Glossary of Hydrologic Terms

The definitions found in this glossary are from, "The Federal Glossary of Selected Terms: Subsurface-Water Flow and Solute Transport": Department of Interior, U.S. Geological Survey, Office of Water Data Coordination, August 1989. References for the definitions can be found at the end of the document.

Absorption - The process by which substances in gaseous, liquid or solid form dissolve or mix with other substances (ASCE, 1985).

Adsorption - Adherence of gas molecules, ions, or molecules in solution to the surface of solids (ASCE, 1985).

Adsorption isotherm - A graphical representation of the relationship between the bulk activity of adsorbate and the amount adsorbed at constant temperature (after Stumm and Morgan, 1981).

Advection - The process whereby solutes are transported by the bulk mass of flowing fluid (Freeze and Cherry, 1979). See also convective transport.

Air-space-ratio - The ratio of (a) the volume of water that can be drained from a saturated soil or rock under the action of force of gravity to (b) the total volume of voids (ASTM, 1980).

Anisotropy - The condition of having different properties in different directions (AGI, 1980).

Anisotropic mass - A mass having different properties in different directions at any given point (ASTM, 1980).

Aquiclude - A hydrogeologic unit which, although porous and capable of storing water, does not transmit it at rates sufficient to furnish an appreciable supply for a well or spring (after WMO, 1974). See preferred term confining unit.

(1) A formation, group of formations, or part of a formation that contains sufficient saturated permeable material to yield significant quantities of water to wells and springs (after Lohman and others, 1972).

(2) A geologic formation, group of formations, or part of a formation capable of yielding a significant amount of groundwater to wells or springs. Any saturated zone created by uranium or thorium recovery operations would not be considered an aquifer unless the zone is or potentially is (1) hydraulically interconnected to a natural aquifer, (2) capable of discharge to surface water, or (3) reasonably accessible because of migration beyond the vertical projection of the boundary of the land transferred for long-term government ownership and care (10 CFR Part 40 Appendix A).

(3) A formation, a group of formations, or a part of a formation that contains sufficient saturated permeable material to yield significant quantities of water to wells and springs (10 CFR Part 960.2).

(4) A zone, stratum, or groups of strata that can store or transmit water in sufficient quantities for a specific use (30 CFR Part 710.5).

(5) Geological formation, groups of formations, or part of a formation, that is capable of yielding a significant amount of water to a well or spring (40 CFR Parts 146.03; 260.10; 270.2).

(6) A geologic formation, group of formations, or portion of a formation capable of yielding usable quantities of groundwater to wells or springs (40 CFR Part 257.3-4).

Aquifer system - A body of permeable and poorly permeable material that functions regionally as a water-yielding unit; it comprises two or more permeable beds separated at least locally by confining beds that impede groundwater movement but do not greatly affect the regional hydraulic continuity of the system; includes both saturated and unsaturated parts of permeable material (after ASCE, 1985).

Aquifer test - A test to determine hydrologic properties of the aquifer involving the withdrawal of measured quantities of water from or addition of water to a well and the measurement of resulting changes in head in the aquifer both during and after the period of discharge or additions (ASCE, 1985).

(1) A hydrogeologic unit which has no interconnected openings and, hence cannot store or transmit water (after WMO, 1974).

(2) A rock which contains no interconnected openings or interstices and therefore neither stores nor transmits water (ASCE, 1985).

Aquitard - A confining bed that retards but does not prevent the flow of water to or from an adjacent aquifer; a leaky confining bed. It does not readily yield water to wells or springs, but may serve as a storage unit for ground water (AGI, 1980). See preferred term confining unit.

Area of influence of a well - The area surrounding a pumping or recharging well within which the potentiometric surface has been changed (after ASCE, 1985).

Artesian well - A well deriving its water from an artesian or confined aquifer (after ASCE, 1985).

Artificial recharge - Recharge at a rate greater than natural, resulting from deliberate or incidental human activities (WRC, 1980).

Base flow - That part of the stream discharge that is not attributable to direct runoff from precipitation or melting snow; it is usually sustained by groundwater discharge (after APHA, 1981).

Baseline monitoring - The establishment and operation of a designed surveillance system for continuous or periodic measurements and recording of existing and changing conditions that will be compared with future observations (after NRC, 1982).

Breakthrough curve - A plot of relative concentration versus time, where relative concentration is defined as C/Co with C as the concentration at a point in the groundwater flow domain, and Co as the source concentration.

Buildup - The vertical distance the water table or potentiometric surface is raised, or the increase of the pressure head due to the addition of water.

Capillary action - The movement of water in the interstices of a porous medium due to capillary forces (after ASTM, 1980).

(1) The property of an unsaturated porous medium to transmit liquid (after AGI, 1980).

(2) Coefficient which expresses the extent to which an unsaturated permeable medium allows flow of water through its interstices, under a unit gradient of capillary potential (after WMO, 1974).

Capillary fringe - The lower subdivision of the unsaturated zone immediately above the water table in which the interstices are filled with water under pressure less than that of the atmosphere, being continuous with the water below the water table but held above it by capillary forces (after ASCE, 1985).

Capillary head - The potential, expressed in head of water, that causes the water to flow by capillary action (ASTM, 1980).

Capillary potential - The scalar quantity that represents the work required to move a unit mass of water from the soil to a chosen reference location and energy state (after SSSA, 1975).

Capillary pressure - The difference in pressure across the interface between two immiscible fluid phases jointly occupying the interstices of a porous medium caused by interfacial tension between the two phases (after AGI, 1980).

Capillary rise - The height above a free water surface to which water will rise by capillary action (ASTM, 1980).

(2) Soil water above hydroscopic moisture and below the field capacity (WMO, 1974).

Cascading water - In reference to wells, groundwater that trickles or pours through cracks or perforations down the casing or uncased borehole above the water level in the well (after Wilson, 1980).

Cation exchange capacity - The sum total of exchangeable cations that a porous medium can absorb. Expressed in moles of ion charge per kilogram of soil (or of other exchanges such as clay) (after SSSA, 1975).

