AIH Professional Hydrologist - Groundwater Examination key terms

A saturated geologic unit that can store and transmit water in quantities sufficient to supply wells or springs. It must have both adequate porosity to hold water and adequate permeability to release it.

A low-permeability layer that retards rather than prevents groundwater flow. It can store water but transmits it too slowly to support production wells, and it controls vertical leakage between adjacent aquifers.

A geologic stratum bounding an aquifer that restricts water movement into or out of it. A confining layer may be an aquitard or an aquiclude, and its presence determines whether an aquifer is confined or unconfined.

An aquifer whose upper boundary is the water table, which rises and falls freely with recharge and discharge. Water is released from storage mainly by gravity drainage of pores, giving it a high specific yield.

An aquifer overlain by a confining layer so that its water is under pressure greater than atmospheric. Pumping lowers the potentiometric surface rather than dewatering the aquifer, and storage is released by compression of water and matrix.

A localized saturated body separated from the main water table by an unsaturated zone above a low-permeability layer. It is typically small, thin, and vulnerable to seasonal drying or contamination.

The upper surface of the saturated zone in an unconfined aquifer, where pore water pressure equals atmospheric pressure. Its shape generally mimics topography in subdued form and defines the top of the phreatic zone.

An imaginary surface representing the level to which water in a confined aquifer would rise in tightly cased wells. It is mapped from water-level measurements and indicates the direction and gradient of groundwater flow.

The total mechanical energy per unit weight of groundwater at a point, expressed as elevation above a datum. Differences in head between two points drive flow, and head is the fundamental variable in groundwater modeling.

The change in hydraulic head per unit distance along a flow path, a dimensionless slope. It is the driving force in Darcy's law and is usually estimated from water-level contours or three-point problems.

The empirical relation stating that specific discharge equals the product of hydraulic conductivity and hydraulic gradient, with flow directed from high to low head. It governs laminar groundwater flow and underpins most aquifer analysis.

A measure of a porous medium's ability to transmit water, combining properties of the fluid and the medium. It has units of velocity and varies over many orders of magnitude among geologic materials.

A property of the porous medium alone, independent of the fluid, describing how readily it transmits any fluid under a pressure gradient. It is related to hydraulic conductivity through fluid density, viscosity, and gravity.

The rate at which water passes through a vertical strip of aquifer of unit width and full saturated thickness under a unit hydraulic gradient. It equals hydraulic conductivity times saturated thickness and controls well yield.

The volume of water an aquifer releases from or takes into storage per unit surface area per unit change in head. In confined aquifers it is small and dominated by compressibility; in unconfined aquifers it approximates specific yield.

The volume of water that drains by gravity from a unit volume of saturated aquifer when the water table drops one unit. It equals porosity minus specific retention and typically ranges from about 0.05 to 0.30.

The fraction of a saturated aquifer's volume that retains water by capillary and surface forces against gravity drainage. It is the complement of specific yield and is larger in fine-grained materials.

The ratio of void volume to total bulk volume of a geologic material, expressed as a fraction or percentage. It indicates storage capacity but not ability to transmit water, which depends on pore connectivity.

The fraction of bulk volume consisting of interconnected pores through which fluid can actually flow. It is always less than total porosity and is used to compute average linear groundwater velocity from specific discharge.

The volume of groundwater flowing per unit time through a unit cross-sectional area of aquifer, including both voids and solids. Also called Darcy velocity, it is a bulk flux and is smaller than actual pore-water velocity.

The decline in water level in a well or aquifer caused by pumping, measured as the difference between static and pumping water levels. Drawdown grows with time and pumping rate and forms the cone of depression.

The roughly conical depression of the water table or potentiometric surface surrounding a pumping well. Its shape reflects aquifer properties and boundaries and can capture nearby contamination or surface water.

The horizontal distance from a pumping well to the outer limit where drawdown becomes negligible. It is an approximate concept used in simple well-hydraulics calculations and grows with pumping rate and duration.

The yield of a well per unit of drawdown, usually expressed as discharge divided by drawdown. It is a practical index of well performance and can be used to estimate transmissivity from pumping-test data.

The transient analytical solution for drawdown around a pumping well in a confined aquifer, based on an analogy to heat flow. It requires transmissivity, storativity, pumping rate, and elapsed time, and uses the exponential integral well function.

