Hooke’s Law: First stated by Robert Hooke in 1660 then published in 1678, his experiments with springs showed that the amount of extension is proportional to the applied stress – so twice the extension requires twice the stress. This converse of this statement, that recovery of extension is also proportional to the reduced level of applied stress. Thus, Hooke’s experiments were the first to demonstrate quantitatively the elastic properties of materials. Elasticity is a fundamental property of many geological materials, particularly during deformation.
Hydraulic conductivity (hydrogeology): This is the proportionality constant in Darcy’s Law. It has dimensions of length/time. Hence it is also called the Darcian velocity. It is a measure of the ease with which a fluid will flow through a porous medium. Importantly, it is a function of the porous medium and the fluid, particularly the fluid viscosity. This means that oil flowing through an aquifer will have a lower hydraulic conductivity than water through the same medium. Hydraulic conductivity is used in all hydrogeological studies. In contrast, the oil and gas industry uses a different proportionality constant – the Darcy, that depends only on the porous medium.
Hydraulic fracturing: Fracturing induced by elevated fluid pressures, such that effective stress decreases to the point where the minimum principal stress is equal to or greater than the tensile strength (also measured as stress). Fractures formed in this way are commonly filled with mineral precipitates from the mobile fluid. The principle is employed to artificially induce fracturing in tight oil-gas bearing rocks – or fracking (see this USGS site).
Hydraulic gradient (hydrogeology): The change in hydraulic head from one location to another can be stated as a gradient, which is the head difference divided by the distance between the two locations. Gradients can also be calculated from contoured potentiometric surface maps. Groundwater always flows towards locations at lower head.
Hydraulic head (hydrogeology): Also called hydraulic potential, is a measure of the potential energy available to drive groundwater flow. From Bernoulli’s equation, the total head is:
HTotal = h (the elevation head) + P (pressure head)/ρg
For which the dimensions are in units of length, or height/depth measured to some datum. The total head is the same anywhere along a line of equal potential (equipotential); however, the elevation and pressure head components change.
Hydraulic head – elevation head (hydrogeology): If the point of measurement is the bottom of a borehole, then the elevation head is the depth from this point to the datum. It is a component of the total head measured at that point; the other component is the pressure head. The point of measurement can be anywhere along the line of the borehole. In most cases, this line will represent an equipotential. For example, if the point of measurement was the watertable, then total head would be made up entirely of the elevation head; the pressure head would be zero.
Hydraulic head – pressure head (hydrogeology): If the point of measurement is the bottom of a borehole, then the pressure head is the depth from this point to the watertable or other equipotential surface. It is a component of the total head measured at that point; the other component is the elevation head. The point of measurement can be anywhere along the line of the borehole. For example, if the point of measurement was the watertable, then total head would be made up entirely of the elevation head; the pressure head would be zero.
Hydraulic potential (hydrogeology): The statement of hydraulic potential derived from Bernoulli’s equation is a statement about the potential energy that drives groundwater flow. Mathematically this simplifies to potential energy E = ρgz + fluid pressure P (ignoring the kinetic energy component), where ρ = fluid density; g = gravity constant; z = depth relative to a datum. The more common expression for this is hydraulic head.
Hydraulic testing: A general description of the tests and procedures used in the field, principally in boreholes, including piezometer nests and dig pits, to determine the hydraulic properties of aquifers, groundwater flow and velocity, evaluation of groundwater supply, tracing and remediation of groundwater contaminants.
Hydraulics: A general term for the conditions promoting flow in water, air, and sediment-water mixtures, and the processes of sediment movement and deposition. Involves consideration of flow velocity, turbulence, laminar flow, frictional drag, and shear stress.
Hydrogeological cross section: A 2-dimensional view of aquifer-aquitard systems, similar to stratigraphic cross sections except the fundamental units are defined primarily by porosity and permeability. Sections are usually oriented in the direction of flow. Hydraulic head data from boreholes and piezometers can be added if the cross section is drawn using local elevations. Other data like chemical compositions (natural and contaminant) can also be mapped in this way.
Hydrogeology: The study of subsurface fluids, particularly groundwater and its utilization,, aquifers and aquitards, fluid chemistry, its influence on rock strength and slope stability, its role in tectonics, hydrocarbon migration and trapping, and mineralization.