Concentration gradient - The change in solute concentration per unit distance in solute. Concentration gradients cause Fickian diffusion (spreading) of solutes from regions of highest to regions of lowest concentrations. In slow moving groundwater, this is the dominant mixing process.

Cone of depression - A depression of the potentiometric surface in the shape of an inverted cone that develops around a well which is being pumped (after ASCE, 1985).

Cone of impression - A rise of the potentiometric surface in the shape of a cone that develops around an injection well.

Confined - A modifier which describes a condition in which the potentiometric surface is above the top of the aquifer.

(1) An aquifer bounded above and below by confining units of distinctly lower permeability than that of the aquifer itself (ASCE, 1985).

(3) An aquifer bounded above and below by impermeable beds or by beds of distinctly lower permeability than that of the aquifer itself; an aquifer containing confined groundwater (40 CFR 260.10).

(2) A body of impermeable or distinctly less permeable material stratigraphically adjacent to one or more aquifers (40 CFR 146.3).

(1) A hydrogeologic unit of impermeable or distinctly less permeable material bounding one or more aquifers and is a general term that replaces aquitard, aquifuge, aquiclude (after AGI, 1980). See definition 2, confining bed.

(2) Means a body of impermeable or distinctly less permeable material stratigraphically adjacent to one or more aquifers (10 CFR Part 960.2).

(2) A geological formation, group of formations, or part of a formation that is capable of limiting fluid movement above an injection zone (40 CFR 146.3).

Connate water - Water entrapped in the interstices of a sedimentary or extrusive igneous rock at the time of its deposition (AGI, 1980).

(1) An undesirable substance not normally present or an unusually high concentration of a naturally occurring substance in water or soil.

(3) Any physical, chemical, biological, or radiological substance or matter in water (40 CFR 142.2; 40 CFR 149.2; 40 CFR 141.2; 40 CFR 143.2; 40 CFR 146.3).

Contamination - The addition to water of any substance or property preventing the use or reducing the usability of the water (AGI, 1980). Sometimes considered synonymous with pollution. Contaminant plume - An elongated body of groundwater containing contaminants, emanating and migrating from a point source within a hydrogeologic unit(s).

(1) The concentration of that substance in the groundwater to exceed the maximum contaminant levels, or

(2) An increase in the concentration of that substance in the ground water where the existing concentration of that substance exceeds the maximum contaminant levels (40 CFR 257.3-4).

Convective transport - The component of movement of heat or mass induced by thermal gradients in groundwater.

Convection - The process whereby heat is carried along with the flowing groundwater (after Freeze and Cherry, 1979).

Darcy's law - An empirical law which states that the velocity of flow through d porous medium is directly proportional to the hydraulic gradient assuming that the flow is laminar and inertia can be neglected (after Darcy, 1856).

Deep percolation - The drainage of soil water downward by gravity below the maximum effective depth of the root zone toward storage in subsurface strata (ASCE, 1985).

Discharge area - An area in which groundwater is discharged to the land surface, surface water, or atmosphere (WRC, 1980). reverse process of sorption. See also sorption.

Differential water capacity - The absolute value of the rate of change of water content with soil water pressure. The water capacity at a given water content will depend on the particular desorption or adsorption curve employed. Distinction should be made between volumetric and specific water capacity (SSSA, 1975). ddd

Diffusion - Process whereby ionic or molecular constituents move under the influence of their kinetic activity in the direction of their concentration gradient (Freeze and Cherry, 1979). Diffusion coefficient - See molecular diffusion, coefficient.

Diffusivity, soil water - The hydraulic conductivity divided by the differential water capacity (care being taken to be consistent with units), or the flux of water per unit gradient of moisture content in the absence of other force fields (SSSA, 1975).

Diffusivity, hydraulic - The ratio of transmissivity divided by the storage coefficient or the hydraulic conductivity divided by the specific storage (Lohman and others, 1972).

(1) A measure of the spreading of a flowing substance due to the nature of the porous medium, with its interconnected channels distributed at random in all directions (ANS, 1980).

(2) The sum of the coefficients of mechanical dispersion and molecular diffusion in a porous medium (Bear, 1979).

Dispersion, longitudinal - Process whereby some of the water molecules and solute molecules travel more rapidly than the average linear velocity and some travel more slowly; spreading of the solute in the direction of the bulk flow (after Freeze and Cherry, 1979).

Dispersion, transverse - Spreading of the solute in directions perpendicular to the bulk flow (after Freeze and Cherry, 1979).

Dispersivity - A geometric property of a porous medium which determines the dispersion characteristics of the medium by relating the components of pore velocity to the dispersion coefficient (ANS, 1980).

(2) A well used for the disposal of waste into a subsurface stratum (40 CFR 146.3).

Distribution coefficient - The quantity of the solute, chemical or radionuclide sorbed by the solid per unit weight of solid divided by the quantity dissolved in the water per unit volume of water (ANS, 1980).

Drainage well (1) A well installed to drain surface water, storm water, or treated waste water into underground strata (after ASCE, 1985). (2) A water well constructed to remove subsurface water or to reduce a hydrogeologic unit's potentiometric surface (after ASCE, 1985).

Drawdown (1) The vertical distance the water elevation is lowered or the reduction of the pressure head due to the removal of water (after ASCE, 1985).

(2) The decline in potentiometric surface at a point caused by the withdrawal of water from a hydrogeologic unit (after Heath, 1984).

Effective hydraulic conductivity - See hydraulic conductivity, effective. effective porosity, (D) - see porosity, effective.

Electrical conductivity - Measure of the ability of material to conduct an electrical current. For water samples, it depends on the concentration and type of ionic constituents in the water and temperature of the water; and it is expressed in siemens per meter.

Equipotential line (or surface) - Line (or surface) along which the potential is constant (WMO, 1974).

Evapotranspiration - The combined loss of water from a given area by evaporation from the land and transpiration from plants (after SSSA, 1975).

Exchange capacity (1) The amount of exchangeable ions measured in moles of ion change per kilogram of solid material at a given pH (after ANS, 1980).