A straight-line approximation of the Theis solution valid at large times or small distances from a pumping well. Plotting drawdown against log time yields slope-based estimates of transmissivity and storativity from aquifer test data.

A controlled field experiment in which a well is pumped or its water level is changed while responses are monitored in observation wells. Analysis of the response yields transmissivity, storativity, and boundary information.

A rapid single-well test in which a known volume of water is suddenly added or removed and the water-level recovery is recorded. It estimates hydraulic conductivity near the well but gives little information about storativity.

A well in a confined aquifer in which the water level rises above the top of the aquifer, and may flow at the surface if the potentiometric surface exceeds land elevation. The term refers to the pressure condition, not to pumping.

The process by which water moves from the surface, typically by infiltration, into the saturated zone and becomes groundwater storage. It replenishes aquifers and is balanced over time by discharge.

A location where groundwater leaves an aquifer and reaches the surface as springs, seeps, baseflow to streams, or evapotranspiration. Discharge areas are where the water table intersects the land surface or a surface-water body.

A stream that receives water from the surrounding aquifer, so that flow increases in the downstream direction. It is typical where the water table lies above the streambed and contributes to baseflow.

A stream that loses water to the underlying aquifer, so that flow decreases downstream. It occurs where the streambed is above the water table or where pumping has lowered the water table beneath the channel.

The sustained portion of streamflow derived from groundwater discharge and other delayed sources rather than direct runoff. It maintains flow during dry periods and is commonly separated from the storm hydrograph.

A graph showing streamflow, stage, or water level plotted against time at a given location. Its shape reflects rainfall input, watershed characteristics, and storage, and is used to separate baseflow from direct runoff.

The direct-runoff hydrograph resulting from one unit of effective rainfall uniformly distributed over a basin during a specified duration. It is used to predict runoff from storms of varying magnitude by scaling and superposition.

The time required for water to travel from the most hydraulically remote point of a watershed to the outlet. It governs the timing of peak discharge and is a key input to the rational method and unit hydrograph theory.

A simple peak-discharge formula in which peak flow equals a runoff coefficient times rainfall intensity times drainage area. It applies mainly to small urban catchments where storage effects are minor.

The downward entry of water through the soil surface into the soil profile. Its rate declines toward a steady value as soils wet and is limited by surface conditions, soil texture, and antecedent moisture.

The unsaturated zone between the land surface and the water table where pores contain both air and water. Water here is held by capillary and adsorptive forces and moves downward under gravity toward the water table.

The zone immediately above the water table where pores are saturated by capillary rise but pressure is less than atmospheric. Its thickness varies inversely with pore size and affects moisture available to shallow-rooted plants.

The combined loss of water to the atmosphere through evaporation from soil and water surfaces and transpiration from plants. It is a major component of the water balance and can deplete shallow groundwater.

An accounting of water inputs, outputs, and storage change for a defined control volume over a period. In hydrology it is typically expressed as precipitation equals evapotranspiration plus runoff plus change in storage.

The fraction of rainfall that becomes surface runoff rather than infiltrating or being stored, used in the rational method. It depends on land use, soil type, slope, and antecedent conditions.

A graphical or analytical procedure for dividing a streamflow hydrograph into baseflow and direct runoff components. Common techniques include fixed-base, variable-slope, and digital filter methods.

The transport of dissolved or suspended substances by the bulk motion of flowing groundwater. It moves contaminants at the average linear velocity and is the dominant transport mechanism at many sites.

The spreading and mixing of a solute in groundwater caused by velocity variations and molecular diffusion. Mechanical dispersion acts along and transverse to flow and produces a plume that widens with distance.

The slowing of solute movement relative to groundwater flow because of sorption onto aquifer solids. It is quantified by a retardation factor that increases with sorption and bulk density and decreases with porosity.

The three-dimensional body of dissolved contamination that migrates from a source through an aquifer. Its shape and extent depend on advection, dispersion, retardation, and degradation processes.

The portion of an aquifer that contributes water to a pumping well under given flow conditions. It is delineated by flow paths ending at the well and is central to wellhead protection and remediation design.

A line on the water table or potentiometric surface from which groundwater flows in opposite directions. It separates adjacent groundwater basins and may shift in response to pumping or recharge changes.

The volume of water released from or added to storage per unit surface area of an aquifer per unit change in head. It is termed storativity for confined aquifers and specific yield for unconfined aquifers.