Hydrologic cycle: The movement and transfer of water through the solid Earth (surface and subsurface), atmosphere, and oceans. It is a complex system of transfer and feedback processes that involves groundwater and geofluids at all depths. Time scales for water transfer in the subsurface are usually orders of magnitude smaller than surface and atmospheric transfer (e.g. rivers, evaporation, precipitation, jet streams).
Hydroperiod: The duration of tidal flooding and inundation over a salt marsh – flooding only occurs during spring tides and storm surges.
Hydrostatic pressure: At any depth, the pressure exerted by a (theoretical) overlying column of water having unit-area cross-section, is calculated from the expression P = ρgz where ρ = density of water, g = gravity constant, and z = depth from some datum, commonly sea level. Note that, assuming a cross-section of unit-area reduces volume to units of depth. It is analogous to lithostatic pressure.
Hydrostratigraphy: The stratigraphy of stratified and non-stratified rock and sediment based on hydraulic parameters, particularly porosity and permeability. In this context, there is no necessary correspondence with formal lithostratigraphy (e.g., formations) or sequence stratigraphy.
Hypersaline: Having salinity greater than seawater (>35 parts/1000). Modern hypersaline environments are most common between the tropics but are found in such diverse places as the Antarctic dry valleys. Plant and animal life require specialized adaptations to survive these conditions. Prolonged hypersalinity may result in evaporite deposits in lakes and seas.
Influent stream: (Losing stream) Describes the relationship between groundwater and river levels. Such rivers ‘lose’ water to the local watertable. It is an aquifer recharge process. This kind of stream is most common in arid and semi-arid regions.
Isotropy: An aquifer or aquitard is considered isotropic if its permeability or hydraulic conductivity is the same in all directions, usually specified by three principal orthogonal axes. Isotropy is often assumed in groundwater modelling as a reasonable simplification. In reality, most porous media are anisotropic.
Joints: Open fractures in hard rock formed by extension. Joints lack displacement – cf faults. Joints commonly occur in three dimensional networks. Joints can form during faulting, folding, or by extension during unloading of the crust, for example during erosion, or melting of ice sheets. Open fractures provide pathways for subsurface fluid flow.
Karst: A landscape of gullies, canyons, and steep-sided pinnacles resulting from intense meteoric diagenesis (dissolution) of thick limestones. The relief on karst landforms ranges from 1-2 m to 100s of metres. The corresponding subterranean structures include sinkholes, caverns and underground streams.
Losing stream: See Influent stream.
Liquefaction: If water-saturated sediment is disturbed, for example by earthquake ground shaking, the grains begin to separate until they are ‘floating’ in the interstitial water. At this point, the fluid now consists not only of water but also the floating grains and a consequence of this is that fluid pressures increase. The sand is now liquefied. It no longer has shear strength and cannot support surface loads. Eventually the grains will settle and at this point the excess water will escape to the surface. Cf. dewatering, fluidization, sand volcanoes.
Lithostatic pressure: At any depth, the pressure exerted by the overlying column of rock and sediment having unit-area cross-section, is calculated from the expression P = ρgh where ρ = density of the rock column, g = gravity constant, and h = depth from some datum, commonly sea level. Note that, assuming a cross-section of unit-area reduces volume to units of depth. Also called overburden pressure. It is analogous to hydrostatic pressure.
Mechanical dispersion: A process of mass transfer involving the mechanical mixing of different fluids (e.g. contaminated and fresh water). It is caused by local variations in fluid velocity around grains and through fractures. Longitudinal dispersion is mixing along a flow line; lateral dispersion is mixing normal to flow lines. Both types of mixing extend the dissolved solute beyond the limits of advective flow.
Meteoric diagenesis (carbonates): Diagenesis of limestone under fresh-water conditions, both in the vadose (unsaturated) zone, and below the watertable. It is largely controlled by the degree of fresh- water seepage and groundwater flow. Vadose zone diagenesis is dominated by dissolution that, if prolonged, produces caverns, sinkholes (dolines), subterranean streams, and spectacular karst landforms. Dissolved calcium carbonate may reprecipitate as cement and fracture-fill in the saturated zone, and as stalactites-stalagmites in caves.