(2) The total ionic charge of the adsorption complex active in the adsorption of ions (SSSA, 1975).

Fickian diffusion - Spreading of solutes from regions of highest to regions of lower concentrations caused by the concentration gradient. In slow moving groundwater, this is the dominant mixing process (after Freeze and Cherry, 1979).

Flow line - The general path that a particle of water follows under laminar flow conditions (after ASTM, 1980).

Flow net - A graphical representation of flow lines and equipotential lines for two-dimensional, steady-state groundwater flow (after ASTM, 1980).

Flow path - The subsurface course a water molecule or solute would follow in a given groundwater velocity field.

Flow, steady - A characteristic of a flow system where the magnitude and direction of specific discharge are constant in time at any point See also flow, unsteady.

Flow, uniform - A characteristic of a flow system where specific discharge has the same magnitude and direction at any point.

Flow, unsteady - A characteristic of a flow system where the magnitude and/or direction of the specific discharge changes with time.

Fluid potential - The mechanical energy per unit mass of a fluid at any given point in space and time with regard to an arbitrary state and datum (Lohman and others, 1972).

Formation fluid - "Fluid" present in a "formation" under natural conditions as opposed to introduced fluids, such as drilling mud (40 CFR 146.3).

Fresh water - Water that contains less than 1,000 milligrams per liter (mg/L) of dissolved solids; generally more than 500 mg/L is undesirable for drinking and many industrial uses (USGS, 1984). See also saline water.

Gaining stream - A stream or reach of a stream whose flow is being increased by inflow of groundwater (ASCE, 1985).

(2) The geohydrologic units within a geologic setting, including any recharge, discharge, interconnections between units, and any natural or human-induced processes or events that could affect groundwater flow within or among those units (10 CFR Part 960.2).

(2) An aquifer, a confining unit, or a combination of aquifers and confining units comprising a framework for a reasonably distinct geohydrologic system (10 CFR Part 960.2).

Gravitational head - The component of total hydraulic head related to the position of a given mass of water relative to an arbitrary datum (Wilson, 1980).

Gravitational water - Water which moves into, through, or out of the soil or rock mass under the influence of gravity (SSSA, 1975).

Groundwater (1) That part of the subsurface water that is in the saturated zone.

(3) All water which occurs below the land surface. It includes both water within the unsaturated and saturated zones (NRC, 1985).

(4) Means water below the land surface in a zone of saturation. For purpose of this appendix, groundwater is the water contained within an aquifer (10 CFR Part 40 Appendix A).

(7) Subsurface water that fills available openings in rock or soil materials to the extent that they are considered water-saturated (30 CFR Part 710.5 and 710.5).

(8) Water below the land surface in a zone of saturation (40 CFR 270.2; 40 CFR 146.3; 40 CFR 144.3).

(9) Water in a saturated zone or stratum beneath the surface of land or water (40 CFR 300.6; 40 CFR 257.3-4).

Groundwater barrier - Rock or artificial material which has a relatively low permeability and which occurs below the land surface where it impedes the movement of ground water and consequently causes a pronounced difference in the potentiometric surface on opposite sides of it (after ASCE, 1985).

Groundwater basin - A general term used to define a groundwater flow system that has defined boundaries and may include permeable materials that are capable of storing or furnishing a significant water supply, the basin includes both the surface area and the permeable materials beneath it (after ASCE, 1985).

Groundwater, confined - Groundwater under pressure significantly greater than atmospheric and whose upper limit is the bottom of a confining unit (after Lohman and others, 1972). See also confined, confining unit, and confined aquifer.

Groundwater divide - A ridge in the water table or other potentiometric surface from which groundwater moves away in both directions normal to the ridge line (WRC, 1980).

(2) The rate of groundwater flow per unit area of porous or fractured media measured perpendicular to the direction of flow (10 CFR Part 960.2).

Groundwater mound - A raised area in a water table or other potentiometric surface created by groundwater recharge.

(2) Unconfined groundwater separated from an underlying body of ground water by an unsaturated zone. Its water table is a perched water table. Perched groundwater is held up by a perching bed whose permeability is so low that water percolating downward through it is not able to bring water in the underlying unsaturated zone above atmospheric pressure (10 CFR Part 960.2).

Groundwater recharge - The process of water addition to the saturated zone or the volume of water added by this process (ANS, 1980).

Groundwater system - A groundwater reservoir and its contained water. Also, the collective hydrodynamic and geochemical processes at work in the reservoir (USGS, 1984).

Groundwater travel time (1) The time required for groundwater to travel between two locations.

(2) The time required for a unit volume of groundwater to travel between two locations. The travel time is the length of the flow path divided by the velocity, where velocity is the average groundwater flux passing through the cross-sectional area of the geologic medium through which flow occurs, perpendicular to the flow direction, divided by the effective porosity along the flow path. If discrete segments of the flow path have different hydrologic properties the total travel time will be the sum of the travel times for each discrete segment (10 CFR Part 960.2).

Groundwater, unconfined - Water in an aquifer that has a water table (Lohman and others, 1972).

Head, static - The height above a standard datum of the surface of a column of water (or other liquid) that can be supported by the static pressure at a given point. The static head is the sum of the elevation head and the pressure head (after Lohman and others, 1972).

Head, total - The total head of a liquid at a given point is the sum of three components: (a) the elevation head, which is equal to the elevation of the point above a datum, (b) the pressure head, which is the height of a column of static water that can be supported by the static pressure at the point, and (c) the velocity head, which is the height to which the kinetic energy of the liquid is capable of lifting the liquid (Lohman and others, 1972).

Heterogeneity - A characteristic of a medium in which material properties vary from point to point (after ANS, 1980).

Homogeneity - A characteristic of a medium in which material properties are identical everywhere (see Lohman and others, 1972).

Hydraulic barrier - A general term referring to modifications of a groundwater flow system to restrict or impede movement of contaminants.

Hydraulic conductivity (1) A proportionality constant relating hydraulic gradient to specific discharge which for an isotropic medium and homogeneous fluid, equals the volume of water at the existing kinematic viscosity that will move in unit time under a unit hydraulic gradient through a unit area measured at right angles to the direction of flow (after ASCE, 1985).

