Tag Archives: hydrogeology

Glossary: Geofluids – hydrogeology

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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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

Effective porosity: The component of porosity that permits significant flow. Microporosity (intergranular, intercrystalline) is commonly excluded from this porosity value.

Elevation head: see hydraulic head

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.

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 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.

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.

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.

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.

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 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).

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).

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 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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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).

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.

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.

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.

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 head: see hydraulic head.

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.

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.

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.

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).

 

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

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Contrails, analogues, and visualizing groundwater flow

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How to picture groundwater flow beneath the surface.

Analogues and analogies.  Standard dictionaries define these as a comparison, correspondence, or similarity between one thing and another, that can apply to concepts, ideas or physical entities. They are tools, used to illustrate concepts, particularly abstract ideas, to help explain phenomena or theories. Science makes frequent use of analogies. It does so because many phenomena that it attempts to investigate and explain extend beyond normal human experience, beyond what is visible to the unaided eye, beyond what we can touch.  Well-chosen analogies can help us understand the universe without, and the universe within. Continue reading

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The Architecture of Connected Holes; A Different Way to Look at the Liquid Earth

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2nd in the Series on Groundwater

lake taupo

We commonly differentiate the solid earth in terms of its architecture, whether it is the foundations of great mountain ranges, or the solidified magmas that underpinned ancient volcanoes.  All rocks, whether layered sedimentary rocks or massive intrusive granites, have unique characteristics that define their physical, chemical and biological make up – their architecture.

WE can also think of groundwater in terms of its own architecture.  The productivity of an aquifer depends first and foremost on its porosity and permeability.  We can use these two fundamental properties to define the architecture of earth materials.

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Whiskey is for drinkin’; water is for fightin!

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San Joaquin Valley land subsidence caused by groundwater miningMark Twain wasn’t far wrong with this sardonic, perceptive quip.  If he was able to comment on the global water situation today, he might add “I guess so, I dunno.

This is the first in a series of posts on underground water, or groundwater.  The posts will outline, with a non-specialist perspective, the science of aquifers, groundwater movement, how groundwater interacts with surface water, water extraction-pumping, and contamination. Continue reading

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