Meteoric flow: Subsurface flow of water or brine that originates at the surface. Most meteoric groundwater flow is driven by topographic gravitational potential. Cf. topography-driven flow, hydraulic potential.
Microporosity: Porosity that is 1-2 µm contributes to the total pore volume of a rock or sediment, but in terms of advective fluid flow it is inefficient. Transfer of dissolved mass probably takes place by diffusion. Common examples are present in pore throats of granular rock, between clay particles in mudrocks, and between pore-filling cements.
Milliequivalents/litre: (Meq) A conversion that normalizes solute concentrations in groundwater chemical analyses. The measure is commonly used to calculate the electroneutrality of sample analyses to check sample integrity. The formula for conversion is: meq = (mg/l) /(formula weight/charge). Mg/l is the measured concentration of an ionic species. Only the charge number is used, regardless of sign (+ or -).
MODFLOW: One of the most commonly used numerical simulation packages for modeling groundwater flow in 2D and 3D, and for solving puzzles such as contaminant sources and sinks. The code was developed by the United States Geological Survey and is generally freely available. Many commercial modeling-simulation software packages use the MODFLOW base. The original version used finite difference solutions to determine aquifer-aquitard characteristics such as equipotential surfaces, flow lines, contaminant flow vectors and travel times, and the influence pumping wells have on flow patterns. Commercial packages have sophisticated graphical interfaces to show 2D cross-sections, equipotential and flow line maps, well locations and hydrostratigraphy, animations of contaminant flow, and topographic overlays.
MPa: Unit of stress – Megapascal; 106 Pa.
Mud volcano: Small cone-shaped buildups associated with erupting mud, ranging from about a metre to 10s of metres high. Eruptions may be quiet where mud flows, slithers and slides down slope, or more violent, reminiscent of lava fire fountains, shooting mud 10s of metres into the air (or water). If methane is present in the mud, the eruptions can ignite. They form on land and on the sea floor.
Newtonian fluid: A rheological class wherein a fluid has no yield strength (cf. plastics), and deforms continuously (strain) with increasing stress, independent of viscosity. Water is the best known example.
No flow boundary: The condition imposed on flow models or flow nets where there fluid flux is not permitted through the boundary. It is an important condition applied to many groundwater flow models. Real world examples include physical boundaries such as walls or dams, impermeable faults, and contacts between aquifers and non-permeable igneous rock bodies.
Observation well: A well installed solely for the purpose of groundwater observation, particularly hydraulic head measurements. See Piezometer and Piezometer nest.
Oil migration: Hydrocarbon production in deeply buried sediments, begins in organic-rich sediment, such as oil shale. Once formed (by a series of complex chemical reactions), the oil (and gas) migrate from the shale or mudstone to more porous and permeable rocks such as sandstones and limestones. Migration is driven buoyancy forces and the flow of deep subsurface groundwater. Migration will continue until the oil is trapped (resulting in an oil field). Oil and gas that isn’t trapped will eventually find its way to the surface or sea floor and escape.
Oil seep: Oil, sometimes accompanied by gases like methane, that leak to the surface via fractures or faults, driven of buoyancy forces, or as a part of spring flow. The hydrocarbons may be sourced from oil-prone porous rock, or from actual subsurface oil pools.
Panne: Shallow ponds on salt marsh platforms. They are usually recharged by saline water during spring tides, but the pond salinity can vary because of precipitation.
Particulate flow: Faulting of soft, non-indurated sediment results in grain rolling and sliding along the fault plane – fault core. This process changes the grain packing geometry and permeability compared with the host sediment.
Permeability: A measure of the ease with which fluids flow through porous sediment and rock. In groundwater studies it is expressed as hydraulic conductivity that has dimensions of distance/time. The hydrocarbon industry uses a dimensionless number for intrinsic permeability, the Darcy, that depends only on the porous medium. The unit reduces mathematically to units of area (ft2, m2). It is basically a measure of pore size.
Permeameter: Used to experimentally measure permeability in porous media. Typically it consists of a cylinder filled with soil, sediment, or core samples, open to flow at both ends to permit flow. In constant head permeameters the fluid level at the top of the sample kept constant by addition; in falling head permeameters an initial water level, or hydraulic head, is allowed to fall (by seepage through the sample) – the rate of fall is measured.