(2) The volume of water that will move through a medium in a unit of time under a unit hydraulic gradient through a unit area measured perpendicular to the direction of flow (10 CFR Part 960.2).

Hydraulic conductivity, effective - The rate of flow of water through a porous medium that contains more than one fluid, such as water and air in the unsaturated zone, and which should be specified in terms of both the fluid type and content and the existing pressure (Lohman and others, 1972).

Hydraulic gradient (1) The change in static head per unit of distance in a given direction. If not specified, the direction generally is understood to be that of the maximum rate of decrease in head.

(3) A change in the static pressure of groundwater, expressed in terms of the height of water above a datum, per unit of distance in a given direction (10 CFR Part 960.2).

Hydraulic head - The height above a datum plane (such as sea level) of the column of water that can be supported by the hydraulic pressure at a given point in a ground water system. For a well, the hydraulic head is equal to the distance between the water level in the well and the datum plane (ASCE, 1985).

Hydrochemical facies - Distinct zones that have cation and anion concentrations of diagnostic chemical character of water solutions in hydrologic systems which is describable within defined composition categories (after Freeze and Cherry, 1979).

Hydrodynamic dispersion - The spreading (at the macroscopic level) of the solute front during transport resulting from both mechanical dispersion and molecular diffusion (Bear, 1979).

Hydrogeologic unit (1) Any soil or rock unit or zone which by virtue of its hydraulic properties has a distinct influence on the storage or movement of groundwater (after ANS, 1980).

(2) Means any soil or rock unit or zone which by virtue of its porosity or permeability, or lack thereof, has a distinct influence on the storage or movement of groundwater (10 CFR Part 61.2).

Hydrograph - A graph relating stage, flow, velocity, or other characteristics of water with respect to time (after ASCE, 1985).

Hydrologic properties - Those properties of a rock that govern the entrance of water and the capacity to hold, transmit, and deliver water, such as porosity, effective porosity, specific retention, permeability, and the directions of maximum and minimum permeabilities (10 CFR Part 960.2).

Immiscible (1) Two or more liquids that are not readily soluble (AGI, 1980).

(2) The chemical property of two or more phases that, at mutual equilibrium, cannot dissolve completely in one another, e.g. oil and water (AGI, 1980).

Impermeable - A characteristic of some geologic material that limits its ability to transmit significant quantities of water under the head differences ordinarily found in the subsurface (after ASCE, 1985).

Infiltration - The downward entry of water into the soil or rock (SSSA, 1975).

Infiltration capacity - The maximum rate at which a soil or rock is capable of absorbing water or limiting infiltration (after ASCE, 1985).

Infiltration rate (1) The rate at which a soil or rock under specified conditions absorbs falling rain, melting snow, or surface water expressed in depth of water per unit time (ASCE, 1985).

(2) A characteristic describing the maximum rate at which water can enter the soil or rock, under specified conditions, including the presence of an excess of water. It has the dimensions of velocity (SSSA, 1975).

(2) A well into which fluids are being injected (40 CFR Parts 144.3; 146.3; and 270.2).

Injection zone - A geological "formation," group of formations, or Dart of a formation receiving fluids through a well (40 CFR Part 146.3).

In-situ density - The density of water measured at its actual depth (AGI, 1980). See potential density.

Interface - The contact zone between two materials of different chemical or physical composition (after USGS, 1984).

Interstice (1) An opening in a rock or soil that is not occupied by solid matter (AGI, 1980).

(2) An opening or space which may be occupied by air, water, or other gaseous or liquid material (ASTM, 1980).

Irrigation return flow - The part of artificially applied water that is not consumed by evapotranspiration and that migrates to an aquifer or surface water body (USGS, 1984).

Isotropic mass - A mass having the same property or properties in all directions (AGI, 1980).

Isotropy - The condition in which the property or properties of interest are the same in all directions.

Laminar flow - Flow in which the head loss is proportional to the first power of the velocity (ASTM, 1980).

(2) A liquid that has percolated through soil, rock or waste and has extracted dissolved or suspended materials (30 CFR Part 710.5).

Leaching (1) The removal of materials in solution from soil, rock, or waste (after SSSA, 1975).

(2) Separation or dissolving out of soluble constituents from a porous medium by percolation of water (McGraw-Hill, 1974).

Leakage (1) The flow of water from one hydrogeologic unit to another. The leakage may be natural, as through semi-impervious confining layer, or human-made, as through an uncased well (APHA, 1981).

(2) The natural loss of water from artificial structures as a result of hydrostatic pressure. 15

Leakance (1) The ratio K'/b', in which K' and b' are the vertical hydraulic conductivity and the thickness, respectively, of the confining beds (Lohman and others, 1972).

(2) The rate of flow across a unit (horizontal) area of a semipervious layer into (or out of) an aquifer under one unit of head difference across this layer (Bear, 1979). Synonymous with coefficient of leakage.

Leaky aquifer - Aquifers, whether artesian or water-table, that lose or gain water through adjacent less permeable layers (after Hantush, 1964).

Losing stream - A stream or reach of a stream in which water flows from the stream bed into the ground (ASCE, 1985). Synonymous with influent stream.

Lysimeter - A device for measuring percolation and leaching losses from a column of soil under controlled conditions (SSSA, 1975).

Matric potential - The energy required to extract water from a porous medium to overcome the capillary and adsorptive forces (after Wilson, 1980).

Mechanical dispersion - The process whereby solutes are mechanically mixed during advective transport caused by the velocity variations at the microscopic level. Synonymous with hydraulic dispersion.

Mechanical dispersion, coefficient - The component of mass transport flux of solutes caused by velocity variations at the microscopic level (after Bear, 1979). Synonymous with convective diffusion.

(1) Said of two or more liquids that are mutually soluble (i.e. they will dissolve in each other) (McGraw-Hill, 1974).

(2) The chemical property of two or more phases that, when brought together, have the ability to mix and form one phase (after AGI, 1980).

