Groundwater Hydrology – Key Vocabulary

Thirty key vocabulary terms covering fundamental concepts, zones, properties, and parameters involved in groundwater hydrology.

Hydrology

The scientific study of the occurrence, distribution, movement, and properties of the waters of the Earth.

Groundwater Hydrology

Branch of hydrology that focuses on the characteristics, occurrence, and movement of water beneath Earth’s surface.

Groundwater

Water that resides in the pores and fractures of soil and rock in the saturated zone below the land surface.

Zone of Saturation

Subsurface region where all pore spaces are completely filled with water; also called the groundwater zone.

Zone of Aeration (Vadose Zone)

Layer between the land surface and the water table where pore spaces contain both air and water.

Water Table

The upper surface of the saturated zone where the pressure is equal to atmospheric pressure.

Capillary Fringe

Narrow zone just above the water table where water rises in pores by capillary action.

Porous Media

Earth materials (soils or rocks) containing interconnected pore spaces that can store and transmit water.

Porosity

Percentage of the total volume of a soil or rock that is occupied by voids or pores.

Primary Porosity

Original pore spaces between grains that formed with the sediment or rock.

Secondary Porosity

Pore space that develops after rock formation, such as fractures, solution cavities, or joints.

Aquifer

A saturated, permeable geologic unit capable of storing and transmitting significant quantities of groundwater to wells and springs.

Unconfined Aquifer

Aquifer whose upper boundary is the water table and is directly recharged by infiltration from the land surface.

Confined Aquifer

Also called artesian or pressure aquifer; bounded above (and often below) by relatively impermeable layers and under pressure greater than atmospheric.

Perched Aquifer

Localized saturated zone that sits above the regional water table on a layer of low-permeability material.

Aquitard

Partly permeable geologic formation that transmits water very slowly; yields to wells are negligible.

Aquiclude

Essentially impermeable geologic unit that can store water but does not transmit it appreciably.

Aquifuge

Geologic formation that is neither porous nor permeable; cannot store or transmit water (e.g., massive unfractured granite).

Piezometric Surface

Imaginary surface that represents the hydraulic head in an aquifer, analogous to contour lines showing water-level elevations in wells.

Specific Yield (Sy)

Fraction of the total volume of a saturated aquifer that can drain by gravity and be extracted as groundwater.

Specific Retention (Sr)

Ratio of the volume of water that cannot be drained (held by molecular and capillary forces) to the total saturated volume of the aquifer.

Hydraulic Conductivity (K)

Measure of the ease with which water can move through pore spaces or fractures; also called the coefficient of permeability.

Transmissivity (T)

Rate at which groundwater is transmitted through a unit width of the entire saturated thickness of an aquifer under a unit hydraulic gradient.

Specific Storage (Ss)

Amount of water per unit volume of a saturated formation that is stored or expelled due to compressibility per unit change in hydraulic head.

Storage Coefficient (S)

Volume of water released from or taken into storage per unit surface area of an aquifer per unit change in head; dimensionless.

Hydraulic Gradient (i)

Slope of the water table or potentiometric surface, calculated as change in head (Δh) divided by distance (ΔL).

Hydraulic Head

Total energy per unit weight of groundwater, represented by the elevation of water in a piezometer or well.

Artesian Well

Well that flows without pumping because groundwater is under sufficient pressure in a confined aquifer to rise above the land surface.

Recharge Area

Zone where water infiltrates from the surface to replenish an aquifer.

Discharge Area

Location where groundwater emerges at the surface or into surface water bodies, such as springs, rivers, or wetlands.