Phase (thermodynamics): The physical state of a substance – gas, liquid, or solid. Plasmas are considered by some as the fourth phase. Phases are also defined by the potential for mixing at the molecular level, and by interfaces between phases. For example, a system containing crystalline calcite and aragonite would have two phases. In aqueous systems two liquids will always mix and hence there is only one phase; this also applies to different gasses. However, a system with aqueous and hydrocarbon liquids would have two phases.
Phreatophytes: Plants that extend their roots into the phreatic groundwater zone or the capillary fringe above this zone. They commonly vegetate areas of groundwater discharge such as lowlands and river flood plains. In arid and semi-arid regions where evapotranspiration is high, the local groundwater can become increasingly saline – the kinds of plants that thrive there are called halophytes.
Piezometer: A relatively simple borehole constructed solely for groundwater pressure observations, specifically hydraulic head. It can be used in confined and unconfined (watertable) aquifers. The borehole may be open at the base or screened – for the latter the screen mid-point depth is taken as the point of measurement for elevation and pressure head calculations. The observation wells are not pumped, but they are used to monitor head changes in nearby pumped boreholes.
Piezometer nest: Several piezometer tubes may be installed in a single borehole, each tube in the nest extending to different depths within an aquifer. Knowing the measurement point for each tube permits the calculation of vertical head gradients within an aquifer (usually confined aquifers).
Piper diagram: A matrix of three triangular plots that map the chemical compositions of water. It is based on normalized percentages of major cations (Calcium, magnesium, potassium, and sodium), and carbonate-bicarbonate, sulphate, and chloride anions. It is useful for tracking the source of groundwater flows in aquifers derived from different rock types, and the evolution of chemical speciation.
Playa lake: From the Spanish word for ‘beach’, its meaning has morphed to a dry lake, usually floored by evaporitic minerals, that intermittently becomes flooded. Cf. Salina.
Pollutant: A chemical or substance introduced into the natural environment by human activity. For example pesticide residues on fruit-vegetables, or excess CO2 in the atmosphere. Cf. contaminant.
Pore pressure: The pressure of fluid in the pore spaces or fractures of sediment and rock; it is usually measured or calculated with reference to the expected hydrostatic pressure at the depth of interest. Pore pressures greater than hydrostatic (over-pressured) reduce the shear strength of sediment and rock. Over-pressuring cannot be maintained unless there is some fluid trapping mechanism.
Pore throat: The narrow passages between grains in contact, that connect the larger intergranular pores. Pore throat sizes are variable, depending in part on the packing arrangement of grains and grain shapes, and range from submillimetre to a few microns. Their size and distribution are a primary control on the characteristics of fluid flow. Pore throats can be blocked and their efficacy reduced by cements, particularly clays.
Porosity – fracture: The void space in hard rock created by joints, fractures, and faults. In rock types such as basalts and granites, this is usually the only kind of porosity that permits fluid flow. Fracture porosity commonly has directionality because of the orientation of the stresses that produce brittle failure.
Porosity – intergranular: the void space between framework clasts within a rock or sediment. It is presented as the ratio of total void space versus total sample volume and is therefore dimensionless. Pore spaces below the watertable are always occupied by fluid – aqueous, or hydrocarbon. The porosity of a clean sand is commonly 30-35% but can be reduced to less than 1% by compaction and cementation. Mud porosity can be as high as 70% at deposition, but this too rapidly decreases during compaction.
Potential energy: This is the energy available for a system to do work. From a mechanical perspective, a drop of water at rest has the potential energy equivalent to the product of the drop mass, it’s height above some datum, and the gravitational constant – written as:
PE = m.h.g
If the drop falls, some of this potential energy is converted into kinetic energy (and possibly some thermal energy) and the change in PE is ΔPE = m.g.(h2 – h1). The choice of a datum is arbitrary – for example the centre of the Earth, sea level, or some other local surface.
Potentiometric surface: In hydrogeology, hydraulic heads, expressed as elevations of water levels in water wells can be mapped as a surface. Each aquifer has its own, unique, potentiometric surface. Each contour represents a line or plane of equal hydraulic head, or equipotential. The map allows prediction of water levels in new wells. It also allows calculation of hydraulic gradients and directions of groundwater flow. For confined aquifers, the potentiometric surface is an imaginary, theoretical surface. In unconfined aquifers it corresponds to the watertable (a real surface).