Miscible displacement - The mutual mixing and movement of two fluids that are soluble in each other (after Freeze and Cherry, 1979). Synonymous with miscible-phase displacement.

(1) A conceptual, mathematical, or physical system obeying certain specified conditions, whose behavior is used to understand the physical system to which it is analogous in some way (after McGrawHill, 1974).

(2) A conceptual description and the associated mathematical representation of a system, subsystem, components, or condition that is used to predict changes from a baseline state as a function of internal and/or external stimuli and as a function of time and space (10 CFR Part 960.2).

Moisture content - The ratio, expressed as a percentage, of either (a) the weight of water to the weight of solid particles expressed as moisture weight percentage or (b) the volume of water to the volume of solid particles expressed as moisture volume percentage in a given volume of porous medium (ASTM, 1980). See water content.

Moisture equivalent - The percentage of water retained in a soil sample 1 cm thick after it has been saturated and subjected to a centrifugal force 1000 times gravity for 30 minutes (SSSA, 1975). Centrifuge moisture equivalent is the water content of a soil after it has been saturated with water and then subjected for 1 hour to a force equal to 1000 times that of gravity (ASTM, 1980).

Moisture tension - The equivalent negative pressure of water in an unsaturated porous medium equal to the pressure that must be applied to the medium to bring the water to hydraulic equilibrium through a porous permeable material with a pool of water of the same composition (after SSSA, 1975).

Moisture volume percentage - The ratio of the volume of water in a soil to the total bulk volume of the soil (SSSA, 1975).

Moisture weight percentage - The moisture content expressed as a percentage of the oven-dry weight of a soil (SSSA, 1975).

Molecular diffusion (diffusion) - The process whereby solutes are transported at the microscopic level due to variations in the solute concentrations within the fluid phases.

Molecular diffusion, coefficient of, - The component of mass transport flux of solutes (at the microscopic level) due to variations in solute concentrations within the fluid phases (after Bear, 1979). Synonymous with diffusion coefficient.

(2) Source originating over broad areas, such as areas of fertilizer and pesticide application and leaking sewer systems, rather than from discrete points (after USGS, 1988).

Partitioning function - A mathematical relation describing the distribution of a reactive solute between solution and other phases.

Peclet number - A relationship between the advective and diffusive components of solute transport expressed as the ratio of the product of the average interstitial velocity, times the characteristic length, divided by the coefficient of molecular diffusion; small values indicate diffusion dominance, large values indicate advection dominance.

Percent saturation - The ratio, expressed as a percentage, of (a) the volume of water to (b) the total volume of intergranular space (voids) in a given porous medium (ASTM, 1980). Synonymous with degree of saturation.

(3) Water that can be extracted by root absorption and evaporation but cannot be moved by gravity or by the unbalanced film forces resulting from localized evaporation and transpiration (after APHA, 1981).

(2) The downward flow of water in saturated or nearly saturated porous medium at hydraulic gradients of the order of 1.0 or less (after SSSA, 1975).

Permeability - The property of a porous medium to transmit fluids under an hydraulic gradient.

Permeability coefficient - The rate of flow of water through a unit cross-sectional area under a unit hydraulic gradient at the prevailing temperature (field permeability coefficient) or adjusted to a temperature of 150C (60-F) (ASCE, 1985).

Permeability, effective - The observed permeability of a porous medium to one fluid phase under conditions of physical interaction between this phase and other fluid phases present (AGI, 1980).

(2) The property of a porous medium itself that expresses the ease with which gases, liquids, or other substances can pass through it (after SSSA, 1975).

Permeability, specific - The permeability measured when the rock contains only one fluid (Thrush, 1968).

Piezometer - A devise used to measure groundwater pressure head at a point in the subsurface.

(2) Any discernible, confined and discrete conveyance, including but not limited to any pipe, ditch, channel, tunnel, conduit, well, discrete fissure, container, rolling stock, concentrated animal feeding operation, or vessel or other floating craft, from which pollutants are or may be discharged. This term does not include agricultural stormwater discharges and return flows from irrigated agriculture [Public Law 95-217, SEC. 502 (14) as amended by Public Law 100-4, SEC. 5031.

(2) Shall include, but not be limited to, any element, substance, compound, or mixture, including disease-causing agents, which after release into the environment and upon exposure, ingestion, inhalation, or assimilation into any organism, either directly from the environment or indirectly by ingestion through food chains, will or may reasonably be anticipated to cause death, disease, behavioral abnormalities, cancer, genetic mutation, physiological malfunctions (including malfunctions in reproduction) or physical deformations, in such organisms or their offspring; except that the term "pollutant" or "contaminant" shall not include petroleum, including crude oil or any fraction thereof which is not otherwise specifically listed or designated as a hazardous substance under subparagraphs (A) through (F) of paragraph (14) (CERCLA) and shall not include natural gas, liquefied natural gas, or synthetic gas of pipeline quality (or mixtures of natural gas and such synthetic gas) [Comprehensive Environmental Response, Compensation, and Liability Act of 1980 (Superfund) (Public Law 96-510) as amended by Superfund Amendment and Reauthorization Act of 1986 (Public Law 99-499)].

(2) The addition to a stored body of water of any material which diminishes the optimal economic use of the water body by the population which it serves, and has an adverse effect on the surrounding environment (APHA, 1981).

Pore space - The total space not occupied by solid soil or rock particles (SSSA, 1975).

(2) The volume percentage of the total bulk not occupied by solid particles (SSSA, 1975).

(2) The ratio of the volume of the voids of a soil or rock mass that can be drained by gravity to the total volume of the mass (ASTM, 1980).

(3) The amount of interconnected pore space and fracture openings available for the transmission of fluids, expressed as the ratio of the volume of interconnected pores and openings to the volume of rock (10 CFR Part 960.2).

Potential - Any of several different scalar quantities, each of which involves energy as a function of position or of condition; e.g. the fluid potential of groundwater (AGI, 1980).

(2) Density that would be reached by a compressible fluid if it were adiabatically compressed or expanded to a standard pressure (McGraw Hill, 1974).

Potential drop - The difference in total head between two equipotential lines (ASTM, 1980).