Pressure (geology): In geology we generally recognise two kinds of forces: surface forces, and body forces such as gravity that act on every part of a sediment, rock, or fluid body. Thus, stress or pressure can be expressed as force per unit area for surface forces, and force per unit mass for body forces. In Earth science we consider stress at the microscopic, single grain or crystal scale up to the scale of entire lithospheric blocks. The standard SI unit is the Pascal (Pa) – Newtons/m2, and dimensions ML-1T-2. The commonly used symbol is σ.
Pressure head: see hydraulic head.
Pumping test: A frequently used field experiment where a pumping well is set to discharge water at a constant known rate and the subsequent drawdown rate of the water level is measured either in the pumping well itself, and/or any number of nearby observation wells and piezometers. There are well-established procedures for converting the drawdown data into the various hydraulic properties of the aquifer, determining the aquifer capacity for supply, and well performance.
Pumping well: A groundwater borehole used to pump water, primarily for potable supply, but also for hydraulic and chemical sample testing. Borehole may tap into a single aquifer or screened across multiple aquifers. cf. observation well.
Recharge: See Groundwater recharge.
Refraction of flow lines: Aquifers systems commonly are made up of multiple layers each having different properties (k, T, S etc). The direction of flow, and therefore equipotential lines will change between layers having different hydraulic conductivities. The refraction of groundwater flow is analogous to refraction of light and obeys the tangent refraction law. Refraction is required if mass is to be conserved when groundwater flows from one hydraulic unit to another (e.g., aquifer to aquitard). It is determined by the ratio of hydraulic conductivities (k) of the two units being equal to the tangent of the angles measured from a line normal to aquifer boundary and the flow lines. For example, flowlines from and aquifer to aquitard (i.e., from high to lower k) will steepen the hydraulic gradient and therefore the flowlines; the opposite happens if flow is from low to high k. Flow nets commonly show very shallow flow lines through aquifers, and very steep flow lines in aquitards.
Residence time: Refers to the time that mass resides in groundwater – or the age of groundwater. Groundwater age generally increases with depth, from days- months in the shallowest levels, and 103 – 106 years at greater depths. Radioactive isotopes like tritium (3H half-life 12.35 years) and carbon 14 (14C half-life 5730 years) are commonly used to date groundwaters; the maximum age for C-14 is 30-40 kyr. Chlorine 36 (36Cl half-life 3.01 x 105 years) is gaining in popularity for older waters.
Riparian zone: The area of land in immediate contact with a river, lake or tidal zone. It is commonly considered to be a buffer zone that is reflected in the type of vegetation, such as marsh or wetland, meadows or forests, as well as a zone of protection and management. or example a well developed riparian vegetation and soil will help trap and sequester land-derived nutrients and sediment.
Saline lake: A terrestrial water body where evaporation exceeds surface freshwater influx and fresh groundwater seepage. Recharge may be seasonal and intermittent. Intense evaporation results in precipitation of salts, commonly halite and gypsum. Lakes may be connected to inflowing and outflowing drainage, or they may be endorheic. See also Playa Lake.
Saline lake brines: Unlike seawater, terrestrial brines have widely variable compositions, depending on local soil and bedrock compositions, groundwater chemistry, and the degree of evaporitic drawdown. Typical brines contain Na+, Ca2+, Mg2+, Cl–, SO42-, HCO3–, CO32-, and SiO2, but concentrations are highly variable. pH ranges from highly alkaline to highly acidic. Evaporation pathways produce a succession of different minerals. See also calcite-gypsum divides.
Saline wedge: (seawater wedge). Seawater that invades a coastal freshwater aquifer; the wedge-shaped intrusion is thickets at the shoreline, thinning landward. It underlies freshwater aquifers because of the density difference. The interface between the saline wedge and freshwater is gradational. The depth to the interface at any position landward of the shoreline is given by the Ghyben-Herzberg formula z = 40bf where z is the depth below sea level to the point on the interface, and bf is the height above sea level to the watertable vertically above the point; z increases landward.
Saturated zone (hydrogeology): The part of an aquifer where pore spaces are permanently filled with water. In unconfined aquifers this occurs below the watertable. Confined aquifers are always completely saturated. Also called the phreatic zone.
Seawater-fresh water interface: The boundary between fresh water and seawater in coastal aquifers, and aquifers that extend beneath a marine shelf. The boundary is diffuse. In coastal aquifers the depth to the interface depends on the watertable elevation above sea level – the depth is governed by the Ghyben-Herzberg principle.
Seawater intrusion: (saline intrusion) The replacement of fresh groundwater by an intruding wedge or lens of seawater. This commonly occurs in coastal aquifers where excessive fresh groundwater withdrawal results in a fall in the local watertable, and a corresponding rise in the fresh water/seawater interface by 40 times the amount the watertable has fallen. Sea water intrusion is, for practical purposes, irreversible. See Ghyben-Herzberg principle.
Screen (borehole): Part of the borehole casing that permits the transit of groundwater through narrow slots. Commonly placed at the base of a borehole, but can be placed at several depths in wells that penetrate multiple aquifers – this permits greater water production from a single borehole. In watertable aquifers the entire borehole depth may be screened. Screen slot size is less that the average or median aquifer grain size to prevent ingress of sediment.
Secondary porosity: Porosity that is created during burial diagenesis by the dissolution of chemically reactive grains such as carbonates and feldspars. Secondary porosity can enhance the overall porosity of a rock, particularly if primary intergranular pore volumes have been occluded by cements. Secondary pores may be larger than those formed during deposition, where entire grains are dissolved. Partial dissolution along twin or cleavage planes in minerals like feldspar, will result in irregular grain boundaries.
Sequestration: Storage of solid, liquid or gas so that it cannot disperse, or escape. Of recent concern is sequestration of carbon in various forms, particularly CO2 and methane. Natural sequestration occurs on rocks (coal, limestones), soils, and permafrost. Artificial sequestration of supercooled CO2 in certain rock formations (such as depleted oil fields) is considered as one means of controlling CO2 emissions.
Sinkhole: Also called Dolines, are collapse structures formed by removal of subsurface rock, either by erosion of dissolution within the vadose and saturated (phreatic) zones, are typical of limestone terrains; they can also occur in landscapes underlain by evaporites. They tend to be circular in cross-section. Collapse usually occurs rapidly into large, subsurface caverns. They are common in karst landscapes.
Slug test: Slug refers to a known volume of water that is added or removed, raising or lowering respectively the water level and increasing or decreasing fluid pressures measured from some depth in the well. The return of the water level or fluid pressure to its steady state condition is monitored. Slug addition is probably the easiest method. This is a common well testing method that does not require a pump but does need a recorder to monitor changing water levels or fluid pressures.
Specific retention: (Sr). The volume of water remaining in an aquifer after it has been drained. The water is present in pore throats, as coatings on grains, and as menisci between grains.
Specific Storage: (Ss) The amount of water per unit volume of saturated aquifer expelled or stored, or the volume of water (L3) per unit surface area of aquifer (L2) per unit aquifer thickness (L) per unit change in head (L). Ss is related to the elastic properties of the formation: Ss = ρg ( α + nβ) where α = compressibility of the aquifer framework (T²L/M), β = compressibility of water (T²L/M), and n =porosity. The dimensions of Ss are L-1. Ss for unconfined aquifers is >> confined aquifers.
Specific Yield: Used for unconfined aquifers – it expresses the volume of water drained by gravity per unit area per unit head decline (total sample volume). It is dimensionless. It is also called effective porosity. It is related to total porosity by adding Sy to Specific Retention (Sr).
Steady state condition: In groundwater analysis, the condition where the volume of water discharged from a pumping well is replaced by flow from the aquifer into the well at an equal rate. Cf. transient drawdown.
Storage: In hydrogeology, the volume of water that can be released or taken up by an aquifer. There are several ways to express storage: Specific Storage (Ss), Storativity (S – also called Storage Coefficient), and Specific Yield (Sy). Unconfined aquifers tend to release greater volumes of groundwater than confined aquifers. For unconfined aquifers water is largely responsible for drainage from pores; the watertable is lowered during drainage and part of the aquifer becomes unsaturated. Drainage from confined aquifers is due to elastic changes such that there is a lowering of the potentiometric surface but the aquifer remains saturated. In most cases the process is reversible. The website for software developer AQTESOLV is a good resource for descriptions of all groundwater variables.