Potentiometric surface - An imaginary surface representing the static head of groundwater and defined by the level to which water will rise in a tightly cased well (after Lohman and others, 1972).

Pressure head - Hydrostatic pressure expressed as the height of a column of water that the pressure can support at the point of measurement. See also head, static, and pressure, hydrostatic.

Pressure, hydrostatic - The pressure exerted by the weight of water at any given point in a body of water at rest (after AGI, 1972).

Radioisotope - An unstable isotope of an element that decays or disintegrates spontaneously, emitting radiation (NRC, 1981).

Radionuclide retardation - The process or processes that cause the time required for a given radionuclide to move between two locations to be greater than the groundwater travel time, because of physical and chemical interactions between the radionuclide and the geohydrologic unit through which the radionuclide travels (10 CFR Part 960.2).

Reaction path modeling - A simulation approach to studying the chemical evolution of a (natural) system.

Recharge - The process of addition of water to the saturated zone; also the water added (USGS, 1984).

Recharge area - An area in which water reaches the zone of saturation by surface infiltration (Heath, 1984).

Recharge capacity - The ability of the soils and underlying materials to allow precipitation and runoff to infiltrate and reach the zone of saturation (30 CFR Parts 701.5 and 710.5).

Retardation factor - The ratio of the average linear velocity of groundwater to the velocity of the retarded constituent at C/Co=0.5 (after Freeze and Cherry, 1979).

Saline water - Water that generally is considered unsuitable for human consumption or for irrigation because of its high content of dissolved solids. Commonly expressed as milligrams per liter (mg/L) of dissolved solids, with 35,000 mg/L defined as equivalent to sea water, slightly saline as 1,000 - 3,000 mg/L, moderately saline as 3,000 - 10,000 mg/L, very saline as 10,000 - 35,000 mg/L, and brine has more than 35,000 mg/L (after USGS, 1984).

Saltwater intrusion - The movement of salt water into fresh water aquifers (ASCE, 1985).

Saturated zone (1) Those parts of the earth's crust in which all voids are filled with water under pressure greater than atmospheric (Lohman and others, 1972).

(2) That part of the earth's crust beneath the regional water table in which all voids, large and small, are filled with water under pressure greater than atmospheric (after NRC, 1985).

(3) Means that part of the earth's crust beneath the regional water table in which all voids, large and small, are ideally filled with water under pressure greater than atmospheric (10 CFR Part 60.2). 22

(4) Means that part of the earth's crust beneath the water table in which all voids, large or small, are ideally filled with water under pressure greater than atmospheric (10 CFR Part 960.2).

Seep (1) An area, generally small, where water or oil percolates slowly to the land surface (see seepage and spring) (AGI, 1980).

Seepage face - A boundary between the saturated flow field and the atmosphere along which groundwater discharges, either by evaporation or movement "downhill" along the land surface or in a well as a thin film in response to the force of gravity (after Franke and others, 1985).

Seepage line (1) The uppermost level at which flowing water emerges along a seepage face (AGI, 1980).

(2) The upper free water surface of the zone of seepage (ASTM, 1980). Synonymous with line of seepage, phreatic line.

Site characterization (1) A general term applied to the investigation activities at a specific location that examines natural phenomena and human-induced conditions important to the resolution of environmental, safety and water-resource issues.

(2) Means the program of exploration and research, both in the laboratory and in the field, undertaken to establish the geologic conditions and the ranges of those parameters of a particular site relevant to the program. Site characterization includes borings, surface excavations, excavation of exploratory shafts, limited subsurface lateral excavations and borings, and in situ testing at depth needed to determine the suitability of the site for a geologic repository but does not include preliminary borings and geophysical testing needed to decide whether site characterization should be undertaken (10 CFR Part 960.2). 23

(3) Activities, whether in the laboratory or the field, undertaken to establish the geologic conditions and the ranges of the parameters of a candidate site relevant to the location of a repository, including borings, surface excavations, excavation of exploratory shafts, limited surface lateral excavations and borings, and in situ testing needed to evaluate the suitability of a candidate site for the location of a repository, but not including preliminary borings and geophysical testing needed to assess whether site characterization should be undertaken (10 CFR Part 960.2).

Soil moisture - Subsurface liquid water in the unsaturated zone expressed as a fraction of the total porous medium volume occupied by water. It is less than or equal to the porosity, n (NRC, 1985).

Soil-water pressure - The pressure (positive or negative), in relation to the external gas pressure on the soil water, to which a solution identical in composition with the soil water must be subjected in order to be in equilibrium through a porous permeable wall with the soil water (SSSA, 1975).

Solubility - The total amount of solute species that will remain indefinitely in a solution maintained at constant temperature and pressure in contact with the solid crystals from which the solutes were derived.

Solute - The substance present in a solution in the smaller amount. For convenience, water is generally considered the solvent even in concentrated solutions with water molecules in the minority.

Solute transport - The net flux of solute through a hydrogeologic unit controlled by the flow of subsurface water and transport mechanisms.

Solution - A homogeneous mixture of two or more components. In ideal solutions, the movement of molecules in charged species are independent of each other. In aqueous solutions charged species interact even at very low concentrations, decreasing the activity of the solutes.

Sorption (1) A general term used to encompass the process of absorption and adsorption.

(2) All processes which remove solutes from the fluid phase and concentrate them on the solid phase of the medium (after ANS, 1980).

Source term - The kinds and amounts of radionuclides that make up the source of a potential release of radioactivity (10 CFR Part 960.2).

Specific capacity - The rate of discharge of water from the well divided by the drawdown of the water level within the well (Lohman and others, 1972).

Specific conductance - A measure of the ability of water to conduct an electrical current expressed in micromhos per centimeter at 25'C (ASCE, 1985).

Specific discharge - The rate of discharge of groundwater per unit area of a porous medium measured at right angle to the direction of flow (Lohman and others, 1972). Synonymous with flow velocity or specific flux.

Specific retention - The ratio of the Volume of water which the porous medium, after being saturated, will retain against the pull of gravity to the volume of the porous medium (Lohman and others, 1972).

Specific storage - The volume of water released from or taken into storage per unit volume of the porous medium per unit change in head (Lohman and others, 1972).

Specific yield - The ratio of the volume of water which the porous medium after being saturated, will yield by gravity to the volume of the porous medium (Lohman and others, 1972).

Spring - A discrete place where groundwater flows naturally from a rock or the soil onto the land surface or into a body of surface water (ASCE, 1985). See also seep.

Storage coefficient - The volume of water an aquifer releases from or takes into storage per unit surface area of the aquifer per unit change in head (virtually equal to the specific yield in an unconfined aquifer) (Lohman and others, 1972).

Subirrigation (1) Irrigation of plants with water delivered to the roots from underneath (30 CFR Part 710.b).

(2) With respect to alluvial valley floors, the supplying of water to plants from underneath or from a semisaturated or saturated subsurface zone where water is available for use by vegetation (30 CFR Part 701.5).

Tensiometer - A device used to measure the moisture tension in the unsaturated zone.

Total dissolved solids (1) The total concentration of dissolved constituents in solution, usually expressed in milligrams per liter.

(2) The total concentration of dissolved material in water [as] ordinarily determined from the weight of the dry residue remaining after evaporation of the volatile portion of an aliquot of the water sample (Hem, 1985).

(3) The total dissolved (filterable) solids as determined by use of the method specified in 40 CFR Part 136 (40 CFR 144.3; 40 CFR 146.3).

Total soil-water potential - The sum of the energy-related components of a soil-water system; i.e., the sum of the gravitational, matric, and osmotic components (Wilson, 1980).

Transient (1) A pulse dampened oscillation or other temporary phenomena occurring in a system prior to reaching a steady-state condition (McGraw-Hill, 1974).

Transmissibility coefficient - (The use of the term transmissibility has been replaced by transmissivity).

Transmissivity - The rate at which water of the prevailing kinematic viscosity is transmitted through a unit width of the aquifer under a unit hydraulic gradient. It is equal to an integration of the hydraulic conductivities across the saturated part of the aquifer perpendicular to the flow paths (Lohman and others, 1972).

Transport - Conveyance of solutes and particulates in flow systems. See also solute transport and particulate transport.

Turbulent flow - The flow condition in which inertial forces predominate over viscous forces and in which head loss is not linearly related to velocity.

Unconfined - A condition in which the upper surface of the zone of saturation forms a water table under atmospheric pressure (after ASCE, 1985).

Underground injection - A "well injection" (40 CFR 144.3; 40 CFR Part 146.3).

Unsaturated flow - The movement of water in a porous medium in which the pore spaces are not filled to capacity with water (after SSSA, 1975).

Unsaturated zone (1) The zone between the land surface and the water table (ASCE, 1985).

(2) The zone between the land surface and the deepest water table which includes the capillary fringe. Water in this zone is generally under less than atmospheric pressure, and some of the voids may contain air or other gases at atmospheric pressure. Beneath flooded areas or in perched water bodies the water pressure locally may be greater than atmospheric (Lohman and others, 1972).

(3) The zone between the land surface and the regional water table. Generally, water in this zone is under less than atmospheric pressure, and some of the voids may contain air or other gases at atmospheric pressure. Beneath flooded areas or in perched water bodies the water pressure locally may be greater than atmospheric (NRC, 1985).

(4) Means the zone between the land surface and the regional water table. Generally, fluid pressure in this zone is less than atmospheric pressure, and some of the voids may contain air or other gases at atmospheric pressure. Beneath flooded areas or in perched water bodies the fluid pressure locally may be greater than atmospheric (10 CFR Part 60.2).

(5) The zone between the land surface and the water table. Generally, water in this zone is under less than atmospheric pressure, and some of the voids may contain air and other gases at atmospheric pressure. Beneath flooded areas or in perched water bodies, the water pressure locally may be greater than atmospheric (10 CFR Part 960.2).

Upconing - Process by which saline water underlying freshwater in an aquifer rises upward into the freshwater zone as a result of pumping water from the freshwater zone (USGS, 1984).

Velocity, average interstitial - The average rate of groundwater flow in interstices expressed as the product of hydraulic conductivity and hydraulic gradient divided by the effective porosity, (after Lohman and others, 1972). Synonymous with average linear groundwater velocity or effective velocity.

Void ratio - The ratio of (a) the volume of void space to (b) the volume f solid particles in a given soil mass (ASTM, 1980).

Volatiles - Substances with relatively large vapor pressures. Many organic substances are almost insoluble in water so that they occur primarily in a gas phase in contact with water, even though their vapor pressure may be very small.

Water content - The amount of water lost from the soil after drying it to constant weight at 1050C, expressed either as the weight of water per unit weight of dry soil or as the volume of water per unit bulk volume of soil (ASTM, 1980). See also moisture content.

Water table (1) The upper surface of a zone of saturation except where that surface is formed by a confining unit (after Lohman, 1972).

(2) The upper surface of the zone of saturation on which the water pressure in the porous medium equals atmospheric pressure.

(3) Means that surface in a groundwater body at which the water pressure is atmospheric (10 CFR Part 60.2).

(4) That surface in a body of groundwater at which the water pressure is atmospheric (10 CFR Part 960.2).

(5) Upper surface of a zone of saturation, where the body of ground water is not confined by an overlying impermeable zone (30 CFR Part 701.5 and 710.5).

Well - A bored, drilled or driven shaft, or a dug hole, whose depth is greater than the largest surface dimension (40 CFR 144.3 and 40 CFR 146.3).

Well injection - The subsurface emplacement of "fluids" through a bored, drilled, or driven "well", or through a dug well, where the depth of the dug well is greater than the largest surface dimension (40 CFR 144.3 and 40 CFR 146.3). 28

(AGI, 1980) American Geological Institute, 1980, Glossary of geology:, Falls Church, Virginia, American Geological Institute.

(ANS, 1980) American Nuclear Society, 1980, American national standard for evaluation of radionuclide transport in groundwater for nuclear power sites: La Grange Park, Illinois, ANSI/ANS-2.17-1980, American Nuclear Society.