Storage Coefficient: another name for Storativity.
Storativity: S is defined as the volume of water released from storage per unit surface area of the aquifer or aquitard per unit decline in hydraulic head. It is used primarily for confined aquifers S = Ssb where b is the saturated thickness; S is dimensionless.
Stress: See Pressure. A term more commonly used in structural geology and tectonics.
Supercritical liquid: A liquid that has properties somewhere between a gas and a liquid. For example, for CO2 these properties include high solubility in oil and water; density similar to the liquid phase but much lower viscosity – the latter property enhances flow through pipes (transport)and through porous rock; low surface tension.
Theis equation: Derived by Charles Theis (1900-1987), it was the first mathematical expression used to calculate aquifer variables like storativity and transmissivity during transient flow of water to a pumping well. It uses an integration function, called the well function, that relates measured drawdown (with time) to pumping rate (known), transmissivity, Storativity, and the radial distance between the pumping well and an observation well. It assumes the aquifer is isotropic, homogenous, and non-leaky. See Domenico and Schwartz, 1997 for a good explanation of the theory).
Thermally driven flow: Advective fluid flow caused by thermal gradients (Hot to cold) or convection. In groundwater systems this is commonly caused by thermal gradients associated with igneous intrusion, and in regions of shallow geothermal activity. Cooling of fluids can also result in mineral precipitation. Convective flow is generated by density and buoyancy differences between hot and cold fluids.
Tidal efficiency: Refers to the elastic response of a confined aquifer to forcing by ocean tides. It is defined as the ratio of the amplitude of a piezometer water level change (in a well) to tidal amplitude. It is a common effect in wells close to shore. The magnitude of the response depends on compressibility of the aquifer medium. The response can also be influenced by barometric pressure changes.
Time-drawdown curve: The standard graphical plot of water level drawdown increments measured from an observation well or piezometer. If the radial distance of the observation well and the pumping discharge are known, then standard variables like transmissivity and storativity can be calculated. See Domenico and Schwartz, 1997 for a description of the method.
Topography driven flow: Groundwater flow that is driven by topographic gravitational potential. It is the dominant mechanism of groundwater flow at shallow levels of Earth’s crust, to depths of 2-3 km. It is usually expressed as hydraulic potential, or hydraulic head (H), where:
HTotal = h (the elevation head) + P (pressure head)/ρg, relative to a datum (commonly taken as sea level).
Transmissivity: The quantity of water flowing horizontally through an area defined by the entire saturated thickness of an aquifer in a column one unit wide and unit hydraulic gradient: written as T = kb where k is hydraulic conductivity and b the thickness.
Unconfined aquifer: see Aquifer-unconfined
Unsaturated zone: The portion of an unconfined aquifer above the watertable where pore spaces are air-filled (and approximately at atmospheric pressure). It is synonymous with vadose zone.
Vadose zone: The portion of an unconfined aquifer above the watertable where pore spaces are air-filled (and approximately at atmospheric pressure). It is synonymous with unsaturated zone.
Viscosity: Viscosity is used to describe a material in which its strength depends on the rate of deformation, or strain rate. From a practical point of view, it is a measure of its resistance to deformation, or flow. It is normally applied to fluids, including rocks that may behave as fluids under high confining pressures and low strain rates. In the Earth sciences, viscosity is applied to phenomena like mud flows and ice sheets, and to rocks in the mantle.
Watertable: The level to which groundwater rises in an unconfined aquifer. It is a special kind of potentiometric surface – it is real in that it can be revealed by drilling or excavation. Watertables always have a gradient, sloping in the direction of groundwater flow. Watertables can be mapped from water level intersections in boreholes. A watertable is at atmospheric pressure for any location. Watertables tend to fluctuate seasonally as a function of recharge and natural discharge. They can also fluctuate as a result of pumping. See Equipotential
Watertable map: A potentiometric contour map of the watertable, contoured with hydraulic head values (using local elevations or elevations relative to a datum). Flow vectors or flow lines are commonly added – drawn at right angles to the head contours assuming isotropic conditions. Keep in mind the map is a 2D representation of a 3D surface. The watertable data must be taken from the same unconfined aquifer.