(APHA, 1981) American Public Health Association, 1981, Glossary, 3d. edition: Washington, D.C., APHA, ASCE, AWWA, WPCF.

(ASCE, 1985) American Society of Civil Engineers, 1985, Manual 40 - Ground water management.

(ASTM, 1980) American Society for Testing and Materials, 1980, Standard definitions of terms and symbols relating to soil and rock mechanics, in (D653-80) 1981 Annual Book of ASTM Standards, Part 19: Philadelphia, Pennsylvania, American Society for Testing Materials, p. 1402-1419.

(Back, 1966) Back, W., 1966, Hydrochemical facies and ground-water flow patterns in northern part of Atlantic Coastal Plain: U.S. Geological Survey Professional Paper 498-A.

(Bear, 1979) Bear, Jacob, 1979 Hydraulics of groundwater: New York, McGraw-Hill.

(CFR, 1988) Code of Federal Regulations, 1988, Title 10—Energy: Nuclear Regulatory Commission (Parts 0-199), Department of Energy (Parts 700-999); Title 30—Mineral Resources-: Office of Surface Mining, Reclamation and Enforcement, Department of the Interior (Parts 700-999); Title 40—Protection of Environment: Environmental Protection Agency (Parts 1-799): Washington, D.C., U.S. Government Printing Office.

(Darcy, 1856) Darcy, H., 1856, Les Fontaines Publiques de la Ville de Dijon: Paris, Victor Dalmont, 647p.

(Franke and others, 1985) Franke O.H., Reilly, T.E. and Bennett, G.D., 1985, Definition of boundary and initial conditions in the analysis of saturated ground-water flow systems—An introduction: U.S. Geological Survey Open-File Report 84-458.

(Freeze and Cherry, 1979) Freeze, R.D., and Cherry, J.A., 1979, Groundwater: Englewood Cliffs, New Jersey, Prentice-Hall, Inc..

(Hantush, 1964) Hantush, Mahdi S., 1964, Hydraulics of wells; Advances in hydroscience, Vol. 1, V.T. Chow, ed.: New York, Academic Press.

(Heath, 1984) Heath, R.C., 1984, Ground-water regions of the United States: U.S. Geological Survey Water-Supply Paper 2242.

(Hem, 1985) Hem, J.D., 1985, Study and interpretation of the chemical characteristics of natural water, 3rd edition: U.S. Geological Survey Water-Supply Paper 2254.

(Johnson and others, 1961) Johnson, A.I., Morris, D.A., and Prill, R.C., 1961, Specific yield and related properties—An annotated bibliography: U.S. Geological Survey Open-File Report.

(Johnson, 1963) Johnson, A.I., 1963, Application of laboratory permeability data: U.S. Geological Survey Open-File Report.

(Johnson, 1981) Johnson, A.I., 1981, "Glossary", permeability and groundwater contaminant transport, in Zimmie, T.F., and Riggs, C.O., eds.,ASTM STP 746,: American Society for Testing and Materials, p. 3-17.

(Lohman, 1972) Lohman, S.W., 1972, Ground-water hydraulics: U.S. Geological Survey Professional Paper 708.

(Lohman and Lohman, S.W., Bennett, R.R., Brown, R.H., Cooper, H.H., others, 1972) Drescher, W.J.,Jr., Ferris, J.G., Johnson, A.I., McGuinness, C.L., Piper, A.M., Rorabaugh, M.I., Stallman, R.W., and Theis, C.V., 1972, Definitions of selected ground-water terms—Revisions and conceptual refinements: U.S. Geological Survey Water-Supply Paper 1988.

(McGraw-Hill, 1974) Lapedes, D.N., Editor-in-Chief, 1974, Dictionary of scientific and technical terms: McGraw-Hill, New York.

(NRC, 1981) U.S. Nuclear Regulatory Commission, 1981, Glossary of terms, Nuclear power and radiation, NUREG-0770: U.S. Nuclear Regulatory Commission, Washington, D.C.

(NRC, 1982) U S. Nuclear Regulatory Commission, 1982, Regulatory Guide 4.17, Standard format and content of site characterization reports for high-level-waste geologic repositories: Washington, D.C., U.S. Nuclear Regulatory Commission,

(NRC, 1985) Hackbarth, C.J., Nicholson, T.J., and Evans, D.D., 1985, Disposal of high-level radioactive wastes in the unsaturated zone—Technical considerations, NUREG-1046: Washington, D.C., U.S. Nuclear Regulatory Commission,

(SSSA, 1975) Soil Science Society of America, 1975, Glossary of soil science terms: Madison, Wisconsin, Soil Science Society of America.

(Stumm and Morgan, 1981) Stumm, Werner, and Morgan, J.J., 1981, Aquatic Chemistry, 2nd edition: New York, John Wiley and Sons.

(Thrush, 1968) Thrush, Paul W., ed., 1968, A dictionary of mining, mineral, and related terms: Washington, D.C., U.S. Department of the Interior, Bureau of Mines, p. 1269.

(USGS, 1984) U.S. Geological Survey, 1985, National water summary 1984—Hydrologic events, selected water-quality trends, and ground-water resources: U.S. Geological Survey Water-Supply Paper 2275, 467p.

(USGS, 1988) U.S. Geological Survey, 1988, National water summary 1986—Hydrologic events and ground-water quality: U.S. Geological Survey Water-Supply Paper 2325, 560p.

(Wilson, 1980) Wilson, L. G., 1980, Monitoring in the vadose zone—A review of technical elements and methods, for the U.S. Environmental Protection Agency: LPA-600/7-80-134, Environmental Monitoring Systems Laboratory, Nevada.

(WMO, 1974) World Meteorological Organization, 1974, International glossary of hydrology: Geneva, Switzerland, World Meteorological Organization No. 385.

(WRC, 1980) U.S. Water Resources Council, 1980, Essentials of groundwater hydrology pertinent to water-resources planning: Washington, D.C., Bulletin 16 (revised).

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GLOSSARY OF HYDROLOGIC TERMS